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TRAINER GUIDE - II
JIGS, FIXTURES & GAUGES
(1ST
SEMESTER)
PGTD / PDTD
VERSION - 0
MSME TOOL ROOM
INDO GERMAN TOOL ROOM
AHMEDABAD
TRAINER GUIDE - II
FOR
ADVANCE DIPLOMA IN
TOOL & DIE MAKING
Subject Area:
Tool Design Theory –
“Jigs, Fixtures & Gauges”
( Part – I & II)
CONTENTS
Chapter
No.
DESCRIPTION
Page
No.
A1. Introduction To Production Toolings
A1.1 Introduction Of Tools Used In Mass Production…………... 1
A2. Introduction To Jigs & Fixtures………………………………… 4
A3 Elements Of Jigs & Fixtures
A3.1 Locators, Locating Methods & Devices……………………. 7
A3.2 Clamps, Clamping Methods & Devices……………………. 35
A3.3 Guiding Elements (Jig Bushings)…………………………... 60
A3.4 Tool Bodies (Jig & Fixture)………………………………….. 81
A3.5 Fasteners (Jig & Fixture)……………………………………. 84
A4 Limit, Fit & Tolerance
A4.1 Introduction…………………………………………………… 96
A4.2 Advantages Of Limits & Fits………………………………… 97
A4.3 Tolerances …………………………………………………… 98
A4.4 Limits………………………………………………………….. 100
A4.5 Fits…………………………………………………………….. 101
A4.6 Types Of Assembly………………………………………….. 108
A4.7 Allowances …………………………………………………… 111
A4.8 Deviation ……………………………………………………... 112
A4.9 Maximum & Minimum Material Condition…………………. 115
CONTENTS
Chapter
No.
DESCRIPTION
Page
No.
A5 Design
A5.1 Design Of Jigs & Fixtures…………………………………… 118
A6 Jigs
A6.1 Introduction…………………………………………………… 137
A6.2 Function Of Jigs & Fixtures…………………………………. 138
A6.3 Factor Characteristics In Jig Design……………………….. 138
A6.4 Jig Support……………………………………………………. 140
A6.5 Jig Bodies And Rigidity……………………………………… 140
A6.6 Classification Of Jigs………………………………………… 140
A6.7 Types Of Jigs & Their Description…………………………. 151
A6.8 Maintenance, Storage & Safety Of Jigs…………………… 152
A7 Fixtures
A7.1 Introduction…………………………………………………… 155
A7.2 Basic Design Consideration………………………………… 155
A7.3 Factors In Fixture Design…………………………………… 156
A7.4 Classification Of Fixture…………………………………….. 158
A7.5 Maintenance, Safety & Storage Of Fixtures………………. 181
A8 Estimation
A8.1 Introduction…………………………………………………… 183
A8.2 Purpose Of Cost Estimating………………………………… 183
A8.3 Elements Of Cost…………………………………………….. 184
A8.4 Cost Structure………………………………………………… 187
A8.5 Estimation Of Cost Elements……………………………….. 188
A8.6 Estimating Tool Cost………………………………………… 192
Chapter
No.
DESCRIPTION
Page
No.
A8.7 Steps In Making A Cost Estimation………………………… 194
A8.8 Chief Factures In Cost Estimation…………………………. 194
A8.9 Numerical Examples………………………………………… 195
A9 Gauges
A9.1 Introduction…………………………………………………… 199
A9.2 Classification Of Gauges……………………………………. 201
A9.3 Design Of Gauges…………………………………………… 219
A9.4 Sub Zero Treatment…………………………………………. 229
A9.5 Maintenance, Safety & Storage Of Gauges………………. 229
A9.6 Numerical Examples………………………………………… 230
INTRODUCTION
A1.1 Introduction of Tools used in Mass Production
Production of quality goods in large quantities at high speeds is the requirement of
the day. To meet this, there have been considerable changes and developments in the
manufacturing industries, with an empha sis on increased efficiency and productivity. As a
sequel to these changes the tool technology has also undergone changes, leading to the
designing and development of special tools, methods and techniques for the benefit of
industry, to ensure quality products at economical rates.
Jigs and fixtures are the special production tools which make the standard machine
tool, more versatile to work as specialised machine tools. They are normally used in large
scale production by semi -skilled operators, however t hey are also used in small scale
production, when interchangeability is important. Manufacturing industries in India, on par
with their counterpart elsewhere, have brought lot of revolution in manufacturing technology,
during the (last 20 years, as a cons equence of which several developments like CNC
Lathes, CNC Machine Centers, Flexible Manufacturing Systems, Fabrications Centre,
Transfer Machines, Robotics, etc. took place). Our Engineers and Technologists are deeply
involved in devising innovative 7 techniques. Lot of modernisation has taken place in Indian
Industry. Even with these advancements in the manufacturing indsutries, there is a
continued use of jigs and fixtures in some form or the other either independently or in
combination with other systems.
CHAPTER OUTLINE
A1.1 – Introduction of tools used in
Mass Production
TOPIC OUTLINE
A1.1a Jigs
A1.1b Fixtures
A1.1c Gauges
A1.1d Press Tools
A1.1e Moulds
The work tooling refers to the hardware necessary to produce a particular product. The most
common classification of types of tooling is as follows :
1. Sheet metal press working tools.
2. Moulds and dies for plastic moulding and die casting.
3. Forging dies for hot and cold forging.
4. Jigs and fixtures for guiding the tool and holding the workpiece.
5. Gauges and measuring instruments.
6. Cutting tools such as drills, reamers, milling cutters, broaches, taps etc.
The tool maker manufactu res the above item from the design supplied to him. On gaining
experience the tool maker will be able to design and manufacture simple tools.
A1.1a Jigs
A jig is a device that locates and holds the workpiece. It also guides and controls
one or more c utting tools. Jigs are fitted with hardened steel bushings for guiding drills or
other tools. Small jigs are not usually clamped to the machine. For holes above 6mm jigs
are usually clamped. Drill jigs are used while drilling reaming counter boring, ta pping,
chamfering etc.
There is hardly a product produced that does not contain one or more holes. The
location finish and size of these holes may be critical as in the case of a component for a
missile or they may be holes like those punched in a templat e for the purpose of hanging it
on the wall when not in use.
Holes are produced and finished in a number of ways. They are drilled, reamed,
bored, punched, ground, flame cut etc. Drilling is by far the most common method.
A1.1b Fixtures
A fixture is a device that locates and holds the workpiece. Setting blocks and feeler
gauges are used for setting the cutter in relation to the workpiece. Fixtures designated for
machining operations always clamped on to the machine.
A fixtures is a device for hold ing a workpiece during machining operations. The
name is derived from the fact that a fixture is always fastened to a machine or bench in a
fixed position.
Many machining operations can be performed by clamping the workpiece to the
machine table without using a fixture, especially when a few parts are to be machined.
However when the number of parts is large enough to justify its cost, a fixture is used for
holding and locating the work. Further, when the profile of the Component is not regular or
when machining has to be done w.r.t. a reference face or bore, application of fixture will be
necessary.
A1.1c Gauges
Modern manufacturing requires extensive uses of gauges for shop work, inspection
and reference.
A gauge can be defined as a device for investigating the dimensional fitness of a part
for a specified function.
A1.1d Press Tools
Press tools are special tools custom built to produce a particular component mainly
out of sheet metal.
The principle op erations of sheet stampings include cutting operations (Shearing,
blanking, piercing etc.) and forming operations (bending, drawing etc.).
Sheet metal items such as automobile parts (roofs fenders, caps etc.) components of
aircraft, parts of business mach ines, household appliances, sheet metal parts of electronic
equipments, precision parts required for horological industry etc. are manufactured by press
tools.
A1.1e Moulds for Plastics
Plastics did not enter our lives with the fanfare of other revolu tionary inventions, but
more by the process of infiltration. Plastics being synthetic materials were at first considered
to be cheap substitute for the better known and more expensive materials. Plastic articles
are not only replacing wood, metal and oth er materials but because of their particulars
qualities they function better than other materials for specific purposes. Through the years
plastics have carved the right as materials themselves and not as substitute for other
materials. Not only are plastics more useful, adaptable and practical than the materials they
have supplemented, but uses for plastics have been found for which no other material can
be used.
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Maximum productivity at minimum cost is the demand of modern industry. To meet this
requirements designing of efficient and accurate jigs and fixtures is required. Quality,
simplicity and economy from the important criteria from the design of jigs and fixtures.
To meet this requirement the designer will have to made an economic analysis for using
jigs and fixtures and has to device certain principles of design, and finally develop a checklist
for the jigs and fixture design.
A5.1a Tool Design Objectives
The main objective of tool design is to lower manufacturing costs while maintaining
quality and increased production. To accomplish this, the tool designer must satisfy the
following objectives:
Design Economics
Maximum productivity at minimal cost is t he demand of the day. Tool designer has
therefore an additional consideration of keeping the cost of these special tools as low as
possible apart from developing designs for efficient and accurate jigs and fixtures. For this
he has to apply the design economy. i.e. to reduce the cost without sacrificing the quality.
The following are some of the considerations involved in the economy design.
1. Simplicity
2. Preformed components
3. Standard components
4. Secondary operations
5. Tolerance and allowances
6. Simplified drawings
TOPIC OUTLINE
A5.1a Tool Design Objectives
A5.1b Design Principles
A5.1c Major factors in design of jigs & fixtures
A5.1d Elements of design (jigs & fixtures)
A5.1e Flow chart for development of design solution
A5.1f Check list for the design of jigs & fixtures
CHAPTER OUTLINE
A5.1 Design of Jigs & Fixtures
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1. Simplicity
Simplicity is essential in the tool design. Every element in the design of jigs and
fixtures should be considered for possible savings in time and materials.
2. Preformed Materials
These materials greatly reduce tooling costs by the elimination of many machining
operations. Wherever practicable, preformed materials, such as drill rods, structural
sections, pre machined bracket materials etc. should be included in the design.
3. Standard Components
Commercially available standard components such as clamps, locators, supports,
drill bushings, pins, screws, bolts, nuts etc. would contribute greatly in improving the tool
quality besides effecting considerable savings in labour cost and time.
5. Secondary Operations
Secondary operations such as grinding, heat treating and some machining should be
as far as possible be eliminated as they involve additional time and cost. If they cannot be
totally eliminated they should be limited to areas necessary for efficient tool operations.
5. Tolerances and Allowances
Generally the tolerances of a jig or a fixture should be between 20 percent and 50
percent of the part tolerance, as unnecessarily close tolerances will be add up to the higher
cost of the tool.
6. Simplified Drawings
Tool drawings will for a sizable part of the total tooling cost, hence it is necessary to
keep them low. This is accomplished by simplifying the drawings as follows :
a) Wherever practicable words should replace drawn details.
b) Elimination of redundant views, projections or details.
c) When possible, replace drawn details with symbols.
d) Reduce the drawing time by using templates and guides.
e) Standard parts should only be drawn for clarity, not detail refer to these by part numbers
or named.
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A5.1b Design Principles
After the economic and design analysis the tool designer must comply with the
following in designing the jigs and fixtures.
1. He has to thoroughly understand the component details, its pre -machined conditions,
reference surface dimensions, accuracies and tolerances to be achieved.
2. He has to know on which machine the operations is likely to be performed. A check has
to be made for constraints on the design parameters.
3. He has to make provision for easy loading and unloading of the work piece.
4. Facility for quick and accurate positioning of work piece be provided.
5. Fool proof method has to be incorporated to avoid wrong position while loading the work
piece.
6. Designer has to take into account the optimum clearance with swarf removal and
cleaning facility.
7. Machined surface are be taken as locating surface preferably.
8. Sharp corners in the locating surface must be avoided.
9. Adjustable locations are to be provided for right surfaces.
10. Locating surfaces should be as small as possible.
11. Locating pins should be tapered and easily accessible and visible to the operator.
12. Designer must have the economic approval to the design considerations.
13. As many degrees of freedom of movement should be arrested as necessary to achieve
the required accuracies. In general 3-2-1 principle to be adopted.
i.e.
3 Points in the first plane
2 Points in the second plane
1 Points in the third plane
14. Make the layout always to a scale, whenever possible.
15. The use of standard items in clamping, locating and fastening elements should be made,
whenever possible.
16. Total engineering data in the drawing to be provided. I.e. material, heat treatment of the
component, geometrical accuracies, toler ances, surface roughness for manufacturing
and inspection purpose etc.
17. Stress to be given to minimise the weight of the jig or fixture for easy handling and to
reduce the fatigue on the operator.
18. Care has to be taken for providing suitable support or guidance for preventing work piece
bending or movement while operation and clamping.
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19. Attention to be given to tightening up of loose items of jig or fixture.
20. In jig and fixture layout a distinction between work piece and jig or fixture component to
be brought by means of chain dotted lines for work piece and full lines for jig and fixture
components.
21. Provision to be made for the setting gauges in fixture.
22. Machining table mounting requirements are to be considered while designing.
23. Bill of material to be provided.
The good design of jigs / fixtures is that which satisfies the following
a) Functional aspect
b) Quality
c) Cost
d) Production schedule
e) Safety
f) Adaptability to the machine
A5.1c Major Factors in the Design of Jigs & Fixtures
In planning jigs and fixtures, it is essential to consider three major factors, which have
a definite influence upon the design of tools.
i) The tool should be designed for efficient operation and for easy manipulation by the
operator.
ii) The tool should be designed so that it will be produce accurate workpieces on a
repetitive basis.
iii) The cost of the tool should usually be governed by the number of parts to be
produced.
1. Efficient Operation
In determining how a jig or fixture can be best designed for its most efficient use by
the operator, the following should be considered :
i) Type of jig or fixture required for the specific part.
ii) Locating and loading of the work
a. Clearances necessary for locating the work.
b. Methods of foolproofing against improper loading
c. Unloading the work
iii) Rapid methods of clamping the work.
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iv) Methods of handling the tool especially when it is large and heavy.
v) Chip clearances and chip removal.
vi) Wearing surfaces and replacement of worn parts.
vii) Selection of materials for the special tool.
viii) Safety in operation.
2. Accurate Workpieces
The design of jigs and fixtures is influenced by the degree of accuracy requiredin the
workpiece. The features of design will vary as the requirements for accuracy vary for a
workpiece. This is one of the major factors to consider in the design of special tools.
When multiple or subsequent operations are necessary, the same locating surface or
surfaces on the workpiece should be used in each of the special tools required for the
manufacture of that part. The accuracy necessary to obtain the propre relationship between
a workpiece and other parts in an assembly is an important consideration in design. Some of
the factors to be considered in this respect are :
1. The accurate relationship of operating surfaces on different parts when they are
assembled?
2. Adequate rigidity to maintain accuracy in jig or fixture.
3. Economy & Cost
The cost of the tool and the number of parts to be produced are other factors in
determining the design. If a small quantity of parts is to be produced, a simple low cost tool
may be satisfactory.
The necessity of keeping the manufacturing cost of a new article as low as possible, or
reducing the present cost of an existing article, usually determines the type of jig or fixture
that is to be made. In some other instances, the cost of an operation may be reduced by
using a more efficien t though more expensive tool. Increased accuracy and
interchangeability secured through the use of a more elaborate tool frequently warrants its
greater coat.
The use of standard accessories requires serious consideration in the economical
construction of jigs and fixtures. The designer should familiarize himself with the possible
used of all types of standard accessories. Pre-fabricated units such as tool bodies, locating
and clamping devices, drill jig bushings, and tool body supports should be used. Standard
parts may be purchased or made in quantities and kept in stock.
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A5.1d Elements of Design (Jigs & Fixtures)
1. The work must be Located Properly
The ease and rapidity with which the workpiece can be located and removed is an
important conside ration in the design of special tools. Therefore, the designer should
become thoroughly familiar with the various methods of locating and clamping before a
design is definitely decided upon.
Parts having rough or irregular surfaces, and parts which varyin size, usually present
special problems in locating. To compensate for such irregularities or variations, adjustable
locators should be used. The design of adjustable locating stops and supports should
provide for positive location and for simple and easily accessible means of adjustment and
locking.
Locating points on jigs and fixtures should be designed so that incorrect loading of
the workpiece is impossible, further more to obtain the proper balance, the locators should
be place as far apart as the shape of the workpiece will allow. Consideration must also be
given to the position of the locators to allow for the necessary clearance in loading and
unloading. The locating points should be made wear resistant in order to maintain accuracy,
especially when non-adjustable stops are used.
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Accuracy
Location should be done on the most accurate surface of the workpiece. A machined
surface is preferable to an unmachined one. When more than one machined surfaces are
available, locate from the most accurate surface. For example, the center of the turned part
can be located from outside diameters 110 or 80 or form central 50 f bore 80f has the
minimum tolerance of 0.05, so the workpiece can be located most accurately from outside
diameter 80f. Location form 50 f bore would be less accur ate than location from 80f but
more precise than location from outside diameter 110f which has a much wider tolerance of
1mm (±0.5mm).
2. The work must be clamped properly
The method of clamping and the design of the clamps depend upon the shape of the
workpiece.
The designer should consider the following fundamentals :
i. The clamps should be positioned to resist the maximum pressure of the cutting
tools.
ii. The clamps should be located over or as near as possible to some bearing point
of the workpiece. This must be done to avoid springing the part.
iii. The clamps should be designed so that they can be quickly and easily unlocked
and shifted out of the way of the workpiece when it is unloaded.
iv. Complicated clamping devices should be avoided if possible. A simple device
has fewer wearing surfaces and will stay in working condition for a longer time.
v. The kind of material in the workpiece should be considered in choosing a design
for the clamps. For example, finished surfaces or soft material require a larger
clamping area than surfaces of hard material. The larger clamping face
distributes the pressure so that the workpiece is not deformed or spoiled. The
work supporting devices opposite the clamps should be large enough to support
the pressure of the clamps.
3. Large tools – Weight & Handling
Special tools designed for large workpieces are often unduly heavy. It is therefore
desirable to make them as light as possible for easy manipulation without impairing their
strength. Large cast iron tool bodies can be made lighter by coring out the metal. It is often
possible to reduce weight by the use of well designed, fabricated and welded tool bodies,
where heavy castings are to be drilled, reamed or bored on several sides, trunnions, thereby
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eliminating the otherwise difficult problem of lifting and turning jig faces up into the operating
position. All corners should be well filleted for strength.
Heavy tools should be provided with handles or holes for bars or eye bolts to facilitate
lifting. In some cases it is often good design to provide smaller jigs with handling devices for
convenience in holding them while the enclosed workpiece is machined. Sharp edges and
corners which might injure the operator should be avoided.
4. Chip clearance
Clearance must be allowed so that the chips will not accumulate and interfere with
the cutting operation or workpiece location. The jigs should be designed so that it ca n be
easily cleaned. Holes or escapes for draining the coolant or cutting lubricant should be
provided.
5. Materials for Jig & Fixture
Jigs and fixtures are made from a variety of materials, some of which can be hardened to
resist wear. It is sometimes necessary to use nonferrous metals like phospher bronze to
reduce wear of the mating parts, or nylons or fibre to prevent damage to the workpiece.
Given below are the materials often used in jigs, fixtures, press tools, collects, etc.
a. High Speed Steels (HSS)
These contain 18% (or 22%) tungsten for toughness and cutting strength, 5.3%
chromium for better hardenability and wear resistance and 1% vandadium for retention of
hardness at high temperature (red hardness) and impact resistance. HSS can be air or oil
hardened to RC 65-65 and are suitable for cutting tools such as drills, reamers and cutters.
b. Die Steels
These are also called high carbon (1.5 -2.3%) high chromium (12%) (HCHC) cold
working steels and are used for cutting press tools and thread forming rolls. Hot die steels
with lesser carbon (0.35%) and chromium (5%) but alloyed with molybdenum (1%) and
vanadium (0.3 -1%) for retention of hardness at high temperature are used for high
temperature work like forging, casting and extrusion.
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c. Carbon Steels
These contain 0.85-1.18% carbon and can be oil hardened to RC62-63. These can be
used for tools for cutting softer materials like wood work, agriculture, etc. and also for hadn
tools such as files, chisels and razors. Theparts of jigs and fixtures like bushes and locators,
which are subjected to heavy wear can also be made from carbon steels and hardened.
d. Collet Steels (Spring Steels)
These contain about 1% carbon and 0.5% Manganese. Spring steels are usually
tempered to RC 57 hardness.
e. Oil Hardening Non-Shrinking Tool Steels (OHNS)
These contain 0.9-1.1% carbon, 0.5-2% tungsten and 0.55-1% carbon. These are used
for fine parts such as taps, hand reamers, milling cutters, engraving tools, and intricate press
tools which cannot be ground after hardening (RC 62).
f. Case Hardening Steels
These can be carburised and case hardened to provide 0.6 -1.5 thick, hard (RC 59 -63)
exterior. 17Mn1Cr95 steel with 1% manganese and 0.95% chromium is widely used.
15Ni2Cr1Mo15 steel with additional nickel (2%) reduces thermal expansion up to 100 0
C.
Case hardening steels are suitable for parts which require only local hardness on small
wearing surfaces where costlier, difficult to machine full hardening tool steels are not
warranted.
g. High Tensile Steels
These can be classified into medium carbon steels with 0.55% - 0.65% carbon (En8-9)
and alloy steels like 50 Ni2Cr1m028 (En25). The tensile strength can be increased up to
125 kg/mm2
(RC50) by tempering.
Medium carbon steels are used widely for fasteners and structural work while alloy
steels are used for high stress applications like press rams.
h. Mild Steel
It is the cheapest and most widely used material in jigs and fixtures. It contains less than
0.3% carbon. It is economical to make parts which are not subjected to much wear and are
not highly stressed from mild steel.
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i. Cast Iron
It contains 2-2.5% carbon. As it can withstand vibrations well, it is used widely in milling
fixtures. Self lubricating properties make cast iron suitable for machine slides and guide
ways. The ingenious shaping of a casting and the pattern can save a lot of machining time.
Although, the strength of cast iron is only half the strength of mild steel, a wide variety of
grades have been developed. Nodular cast iron is as strong as mild steel, while meehanite
castings have heat resistant, wear resistant, and corrosion resistant grades.
j. Steel Castings
These combine the strength of steel and shapabilly of a casting.
k. Nylon and Fibre
These are usually used as soft lining for clamps to prevent denting or damage to the
workpiece under high clamping force. Nylon of fibre padsare screwed of stuck to mild steel
clamps.
l. Phospher Bronze
It is widely used for replaceable nuts in screw operated feeding and clamping systems.
Generally screw making process is time consuming and costly. So, their wear is minimised
by using softer, shorter phospher bronze mating nuts. These can be replaced periodically.
Phospher bronze is also used in applications calling for corrosion resistance, like boiler
valves.
6. Construction of Jigs and Fixtures
Jigs and fixtures bodies may be made of c ast iron, or they may be built up of steel
plates or structural forms held together by screws and dowels or welded joints. Welded
constructions are proving desirable because the bodies are strong and light, and addition
alterations and additions to the tool can be effectively accomplished.
The size of a tool, the quantity of workpieces to be made, and the cost of construction
are important considerations in planning a design.
7. Replacement of Worn Parts
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Some parts of tools are subjected to so much wear that the accuracy of the tool may
be impaired. For such tool parts, material that can be made wear resistant must be selected
and the tool must be constructed so that worn out parts are easily replaceable.
8. Safety in Operation
One of the most important considerations affecting the design of tools is the safety of
the operator. Any features, which might cause injury, must be eliminated. Adequate
operating accessories, such as suitable and efficient levers and locks are essential for safety
in operation. The design of jigs and fixtures should provide means of clamping to the
machine table if large tools are used or when tooling need not be shifted. Convenient
holding devices should be provided as a safety factor whenever necessary.
The tool must be designed so that it can be easily set up, adjusted, operated, and
cleaned. Features, which safeguard the equipment against misuse, are also very important
elements of design.
A5.1e Flow chart for development of design solution
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FLOW CHART FOR DEVELOPMENT OF DESIGN SOLUTIONS
Initial Design Concept
Design procedure
1. Statement of the problem
eg. To design a drill jig to hold a support bracket while drilling 3 – 6mm holes
To design a lathe fixture for holding a pump housing for drilling and boring of bearing
holes.
Part Details Operation
Classification
Equipment
Selection
Operator
Criteria
Select Pertinent
Items
Select Pertinent
Items
Select Pertinent
Items
Select Pertinent
Items
Discarded
ideas
Preliminary
Tool Design
Cost Analysis &
Evaluation
Primary Tool
Design
Alternate 1 Tool
Design
Alternate 2 Tool
Design
Evaluation and Final Decision
Completion of Design, Execution of Shop Drawings
Phase 1
Phase 2
Phase 3
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2. Need Analysis: (Who, why, how, when, what and where about functional
requirements)
3. Ideation (sketches)
Information collection.
4. Analysis and synthesis
5. Tentative design solutions
6. Evaluation and testing
7. The finished design
A5.1f Checklist for the Design of Jigs & Fixtures
The following list of checkpoints should be consi dered before any design of jig or fixture is
released for manufacturing.
¨ Check List for Tooling Layout
1. Is the tool layout the latest issued?
2. Is the part drawing the latest issued?
3. Is the part correctly shown on the layout?
4. Are the locating points provided using the thumb rule 3-2-1- -Three points in first plane
- Two points in second plane
- One point in third plane
5. Are practical considerations made in locating and clamping a part in jig or fixture?
6. Can locators be easily cleaned or replaced?
7. Is the jig or fixture of sound design? Are rigidity and simplicity taken into consideration?
8. Are the locators accessible for cleaning?
9. Are all the clamping requirements properly considered?
10. Are the individual components designed from the point of minimizing machining?
11. Is the layout made to scale?
12. Are the standard items used indicated?
13. Does using separate numbers with leaders and arrows pointing to the details identify the
different parts?
14. Is the bill of materials provided in the drawing?
15. Are the details of operations, such as heat treatment and the surface roughness
indicated in the drawings?
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¨ The machine and Set Up
1. Will the jig or fixture fit into the machine for which it is intend?
2. Will the clamping slots or holes in the jig or fixture line up with the T-slots in the table?
3. Will the fixture, when in place, overhand at the end of the table?
4. Will the jig or fixture interfere with any other fixture next to it in the case of multispindle
machine?
5. Can a set up operator see whether the cutter/drill are correctly set?
6. Can the cutting tools be adjusted and removed easily for sharpening when the fixture is
in place?
7. Can setting blocks, bushings, stops, or collars be used in setting up the cutting tools?
8. Are suitable locating plugs for setting up, provided?
9. Does the set up operator need more than one size of wrench?
10. Are the hold down bolts to make tightening easy provided?
¨ Method of Location
1. Are the locating points widely placed?
2. Are the centralized means required to compensate for variations in the work piece.
3. Is the tolerance on locating points sufficiently close to obtain the specified operational
accuracy?
4. Are the locating points as small in area as possible?
5. Are locators safe from damage by cutters?
¨ Method of Clamping
1. Are the loads static or dynamic?
2. Is the work piece supported as closely as possible to the point of load applications?
3. Is the cutting force resisted by a solid support and not by the clamp?
4. Can the cutting force be used conveniently to help securing the work piece?
5. Has the clamp sufficient range to accommodate allowable work piece variations?
6. Is the work piece directly supported under clamping points?
7. Will the clamping force unduly distort the work piece?
8. Will the clamp tend to loosen under cutter chatter or vibration?
9. Can it be planned to have a single standard wrench to tighten all the clamps?
10. Does the work piece size, the required clamping force, or the required speed of action
warrant used of pneumatic or hydraulic clamping?
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¨ Handling
1. Is the part within handling capacities by hand?
2. Is by hoist, are the facilities and necessary sling clearance, loading skills provided to
ensure easy handling?
3. If by conveyer, is the correct height maintained?
¨ Loading / Unloading Work Pieces
1. Will cutters such as long drills interfere with work piece while loading or locating?
2. Will clamp interfere while loading or unloading?
3. Is the clearance sufficient to permit the work piece to be easily lifted over or into locating
and centering devices?
4. Have any sliding pins or other hand -operated locators been provided with comfortable
handles?
5. Are movable locators and adjustments on the side of the fixture nearest the operator?
6. Can the fixture be loaded with one hand while the other hand is used for loadi ng the
completed work piece?
7. Are any burrs likely to interfere with unloading?
8. Should an ejector be provided?
¨ Thrust and Torque
1. Can satisfactory blockings be arranged to withstand cutter feed strains and distortion?
2. Are clamps carrying thrust load avoided?
¨ Chips
1. Are channels to allow the collate to wash the chips away provided?
2. Are blind spots and traps avoided?
¨ Capacity
1. Will the proposed design come within the column clearance capacity, table spindle travel
of machine?
2. Are the jigs fit large enough to span T-slots in the machine table?
3. Is there sufficient clearance between tools and the work piece for easy gauging?
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¨ Lubrication
1. Is the machine equipped for coolants?
2. Can suitable coolant reservoir be provided?
¨ Tool
1. Is the design ensures the use of standard tools (drills, reamers, milling cutters)?
2. Can use of existing special tools be incorporated?
3. Is provision made to secure all special tools needed?
4. Can the cutter flutes discharge chips even when covered up?
¨ Standard Parts
1. Does the proposed design incorporated fully the use of all standard parts carried I stock
for jigs or fixtures?
¨ Loose Pieces
1. Can the loose parts (if inavoidable) be attached to the fixture with keeper screws or light
chains?
2. Is the proper identification or storage facilities provided for the loose parts (Clamps,
removable locating plugs and adapters)?
¨ Balance
1. Is the uniform mass distribution in design ensured?
¨ Progressive Experience
1. Does similar type of equipment exist if so what is your experience?
2. Are they suggestions from machine shop or tool room been considered and
incorporated?
¨ Manufacturing Considerations
a) Built up
1. Can the jig or fixture be built? Does the proposed design includes any impossible
(difficult impractical) machining problems?
2. Does it confirm to all known machine capacities?
b) Castings
1. Does the design land itself to good pattern making practices and economical castings?
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c) Welded Construction
1. Would welded steel construction offer any advantages?
d) Material
1. Have proper steels / materials been selected to make the miscellaneous details?
¨ Production Requirement
1. Is the jig/fixture best suited to meet the requirements?
2. Will available machine burden, require manual semi or full automatic, single or multiple
set up?
¨ Safety
1. Does the fixture design protect the operator from coolant spray or flying chips?
2. Is the designed tool safe to operate with?
¨ Sundry Requirements
1. Will the fixture design keep the length of the cutter travel to a minimum?
2. Will the operator, when positioning the jig, clearly able to see all bushings or cutter
guides?
3. Are the bushings long enough to provide adequate tool support?
4. Do the tool need guiding for a second operations?
5. Can the use of slip bushings be avoided by used of stepped drills?
6. If the slip bushings are must, are the heads large enough, fluted for easy gr ipping, and
provided with locking means?
7. Do all supporting pads and pins stand well clear of chip collecting surfaces?
8. Is the chip fouling the clamp lifting springs?
9. If the work piece is to be measured while still in the fixture can it be easily cleaned?
10. Is there sufficient clearance between tools and the work piece for easy gazing?
11. Can the tools be damaged or made inaccurate through incorrect insertion of the work
piece?
12. Have breather holes been provided to allow escape of air from clo se firing plunger
holes?
13. Provided the production and economics warrant, have all wearing parts been specified to
be hardened?
14. Have means of setting the cutter or cutters in the correct position been provided?
15. In case of revolving features
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a) Has sufficient metal been left on jig to form an integral balance weight or provision for
balance weight?
b) Have arrangements been made for swarf to be shaken out or be blown out from
interior of jig?
c) Are projecting screws etc. covered in order to eliminate risk of injury to the operator
(counter bored/counter sunk)?
d) Have pilot made fool proof, if arranged so that the work cannot be inserted except in
the correct way?
16. Is core out of unnecessary metal, making the fixture as light as possible, consistent with
rigidity and stiffness made?
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Summary
Quality and higher productivity are aimed through the use of jigs and fixtures in any
production process. The jigs and fixtures add to the cost of tooling, as such their use in
production should be justified economically. The design of jigs and fixtures need to satisfy,
functional, qualitative, safety and adaptability aspects to a production techniques. Tool
engineer has to adopt certain design principles for the jigs and fixtures. A check list for the
design of jigs and fixtures before releasing it for manufacture will greatly help the production
process with considerable saving in production time and cost.
Questions
1. What are the different elements of design?
2. Explain the design steps for designing of jigs and fixtures?
3. How does the checklist help the tool designer in designing of good jig or fixture?
4. What are the major factors to be considered in design of jigs & fixtures?
5. Explain, why jig has a four feet not three?
6. What are the design principles to be followed while designing of jigs & fixtures?
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A8.1 Introduction
Cost estimating may be defined as the process of forecasting the expenses that must
be incurred to manufacture a product. These expenses take into consideration all
expenditures involved in design and manufacturing, with all the related service facilities such
as pattern making, tool making, as well as a portion of the general administration and selling
costs.
Cost estimates are the joint product of the engineer and the cost accountant, and
involves two factors : physical data and cost ing data. The engineer as part of his job of
planning manufacturing determines the physical data. The cost accountant compiles and
applies the costing data.
A8.2 Purpose of Cost Estimation
Cost is the background of almost every decision the tool engin eer makes in
organizing manufacturing operations and in selecting materials, methods, tooling and
facilities. An understanding of cost determination is essential to ensure that these decisions
are based on sound and dependable estimates of cost.
Estimates of cost must be reasonably accurate if a venture is to be successful
(realistic cost estimate). If a job is overpriced, it is lost to a competitor. If it is underestimated,
it results in financial loss.
Detailed cost estimates are prepared to:
CHAPTER OUTLINE
A8.1 Introduction
A8.2 Purpose of cost estimation
A8.3 Elements of cost
A8.4 Cost structure
A8.5 Estimation of cost elements
A8.6 Estimating tool cost
A8.8 Steps in making of cost
estimation
A8.8 Chip factors in cost estimation
A8.9 Numerical examples
CHAPTER OUTLINE
A8.3a Material cost
A8.3b Labour cost
A8.3c Expenses
A8.5a Direct Material cost
A8.5b Direct labour cost
A8.5c Indirect expenses
A8.5d Direct expenses
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1. Determine the selling price of a product for a quotation or contract, so as to ensure a
reasonable profit to the company.
2. Check the quotations supplied by the vendors.
3. Decide whether a part or assembly is economical to be manufactured in the plant or
is to be purchased from outside.
4. Determine the most economical process or material to manufacture a product.
5. Initiate means of cost reduction in existing production facilities by using new
materials, which result in savings due to lower scrap loss and re vised methods of
tooling and processing.
6. To determine standards of production performance that may be used to control
costs.
A8.3 Elements of Cost
The constituents of cost of a product or the “cost elements” are : Material cost,
Labour cost and Expenses. We shall discuss each element in turn.
a) Material cost
Material is divided into two basic categories: (a) material for fabricated parts (b)
standard purchased parts. The total cost of thesetwo will give the material cost. Again there
are two kinds of materials, which comprise the factory cost of a product. These are : Direct
material and Indirect material.
i) Direct Material
The direct material is the raw material, which is processed in the plant and finally forms
the finished product. Any standard part, which also becomes a part of the finished product,
will also come under the category of direct material.
ii) Indirect Material
Indirect materials are those, which hel p in the processing of direct materials into the
finished product. These materials don’t form a part of the finished product. Indirect
materials include: Shop supplies such as cotton waste, lubricating oil, cutting fluids, coal, oil,
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gas, shielding gases used in Arc welding, Emery paper used for polishing, quenching oils for
heat treatment etc. Indirect materials form the part of on cost or overheads.
b) Labour Cost
Labour, which enters into the manufacture of a product, is of two categories : Direct
Labour and Indirect Labour.
i) Direct Labour
The operator or operators, which actually process the raw materials either on machines
or manually, form the direct labour.
ii) Indirect Labour
All the staff excepting administrative and sales office staff, wh ich helps in running the
plant, comes under the category of indirect labour. Indirect labour includes: Foremen,
supervisors, maintenance staff, stores personnel, time office staff, drawing office staff, etc.
Indirect labour forms a part of overheads.
c) Expenses
Total cost of the product minus the costs of direct material and direct labour constitutes
the ‘Expenses’. Expenses may also be either direct or indirect.
i) Direct Expenses
These expenses like the direct material and direct labour are directly chargeable to
the finished product. These are also known as “chargeable expenses”. These include:
a) Cost of patterns, jigs, fixtures, dies, drawings or designs specifically prepared for a
particular product, which cannot be used for other purposes.
b) Cost of any experimental work done specially for a particular product.
c) Cost of inward carriage or freight incurred on supply of special material needed for the
particular product.
d) Hire of special or single purpose tools or equipment for a particular product.
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ii) Indirect Expenses
These are also called “oncosts” “overheads” or “burden”. These include: cost of
indirect material, cost of indirect labour and other expenses that cannot be conveniently
charged directly to a particular job. Indirect expenses may be divided into:
a) Factory expenses or overheads.
b) Office and Administrative expenses or overheads.
c) Selling and Distribution expenses or overheads.
¨ Factory expenses
These expenses include : indirect materials, indirect labour, expenses, insurance,
maintenance and depreciation of machine, power etc.
¨ Office and administrative expenses
These expenses consist of all expenses incurred in the direction, control and
administration of an undertaking. These expenses include : rent and rates of office
premises, salaries of office staff, printing and stationery, postage, salaries of high officers,
depreciation of office equipment and insurance on office equipment.
¨ Selling and distribution expenses
These expenses include : salaries of sales staff, publicity and advertisement,
catalogues, leaflets and price lists, packing and forwarding charges, godown rent,
commission to salesmen etc.
The overheads may be grouped into two main categories:
1. Fixed overheads or constant overheads
These are such items of indirect expenses, which remain constant or fixed irrespective of
volume of production. These items include : salaries of higher officers (administrative and
management executives), capital taxes, insurance charges, depreciation of building, plant
machinery etc., rent of buildings.
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2. Variable or floating overheads
These are such items of overheads, which vary, with the volume of production. Such
items are : internal tran sport charges, power, fuel, stores expense, factory lighting and
heating and sales office expenses and repairs of machine tools.
Since fixed overheads remain constant irrespective of volume of production, production
should be increased to reduce the cost o f the part. There should be some minimum
production to meet the fixed expenses and start earning profit.
A8.4 Cost Structure
The elements of cost can be combined to give following types of cost:
1. Prime cost. Prime cost or direct cost is given as :
Prime cost = Direct material + Direct labour + Direct expenses (if any)
2. Factory cost. This cost is given as:
Factory cost = Prime cost + Factory expenses.
Factory cost is also called as “Works cost”.
3. Manufacturing cost. Manufacturing cost or cost of production is given as:
Manufacturing cost = Factory cost + Administrative expenses.
4. Total cost. Total cost is given as:
Total cost = Manufacturing cost + Selling and Distribution expenses.
5. Selling price. Selling price is given as:
Selling price = Total cost + Profit.
The above mentioned cost structure is explained with the help of a block diagram as follows:
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Selling price
Total cost Profit
Manufacturing
cost
Selling
expenses
Factory cost
Administration
expenses
Prime cost
Factory
expenses
Directmaterial
+
Directlabour
+
Directexpenses
(ifany)
A8.5 Estimation of Cost Elements
Directly material cost, direct labour cost and directs expenses can be found out most
accurately by the estimat ing procedure. The indirect expenses items, which are so
numerous, are determined by cost accounting section only, which furnishes the figure
department wise to the estimator. The various cost elements are estimated in the manner
give below.
a) Direct Material Cost
The cost of standard purchased parts can be obtained from the purchasing section. The
raw material chargeable to a product is that in the rough state and includes all scrap
removed. Material can be in the form of sheet metal, bar stock, forgings or castings, plastic
etc. The weight of the material can be determined from the drawing of the part. An irregular
part is divided into simple sections to calculate its volume. Volume is multiplied by density of
the material to find its weight. The weight of a part multiplied by the unit cost of the material
gives the material cost per piece. If the unit cost covers only the purchase price of the
material, the material cost is multiplied by one or more additional factors to account for bulk
losses, purchasing and handling costs.
b) Direct Labour Cost
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For estimating the direct labour cost of a product, the job is divided into operations
needed to machine it, and then estimating the operation time for each operation. Total time
multiplied by a labour rate gives the direct labour cost. The total time required to perform an
operation may be divided into the following parts:
i) Set-up time
This is the time needed to prepare for the operation and may include : time of study the
blueprint or to do paper work, time to get tools from the crib and the time to install the tools
also on the machine.
ii) Man or handling time
This is the time the operator spends loading and unloading the work, manipulating the
tools and the machine and making measurements during each cycle of operation.
iii) Machining time
The elements comprising the machining time are those, which are performed by the
machine. This is the time during each cycle of operation that the machine is working or the
tools are cutting.
iv) Tear down time
This is the time required to remove the tools from the machine and to clean the tools and
the machine after the last component of the batch has been machined.
Tear down time is usually small. It will seldom run over 10 minutes on the aver age
machine in the stop. It may require only a few minutes to tear down a set up on a drilling
press and 10 to 15 minutes on the average miller or turret lathe. In exceptional cases, it may
go upto as high as 30 minutes on very large boring mills and large milling machines.
v) Down or lost time
This is the unavoidable time lost by the operator due to breakdowns, waiting for the tools
and materials etc.
vi) Allowance
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The total time to perform an operation also includes time for personal needs of the
operator, time to change or resharpen the tools etc. The time all these allowances are taken
to be about 20 percent of the sum of all other times and then the total time for the operation
is obtained.
(a) Personal allowances. This is time taken by the operator to attend to his personal
needs such as going to lavatory, taking a cup of tea, smoking etc. The time for this is usually
taken to be about 5 percent of the total time.
(b) Fatigue. The efficiency of the worker decreases due to fatigue or working at a
stretch and also due to working conditions such as poor lighting, heating or ventilation. The
efficiency is also affected by the psychology of the worker, which may be due to domestic
worries, job security etc. For normal work, the allowance for fatigue is about 5 percent of the
other times. This allowance can be increase depending upon the type and nature of work
and working conditions.
(c) Time to change or resharpen tools. Some allowances should also be provided for
the time taken by the operator to get the tools changed or to resharpen the tools. This time
varies from machine to machine.
(d) Inspection or checking allowance. To maintain the uniform quality of the parts,
the dimensions of the parts should be checked or inspected at regular intervals depending
upon the closeness of tolerances. The checking times for the various instruments are given
below, to check one dimension :
With rule 0.10 minute
Vernier caliper 0.50 minute
Inside caliper 0.10 minute
Outside caliper 0.05 minute
Inside micrometer 0.30 minute
Outside micrometer 0.15 minute
Depth micrometer 0.20 minute
Dial micrometer 0.30 minute
Thread micrometer 0.025 minute
Plug gauge 0.20 minute
Snap gauge 0.10 minute
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Set up time and tear down time are perfor med usually once for each lot or batch of
parts. Set up time per piece is obtained by dividing the set up time for the machine by the
number of pieces produced in lot. Set up time, handling time and tear down time are
estimated from previous performanceson similar operations. All work on a particular type of
machining tool consists of a limited number of elements. These elements can be
standardized, measured and recorded. This is done under Time and Motion study.
Standard data is available for set uptime, tear down time and handling time. Machining time
is obtained with the help of formulas for each machining operation, which takes into account
speeds of cutting, feeds, and depth of cut and tool travel. The actual amount of down or lost
time that will occur in a particular operation can scarcely be predicted. Some operations will
run smoothly, others may be beset by troubles. The sum of machining time and the handling
time is called ‘run time’ or ‘unit of operation time’.
The total time to manufa cture a product (from which the direct labour cost will be
estimated) may be divided into the following major groups :
1. Set up time
2. Machining time
3. Non machining time
4. Down time
The tear down time discussed earlier may be included in the set up t ime itself. The
non-machining times will be man or handling time, personal needs, fatigue, cutter or tool
sharpening and inspection. The man or handling time, as already discussed, includes :
loading and clamping the part, unloading the part, advancing or retracting the cutting tool,
tightening a chuck, a trial cut, trial gauging, debarring the machine, cleaning the fixture etc.
c) Indirect Expenses
Indirect expenses or overheads are those charges which vary in proportion to the
production rate, but which are not easily attributable directly to a given operation or part.
These expenses are apportioned among the operational units (machines, plants etc.)
according to some weighting factor.
1. Percentage of direct labour cost
2. Percentage of direct labour hours
3. Machine hour method
4. Direct material method
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5. Unit of production method
6. Space rate method
d) Direct Expenses
The direct expenses are estimated in the following manner :
The engineering (preparation of drawings, blue prints, drafting etc.) and design cost of a
product is calculated as a flat hourly rate for each estimated hour of design and engineering
time. On the same lines, the cost of any experimental work done specifically for a particular
product, can be estimated. The cost of patterns and special tools such as jigs, fixtures, dies
and gauges can be estimated as outlined below:
¨ Tool cost
Generally tool cost estimating is concerned with tool and other special equipment to
be used for production. Much product cost estimating depends upon tooling cost estimating.
Therefore, tool costs are often treated separately during a product cost study.
Tooling costs are estimated to :
1. Determine how much must be invested i n tools and equipment to manufacture a
product.
2. Determine the cost of alternate methods of tooling to help in selecting the more
economical method.
3. Find the cost of a proposed machine or tool that promises to produce more
economically method.
4. Determine the reasonable cost of a special machine or tool to gauge whether tool
room performance is efficient or vendor’s prices are reasonable.
The cost of cutting tools, both special and standard, jigs, fixtures, dies and gauges and
other special equipment is often separated from the cost of machine tools. This is done to
determine what portion of the tooling cost is to be charged directly to the project or proposal
and what portion is to be capitalized.
A8.6 Estimating tool cost
Tooling cost is estimated in the same manner as the cost of manufacturing of a product
is estimated. There are four major factors in the cost of any tool. These are: material, labour
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and overheads or burden as in manufacturing costs. The fourth factor is the cost to engineer
and design the tool, that is, the cost of designing and drafting the tool.
i) Engineering and design cost
The engineering and design cost represents a large portion of the total cost of the tool,
often as high as 20 to 30 percent. This cost can be directlycharged to the individual tool and
as a consequence are always considered first in establishing an estimate, the engineering
and design cost is applied to the tool cost estimate as a flat hourly rate for each estimated
hour of design and engineering time.
ii) Tool materials cost
The cost of the material for a proposed tool may be calculated and therefore becomes
more of an actual cost than an estimated cost. It is the most accurate item in the tool cost
estimate and is determined as discussed before. Sta ndard parts such as knobs, hand
wheels, bushings, bolts, screws, springs, and similar items that complete the tool bill of
material can be accurately priced from catalogues, price lists or invoices.
iii) Tool labour cost
The labour involved in the machining, assembling, fitting, and tryout of tools is always
difficult to estimate accurately, even for the most experienced estimator. The machining
time can be calculated. The other times cannot be calculated and must b esti mated. It is
difficult to foresee all the problems that may develop in fitting assembly and try out
operations, even under most favourable conditions. Therefore, the estimate must include a
liberal factor of safety for lost time, which is impossible to an ticipate and will always be
present. The sum of all the times will be the labour time for the tool. When multiplied by the
toolmakers hourly rate, the estimated labour cost is determined.
iv) Burden or overhead
The tool room may be considered as a depar tment, and therefore may have an
established burden rate, just as production department has. It general, the man -hour or
direct labour cost method of burden distribution is applied as discussed under point 3 above.
A8.8 Steps in making a cost estimation
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The cost of a new product may be estimated by following the basic steps given below :
1. Make a complete and thorough analysis of the cost request to understand it fully.
2. Make an analysis of the part or product and make separate lists of standard partsand the
parts to be fabricated within the plant.
3. Make a manufacturing process plan for the parts to be fabricated.
4. Determine the material costs for the standard and the fabricated parts.
5. Estimate the total production time for each operation listed in step 3.
6. Apply the labour and burden rates to each operation.
7. Add the material costs (step 4) and the labour and burden costs (step 6). This will give
the total manufacturing cost.
8. Apply the profit factors to arrive at the selling price.
A8.8 Chief factors in cost estimation
Each cost estimate may not be exactly the same as the actual manufacturing cost. The
most significant causes for the cost deviations can be : Fluctuations in ma terial and labour
costs, incomplete design information at the time of estimate, unexpected delays resulting in
premiums paid for overtimes and materials and the unexpected machining or assembly
problems. However, the average of cost estimates over a perio d of time should be
reasonably close to the actual manufacturing costs. For this, the following factors should be
considered for arriving at an accurate and complete cost estimate :
1. Each estimate should contain complete costs of direct material, direct labour, factory
overheads, spoilage, engineering, administration and selling.
2. If the cost of a new product is estimated on the basis of previous estimates of
comparable parts, detailed estimating should be used. It is necessary to make
substitutes in the past estimates for individual operations, individual parts or individual
sub assemblies.
3. The period of time between the cost estimating and the actual production of the part
affects the determination of unit prices. During the intervening period, the basic material
and labour rates may rise or fall. Therefore, an estimate of what the cost will be at the
time of actual production is what is really needed. Thus the estimator should have the
ability to project thinking and reasoning into the future.
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4. The volume of the pieces to be produced also affects the costing rates since the time and
therefore the cost of performing an operation decreases as the number of units produced
is increased.
5. The addition of new type of equipment and special buildings require the development of
new overhead rates etc.
A8.9 Numerical Examples
From the following data, calculate the total cost and selling price for a job :
Direct material = Rs. 5500
Manufacturing wages = Rs. 3000
Factory overheads to manufacturing wages = 100%
Non manufacturing overheads to factory cost = 15%
Profit on total cost = 12%
Solution. Direct material = Rs. 5500
Manufacturing wages (Direct labour) = Rs. 3000
Factory overheads = 100% of Rs. 3000
= Rs. 3000
 Factory cost = Direct material + Direct labour + Factory overheads
= Rs. 5500 + 3000 + 3000
= Rs. 11,500.
Non-manufacturing overheads, i.e., administrative and selling overheads
= 15% of Rs. 11,500
= Rs. 1825
Total cost = Factory cost + Rs. 1825
= Rs. 11,500 + Rs. 1825
= Rs. 13,225
Profit = 12% of total cost
= Rs. 1588
 Selling price = Total cost + Profit
= Rs. 14,81
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Table 1 - Weight of Rolled Steel (8.843 gm/cc)
Size S
Round S f kg/metre
length
Square S Sq.
Hexagonal S
A/F
Sheet S Thick
kg/sq. metre
5
5.5
6
8
8
9
10
11
12
14
16
18
18
19
20
22
25
28
28
32
35
36
38
40
41
45
50
55
56
60
63
65
80
81
85
80
0.154
0.19
0.222
0.302
0.395
0.50
0.62
0.85
0.89
1.21
1.58
2.00
2.48
2.98
3.85
4.83
6.31
8.99
9.86
12.49
15.41
18.8
19.34
22.2
24.48
26.0
30.2
31.08
34.8
39.46
0.20
0.24
0.28
0.38
0.502
0.64
0.885
0.95
1.13
1.54
2.01
2.54
2.83
3.14
3.80
4.91
5.82
6.15
8.04
10.18
12.56
15.90
19.62
23.8
24.62
28.3
31.16
33.2
38.5
39.58
44.2
50.24
0.180
0.206
0.245
0.333
0.435
0.551
0.68
0.823
1.33
1.96
2.45
3.29
4.96
6.96
8.81
11.4
18.0
20.6
24.5
28.8
33.3
38.2
43.5
39.2
55
88.5
94.2
109.9
125.6
141.3
182.8
196.2
219.84
251.14
284.68
298.22
313.92
331.36
364.46
398.63
430.88
463.9
498.04
530.18
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Table 2 - Thumb Rules for Estimation
Group
No.
Operations involved Total costs
1. Only shaping, turning drilling and
fitting.
Three times the raw materials cost.
2. Shaping / turning, drilling, fitting
and milling.
Four times the raw materials costs.
3. Above operations plus heat
treatment.
Five times the raw materials costs.
4. Above operations plus precision
grinding or lapping.
Six times the raw materials costs.
Example
Find the manufacturing cost of 14 f bore collared bush shown in figure. The bush is to be
manufactured form 28 f x 35 long alloy steel bar which costs Rs.80 / kg. After rough turning,
the bush is to be hardened and finished by grinding.
Solution: Referring to Table1, we note that 28 f
Steel bar weights 4.83 kg/metre.
 Wt. Of a 35 long piece = 83.4x
100
35
= 0.169 kg.
Cost of material at Rs.80 = 0.169 x 80
kg = Rs. 13.52
Machining involves turning, hardening, and grinding.
Referring to table 2, we notice that the bush falls in Group 4, for which the total cost is
approximately six times the raw material cost.
 Total cost = 13.52 x 6
= Rs. 81.12
If the bush is to be sold, profit should be added.
Selling cost at 30% profit = 81.12 x 1.3
= Rs. 105.5 (min.)
Selling cost at 100% profit = 81.12 x 2
= Rs. 162.24 (max.)
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SUMMARY
Quality and higher productivity are aimed through the use of jigs and fixtures in any
production process. The jigs and fixture s add to the cost of tooling, as such their use in
production should be justified economically. The design of jigs and fixtures need to satisfy,
functional, qualitative safety and adaptability aspects to a production techniques. Tool
engineer has to adopt certain design principles for the jigs and fixtures. A check list for the
design of jigs or fixtures before releasing it for manufacture will greatly help the production
process with considerable saving in production time and cost.
Questions
1) Define cost estimating.
2) What is the purpose of cost estimating?
3) Name the various constituents of cost.
4) Name the indirect material and indirect labour.
5) What are : direct expenses, indirect expenses, factory expenses, office and
administrative expenses, selling and distribution expenses.
6) Define : Prime cost, Factory cost, Manufacturing cost. Total cost and Selling price.
7) Define : Set up time, Handling time, Machining time. Tear down time and down or
lost time.
8) List the various steps of cost estimating.
9) Discuss the chief factors in cost estimating.
10) Find the manufacturing cost of 14 f bore collared bush shown in figure. The bush is
to be manufactured form 28 f x 35 long alloy steel bar which costs Rs.80 / kg. After
rough turning, the bush is to be hardened and finished by grinding.
11) From the following data, calculate the total cost and selling price for a job :
Direct material = Rs. 5500
Manufacturing wages = Rs. 3000
Factory overheads to manufacturing wages = 100%
Non manufacturing overheads to factory cost = 15%
Profit on total cost = 12%
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A7.1 Introduction
Maximum productivity at minimum cost is the demand of modern industry. To meet
this requirement designing of efficient and accurate jigs and fixtures is required. Quality,
simplicity and economy from the important criteria for the design of jigs and fixtures.
To meet this requirement the designer will have to make an economic analysis for
using jigs and fixtures and has to device certain principles of design, and finally develop a
checklist for the jigs and fixture design.
A7.2 Basic Design Considerations
In addition to locating and holding the part, the designer must also consider several
other factors before a welding jig or fixture can be designed.
Heat dissipation is an important consideration with any welding tool. Several
methods can be usedto insure that proper heat is maintained in the weld area. The primary
factor that determines the amount of heat required is the metal being joined.
When metals such as steel and other poor heat conductors are joined, the excess
heat should be carried of f to prevent overheating the weld. To do this, backup bars of
copper, titanium, or beryllium can be used. For metals that are good conductors or heat,
such as copper or aluminum, too rapid cooling becomes the problem. To prevent this, the
fixture or jig must be made to contact the part in as small an area as possible.
Clamping supports must be provided to prevent distorting the work while it is in a
heated condition. Whenever possible, place clamps directly over the supporting elements.
Locators should be positioned so that the distortion will cause the part to loosen
rather then tighten against the locators. If this is not possible, either power or manual
ejectors should be built into the tool.
Foolproofing is one feature that is necessary for any t ype of welding jig or fixture.
Each tool must be designed so the part will only fit into its proper position.
CHAPTER OUTLINE
A7.1 Introduction
A7.2 Basic Design consideration
A7.3 Factors in fixture design
A7.4 Classification of fixtures
A7.5 Maintenance, Safety and Storage
TOPIC OUTLINE
A7.4a Types of fixtures based on how the
tool is built
A7.4b Types of fixtures based on the type of
machine on which they are used
A7.4c Types of fixtures & their descriptions
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A7.3 Factors in Fixture Design
The major difference between a drill jig and a fixture is that a jig has hardened
bushings which guide the dril l, while a fixture for a machining operation is attached to the
work table of a machine to hold the workpiece in a fixed position for the action of milling
cutters, broaches, or other of cutting tools.
Work may be entirely enclosed in a drill jig and is r eached for the drilling or boring
through bushings provided for that purpose. However, to enclose the work in a milling fixture
would defeat the purpose for which the fixture designed that of securely holding the work in
position for the action of cutting tools. The clamps used in fixtures are applied so that the
work surface is clear for machining.
Milling fixtures are usually fastened to the table of the machine upon which they are
to be used. As a rule, they are flat on the bottom so that they will r est securely against the
table upon which they are clamped. Clamping lugs or other clamping surfaces are generally
provided. The cutter operating with a fixture is usually in a fixed position. The work held in
the fixture is “fed” to the revolving cutter by means of the movable table. Broaching fixtures,
on the other hand, are usually stationary with the broach travelling through the work.
Cutting tool chatter can be greatly reduced by carefully designed work holding
devices. These must be designed so that they are properly proportioned and sufficient in
number to support and hold the work rigidly in the fixture.
One of the major factors in the design of a milling fixture is providing a place in the
fixture for the workpiece to resist the thrust of the cutter. The thrust of the cutter should be
against the body of the fixture, rather than against the clamps. The direction of rotating of
the cutter often governs the placing of the work. An attempt must be made, therefore, to
design the tool so that the body of the fixture takes this thrust. The design of fixtures should
also permit the use of the clamping collars used on the cutter arbor. Often, though, it is not
good economy to use small diameter cutters that allow only a minimum clearance of 3mm or
less between the arbor and the work or a projecting part of the fixture, because of the
necessity of sharpening the cutter which will reduce the clearance.
¨ Design Points
Are all parts well designed to take the loads imposed on them in service?
Is the tool study enough to stand considerable abuse?
Is the fixture amply proportioned to damp out vibration and chatter? This applies especially
to milling fixtures.
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Is the design of all parts and mechanisms as simple as possible?
Have cylindrical plungers and holes been used in preference to square or polymer once?
Have holes for headed pressed in parts (such as for accurate location of resigns) been
countersunk to allow any excess press lubricant to collect in the countersink (allowing the
rest pin to vibrate slightly in service) instead of gradually squeezing out under the head?
Have spring pocket holes been countersunk on their open ends?
Are the dowel pins in each part as widely spaced as practicable?
Where detachable parts need very accurate location, have register keys or pins been used
instead of dowels?
Is the accuracy of the operation such that the base of the fixture should be scraped to fit the
machine table?
Have breather holes been drilled to allow air to escape from lose fitting plunger holes?
Is it possible to forecast any part design changes and to make allowance for them in the
design of the fixture?
¨ The procedure in developing designs for fixtures is similar to the procedure
followed in designing jigs.
A7.4 Classification of Fixture
Fixture
Types of Fixtures based on how
the tool is built
Types of Fixtures based on the type of
machine on which they are used
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The jigs and fixtures used for welding can generally be limited to three basic types;
tacking, welding, and holding.
Types of Fixtures based on
how the tool is built
Plate fixture
Angle plate fixture
Vice jaw fixture
Indexing fixture
Multistation fixture
Profile fixture
Types of Fixtures based on the type
of machine on which they are used
Turning
fixture
Milling
fixture
Planning
fixture
Broaching
fixture
Grinding
fixture
Shaping
fixture
Shaving
fixture
Forming
fixture
Stamping
fixture
Welding, brazing,
soldering fixture
Assembly
fixture
Inspection
fixture
Testing
fixture
Heat treatment
fixture
Honing
fixture
Lapping
fixture
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Tacking jigs and fixtures are used to hold the parts of an assembly in their proper
position so they canbe tack welded together. These tools are generally used for assemblies
that must be held together in several places to prevent warping or distortion when welding is
complete. Parts assembled in a tacking jig or fixture are removed after tacking and eith er
finished without special tools or transferred to a holding jig or fixture.
Welding jigs or fixtures are used to hold the parts of an assembly in position for
welding. The difference between welding and tacking is the amount of welding performed.
The tacking tool is used only when the part is to be tack welded. When the part is to be
completely welded together, a welding jig or fixture is used. Welding jigs and fixtures are
normally built heavier than tacking tools to resist the added forces caused by the heat within
the part.
Holding jigs and fixtures are used to finish tack welded assemblies. Like welding
tools, holding jigs and fixtures must be made rigid enough to prevent distortion and warping.
Generally fixtures are classified as -
1) How tool is built
a) Plate Fixture
b) Angle plate
c) Vice jaw
d) Indexing
e) Multistation
f) Profiling
2) Machine on which they used
a) Turning
b) Milling
c) Planning, shaping & slotting
d) Broaching
e) Grinding
f) Holding, Broaching and soldering
g) Assembly
h) Inspection
Types of Fixtures
The fixtures are classified based on
i. How the tool is built
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ii. The type of machine on which they are used
Though jigs and fixtures are made basically the same way, due the increased tool forces the
fixtures are built stronger a heavier than jigs.
7.4a Types of Fixtures based on how the tool is built
1. Plate Fixtures
These are the simplest form of fixtures used for most machining operations. This fixtures
consists of flat plate with variety of clamps and locators. Its adaptability to several machining
operations makes it a popular type of fixtures.
2. Angle Plate Fixtures
These fixtures are used to machine at right angles to its locator. If machining is to be
carried out at other angles a modified angle plate fixtures can be used.
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3. Vise jaw Fixtures
These are used for machining small parts. With this type of tool, the standard vise jaws
are replaced with jaws which are formed to fit the part. These are least expansible and are
limited by the size of vises available.
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4. Indexing Fixtures
These are similar to indexing jigs and are used for machining parts which must have
machined details evenly placed.
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5. Multistation Fixtures
These are intended for high speed, high vo lume production runs, where the machining
cycle is continuous.
Duplex fixtures are two station fixtures and are the simplest of the multistation fixtures.
This form of fixture allows both the loading and unloading operations while the machining
operation is in progress.
For example, once the machining operation is complete at station one, the tool is
revolved and the cycle repeated at station two. At the same time, the part is unloaded at
station one and a fresh part loaded.
6. Profiling Fixtures
These are used to guide tools for machining contours which the machine cannot
normally follow. These contours can be either internal or external. Since the fixtures
continuously contacts the tool an incorrectly cut shape is almost impossible.
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7.4b Types of Fixtures Based on The Type Of Machine on Which They Are Used
Fixtures are generally clas sified based on the machining on which they are used.
The following are some of the common production operations in which fixtures are used :
1.Turning
2.Milling
3.Grinding
4.Welding, brazing, soldering fixtures
5.Assembly fixtures / Inspection fixtures
1. Turning Fixtures
These fixtures are used for turning, facing and boring operations, and mainly consist of
workpiece locating and clamping elements. The standard fixtures that are used in a Lathe
machines are, three jaw and four jaw chucks, collectsface plate etc. These jaw chucks are
used for holding round hexagonal or other symmetrical works. Collects are used for bar
stock. Special jaw chucks, face plates with clamping devices are used for holding irregular
shaped turning jobs.
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CLASSIFICATION OF LATHE FIXTURES
Following points are to be noted while designing turning fixtures.
i) Grip the rotating work piece to the fixture to resist torsional forces.
ii) The fixture should be rigid with minimum possible overhang.
iii) Locate the work piece on critical surfaces, which are the areas from which all or
major dimensional or angular tolerances are taken.
iv) Provides adequate support for frail sections or sections or sections under pressure
from turning tools.
v) Balance the fixtures to avoid vibrations.
vi) Fixtures should not have any projections, as they will cause injury to the operator.
vii) A pilot bush for supporting tools should be provided where extreme accuracy is
required in boring operations.
Figure shows a typical turning fixtures. The fixture body is located on the machine spindle
and bolted in position, it carries the work piece location and clamping system.
Figure shows yet another special turning fixture in which work piece is located and clamped
to a shelf that projects from the fixture body. The fixture incorporates a balance weight (the
fixture would other wise be out of balance) and a pilot bush to guide the boring bar.
Lathe Fixtures
Chuck Type Face Plate Arbor Special
Collet Type Pot TypeMandrel
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2. Milling Fixtures
Milling fixtures are the work holding device which are firmly clamped to the table of
milling machine. They hold the work piece in correct position as the table movement carries
it past the cutter or cutters.
The essential features of a milling fixture are
a) Base
b) Location elements
c) Clamping elements and
d) Setting blocks
These fixtures are classified based on
i) Type of operation performed
ii) Method of milling
iii) Method of clamping the work piece
MILLING FIXTURE
Cradle
fixture
Rotary
fixture
Drum
fixture
Indexing
fixture
Magnetic
chucks
Vacuum
chucks
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Following design principles be adopted for milling fixtures
1. Pressure of cut should always be against the solid part of the fixtures.
2. Clamps should always operate from the front of the fixture.
3. Work piece should be supported as near the tool thrust as possible.
A milling fixture is located accurately on machine tube and then bolted in position. The tube
is positioned relative to the cutter with the air of setting blocks. The location an d clamping
systems are similar to those used for drill jigs, but as the cutting forces are high, interrupted
and tend to lift the workpiece, the clamping forces must be big, hexagonal nuts are usually
used to clamp the work piece rather than hand nuts.
Fig. show a simple milling fixture and a line or string milling fixtures. The line or string milling
fixture shown is used to mill a slot in the end of each of the five cylindrical work pieces
arranged in line. This arrangement facilities all the work pieces to be located as required and
clamped with one screw.
Fig. shows an index milling fixture having a number of surfaces to be milled by successive
positioning of a single fixture provided with an indexing arrangement.
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3. Grinding Fixtures
Fixtures used in grinding depend upon the type of grinding operation and the machine
used.
GRINDING FIXTURE
Angle Plat Fixture Automotive
Grinding Fixture
Magnetic Chuck
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Following table gives some commonly used grinding fixtures
Sr.
No.
TYPE OF OPERATION FIXTURES USED
1 External Grinding Mandrel - Taper
- Straight
- Combined
- Straight
- And taper
2 Internal Grinding Chucks, special jaw chucks or special fixtures
as the lathe fixtures.
3 Surface Grinding Clamped on machine table, held in vise, held in
magnetic, vacuum and special features.
4. Welding Brazing and Soldering Fixtures
These fixtures comprise of usual locating and clamping elements used in other fixtures.
However the effects of heat and prevalence of welding spatter will have to be taken into
account while designing them.
Some of the consideration are as follows :
i) Expansion of heated work pieces and resulting distortion should be taken care of by
providing adequate clearances between work piece and locators.
ii) Handles subject to heating should be properly insulated.
iii) The welding spatter should not be allowed to face on the threaded parts of clamping
elements.
iv) Parts near the welding area should not be threaded.
v) Spatter grooves must be provided below the line of welding of work pieces to the
base plate with the weld spatter.
vi) Care should be taken to prevent locking of joined work pieces in the welding fixture
after welding.
vii) Provision for easy tilting or rotation be made to ease welding from various dies.
Toggle clamps, without threaded elements are widely used in welding fixtures.
Welding and inspection are everyday operations in manufacturing. Like many other areas,
these operations can be simplified and improved through the use of appropriate jigs and
fixtures. Although welding is specified in the examples, the methods and techniques listed
will apply equally well to other assembly operations such as brazing, soldering, riveting, and
stapling.
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5. Broaching Fixtures
As broaching is a fast and accurate method of metal cutting involving high cutting forces,
broaching fixtures are required to perform one or more of the following functions:
a) Hold the workpiece rigidly.
b) Locate the workpiece in correct position relative to the tool or the machine table.
c) Guide the broach in relation to the workpiece.
d) Move the workpiece into and out of the cutting position.
e) Index the workpiece between the cuts.
Fixtures are used for both internal and external broaching. The fixtures used for internal
broaching are the simplest and for many operations consist of a face plate or support place
on the broaching machine. The fixtures for external broaching are made quite ri gid so that
the workpiece does not move during the broaching action.
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6. Assembly Fixtures
These are used to hold various components in their correct position, while they are
assembled; Assembly operations often involve pressing interference fit pins, bushes and
other parts in housings. The assembly fixtures need to be of light construction with adequate
rigidity to ensure relative positional relationships of the various components. They may be
built up from light castings, steel section or completely from steel.
Assembly fixtures generally are of two types :
a) Mechanical assembly fixtures used for operations generally performed at ordinary room
temperatures with mechanical means.
Eg, Reverting Fixtures.
b) Fixtures for hot joining methods of assembly work using energy in the form of heat.
Eg. Welding, brazing and soldering fixtures.
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7. Inspection Fixtures
Every part made must meet a standard for size and shape if it is to perform its design
function. While it is quite possible to measure each dimension separately, this is not the
most cost effective means to insure part quality and conformity to dimension. To satisfy the
requirements of speed and accuracy, gauging or inspection fixtures are used.
The main requirement of an inspection fixture is accuracy. Each inspection fixture
should contain only those elements needed to check the specified sizes of forms. Individual
gauges that only check one size are preferred over complicated tooling if the dimension
being gauged is independent of other part features. An example of this is the size of the
threads in a hole.
While the location of the hole is important to the part, the size of the thread is
independent of the location.
There are two general types of inspection fixtures; gauging and measuring.
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A7.4c Types of fixture & there descriptions
Types of fixture Description
Vise Fixtures
Standard machine vises adopted with special jaws provided
on easy way of holding parts for machining
Lathe Fixtures
Are used on vertical and horizontal turret lathes and
high-speed production lathes
Chuck fixtures
The cheapest type of lathe fixture is the standard lathe
chuck with special jaws or inserts machined to fit the part
Face Plate Fixtures Is used to machinelarge diameter parts on the vertical lathe
Mandrel and arbor type
fixtures
These fixtures will centre, locate and grip the work from the
inside and are normally used for parts that already have
machined internal surface
Miscellaneous fixtures
Lathe operations on parts that are unusual because of their
shape or dimensions, at the time fixtures are complicated
and expensive yet they are always efficient with respect to
the saving of time and the improvement of quality
Milling fixtures
A mill fixture holds the part in the correct relation to the
milling cutter as the table movement carries the part
through cutters
a) Cradle fixture
The work piece is rocked or rotated within a given angle
during milling
b) Rotary fixture The work piece is rotated under the cutter
c) Drum fixtures
The work piece is mounted on the periphery of a rotating
drum
Indexing fixtures
Where the work piece is indexed in to the next position
during the machining cycle of mill
Rise and fall fixtures
Which allow raising and lowering of the work piece in
conjunction with the mil feed
Magnetic chucks
Are used to hold ferromagnetic materials in production
milling operation
Types of fixture Description
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Vacuum chuck
Are being used for holding nonferrous and non magnetic
parts for milling operations
Boring fixtures
Differ from drill jigs in that they are to be used with boring
bars
Line boring fixture
The distance between the holes requires a line boring bar,
consequently their type of fixture is called a line boring fixture
Stationery fixture
Is designed to mount on the table and present the work piece
to the boring bar in proper location for matching
Universal fixture
May be obtained commercially from the manufacturers of
boring machines.
Indexing fixture
Consist of a base with a rotary table or rotating indexing
plate mounted on it
Automatic loading fixture
Is suitable only for long production tuns of a particular work
piece
Broaching fixture
External broaching usually requires a special fixture for each
job
Grinding fixture
Must allow for the unrestricted access of coolant to the work
The structural design of grinding fixtures is very similar to
that of other fixtures
Angle plate fixtures Is used for internal grinding
Automotive grinding
fixtures
Are used in the automotive industries
Trunk pin grinding fixtures, cam grinding fixtures, cam shaft
grinding fixtures etc.
Magnetic chuck Is used to hold work pieces in surface grinding operation
Planning fixtures
Can be economically applied to smaller parts when they are
clamped in a gang fixture
Welding fixtures
Their purpose is to locate and hold the parts in correct
relative position for joining to reduce distortion
A7.5 Maintenance, Safety & Storage
Provision for Maintenance
Has provision been made for lubricating the tool mechanisms?
55
INDO-GERMAN TOOL ROOM, AHMEDABAD
TG2CHAPTER : A7 – FIXTURES
JIGS, FIXTURES & GAUGES
181
Have all wearing parts been hardened?
Are these parts easily made and replaced?
Have correct materials and heat treatment been specified?
Has provision been made for easy removal or pressed in parts?
Can vulnerable parts be removed and replaced quickly without disturbing the set up of the
fixture on the machine?
Safety
1. Does the fixture design protect the operator from coolant spray or flying chips?
2. Is the designed tool safe to operate with?
Handling and Storage
Lifting Aids
Have lifting lugs, eyebolts, or chain slots been provided for slinging heavy tools?
Have lifting handles been attached to all awkward or heavy loose parts of the fixture?
Loose Parts
If loose parts such as spacing pieces, wrenches, or locating pins are unavoidable, can they
be attached to the fixture with keeper screws or light chains to prevent loss in storage?
Fragile Parts
Is there any fragile part of the jig which needs a protective cover in storage?
Is the tool so delicate or highly finished as to require a special case, cover, or box to protect
it in storage?
Identification
Has the tool, and all loose items belonging to it, been marked clearly with identification
numbers or symbols?
Storage Aids
Can the tool be stowed safety without danger of tipping over?
Is a special storage stand or rack desirable for safe and convenient storage?
SUMMARY:
INDO-GERMAN TOOL ROOM, AHMEDABAD
TG2CHAPTER : A7 – FIXTURES
JIGS, FIXTURES & GAUGES
182
The use of fixtures is extending and developing very fast. The quality, type and
complexity of fixtures used depend upon the type of job and its method of production.
Fixtures are classified into two types depending on how they are builtand based on the type
of machine on which they are used.
QUESTIONS:
1. What are the two types of classifying fixtures?
2. What is an indexing fixture?
3. What type of fixture is used for machining contours, which the machine cannot
normally follow?
4. What is an assembly fixture?
5. What are the rules for selecting clamps of work piece in fixtures?
6. What are the principles to be following in designing of fixtures?
7. Describe the various grinding fixtures.
8. Describe the design principles for a lathe fixture.
9. Name the various work holding devices use on a lathe.
10. How are cutters set in relation to the work in milling fixture?
11. Name the essential features of a milling fixture.
12. Why the proper disposal of swarf or burr is very important in jigs & fixtures design?
13. What provisions can be made to ease the handling of heavy jigs & fixtures?
14. Explain the advantages to be obtained from the use of pneumatic & hydraulic
clamping devices.
15. How can a lathe fixture be clamped to the lathe?
16. Write short notes on “Broaching fixtures”, “Assembly fixtures”.
17. What are the checks to be made for fixtures for (a) provision for maintenance,
(b) manufacturing & maintenance cost, (c) handling, (d) loading & unloading, (e)
storage, (f) human factors?
INDO-GERMAN TOOL ROOM, AHMEDABAD
TG2CHAPTER : A9 – GAUGES
JIGS, FIXTURES & GAUGES
199
A9.1 Introduction
Gauges are inspection tools of rigid design, without a scale, which serve to check the
dimensions of manufactured parts. Gauges do not indicate the actual valueof the inspected
dimension on the work. They can only be used for determining as to whether the inspected
parts are made within the specified limits. A workman checking a component with a gauge
does not have to make any calculations or to determine the a ctual dimensions of the part.
Gauges are easy to employ. This is one reason for their wide application in engineering.
Gauges differ from measuring instruments in the following respects :
(a) No adjustment in necessary in their use.
(b) They usually are not general-purpose instruments but are specially made for some
particular part, which is to be produced in sufficiently large quantities.
Gauging is used in preference to measuring when quantities are sufficiently high,
because it is faster and easier with resulting lower costs.
A9.1a Advantages and Disadvantages
Modern manufacturing requires extensive use of gauges for shop work, inspection, and
reference. Shop gauges are used by workmen. Inspection gauges are used by inspectors
to check manufactured product, and reference gauges are reserved for checking the other
two types.
A gauge is defined by the Sheffield Corporation as “a device for investigating the
dimensional fitness of a part for a specified function”. Gauging is defined bythe ANSI as “a
process of measuring manufactured materials to assure the specified uniformity of size and
contour required by industries.”
CHAPTER OUTLINE
A9.1 Introduction
A9.2 Classification of Gauges
TOPIC OUTLINE
A9.1a Advantages & disadvantages
of gauges
A9.2a Fixed gauges
A9.2b Advantages of fixed gauges
A9.2c Classification of fixed gauges
A9.2d Indicating gauges
A9.2e Special gauges
A9.2f Classification of Plain Gauges
INDO-GERMAN TOOL ROOM, AHMEDABAD
TG2CHAPTER : A9 – GAUGES
JIGS, FIXTURES & GAUGES
200
Basically, gauging accomplishes two things: (1) it controls the dimensions of a product
within the prescribed limitat ions, and (2) it segregates or rejects products that are outside
these limitations.
Gauging devices and gauging methods, like other phases of tooling in modern
manufacturing, have become standardized. Generally speaking, standardized components
that can be obtained commercially are assembled into a unit to gauge a particular product. It
is therefore quite important that the tool designer be familiar with gauging equipment and
practice.
It may be necessary to design special gauges for checking dimensions that do not
readily adapt to standard gauges. A gauge of this type may be quite simple, as shown in Fig.
Frequently time can be saved by the use of a simple length gauge in place of a machinist’s
rule when a quantity of workpieces is involved. It shoul d not be assumed that special
gauges are necessarily elaborate or that they are used only to measure close tolerances.
The tolerance of the workpiece in Figure may be as large as ±1/64 in., and a machinist’ s
scale would be sufficiently accurate for the job; however, it would take a little longer to read.
There are many gauging methods used to determine when a product conforms
dimensionally with drawings, specifications, or other prescribed requirements. Ho wever,
when these methods are analyzed, it will be found that they are designed to check one of the
seven basic elements of workpiece geometry :
Distance
Used to specify the relative location of the various components and elements of the
workpiece. Distance is measured by comparison to a known standard.
Flatness
Used to ensure that every element of a surface is within a specified distance from a nominal
surface plane. Determines straightness and alignment of a product.
Parallelism
Used to ensure that two flat surfaces are parallel to each other.
Perpendicularity (squareness)
Used to determine that two flat surfaces are normal to each other.
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
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Jig, fixture & guages theory
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Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
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Jig, fixture & guages theory
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Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
Jig, fixture & guages theory
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Jig, fixture & guages theory
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Jig, fixture & guages theory
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Jig, fixture & guages theory
Jig, fixture & guages theory

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Jig, fixture & guages theory

  • 1. TRAINER GUIDE - II JIGS, FIXTURES & GAUGES (1ST SEMESTER) PGTD / PDTD VERSION - 0 MSME TOOL ROOM INDO GERMAN TOOL ROOM AHMEDABAD
  • 2. TRAINER GUIDE - II FOR ADVANCE DIPLOMA IN TOOL & DIE MAKING Subject Area: Tool Design Theory – “Jigs, Fixtures & Gauges” ( Part – I & II)
  • 3. CONTENTS Chapter No. DESCRIPTION Page No. A1. Introduction To Production Toolings A1.1 Introduction Of Tools Used In Mass Production…………... 1 A2. Introduction To Jigs & Fixtures………………………………… 4 A3 Elements Of Jigs & Fixtures A3.1 Locators, Locating Methods & Devices……………………. 7 A3.2 Clamps, Clamping Methods & Devices……………………. 35 A3.3 Guiding Elements (Jig Bushings)…………………………... 60 A3.4 Tool Bodies (Jig & Fixture)………………………………….. 81 A3.5 Fasteners (Jig & Fixture)……………………………………. 84 A4 Limit, Fit & Tolerance A4.1 Introduction…………………………………………………… 96 A4.2 Advantages Of Limits & Fits………………………………… 97 A4.3 Tolerances …………………………………………………… 98 A4.4 Limits………………………………………………………….. 100 A4.5 Fits…………………………………………………………….. 101 A4.6 Types Of Assembly………………………………………….. 108 A4.7 Allowances …………………………………………………… 111 A4.8 Deviation ……………………………………………………... 112 A4.9 Maximum & Minimum Material Condition…………………. 115
  • 4. CONTENTS Chapter No. DESCRIPTION Page No. A5 Design A5.1 Design Of Jigs & Fixtures…………………………………… 118 A6 Jigs A6.1 Introduction…………………………………………………… 137 A6.2 Function Of Jigs & Fixtures…………………………………. 138 A6.3 Factor Characteristics In Jig Design……………………….. 138 A6.4 Jig Support……………………………………………………. 140 A6.5 Jig Bodies And Rigidity……………………………………… 140 A6.6 Classification Of Jigs………………………………………… 140 A6.7 Types Of Jigs & Their Description…………………………. 151 A6.8 Maintenance, Storage & Safety Of Jigs…………………… 152 A7 Fixtures A7.1 Introduction…………………………………………………… 155 A7.2 Basic Design Consideration………………………………… 155 A7.3 Factors In Fixture Design…………………………………… 156 A7.4 Classification Of Fixture…………………………………….. 158 A7.5 Maintenance, Safety & Storage Of Fixtures………………. 181 A8 Estimation A8.1 Introduction…………………………………………………… 183 A8.2 Purpose Of Cost Estimating………………………………… 183 A8.3 Elements Of Cost…………………………………………….. 184 A8.4 Cost Structure………………………………………………… 187 A8.5 Estimation Of Cost Elements……………………………….. 188 A8.6 Estimating Tool Cost………………………………………… 192
  • 5. Chapter No. DESCRIPTION Page No. A8.7 Steps In Making A Cost Estimation………………………… 194 A8.8 Chief Factures In Cost Estimation…………………………. 194 A8.9 Numerical Examples………………………………………… 195 A9 Gauges A9.1 Introduction…………………………………………………… 199 A9.2 Classification Of Gauges……………………………………. 201 A9.3 Design Of Gauges…………………………………………… 219 A9.4 Sub Zero Treatment…………………………………………. 229 A9.5 Maintenance, Safety & Storage Of Gauges………………. 229 A9.6 Numerical Examples………………………………………… 230
  • 6. INTRODUCTION A1.1 Introduction of Tools used in Mass Production Production of quality goods in large quantities at high speeds is the requirement of the day. To meet this, there have been considerable changes and developments in the manufacturing industries, with an empha sis on increased efficiency and productivity. As a sequel to these changes the tool technology has also undergone changes, leading to the designing and development of special tools, methods and techniques for the benefit of industry, to ensure quality products at economical rates. Jigs and fixtures are the special production tools which make the standard machine tool, more versatile to work as specialised machine tools. They are normally used in large scale production by semi -skilled operators, however t hey are also used in small scale production, when interchangeability is important. Manufacturing industries in India, on par with their counterpart elsewhere, have brought lot of revolution in manufacturing technology, during the (last 20 years, as a cons equence of which several developments like CNC Lathes, CNC Machine Centers, Flexible Manufacturing Systems, Fabrications Centre, Transfer Machines, Robotics, etc. took place). Our Engineers and Technologists are deeply involved in devising innovative 7 techniques. Lot of modernisation has taken place in Indian Industry. Even with these advancements in the manufacturing indsutries, there is a continued use of jigs and fixtures in some form or the other either independently or in combination with other systems. CHAPTER OUTLINE A1.1 – Introduction of tools used in Mass Production TOPIC OUTLINE A1.1a Jigs A1.1b Fixtures A1.1c Gauges A1.1d Press Tools A1.1e Moulds
  • 7. The work tooling refers to the hardware necessary to produce a particular product. The most common classification of types of tooling is as follows : 1. Sheet metal press working tools. 2. Moulds and dies for plastic moulding and die casting. 3. Forging dies for hot and cold forging. 4. Jigs and fixtures for guiding the tool and holding the workpiece. 5. Gauges and measuring instruments. 6. Cutting tools such as drills, reamers, milling cutters, broaches, taps etc. The tool maker manufactu res the above item from the design supplied to him. On gaining experience the tool maker will be able to design and manufacture simple tools. A1.1a Jigs A jig is a device that locates and holds the workpiece. It also guides and controls one or more c utting tools. Jigs are fitted with hardened steel bushings for guiding drills or other tools. Small jigs are not usually clamped to the machine. For holes above 6mm jigs are usually clamped. Drill jigs are used while drilling reaming counter boring, ta pping, chamfering etc. There is hardly a product produced that does not contain one or more holes. The location finish and size of these holes may be critical as in the case of a component for a missile or they may be holes like those punched in a templat e for the purpose of hanging it on the wall when not in use. Holes are produced and finished in a number of ways. They are drilled, reamed, bored, punched, ground, flame cut etc. Drilling is by far the most common method. A1.1b Fixtures A fixture is a device that locates and holds the workpiece. Setting blocks and feeler gauges are used for setting the cutter in relation to the workpiece. Fixtures designated for machining operations always clamped on to the machine. A fixtures is a device for hold ing a workpiece during machining operations. The name is derived from the fact that a fixture is always fastened to a machine or bench in a fixed position. Many machining operations can be performed by clamping the workpiece to the machine table without using a fixture, especially when a few parts are to be machined.
  • 8. However when the number of parts is large enough to justify its cost, a fixture is used for holding and locating the work. Further, when the profile of the Component is not regular or when machining has to be done w.r.t. a reference face or bore, application of fixture will be necessary. A1.1c Gauges Modern manufacturing requires extensive uses of gauges for shop work, inspection and reference. A gauge can be defined as a device for investigating the dimensional fitness of a part for a specified function. A1.1d Press Tools Press tools are special tools custom built to produce a particular component mainly out of sheet metal. The principle op erations of sheet stampings include cutting operations (Shearing, blanking, piercing etc.) and forming operations (bending, drawing etc.). Sheet metal items such as automobile parts (roofs fenders, caps etc.) components of aircraft, parts of business mach ines, household appliances, sheet metal parts of electronic equipments, precision parts required for horological industry etc. are manufactured by press tools. A1.1e Moulds for Plastics Plastics did not enter our lives with the fanfare of other revolu tionary inventions, but more by the process of infiltration. Plastics being synthetic materials were at first considered to be cheap substitute for the better known and more expensive materials. Plastic articles are not only replacing wood, metal and oth er materials but because of their particulars qualities they function better than other materials for specific purposes. Through the years plastics have carved the right as materials themselves and not as substitute for other materials. Not only are plastics more useful, adaptable and practical than the materials they have supplemented, but uses for plastics have been found for which no other material can be used.
  • 9. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 118 Maximum productivity at minimum cost is the demand of modern industry. To meet this requirements designing of efficient and accurate jigs and fixtures is required. Quality, simplicity and economy from the important criteria from the design of jigs and fixtures. To meet this requirement the designer will have to made an economic analysis for using jigs and fixtures and has to device certain principles of design, and finally develop a checklist for the jigs and fixture design. A5.1a Tool Design Objectives The main objective of tool design is to lower manufacturing costs while maintaining quality and increased production. To accomplish this, the tool designer must satisfy the following objectives: Design Economics Maximum productivity at minimal cost is t he demand of the day. Tool designer has therefore an additional consideration of keeping the cost of these special tools as low as possible apart from developing designs for efficient and accurate jigs and fixtures. For this he has to apply the design economy. i.e. to reduce the cost without sacrificing the quality. The following are some of the considerations involved in the economy design. 1. Simplicity 2. Preformed components 3. Standard components 4. Secondary operations 5. Tolerance and allowances 6. Simplified drawings TOPIC OUTLINE A5.1a Tool Design Objectives A5.1b Design Principles A5.1c Major factors in design of jigs & fixtures A5.1d Elements of design (jigs & fixtures) A5.1e Flow chart for development of design solution A5.1f Check list for the design of jigs & fixtures CHAPTER OUTLINE A5.1 Design of Jigs & Fixtures
  • 10. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 119 1. Simplicity Simplicity is essential in the tool design. Every element in the design of jigs and fixtures should be considered for possible savings in time and materials. 2. Preformed Materials These materials greatly reduce tooling costs by the elimination of many machining operations. Wherever practicable, preformed materials, such as drill rods, structural sections, pre machined bracket materials etc. should be included in the design. 3. Standard Components Commercially available standard components such as clamps, locators, supports, drill bushings, pins, screws, bolts, nuts etc. would contribute greatly in improving the tool quality besides effecting considerable savings in labour cost and time. 5. Secondary Operations Secondary operations such as grinding, heat treating and some machining should be as far as possible be eliminated as they involve additional time and cost. If they cannot be totally eliminated they should be limited to areas necessary for efficient tool operations. 5. Tolerances and Allowances Generally the tolerances of a jig or a fixture should be between 20 percent and 50 percent of the part tolerance, as unnecessarily close tolerances will be add up to the higher cost of the tool. 6. Simplified Drawings Tool drawings will for a sizable part of the total tooling cost, hence it is necessary to keep them low. This is accomplished by simplifying the drawings as follows : a) Wherever practicable words should replace drawn details. b) Elimination of redundant views, projections or details. c) When possible, replace drawn details with symbols. d) Reduce the drawing time by using templates and guides. e) Standard parts should only be drawn for clarity, not detail refer to these by part numbers or named.
  • 11. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 120 A5.1b Design Principles After the economic and design analysis the tool designer must comply with the following in designing the jigs and fixtures. 1. He has to thoroughly understand the component details, its pre -machined conditions, reference surface dimensions, accuracies and tolerances to be achieved. 2. He has to know on which machine the operations is likely to be performed. A check has to be made for constraints on the design parameters. 3. He has to make provision for easy loading and unloading of the work piece. 4. Facility for quick and accurate positioning of work piece be provided. 5. Fool proof method has to be incorporated to avoid wrong position while loading the work piece. 6. Designer has to take into account the optimum clearance with swarf removal and cleaning facility. 7. Machined surface are be taken as locating surface preferably. 8. Sharp corners in the locating surface must be avoided. 9. Adjustable locations are to be provided for right surfaces. 10. Locating surfaces should be as small as possible. 11. Locating pins should be tapered and easily accessible and visible to the operator. 12. Designer must have the economic approval to the design considerations. 13. As many degrees of freedom of movement should be arrested as necessary to achieve the required accuracies. In general 3-2-1 principle to be adopted. i.e. 3 Points in the first plane 2 Points in the second plane 1 Points in the third plane 14. Make the layout always to a scale, whenever possible. 15. The use of standard items in clamping, locating and fastening elements should be made, whenever possible. 16. Total engineering data in the drawing to be provided. I.e. material, heat treatment of the component, geometrical accuracies, toler ances, surface roughness for manufacturing and inspection purpose etc. 17. Stress to be given to minimise the weight of the jig or fixture for easy handling and to reduce the fatigue on the operator. 18. Care has to be taken for providing suitable support or guidance for preventing work piece bending or movement while operation and clamping.
  • 12. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 121 19. Attention to be given to tightening up of loose items of jig or fixture. 20. In jig and fixture layout a distinction between work piece and jig or fixture component to be brought by means of chain dotted lines for work piece and full lines for jig and fixture components. 21. Provision to be made for the setting gauges in fixture. 22. Machining table mounting requirements are to be considered while designing. 23. Bill of material to be provided. The good design of jigs / fixtures is that which satisfies the following a) Functional aspect b) Quality c) Cost d) Production schedule e) Safety f) Adaptability to the machine A5.1c Major Factors in the Design of Jigs & Fixtures In planning jigs and fixtures, it is essential to consider three major factors, which have a definite influence upon the design of tools. i) The tool should be designed for efficient operation and for easy manipulation by the operator. ii) The tool should be designed so that it will be produce accurate workpieces on a repetitive basis. iii) The cost of the tool should usually be governed by the number of parts to be produced. 1. Efficient Operation In determining how a jig or fixture can be best designed for its most efficient use by the operator, the following should be considered : i) Type of jig or fixture required for the specific part. ii) Locating and loading of the work a. Clearances necessary for locating the work. b. Methods of foolproofing against improper loading c. Unloading the work iii) Rapid methods of clamping the work.
  • 13. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 122 iv) Methods of handling the tool especially when it is large and heavy. v) Chip clearances and chip removal. vi) Wearing surfaces and replacement of worn parts. vii) Selection of materials for the special tool. viii) Safety in operation. 2. Accurate Workpieces The design of jigs and fixtures is influenced by the degree of accuracy requiredin the workpiece. The features of design will vary as the requirements for accuracy vary for a workpiece. This is one of the major factors to consider in the design of special tools. When multiple or subsequent operations are necessary, the same locating surface or surfaces on the workpiece should be used in each of the special tools required for the manufacture of that part. The accuracy necessary to obtain the propre relationship between a workpiece and other parts in an assembly is an important consideration in design. Some of the factors to be considered in this respect are : 1. The accurate relationship of operating surfaces on different parts when they are assembled? 2. Adequate rigidity to maintain accuracy in jig or fixture. 3. Economy & Cost The cost of the tool and the number of parts to be produced are other factors in determining the design. If a small quantity of parts is to be produced, a simple low cost tool may be satisfactory. The necessity of keeping the manufacturing cost of a new article as low as possible, or reducing the present cost of an existing article, usually determines the type of jig or fixture that is to be made. In some other instances, the cost of an operation may be reduced by using a more efficien t though more expensive tool. Increased accuracy and interchangeability secured through the use of a more elaborate tool frequently warrants its greater coat. The use of standard accessories requires serious consideration in the economical construction of jigs and fixtures. The designer should familiarize himself with the possible used of all types of standard accessories. Pre-fabricated units such as tool bodies, locating and clamping devices, drill jig bushings, and tool body supports should be used. Standard parts may be purchased or made in quantities and kept in stock.
  • 14. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 123 A5.1d Elements of Design (Jigs & Fixtures) 1. The work must be Located Properly The ease and rapidity with which the workpiece can be located and removed is an important conside ration in the design of special tools. Therefore, the designer should become thoroughly familiar with the various methods of locating and clamping before a design is definitely decided upon. Parts having rough or irregular surfaces, and parts which varyin size, usually present special problems in locating. To compensate for such irregularities or variations, adjustable locators should be used. The design of adjustable locating stops and supports should provide for positive location and for simple and easily accessible means of adjustment and locking. Locating points on jigs and fixtures should be designed so that incorrect loading of the workpiece is impossible, further more to obtain the proper balance, the locators should be place as far apart as the shape of the workpiece will allow. Consideration must also be given to the position of the locators to allow for the necessary clearance in loading and unloading. The locating points should be made wear resistant in order to maintain accuracy, especially when non-adjustable stops are used.
  • 15. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 124 Accuracy Location should be done on the most accurate surface of the workpiece. A machined surface is preferable to an unmachined one. When more than one machined surfaces are available, locate from the most accurate surface. For example, the center of the turned part can be located from outside diameters 110 or 80 or form central 50 f bore 80f has the minimum tolerance of 0.05, so the workpiece can be located most accurately from outside diameter 80f. Location form 50 f bore would be less accur ate than location from 80f but more precise than location from outside diameter 110f which has a much wider tolerance of 1mm (±0.5mm). 2. The work must be clamped properly The method of clamping and the design of the clamps depend upon the shape of the workpiece. The designer should consider the following fundamentals : i. The clamps should be positioned to resist the maximum pressure of the cutting tools. ii. The clamps should be located over or as near as possible to some bearing point of the workpiece. This must be done to avoid springing the part. iii. The clamps should be designed so that they can be quickly and easily unlocked and shifted out of the way of the workpiece when it is unloaded. iv. Complicated clamping devices should be avoided if possible. A simple device has fewer wearing surfaces and will stay in working condition for a longer time. v. The kind of material in the workpiece should be considered in choosing a design for the clamps. For example, finished surfaces or soft material require a larger clamping area than surfaces of hard material. The larger clamping face distributes the pressure so that the workpiece is not deformed or spoiled. The work supporting devices opposite the clamps should be large enough to support the pressure of the clamps. 3. Large tools – Weight & Handling Special tools designed for large workpieces are often unduly heavy. It is therefore desirable to make them as light as possible for easy manipulation without impairing their strength. Large cast iron tool bodies can be made lighter by coring out the metal. It is often possible to reduce weight by the use of well designed, fabricated and welded tool bodies, where heavy castings are to be drilled, reamed or bored on several sides, trunnions, thereby
  • 16. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 125 eliminating the otherwise difficult problem of lifting and turning jig faces up into the operating position. All corners should be well filleted for strength. Heavy tools should be provided with handles or holes for bars or eye bolts to facilitate lifting. In some cases it is often good design to provide smaller jigs with handling devices for convenience in holding them while the enclosed workpiece is machined. Sharp edges and corners which might injure the operator should be avoided. 4. Chip clearance Clearance must be allowed so that the chips will not accumulate and interfere with the cutting operation or workpiece location. The jigs should be designed so that it ca n be easily cleaned. Holes or escapes for draining the coolant or cutting lubricant should be provided. 5. Materials for Jig & Fixture Jigs and fixtures are made from a variety of materials, some of which can be hardened to resist wear. It is sometimes necessary to use nonferrous metals like phospher bronze to reduce wear of the mating parts, or nylons or fibre to prevent damage to the workpiece. Given below are the materials often used in jigs, fixtures, press tools, collects, etc. a. High Speed Steels (HSS) These contain 18% (or 22%) tungsten for toughness and cutting strength, 5.3% chromium for better hardenability and wear resistance and 1% vandadium for retention of hardness at high temperature (red hardness) and impact resistance. HSS can be air or oil hardened to RC 65-65 and are suitable for cutting tools such as drills, reamers and cutters. b. Die Steels These are also called high carbon (1.5 -2.3%) high chromium (12%) (HCHC) cold working steels and are used for cutting press tools and thread forming rolls. Hot die steels with lesser carbon (0.35%) and chromium (5%) but alloyed with molybdenum (1%) and vanadium (0.3 -1%) for retention of hardness at high temperature are used for high temperature work like forging, casting and extrusion.
  • 17. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 126 c. Carbon Steels These contain 0.85-1.18% carbon and can be oil hardened to RC62-63. These can be used for tools for cutting softer materials like wood work, agriculture, etc. and also for hadn tools such as files, chisels and razors. Theparts of jigs and fixtures like bushes and locators, which are subjected to heavy wear can also be made from carbon steels and hardened. d. Collet Steels (Spring Steels) These contain about 1% carbon and 0.5% Manganese. Spring steels are usually tempered to RC 57 hardness. e. Oil Hardening Non-Shrinking Tool Steels (OHNS) These contain 0.9-1.1% carbon, 0.5-2% tungsten and 0.55-1% carbon. These are used for fine parts such as taps, hand reamers, milling cutters, engraving tools, and intricate press tools which cannot be ground after hardening (RC 62). f. Case Hardening Steels These can be carburised and case hardened to provide 0.6 -1.5 thick, hard (RC 59 -63) exterior. 17Mn1Cr95 steel with 1% manganese and 0.95% chromium is widely used. 15Ni2Cr1Mo15 steel with additional nickel (2%) reduces thermal expansion up to 100 0 C. Case hardening steels are suitable for parts which require only local hardness on small wearing surfaces where costlier, difficult to machine full hardening tool steels are not warranted. g. High Tensile Steels These can be classified into medium carbon steels with 0.55% - 0.65% carbon (En8-9) and alloy steels like 50 Ni2Cr1m028 (En25). The tensile strength can be increased up to 125 kg/mm2 (RC50) by tempering. Medium carbon steels are used widely for fasteners and structural work while alloy steels are used for high stress applications like press rams. h. Mild Steel It is the cheapest and most widely used material in jigs and fixtures. It contains less than 0.3% carbon. It is economical to make parts which are not subjected to much wear and are not highly stressed from mild steel.
  • 18. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 127 i. Cast Iron It contains 2-2.5% carbon. As it can withstand vibrations well, it is used widely in milling fixtures. Self lubricating properties make cast iron suitable for machine slides and guide ways. The ingenious shaping of a casting and the pattern can save a lot of machining time. Although, the strength of cast iron is only half the strength of mild steel, a wide variety of grades have been developed. Nodular cast iron is as strong as mild steel, while meehanite castings have heat resistant, wear resistant, and corrosion resistant grades. j. Steel Castings These combine the strength of steel and shapabilly of a casting. k. Nylon and Fibre These are usually used as soft lining for clamps to prevent denting or damage to the workpiece under high clamping force. Nylon of fibre padsare screwed of stuck to mild steel clamps. l. Phospher Bronze It is widely used for replaceable nuts in screw operated feeding and clamping systems. Generally screw making process is time consuming and costly. So, their wear is minimised by using softer, shorter phospher bronze mating nuts. These can be replaced periodically. Phospher bronze is also used in applications calling for corrosion resistance, like boiler valves. 6. Construction of Jigs and Fixtures Jigs and fixtures bodies may be made of c ast iron, or they may be built up of steel plates or structural forms held together by screws and dowels or welded joints. Welded constructions are proving desirable because the bodies are strong and light, and addition alterations and additions to the tool can be effectively accomplished. The size of a tool, the quantity of workpieces to be made, and the cost of construction are important considerations in planning a design. 7. Replacement of Worn Parts
  • 19. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 128 Some parts of tools are subjected to so much wear that the accuracy of the tool may be impaired. For such tool parts, material that can be made wear resistant must be selected and the tool must be constructed so that worn out parts are easily replaceable. 8. Safety in Operation One of the most important considerations affecting the design of tools is the safety of the operator. Any features, which might cause injury, must be eliminated. Adequate operating accessories, such as suitable and efficient levers and locks are essential for safety in operation. The design of jigs and fixtures should provide means of clamping to the machine table if large tools are used or when tooling need not be shifted. Convenient holding devices should be provided as a safety factor whenever necessary. The tool must be designed so that it can be easily set up, adjusted, operated, and cleaned. Features, which safeguard the equipment against misuse, are also very important elements of design. A5.1e Flow chart for development of design solution
  • 20. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 129 FLOW CHART FOR DEVELOPMENT OF DESIGN SOLUTIONS Initial Design Concept Design procedure 1. Statement of the problem eg. To design a drill jig to hold a support bracket while drilling 3 – 6mm holes To design a lathe fixture for holding a pump housing for drilling and boring of bearing holes. Part Details Operation Classification Equipment Selection Operator Criteria Select Pertinent Items Select Pertinent Items Select Pertinent Items Select Pertinent Items Discarded ideas Preliminary Tool Design Cost Analysis & Evaluation Primary Tool Design Alternate 1 Tool Design Alternate 2 Tool Design Evaluation and Final Decision Completion of Design, Execution of Shop Drawings Phase 1 Phase 2 Phase 3
  • 21. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 130 2. Need Analysis: (Who, why, how, when, what and where about functional requirements) 3. Ideation (sketches) Information collection. 4. Analysis and synthesis 5. Tentative design solutions 6. Evaluation and testing 7. The finished design A5.1f Checklist for the Design of Jigs & Fixtures The following list of checkpoints should be consi dered before any design of jig or fixture is released for manufacturing. ¨ Check List for Tooling Layout 1. Is the tool layout the latest issued? 2. Is the part drawing the latest issued? 3. Is the part correctly shown on the layout? 4. Are the locating points provided using the thumb rule 3-2-1- -Three points in first plane - Two points in second plane - One point in third plane 5. Are practical considerations made in locating and clamping a part in jig or fixture? 6. Can locators be easily cleaned or replaced? 7. Is the jig or fixture of sound design? Are rigidity and simplicity taken into consideration? 8. Are the locators accessible for cleaning? 9. Are all the clamping requirements properly considered? 10. Are the individual components designed from the point of minimizing machining? 11. Is the layout made to scale? 12. Are the standard items used indicated? 13. Does using separate numbers with leaders and arrows pointing to the details identify the different parts? 14. Is the bill of materials provided in the drawing? 15. Are the details of operations, such as heat treatment and the surface roughness indicated in the drawings?
  • 22. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 131 ¨ The machine and Set Up 1. Will the jig or fixture fit into the machine for which it is intend? 2. Will the clamping slots or holes in the jig or fixture line up with the T-slots in the table? 3. Will the fixture, when in place, overhand at the end of the table? 4. Will the jig or fixture interfere with any other fixture next to it in the case of multispindle machine? 5. Can a set up operator see whether the cutter/drill are correctly set? 6. Can the cutting tools be adjusted and removed easily for sharpening when the fixture is in place? 7. Can setting blocks, bushings, stops, or collars be used in setting up the cutting tools? 8. Are suitable locating plugs for setting up, provided? 9. Does the set up operator need more than one size of wrench? 10. Are the hold down bolts to make tightening easy provided? ¨ Method of Location 1. Are the locating points widely placed? 2. Are the centralized means required to compensate for variations in the work piece. 3. Is the tolerance on locating points sufficiently close to obtain the specified operational accuracy? 4. Are the locating points as small in area as possible? 5. Are locators safe from damage by cutters? ¨ Method of Clamping 1. Are the loads static or dynamic? 2. Is the work piece supported as closely as possible to the point of load applications? 3. Is the cutting force resisted by a solid support and not by the clamp? 4. Can the cutting force be used conveniently to help securing the work piece? 5. Has the clamp sufficient range to accommodate allowable work piece variations? 6. Is the work piece directly supported under clamping points? 7. Will the clamping force unduly distort the work piece? 8. Will the clamp tend to loosen under cutter chatter or vibration? 9. Can it be planned to have a single standard wrench to tighten all the clamps? 10. Does the work piece size, the required clamping force, or the required speed of action warrant used of pneumatic or hydraulic clamping?
  • 23. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 132 ¨ Handling 1. Is the part within handling capacities by hand? 2. Is by hoist, are the facilities and necessary sling clearance, loading skills provided to ensure easy handling? 3. If by conveyer, is the correct height maintained? ¨ Loading / Unloading Work Pieces 1. Will cutters such as long drills interfere with work piece while loading or locating? 2. Will clamp interfere while loading or unloading? 3. Is the clearance sufficient to permit the work piece to be easily lifted over or into locating and centering devices? 4. Have any sliding pins or other hand -operated locators been provided with comfortable handles? 5. Are movable locators and adjustments on the side of the fixture nearest the operator? 6. Can the fixture be loaded with one hand while the other hand is used for loadi ng the completed work piece? 7. Are any burrs likely to interfere with unloading? 8. Should an ejector be provided? ¨ Thrust and Torque 1. Can satisfactory blockings be arranged to withstand cutter feed strains and distortion? 2. Are clamps carrying thrust load avoided? ¨ Chips 1. Are channels to allow the collate to wash the chips away provided? 2. Are blind spots and traps avoided? ¨ Capacity 1. Will the proposed design come within the column clearance capacity, table spindle travel of machine? 2. Are the jigs fit large enough to span T-slots in the machine table? 3. Is there sufficient clearance between tools and the work piece for easy gauging?
  • 24. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 133 ¨ Lubrication 1. Is the machine equipped for coolants? 2. Can suitable coolant reservoir be provided? ¨ Tool 1. Is the design ensures the use of standard tools (drills, reamers, milling cutters)? 2. Can use of existing special tools be incorporated? 3. Is provision made to secure all special tools needed? 4. Can the cutter flutes discharge chips even when covered up? ¨ Standard Parts 1. Does the proposed design incorporated fully the use of all standard parts carried I stock for jigs or fixtures? ¨ Loose Pieces 1. Can the loose parts (if inavoidable) be attached to the fixture with keeper screws or light chains? 2. Is the proper identification or storage facilities provided for the loose parts (Clamps, removable locating plugs and adapters)? ¨ Balance 1. Is the uniform mass distribution in design ensured? ¨ Progressive Experience 1. Does similar type of equipment exist if so what is your experience? 2. Are they suggestions from machine shop or tool room been considered and incorporated? ¨ Manufacturing Considerations a) Built up 1. Can the jig or fixture be built? Does the proposed design includes any impossible (difficult impractical) machining problems? 2. Does it confirm to all known machine capacities? b) Castings 1. Does the design land itself to good pattern making practices and economical castings?
  • 25. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 134 c) Welded Construction 1. Would welded steel construction offer any advantages? d) Material 1. Have proper steels / materials been selected to make the miscellaneous details? ¨ Production Requirement 1. Is the jig/fixture best suited to meet the requirements? 2. Will available machine burden, require manual semi or full automatic, single or multiple set up? ¨ Safety 1. Does the fixture design protect the operator from coolant spray or flying chips? 2. Is the designed tool safe to operate with? ¨ Sundry Requirements 1. Will the fixture design keep the length of the cutter travel to a minimum? 2. Will the operator, when positioning the jig, clearly able to see all bushings or cutter guides? 3. Are the bushings long enough to provide adequate tool support? 4. Do the tool need guiding for a second operations? 5. Can the use of slip bushings be avoided by used of stepped drills? 6. If the slip bushings are must, are the heads large enough, fluted for easy gr ipping, and provided with locking means? 7. Do all supporting pads and pins stand well clear of chip collecting surfaces? 8. Is the chip fouling the clamp lifting springs? 9. If the work piece is to be measured while still in the fixture can it be easily cleaned? 10. Is there sufficient clearance between tools and the work piece for easy gazing? 11. Can the tools be damaged or made inaccurate through incorrect insertion of the work piece? 12. Have breather holes been provided to allow escape of air from clo se firing plunger holes? 13. Provided the production and economics warrant, have all wearing parts been specified to be hardened? 14. Have means of setting the cutter or cutters in the correct position been provided? 15. In case of revolving features
  • 26. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 135 a) Has sufficient metal been left on jig to form an integral balance weight or provision for balance weight? b) Have arrangements been made for swarf to be shaken out or be blown out from interior of jig? c) Are projecting screws etc. covered in order to eliminate risk of injury to the operator (counter bored/counter sunk)? d) Have pilot made fool proof, if arranged so that the work cannot be inserted except in the correct way? 16. Is core out of unnecessary metal, making the fixture as light as possible, consistent with rigidity and stiffness made?
  • 27. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 136 Summary Quality and higher productivity are aimed through the use of jigs and fixtures in any production process. The jigs and fixtures add to the cost of tooling, as such their use in production should be justified economically. The design of jigs and fixtures need to satisfy, functional, qualitative, safety and adaptability aspects to a production techniques. Tool engineer has to adopt certain design principles for the jigs and fixtures. A check list for the design of jigs and fixtures before releasing it for manufacture will greatly help the production process with considerable saving in production time and cost. Questions 1. What are the different elements of design? 2. Explain the design steps for designing of jigs and fixtures? 3. How does the checklist help the tool designer in designing of good jig or fixture? 4. What are the major factors to be considered in design of jigs & fixtures? 5. Explain, why jig has a four feet not three? 6. What are the design principles to be followed while designing of jigs & fixtures?
  • 28. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 183 A8.1 Introduction Cost estimating may be defined as the process of forecasting the expenses that must be incurred to manufacture a product. These expenses take into consideration all expenditures involved in design and manufacturing, with all the related service facilities such as pattern making, tool making, as well as a portion of the general administration and selling costs. Cost estimates are the joint product of the engineer and the cost accountant, and involves two factors : physical data and cost ing data. The engineer as part of his job of planning manufacturing determines the physical data. The cost accountant compiles and applies the costing data. A8.2 Purpose of Cost Estimation Cost is the background of almost every decision the tool engin eer makes in organizing manufacturing operations and in selecting materials, methods, tooling and facilities. An understanding of cost determination is essential to ensure that these decisions are based on sound and dependable estimates of cost. Estimates of cost must be reasonably accurate if a venture is to be successful (realistic cost estimate). If a job is overpriced, it is lost to a competitor. If it is underestimated, it results in financial loss. Detailed cost estimates are prepared to: CHAPTER OUTLINE A8.1 Introduction A8.2 Purpose of cost estimation A8.3 Elements of cost A8.4 Cost structure A8.5 Estimation of cost elements A8.6 Estimating tool cost A8.8 Steps in making of cost estimation A8.8 Chip factors in cost estimation A8.9 Numerical examples CHAPTER OUTLINE A8.3a Material cost A8.3b Labour cost A8.3c Expenses A8.5a Direct Material cost A8.5b Direct labour cost A8.5c Indirect expenses A8.5d Direct expenses
  • 29. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 184 1. Determine the selling price of a product for a quotation or contract, so as to ensure a reasonable profit to the company. 2. Check the quotations supplied by the vendors. 3. Decide whether a part or assembly is economical to be manufactured in the plant or is to be purchased from outside. 4. Determine the most economical process or material to manufacture a product. 5. Initiate means of cost reduction in existing production facilities by using new materials, which result in savings due to lower scrap loss and re vised methods of tooling and processing. 6. To determine standards of production performance that may be used to control costs. A8.3 Elements of Cost The constituents of cost of a product or the “cost elements” are : Material cost, Labour cost and Expenses. We shall discuss each element in turn. a) Material cost Material is divided into two basic categories: (a) material for fabricated parts (b) standard purchased parts. The total cost of thesetwo will give the material cost. Again there are two kinds of materials, which comprise the factory cost of a product. These are : Direct material and Indirect material. i) Direct Material The direct material is the raw material, which is processed in the plant and finally forms the finished product. Any standard part, which also becomes a part of the finished product, will also come under the category of direct material. ii) Indirect Material Indirect materials are those, which hel p in the processing of direct materials into the finished product. These materials don’t form a part of the finished product. Indirect materials include: Shop supplies such as cotton waste, lubricating oil, cutting fluids, coal, oil,
  • 30. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 185 gas, shielding gases used in Arc welding, Emery paper used for polishing, quenching oils for heat treatment etc. Indirect materials form the part of on cost or overheads. b) Labour Cost Labour, which enters into the manufacture of a product, is of two categories : Direct Labour and Indirect Labour. i) Direct Labour The operator or operators, which actually process the raw materials either on machines or manually, form the direct labour. ii) Indirect Labour All the staff excepting administrative and sales office staff, wh ich helps in running the plant, comes under the category of indirect labour. Indirect labour includes: Foremen, supervisors, maintenance staff, stores personnel, time office staff, drawing office staff, etc. Indirect labour forms a part of overheads. c) Expenses Total cost of the product minus the costs of direct material and direct labour constitutes the ‘Expenses’. Expenses may also be either direct or indirect. i) Direct Expenses These expenses like the direct material and direct labour are directly chargeable to the finished product. These are also known as “chargeable expenses”. These include: a) Cost of patterns, jigs, fixtures, dies, drawings or designs specifically prepared for a particular product, which cannot be used for other purposes. b) Cost of any experimental work done specially for a particular product. c) Cost of inward carriage or freight incurred on supply of special material needed for the particular product. d) Hire of special or single purpose tools or equipment for a particular product.
  • 31. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 186 ii) Indirect Expenses These are also called “oncosts” “overheads” or “burden”. These include: cost of indirect material, cost of indirect labour and other expenses that cannot be conveniently charged directly to a particular job. Indirect expenses may be divided into: a) Factory expenses or overheads. b) Office and Administrative expenses or overheads. c) Selling and Distribution expenses or overheads. ¨ Factory expenses These expenses include : indirect materials, indirect labour, expenses, insurance, maintenance and depreciation of machine, power etc. ¨ Office and administrative expenses These expenses consist of all expenses incurred in the direction, control and administration of an undertaking. These expenses include : rent and rates of office premises, salaries of office staff, printing and stationery, postage, salaries of high officers, depreciation of office equipment and insurance on office equipment. ¨ Selling and distribution expenses These expenses include : salaries of sales staff, publicity and advertisement, catalogues, leaflets and price lists, packing and forwarding charges, godown rent, commission to salesmen etc. The overheads may be grouped into two main categories: 1. Fixed overheads or constant overheads These are such items of indirect expenses, which remain constant or fixed irrespective of volume of production. These items include : salaries of higher officers (administrative and management executives), capital taxes, insurance charges, depreciation of building, plant machinery etc., rent of buildings.
  • 32. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 187 2. Variable or floating overheads These are such items of overheads, which vary, with the volume of production. Such items are : internal tran sport charges, power, fuel, stores expense, factory lighting and heating and sales office expenses and repairs of machine tools. Since fixed overheads remain constant irrespective of volume of production, production should be increased to reduce the cost o f the part. There should be some minimum production to meet the fixed expenses and start earning profit. A8.4 Cost Structure The elements of cost can be combined to give following types of cost: 1. Prime cost. Prime cost or direct cost is given as : Prime cost = Direct material + Direct labour + Direct expenses (if any) 2. Factory cost. This cost is given as: Factory cost = Prime cost + Factory expenses. Factory cost is also called as “Works cost”. 3. Manufacturing cost. Manufacturing cost or cost of production is given as: Manufacturing cost = Factory cost + Administrative expenses. 4. Total cost. Total cost is given as: Total cost = Manufacturing cost + Selling and Distribution expenses. 5. Selling price. Selling price is given as: Selling price = Total cost + Profit. The above mentioned cost structure is explained with the help of a block diagram as follows:
  • 33. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 188 Selling price Total cost Profit Manufacturing cost Selling expenses Factory cost Administration expenses Prime cost Factory expenses Directmaterial + Directlabour + Directexpenses (ifany) A8.5 Estimation of Cost Elements Directly material cost, direct labour cost and directs expenses can be found out most accurately by the estimat ing procedure. The indirect expenses items, which are so numerous, are determined by cost accounting section only, which furnishes the figure department wise to the estimator. The various cost elements are estimated in the manner give below. a) Direct Material Cost The cost of standard purchased parts can be obtained from the purchasing section. The raw material chargeable to a product is that in the rough state and includes all scrap removed. Material can be in the form of sheet metal, bar stock, forgings or castings, plastic etc. The weight of the material can be determined from the drawing of the part. An irregular part is divided into simple sections to calculate its volume. Volume is multiplied by density of the material to find its weight. The weight of a part multiplied by the unit cost of the material gives the material cost per piece. If the unit cost covers only the purchase price of the material, the material cost is multiplied by one or more additional factors to account for bulk losses, purchasing and handling costs. b) Direct Labour Cost
  • 34. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 189 For estimating the direct labour cost of a product, the job is divided into operations needed to machine it, and then estimating the operation time for each operation. Total time multiplied by a labour rate gives the direct labour cost. The total time required to perform an operation may be divided into the following parts: i) Set-up time This is the time needed to prepare for the operation and may include : time of study the blueprint or to do paper work, time to get tools from the crib and the time to install the tools also on the machine. ii) Man or handling time This is the time the operator spends loading and unloading the work, manipulating the tools and the machine and making measurements during each cycle of operation. iii) Machining time The elements comprising the machining time are those, which are performed by the machine. This is the time during each cycle of operation that the machine is working or the tools are cutting. iv) Tear down time This is the time required to remove the tools from the machine and to clean the tools and the machine after the last component of the batch has been machined. Tear down time is usually small. It will seldom run over 10 minutes on the aver age machine in the stop. It may require only a few minutes to tear down a set up on a drilling press and 10 to 15 minutes on the average miller or turret lathe. In exceptional cases, it may go upto as high as 30 minutes on very large boring mills and large milling machines. v) Down or lost time This is the unavoidable time lost by the operator due to breakdowns, waiting for the tools and materials etc. vi) Allowance
  • 35. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 190 The total time to perform an operation also includes time for personal needs of the operator, time to change or resharpen the tools etc. The time all these allowances are taken to be about 20 percent of the sum of all other times and then the total time for the operation is obtained. (a) Personal allowances. This is time taken by the operator to attend to his personal needs such as going to lavatory, taking a cup of tea, smoking etc. The time for this is usually taken to be about 5 percent of the total time. (b) Fatigue. The efficiency of the worker decreases due to fatigue or working at a stretch and also due to working conditions such as poor lighting, heating or ventilation. The efficiency is also affected by the psychology of the worker, which may be due to domestic worries, job security etc. For normal work, the allowance for fatigue is about 5 percent of the other times. This allowance can be increase depending upon the type and nature of work and working conditions. (c) Time to change or resharpen tools. Some allowances should also be provided for the time taken by the operator to get the tools changed or to resharpen the tools. This time varies from machine to machine. (d) Inspection or checking allowance. To maintain the uniform quality of the parts, the dimensions of the parts should be checked or inspected at regular intervals depending upon the closeness of tolerances. The checking times for the various instruments are given below, to check one dimension : With rule 0.10 minute Vernier caliper 0.50 minute Inside caliper 0.10 minute Outside caliper 0.05 minute Inside micrometer 0.30 minute Outside micrometer 0.15 minute Depth micrometer 0.20 minute Dial micrometer 0.30 minute Thread micrometer 0.025 minute Plug gauge 0.20 minute Snap gauge 0.10 minute
  • 36. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 191 Set up time and tear down time are perfor med usually once for each lot or batch of parts. Set up time per piece is obtained by dividing the set up time for the machine by the number of pieces produced in lot. Set up time, handling time and tear down time are estimated from previous performanceson similar operations. All work on a particular type of machining tool consists of a limited number of elements. These elements can be standardized, measured and recorded. This is done under Time and Motion study. Standard data is available for set uptime, tear down time and handling time. Machining time is obtained with the help of formulas for each machining operation, which takes into account speeds of cutting, feeds, and depth of cut and tool travel. The actual amount of down or lost time that will occur in a particular operation can scarcely be predicted. Some operations will run smoothly, others may be beset by troubles. The sum of machining time and the handling time is called ‘run time’ or ‘unit of operation time’. The total time to manufa cture a product (from which the direct labour cost will be estimated) may be divided into the following major groups : 1. Set up time 2. Machining time 3. Non machining time 4. Down time The tear down time discussed earlier may be included in the set up t ime itself. The non-machining times will be man or handling time, personal needs, fatigue, cutter or tool sharpening and inspection. The man or handling time, as already discussed, includes : loading and clamping the part, unloading the part, advancing or retracting the cutting tool, tightening a chuck, a trial cut, trial gauging, debarring the machine, cleaning the fixture etc. c) Indirect Expenses Indirect expenses or overheads are those charges which vary in proportion to the production rate, but which are not easily attributable directly to a given operation or part. These expenses are apportioned among the operational units (machines, plants etc.) according to some weighting factor. 1. Percentage of direct labour cost 2. Percentage of direct labour hours 3. Machine hour method 4. Direct material method
  • 37. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 192 5. Unit of production method 6. Space rate method d) Direct Expenses The direct expenses are estimated in the following manner : The engineering (preparation of drawings, blue prints, drafting etc.) and design cost of a product is calculated as a flat hourly rate for each estimated hour of design and engineering time. On the same lines, the cost of any experimental work done specifically for a particular product, can be estimated. The cost of patterns and special tools such as jigs, fixtures, dies and gauges can be estimated as outlined below: ¨ Tool cost Generally tool cost estimating is concerned with tool and other special equipment to be used for production. Much product cost estimating depends upon tooling cost estimating. Therefore, tool costs are often treated separately during a product cost study. Tooling costs are estimated to : 1. Determine how much must be invested i n tools and equipment to manufacture a product. 2. Determine the cost of alternate methods of tooling to help in selecting the more economical method. 3. Find the cost of a proposed machine or tool that promises to produce more economically method. 4. Determine the reasonable cost of a special machine or tool to gauge whether tool room performance is efficient or vendor’s prices are reasonable. The cost of cutting tools, both special and standard, jigs, fixtures, dies and gauges and other special equipment is often separated from the cost of machine tools. This is done to determine what portion of the tooling cost is to be charged directly to the project or proposal and what portion is to be capitalized. A8.6 Estimating tool cost Tooling cost is estimated in the same manner as the cost of manufacturing of a product is estimated. There are four major factors in the cost of any tool. These are: material, labour
  • 38. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 193 and overheads or burden as in manufacturing costs. The fourth factor is the cost to engineer and design the tool, that is, the cost of designing and drafting the tool. i) Engineering and design cost The engineering and design cost represents a large portion of the total cost of the tool, often as high as 20 to 30 percent. This cost can be directlycharged to the individual tool and as a consequence are always considered first in establishing an estimate, the engineering and design cost is applied to the tool cost estimate as a flat hourly rate for each estimated hour of design and engineering time. ii) Tool materials cost The cost of the material for a proposed tool may be calculated and therefore becomes more of an actual cost than an estimated cost. It is the most accurate item in the tool cost estimate and is determined as discussed before. Sta ndard parts such as knobs, hand wheels, bushings, bolts, screws, springs, and similar items that complete the tool bill of material can be accurately priced from catalogues, price lists or invoices. iii) Tool labour cost The labour involved in the machining, assembling, fitting, and tryout of tools is always difficult to estimate accurately, even for the most experienced estimator. The machining time can be calculated. The other times cannot be calculated and must b esti mated. It is difficult to foresee all the problems that may develop in fitting assembly and try out operations, even under most favourable conditions. Therefore, the estimate must include a liberal factor of safety for lost time, which is impossible to an ticipate and will always be present. The sum of all the times will be the labour time for the tool. When multiplied by the toolmakers hourly rate, the estimated labour cost is determined. iv) Burden or overhead The tool room may be considered as a depar tment, and therefore may have an established burden rate, just as production department has. It general, the man -hour or direct labour cost method of burden distribution is applied as discussed under point 3 above. A8.8 Steps in making a cost estimation
  • 39. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 194 The cost of a new product may be estimated by following the basic steps given below : 1. Make a complete and thorough analysis of the cost request to understand it fully. 2. Make an analysis of the part or product and make separate lists of standard partsand the parts to be fabricated within the plant. 3. Make a manufacturing process plan for the parts to be fabricated. 4. Determine the material costs for the standard and the fabricated parts. 5. Estimate the total production time for each operation listed in step 3. 6. Apply the labour and burden rates to each operation. 7. Add the material costs (step 4) and the labour and burden costs (step 6). This will give the total manufacturing cost. 8. Apply the profit factors to arrive at the selling price. A8.8 Chief factors in cost estimation Each cost estimate may not be exactly the same as the actual manufacturing cost. The most significant causes for the cost deviations can be : Fluctuations in ma terial and labour costs, incomplete design information at the time of estimate, unexpected delays resulting in premiums paid for overtimes and materials and the unexpected machining or assembly problems. However, the average of cost estimates over a perio d of time should be reasonably close to the actual manufacturing costs. For this, the following factors should be considered for arriving at an accurate and complete cost estimate : 1. Each estimate should contain complete costs of direct material, direct labour, factory overheads, spoilage, engineering, administration and selling. 2. If the cost of a new product is estimated on the basis of previous estimates of comparable parts, detailed estimating should be used. It is necessary to make substitutes in the past estimates for individual operations, individual parts or individual sub assemblies. 3. The period of time between the cost estimating and the actual production of the part affects the determination of unit prices. During the intervening period, the basic material and labour rates may rise or fall. Therefore, an estimate of what the cost will be at the time of actual production is what is really needed. Thus the estimator should have the ability to project thinking and reasoning into the future.
  • 40. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 195 4. The volume of the pieces to be produced also affects the costing rates since the time and therefore the cost of performing an operation decreases as the number of units produced is increased. 5. The addition of new type of equipment and special buildings require the development of new overhead rates etc. A8.9 Numerical Examples From the following data, calculate the total cost and selling price for a job : Direct material = Rs. 5500 Manufacturing wages = Rs. 3000 Factory overheads to manufacturing wages = 100% Non manufacturing overheads to factory cost = 15% Profit on total cost = 12% Solution. Direct material = Rs. 5500 Manufacturing wages (Direct labour) = Rs. 3000 Factory overheads = 100% of Rs. 3000 = Rs. 3000 Factory cost = Direct material + Direct labour + Factory overheads = Rs. 5500 + 3000 + 3000 = Rs. 11,500. Non-manufacturing overheads, i.e., administrative and selling overheads = 15% of Rs. 11,500 = Rs. 1825 Total cost = Factory cost + Rs. 1825 = Rs. 11,500 + Rs. 1825 = Rs. 13,225 Profit = 12% of total cost = Rs. 1588 Selling price = Total cost + Profit = Rs. 14,81
  • 41. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 196 Table 1 - Weight of Rolled Steel (8.843 gm/cc) Size S Round S f kg/metre length Square S Sq. Hexagonal S A/F Sheet S Thick kg/sq. metre 5 5.5 6 8 8 9 10 11 12 14 16 18 18 19 20 22 25 28 28 32 35 36 38 40 41 45 50 55 56 60 63 65 80 81 85 80 0.154 0.19 0.222 0.302 0.395 0.50 0.62 0.85 0.89 1.21 1.58 2.00 2.48 2.98 3.85 4.83 6.31 8.99 9.86 12.49 15.41 18.8 19.34 22.2 24.48 26.0 30.2 31.08 34.8 39.46 0.20 0.24 0.28 0.38 0.502 0.64 0.885 0.95 1.13 1.54 2.01 2.54 2.83 3.14 3.80 4.91 5.82 6.15 8.04 10.18 12.56 15.90 19.62 23.8 24.62 28.3 31.16 33.2 38.5 39.58 44.2 50.24 0.180 0.206 0.245 0.333 0.435 0.551 0.68 0.823 1.33 1.96 2.45 3.29 4.96 6.96 8.81 11.4 18.0 20.6 24.5 28.8 33.3 38.2 43.5 39.2 55 88.5 94.2 109.9 125.6 141.3 182.8 196.2 219.84 251.14 284.68 298.22 313.92 331.36 364.46 398.63 430.88 463.9 498.04 530.18
  • 42. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 197 Table 2 - Thumb Rules for Estimation Group No. Operations involved Total costs 1. Only shaping, turning drilling and fitting. Three times the raw materials cost. 2. Shaping / turning, drilling, fitting and milling. Four times the raw materials costs. 3. Above operations plus heat treatment. Five times the raw materials costs. 4. Above operations plus precision grinding or lapping. Six times the raw materials costs. Example Find the manufacturing cost of 14 f bore collared bush shown in figure. The bush is to be manufactured form 28 f x 35 long alloy steel bar which costs Rs.80 / kg. After rough turning, the bush is to be hardened and finished by grinding. Solution: Referring to Table1, we note that 28 f Steel bar weights 4.83 kg/metre. Wt. Of a 35 long piece = 83.4x 100 35 = 0.169 kg. Cost of material at Rs.80 = 0.169 x 80 kg = Rs. 13.52 Machining involves turning, hardening, and grinding. Referring to table 2, we notice that the bush falls in Group 4, for which the total cost is approximately six times the raw material cost. Total cost = 13.52 x 6 = Rs. 81.12 If the bush is to be sold, profit should be added. Selling cost at 30% profit = 81.12 x 1.3 = Rs. 105.5 (min.) Selling cost at 100% profit = 81.12 x 2 = Rs. 162.24 (max.)
  • 43. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 198 SUMMARY Quality and higher productivity are aimed through the use of jigs and fixtures in any production process. The jigs and fixture s add to the cost of tooling, as such their use in production should be justified economically. The design of jigs and fixtures need to satisfy, functional, qualitative safety and adaptability aspects to a production techniques. Tool engineer has to adopt certain design principles for the jigs and fixtures. A check list for the design of jigs or fixtures before releasing it for manufacture will greatly help the production process with considerable saving in production time and cost. Questions 1) Define cost estimating. 2) What is the purpose of cost estimating? 3) Name the various constituents of cost. 4) Name the indirect material and indirect labour. 5) What are : direct expenses, indirect expenses, factory expenses, office and administrative expenses, selling and distribution expenses. 6) Define : Prime cost, Factory cost, Manufacturing cost. Total cost and Selling price. 7) Define : Set up time, Handling time, Machining time. Tear down time and down or lost time. 8) List the various steps of cost estimating. 9) Discuss the chief factors in cost estimating. 10) Find the manufacturing cost of 14 f bore collared bush shown in figure. The bush is to be manufactured form 28 f x 35 long alloy steel bar which costs Rs.80 / kg. After rough turning, the bush is to be hardened and finished by grinding. 11) From the following data, calculate the total cost and selling price for a job : Direct material = Rs. 5500 Manufacturing wages = Rs. 3000 Factory overheads to manufacturing wages = 100% Non manufacturing overheads to factory cost = 15% Profit on total cost = 12%
  • 44. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 155 A7.1 Introduction Maximum productivity at minimum cost is the demand of modern industry. To meet this requirement designing of efficient and accurate jigs and fixtures is required. Quality, simplicity and economy from the important criteria for the design of jigs and fixtures. To meet this requirement the designer will have to make an economic analysis for using jigs and fixtures and has to device certain principles of design, and finally develop a checklist for the jigs and fixture design. A7.2 Basic Design Considerations In addition to locating and holding the part, the designer must also consider several other factors before a welding jig or fixture can be designed. Heat dissipation is an important consideration with any welding tool. Several methods can be usedto insure that proper heat is maintained in the weld area. The primary factor that determines the amount of heat required is the metal being joined. When metals such as steel and other poor heat conductors are joined, the excess heat should be carried of f to prevent overheating the weld. To do this, backup bars of copper, titanium, or beryllium can be used. For metals that are good conductors or heat, such as copper or aluminum, too rapid cooling becomes the problem. To prevent this, the fixture or jig must be made to contact the part in as small an area as possible. Clamping supports must be provided to prevent distorting the work while it is in a heated condition. Whenever possible, place clamps directly over the supporting elements. Locators should be positioned so that the distortion will cause the part to loosen rather then tighten against the locators. If this is not possible, either power or manual ejectors should be built into the tool. Foolproofing is one feature that is necessary for any t ype of welding jig or fixture. Each tool must be designed so the part will only fit into its proper position. CHAPTER OUTLINE A7.1 Introduction A7.2 Basic Design consideration A7.3 Factors in fixture design A7.4 Classification of fixtures A7.5 Maintenance, Safety and Storage TOPIC OUTLINE A7.4a Types of fixtures based on how the tool is built A7.4b Types of fixtures based on the type of machine on which they are used A7.4c Types of fixtures & their descriptions
  • 45. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 156 A7.3 Factors in Fixture Design The major difference between a drill jig and a fixture is that a jig has hardened bushings which guide the dril l, while a fixture for a machining operation is attached to the work table of a machine to hold the workpiece in a fixed position for the action of milling cutters, broaches, or other of cutting tools. Work may be entirely enclosed in a drill jig and is r eached for the drilling or boring through bushings provided for that purpose. However, to enclose the work in a milling fixture would defeat the purpose for which the fixture designed that of securely holding the work in position for the action of cutting tools. The clamps used in fixtures are applied so that the work surface is clear for machining. Milling fixtures are usually fastened to the table of the machine upon which they are to be used. As a rule, they are flat on the bottom so that they will r est securely against the table upon which they are clamped. Clamping lugs or other clamping surfaces are generally provided. The cutter operating with a fixture is usually in a fixed position. The work held in the fixture is “fed” to the revolving cutter by means of the movable table. Broaching fixtures, on the other hand, are usually stationary with the broach travelling through the work. Cutting tool chatter can be greatly reduced by carefully designed work holding devices. These must be designed so that they are properly proportioned and sufficient in number to support and hold the work rigidly in the fixture. One of the major factors in the design of a milling fixture is providing a place in the fixture for the workpiece to resist the thrust of the cutter. The thrust of the cutter should be against the body of the fixture, rather than against the clamps. The direction of rotating of the cutter often governs the placing of the work. An attempt must be made, therefore, to design the tool so that the body of the fixture takes this thrust. The design of fixtures should also permit the use of the clamping collars used on the cutter arbor. Often, though, it is not good economy to use small diameter cutters that allow only a minimum clearance of 3mm or less between the arbor and the work or a projecting part of the fixture, because of the necessity of sharpening the cutter which will reduce the clearance. ¨ Design Points Are all parts well designed to take the loads imposed on them in service? Is the tool study enough to stand considerable abuse? Is the fixture amply proportioned to damp out vibration and chatter? This applies especially to milling fixtures.
  • 46. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 157 Is the design of all parts and mechanisms as simple as possible? Have cylindrical plungers and holes been used in preference to square or polymer once? Have holes for headed pressed in parts (such as for accurate location of resigns) been countersunk to allow any excess press lubricant to collect in the countersink (allowing the rest pin to vibrate slightly in service) instead of gradually squeezing out under the head? Have spring pocket holes been countersunk on their open ends? Are the dowel pins in each part as widely spaced as practicable? Where detachable parts need very accurate location, have register keys or pins been used instead of dowels? Is the accuracy of the operation such that the base of the fixture should be scraped to fit the machine table? Have breather holes been drilled to allow air to escape from lose fitting plunger holes? Is it possible to forecast any part design changes and to make allowance for them in the design of the fixture? ¨ The procedure in developing designs for fixtures is similar to the procedure followed in designing jigs. A7.4 Classification of Fixture Fixture Types of Fixtures based on how the tool is built Types of Fixtures based on the type of machine on which they are used
  • 47. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 158 The jigs and fixtures used for welding can generally be limited to three basic types; tacking, welding, and holding. Types of Fixtures based on how the tool is built Plate fixture Angle plate fixture Vice jaw fixture Indexing fixture Multistation fixture Profile fixture Types of Fixtures based on the type of machine on which they are used Turning fixture Milling fixture Planning fixture Broaching fixture Grinding fixture Shaping fixture Shaving fixture Forming fixture Stamping fixture Welding, brazing, soldering fixture Assembly fixture Inspection fixture Testing fixture Heat treatment fixture Honing fixture Lapping fixture
  • 48. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 159 Tacking jigs and fixtures are used to hold the parts of an assembly in their proper position so they canbe tack welded together. These tools are generally used for assemblies that must be held together in several places to prevent warping or distortion when welding is complete. Parts assembled in a tacking jig or fixture are removed after tacking and eith er finished without special tools or transferred to a holding jig or fixture. Welding jigs or fixtures are used to hold the parts of an assembly in position for welding. The difference between welding and tacking is the amount of welding performed. The tacking tool is used only when the part is to be tack welded. When the part is to be completely welded together, a welding jig or fixture is used. Welding jigs and fixtures are normally built heavier than tacking tools to resist the added forces caused by the heat within the part. Holding jigs and fixtures are used to finish tack welded assemblies. Like welding tools, holding jigs and fixtures must be made rigid enough to prevent distortion and warping. Generally fixtures are classified as - 1) How tool is built a) Plate Fixture b) Angle plate c) Vice jaw d) Indexing e) Multistation f) Profiling 2) Machine on which they used a) Turning b) Milling c) Planning, shaping & slotting d) Broaching e) Grinding f) Holding, Broaching and soldering g) Assembly h) Inspection Types of Fixtures The fixtures are classified based on i. How the tool is built
  • 49. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 160 ii. The type of machine on which they are used Though jigs and fixtures are made basically the same way, due the increased tool forces the fixtures are built stronger a heavier than jigs. 7.4a Types of Fixtures based on how the tool is built 1. Plate Fixtures These are the simplest form of fixtures used for most machining operations. This fixtures consists of flat plate with variety of clamps and locators. Its adaptability to several machining operations makes it a popular type of fixtures. 2. Angle Plate Fixtures These fixtures are used to machine at right angles to its locator. If machining is to be carried out at other angles a modified angle plate fixtures can be used.
  • 50. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 161 3. Vise jaw Fixtures These are used for machining small parts. With this type of tool, the standard vise jaws are replaced with jaws which are formed to fit the part. These are least expansible and are limited by the size of vises available.
  • 51. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 162 4. Indexing Fixtures These are similar to indexing jigs and are used for machining parts which must have machined details evenly placed.
  • 52. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 163 5. Multistation Fixtures These are intended for high speed, high vo lume production runs, where the machining cycle is continuous. Duplex fixtures are two station fixtures and are the simplest of the multistation fixtures. This form of fixture allows both the loading and unloading operations while the machining operation is in progress. For example, once the machining operation is complete at station one, the tool is revolved and the cycle repeated at station two. At the same time, the part is unloaded at station one and a fresh part loaded. 6. Profiling Fixtures These are used to guide tools for machining contours which the machine cannot normally follow. These contours can be either internal or external. Since the fixtures continuously contacts the tool an incorrectly cut shape is almost impossible.
  • 53. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 164 7.4b Types of Fixtures Based on The Type Of Machine on Which They Are Used Fixtures are generally clas sified based on the machining on which they are used. The following are some of the common production operations in which fixtures are used : 1.Turning 2.Milling 3.Grinding 4.Welding, brazing, soldering fixtures 5.Assembly fixtures / Inspection fixtures 1. Turning Fixtures These fixtures are used for turning, facing and boring operations, and mainly consist of workpiece locating and clamping elements. The standard fixtures that are used in a Lathe machines are, three jaw and four jaw chucks, collectsface plate etc. These jaw chucks are used for holding round hexagonal or other symmetrical works. Collects are used for bar stock. Special jaw chucks, face plates with clamping devices are used for holding irregular shaped turning jobs.
  • 54. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 165 CLASSIFICATION OF LATHE FIXTURES Following points are to be noted while designing turning fixtures. i) Grip the rotating work piece to the fixture to resist torsional forces. ii) The fixture should be rigid with minimum possible overhang. iii) Locate the work piece on critical surfaces, which are the areas from which all or major dimensional or angular tolerances are taken. iv) Provides adequate support for frail sections or sections or sections under pressure from turning tools. v) Balance the fixtures to avoid vibrations. vi) Fixtures should not have any projections, as they will cause injury to the operator. vii) A pilot bush for supporting tools should be provided where extreme accuracy is required in boring operations. Figure shows a typical turning fixtures. The fixture body is located on the machine spindle and bolted in position, it carries the work piece location and clamping system. Figure shows yet another special turning fixture in which work piece is located and clamped to a shelf that projects from the fixture body. The fixture incorporates a balance weight (the fixture would other wise be out of balance) and a pilot bush to guide the boring bar. Lathe Fixtures Chuck Type Face Plate Arbor Special Collet Type Pot TypeMandrel
  • 55. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 166
  • 56. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 167
  • 57. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 168 2. Milling Fixtures Milling fixtures are the work holding device which are firmly clamped to the table of milling machine. They hold the work piece in correct position as the table movement carries it past the cutter or cutters. The essential features of a milling fixture are a) Base b) Location elements c) Clamping elements and d) Setting blocks These fixtures are classified based on i) Type of operation performed ii) Method of milling iii) Method of clamping the work piece MILLING FIXTURE Cradle fixture Rotary fixture Drum fixture Indexing fixture Magnetic chucks Vacuum chucks
  • 58. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 169 Following design principles be adopted for milling fixtures 1. Pressure of cut should always be against the solid part of the fixtures. 2. Clamps should always operate from the front of the fixture. 3. Work piece should be supported as near the tool thrust as possible. A milling fixture is located accurately on machine tube and then bolted in position. The tube is positioned relative to the cutter with the air of setting blocks. The location an d clamping systems are similar to those used for drill jigs, but as the cutting forces are high, interrupted and tend to lift the workpiece, the clamping forces must be big, hexagonal nuts are usually used to clamp the work piece rather than hand nuts. Fig. show a simple milling fixture and a line or string milling fixtures. The line or string milling fixture shown is used to mill a slot in the end of each of the five cylindrical work pieces arranged in line. This arrangement facilities all the work pieces to be located as required and clamped with one screw. Fig. shows an index milling fixture having a number of surfaces to be milled by successive positioning of a single fixture provided with an indexing arrangement.
  • 59. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 170
  • 60. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 171
  • 61. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 172 3. Grinding Fixtures Fixtures used in grinding depend upon the type of grinding operation and the machine used. GRINDING FIXTURE Angle Plat Fixture Automotive Grinding Fixture Magnetic Chuck
  • 62. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 173 Following table gives some commonly used grinding fixtures Sr. No. TYPE OF OPERATION FIXTURES USED 1 External Grinding Mandrel - Taper - Straight - Combined - Straight - And taper 2 Internal Grinding Chucks, special jaw chucks or special fixtures as the lathe fixtures. 3 Surface Grinding Clamped on machine table, held in vise, held in magnetic, vacuum and special features. 4. Welding Brazing and Soldering Fixtures These fixtures comprise of usual locating and clamping elements used in other fixtures. However the effects of heat and prevalence of welding spatter will have to be taken into account while designing them. Some of the consideration are as follows : i) Expansion of heated work pieces and resulting distortion should be taken care of by providing adequate clearances between work piece and locators. ii) Handles subject to heating should be properly insulated. iii) The welding spatter should not be allowed to face on the threaded parts of clamping elements. iv) Parts near the welding area should not be threaded. v) Spatter grooves must be provided below the line of welding of work pieces to the base plate with the weld spatter. vi) Care should be taken to prevent locking of joined work pieces in the welding fixture after welding. vii) Provision for easy tilting or rotation be made to ease welding from various dies. Toggle clamps, without threaded elements are widely used in welding fixtures. Welding and inspection are everyday operations in manufacturing. Like many other areas, these operations can be simplified and improved through the use of appropriate jigs and fixtures. Although welding is specified in the examples, the methods and techniques listed will apply equally well to other assembly operations such as brazing, soldering, riveting, and stapling.
  • 63. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 174 5. Broaching Fixtures As broaching is a fast and accurate method of metal cutting involving high cutting forces, broaching fixtures are required to perform one or more of the following functions: a) Hold the workpiece rigidly. b) Locate the workpiece in correct position relative to the tool or the machine table. c) Guide the broach in relation to the workpiece. d) Move the workpiece into and out of the cutting position. e) Index the workpiece between the cuts. Fixtures are used for both internal and external broaching. The fixtures used for internal broaching are the simplest and for many operations consist of a face plate or support place on the broaching machine. The fixtures for external broaching are made quite ri gid so that the workpiece does not move during the broaching action.
  • 64. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 175
  • 65. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 176
  • 66. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 177 6. Assembly Fixtures These are used to hold various components in their correct position, while they are assembled; Assembly operations often involve pressing interference fit pins, bushes and other parts in housings. The assembly fixtures need to be of light construction with adequate rigidity to ensure relative positional relationships of the various components. They may be built up from light castings, steel section or completely from steel. Assembly fixtures generally are of two types : a) Mechanical assembly fixtures used for operations generally performed at ordinary room temperatures with mechanical means. Eg, Reverting Fixtures. b) Fixtures for hot joining methods of assembly work using energy in the form of heat. Eg. Welding, brazing and soldering fixtures.
  • 67. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 178 7. Inspection Fixtures Every part made must meet a standard for size and shape if it is to perform its design function. While it is quite possible to measure each dimension separately, this is not the most cost effective means to insure part quality and conformity to dimension. To satisfy the requirements of speed and accuracy, gauging or inspection fixtures are used. The main requirement of an inspection fixture is accuracy. Each inspection fixture should contain only those elements needed to check the specified sizes of forms. Individual gauges that only check one size are preferred over complicated tooling if the dimension being gauged is independent of other part features. An example of this is the size of the threads in a hole. While the location of the hole is important to the part, the size of the thread is independent of the location. There are two general types of inspection fixtures; gauging and measuring.
  • 68. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 179 A7.4c Types of fixture & there descriptions Types of fixture Description Vise Fixtures Standard machine vises adopted with special jaws provided on easy way of holding parts for machining Lathe Fixtures Are used on vertical and horizontal turret lathes and high-speed production lathes Chuck fixtures The cheapest type of lathe fixture is the standard lathe chuck with special jaws or inserts machined to fit the part Face Plate Fixtures Is used to machinelarge diameter parts on the vertical lathe Mandrel and arbor type fixtures These fixtures will centre, locate and grip the work from the inside and are normally used for parts that already have machined internal surface Miscellaneous fixtures Lathe operations on parts that are unusual because of their shape or dimensions, at the time fixtures are complicated and expensive yet they are always efficient with respect to the saving of time and the improvement of quality Milling fixtures A mill fixture holds the part in the correct relation to the milling cutter as the table movement carries the part through cutters a) Cradle fixture The work piece is rocked or rotated within a given angle during milling b) Rotary fixture The work piece is rotated under the cutter c) Drum fixtures The work piece is mounted on the periphery of a rotating drum Indexing fixtures Where the work piece is indexed in to the next position during the machining cycle of mill Rise and fall fixtures Which allow raising and lowering of the work piece in conjunction with the mil feed Magnetic chucks Are used to hold ferromagnetic materials in production milling operation Types of fixture Description
  • 69. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 180 Vacuum chuck Are being used for holding nonferrous and non magnetic parts for milling operations Boring fixtures Differ from drill jigs in that they are to be used with boring bars Line boring fixture The distance between the holes requires a line boring bar, consequently their type of fixture is called a line boring fixture Stationery fixture Is designed to mount on the table and present the work piece to the boring bar in proper location for matching Universal fixture May be obtained commercially from the manufacturers of boring machines. Indexing fixture Consist of a base with a rotary table or rotating indexing plate mounted on it Automatic loading fixture Is suitable only for long production tuns of a particular work piece Broaching fixture External broaching usually requires a special fixture for each job Grinding fixture Must allow for the unrestricted access of coolant to the work The structural design of grinding fixtures is very similar to that of other fixtures Angle plate fixtures Is used for internal grinding Automotive grinding fixtures Are used in the automotive industries Trunk pin grinding fixtures, cam grinding fixtures, cam shaft grinding fixtures etc. Magnetic chuck Is used to hold work pieces in surface grinding operation Planning fixtures Can be economically applied to smaller parts when they are clamped in a gang fixture Welding fixtures Their purpose is to locate and hold the parts in correct relative position for joining to reduce distortion A7.5 Maintenance, Safety & Storage Provision for Maintenance Has provision been made for lubricating the tool mechanisms? 55
  • 70. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 181 Have all wearing parts been hardened? Are these parts easily made and replaced? Have correct materials and heat treatment been specified? Has provision been made for easy removal or pressed in parts? Can vulnerable parts be removed and replaced quickly without disturbing the set up of the fixture on the machine? Safety 1. Does the fixture design protect the operator from coolant spray or flying chips? 2. Is the designed tool safe to operate with? Handling and Storage Lifting Aids Have lifting lugs, eyebolts, or chain slots been provided for slinging heavy tools? Have lifting handles been attached to all awkward or heavy loose parts of the fixture? Loose Parts If loose parts such as spacing pieces, wrenches, or locating pins are unavoidable, can they be attached to the fixture with keeper screws or light chains to prevent loss in storage? Fragile Parts Is there any fragile part of the jig which needs a protective cover in storage? Is the tool so delicate or highly finished as to require a special case, cover, or box to protect it in storage? Identification Has the tool, and all loose items belonging to it, been marked clearly with identification numbers or symbols? Storage Aids Can the tool be stowed safety without danger of tipping over? Is a special storage stand or rack desirable for safe and convenient storage? SUMMARY:
  • 71. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 182 The use of fixtures is extending and developing very fast. The quality, type and complexity of fixtures used depend upon the type of job and its method of production. Fixtures are classified into two types depending on how they are builtand based on the type of machine on which they are used. QUESTIONS: 1. What are the two types of classifying fixtures? 2. What is an indexing fixture? 3. What type of fixture is used for machining contours, which the machine cannot normally follow? 4. What is an assembly fixture? 5. What are the rules for selecting clamps of work piece in fixtures? 6. What are the principles to be following in designing of fixtures? 7. Describe the various grinding fixtures. 8. Describe the design principles for a lathe fixture. 9. Name the various work holding devices use on a lathe. 10. How are cutters set in relation to the work in milling fixture? 11. Name the essential features of a milling fixture. 12. Why the proper disposal of swarf or burr is very important in jigs & fixtures design? 13. What provisions can be made to ease the handling of heavy jigs & fixtures? 14. Explain the advantages to be obtained from the use of pneumatic & hydraulic clamping devices. 15. How can a lathe fixture be clamped to the lathe? 16. Write short notes on “Broaching fixtures”, “Assembly fixtures”. 17. What are the checks to be made for fixtures for (a) provision for maintenance, (b) manufacturing & maintenance cost, (c) handling, (d) loading & unloading, (e) storage, (f) human factors?
  • 72. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 199 A9.1 Introduction Gauges are inspection tools of rigid design, without a scale, which serve to check the dimensions of manufactured parts. Gauges do not indicate the actual valueof the inspected dimension on the work. They can only be used for determining as to whether the inspected parts are made within the specified limits. A workman checking a component with a gauge does not have to make any calculations or to determine the a ctual dimensions of the part. Gauges are easy to employ. This is one reason for their wide application in engineering. Gauges differ from measuring instruments in the following respects : (a) No adjustment in necessary in their use. (b) They usually are not general-purpose instruments but are specially made for some particular part, which is to be produced in sufficiently large quantities. Gauging is used in preference to measuring when quantities are sufficiently high, because it is faster and easier with resulting lower costs. A9.1a Advantages and Disadvantages Modern manufacturing requires extensive use of gauges for shop work, inspection, and reference. Shop gauges are used by workmen. Inspection gauges are used by inspectors to check manufactured product, and reference gauges are reserved for checking the other two types. A gauge is defined by the Sheffield Corporation as “a device for investigating the dimensional fitness of a part for a specified function”. Gauging is defined bythe ANSI as “a process of measuring manufactured materials to assure the specified uniformity of size and contour required by industries.” CHAPTER OUTLINE A9.1 Introduction A9.2 Classification of Gauges TOPIC OUTLINE A9.1a Advantages & disadvantages of gauges A9.2a Fixed gauges A9.2b Advantages of fixed gauges A9.2c Classification of fixed gauges A9.2d Indicating gauges A9.2e Special gauges A9.2f Classification of Plain Gauges
  • 73. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 200 Basically, gauging accomplishes two things: (1) it controls the dimensions of a product within the prescribed limitat ions, and (2) it segregates or rejects products that are outside these limitations. Gauging devices and gauging methods, like other phases of tooling in modern manufacturing, have become standardized. Generally speaking, standardized components that can be obtained commercially are assembled into a unit to gauge a particular product. It is therefore quite important that the tool designer be familiar with gauging equipment and practice. It may be necessary to design special gauges for checking dimensions that do not readily adapt to standard gauges. A gauge of this type may be quite simple, as shown in Fig. Frequently time can be saved by the use of a simple length gauge in place of a machinist’s rule when a quantity of workpieces is involved. It shoul d not be assumed that special gauges are necessarily elaborate or that they are used only to measure close tolerances. The tolerance of the workpiece in Figure may be as large as ±1/64 in., and a machinist’ s scale would be sufficiently accurate for the job; however, it would take a little longer to read. There are many gauging methods used to determine when a product conforms dimensionally with drawings, specifications, or other prescribed requirements. Ho wever, when these methods are analyzed, it will be found that they are designed to check one of the seven basic elements of workpiece geometry : Distance Used to specify the relative location of the various components and elements of the workpiece. Distance is measured by comparison to a known standard. Flatness Used to ensure that every element of a surface is within a specified distance from a nominal surface plane. Determines straightness and alignment of a product. Parallelism Used to ensure that two flat surfaces are parallel to each other. Perpendicularity (squareness) Used to determine that two flat surfaces are normal to each other.