2. Jigs & Fixtures
Fixture : Mechanical device (Strong & Rigid) designed and used in M/c shops to enable
easy, quick & consistently accurate locating , supporting and Clamping Workpiece against
tool for higher productivity and quality in batch Production.
Jig : Jig is a fixture having additional system of tool guidance.
3. Advantages of Jigs and Fixtures :
1. Minimize tool breakage, guidance of cutting tool, drill, reamer
2. Minimize the possibility of human error, partially automates the M/c tool.
3. Permit the use of less skilled labor
4. Reduce manufacturing time, cost
4. Design of Jigs & Fixtures :
Major Parts comprising jig & fixtures // elements to be designed :
1. Base and Body or Frame : Strong and rigid
2. Locating elements /System : for positioning and orientation of the blank
3. Supporting System
4. Clamping System/elements
5. Tool guides- bushings
6. Indexing Mechanism –plates / systems
7. Auxiliary elements : fasteners
5. 12 : Degrees of Freedom
3-2-1, 4-2-1,Principle of Location:
Pins A, B, C : Restrict 1,2,3,4,7 , no of D.O F. ( Rotation about X and Y axis and translation along -ve Z- Axis)
Pins D, E : Restrict 5,6, 12 , no of D.O.F. (Rotation about Z axis and , Translation along +ve Y – axis )
Pin F : Restrict 10 , no of D.O.F. ( Translation along –ve X-axis)
Remaining D.O.F are : 8 ,9,11, Which can be arrested by Locating and Clamping
(Translation along +ve, Z-axis, +ve , X-axis, -ve , Y-axis)
F
8
D E
A
B C
F
7. Design of Jigs : Element / Part-2 & 3,
Locating & Supporting Devices Rules / Principles :
Locating :
1. One or more surfaces preferably machined are to be taken for reference.
2. Locating should be easy , quick and accurate
3. The locating pins should be as far as possible
4. System should be Fool proof and redundancy should be avoided
5. Vee block and cones should be used for self locating solid and hollow
cylindrical jobs
Supporting :
1. Supporting should be provided at least at three points
2. Unsupported span should not be large to cause sagging
3. Proper recess should be provided for supporting large flat area
4. If more than three pins are required for supporting large work pieces then the
additional pins are to be spring loaded or adjustable
10. Locators / Methods :
Locating by Flat surfaces:
Solid Supports : Easiest and Cheapest, Less accurate, used where m/c surface act
as locating point.
14. Locators / Methods :
Locating on mandrel or plug :
Nesting / Profile Locators :
15. Locators / Methods :
Nesting / Profile Locators :
Used for Cylindrical W/p
No need of supplementary locating devices
But it is difficult to lift out of the cavity
Some time finger slots are provided
It encloses the w/p fully
16. Locators / Methods :
Nesting / Profile Locators :
Full Nest for work pieces other than cylinders
Encloses the work completely
For large W/p , partial nests are used to
Enclose certain contours of the W/p
17. Design of Jigs : Element/Part -4, Clamping Principles :
Clamping must be strong , rigid and Stable, unaffected by vibrations, easy, quick.
Should not hinder loading and unloading work, interfere with operator of m/c tool.
Clamping force should act against rigid part and not damage the job.
Use minimum Standard parts , Use pads of softer material such as nylon or fiber
Wearing parts have to be hard and replaceable
Clamping should be fool proof, Reliablity, safety and low cost
25. Design of Jigs : Element /Part-5, Tool Guides Bushings :
Tool Guides / Bushes are Provided :
(0.00025 to 0.0025 larger than drill size , can drill 10,000 to 15, 000 holes )
1. To eliminate elastic spring back ,to locate & guide tool relative to the work.
2. No need of marking,
3. Reduce tool run out, off
4. No hole expansion ovalisation
5. Reduces tool breakage
26. Types of Bushes :
1. Press Fit Bushings : Installed directly in jig body, Less Accuracy.
2. Liner Bushings : Master Bushings, Permanently fixed to jig body. guide to
renewable type of bushings:
27. Types of Bushes :
3. Renewable Bushings :
For periodic replacement
In large production , due to wear
Sliding fit into liner bush
Retaining screw for preventing rotation lift
Can be removed only after removing retaining screw
4. Slip Bushings:
For two or more tools to cut
On same axis, different diameter
Drilling followed by reaming
Slip bush is used for each tool
Retaining screw for preventing rotation lift
28. Types of Bushes :
Screw Bush
Holds the bush in place
Makes bush adjustable
Can be used for locating
Can be used to clamp
29. Design of Jigs : Element/Part-6, Indexing Mechanism
Used to move the work piece relative to machine table or spindle.
Used for quick, accurate location of work piece in number of specific positions.
32. Types of Jigs : Template Jig :
Based on construction and method of operation Drill Jigs are Classified as
Simple plate of metal or wood which carries correct location of holes to be
made in the workpiece. Size of template should be same as that of workpiece.
Directly placed on the part , No Clamping arrangement
Eliminates marking need completely, Least expensive
33. Types of Jigs : Plate Jig :
A Template jig with work piece clamping system
Located and Clamped directly on the work piece and bolted in position
36. Design of Jigs :
Principles :
1. Should allow easy and quick loading and unloading of work piece
2. Should be as open as possible to minimize chip or burr accumulations
3. Fool proofing provision should be there.
4. The use of ejection devices to force the work piece out from the jig or fixture
5. Design for safety : avoid sharp corners , bolts and nuts should be inside the
body of jig or fixtures.
57. Design of Fixtures :
Fixtures name is derived /fact /it is always fastened to a m/c or bench
Fixtures are designed specifically for an operation and so named like
Milling Fixtures
Turning Fixtures
Tapping Fixtures
Assembling Fixtures
Fixtures for grinding
Fixtures for broaching
Fixtures for boring/ drilling
Fixtures for welding
Difference between Jig and Fixtures
1. A fixtures holds and position the work but does not guide like jig
2. Fixtures are heavier in construction, Jigs are lighter in construction
3. Bolted rigidly on machine table, Jigs are not bolted
4. Subjected to higher cutting forces, Clamping must be big, Hexagonal
nuts used to clamp, Jigs to lighter forces, Hand nuts used.
5. Fixtures can be used for all machining applications, Jig not for all.
58. Design of Fixtures : Milling Fixtures
Fixture components :
• Base : Securely fastened to milling machine table
• Setting Block :
Motion of cutting tool is more complex, movement in two different axes
Used for tool setting/ locating cutting tool in relation to w/p surface
59. Design of Fixtures : Milling Fixtures
Setting block fixed to the fixture, feeler gauges are placed between the cutter and
reference plane on the setting block so that the correct depth of cut and correct
lateral setting is obtained.
60. Design of Fixtures : Milling Fixtures/ Example-1 & 2
Design a String milling fixture to cut slot 3*3 mm in a cylindrical mild steel
component
61. Design of Fixtures : Milling Fixtures/ Example-1
1. Base Plate
2. Support Block
3. Support Block
4. Spindle
5. Holding Block
6. Thrust Plate
7. Stepped V- Block
8. Std V- Block
9. Guide Block
10. Work Plate
11. Allen Screw
12. Allen Screw
13. Handle
66. References :
1. Introduction to Jigs and Fixtures , 3rd Edition, M.H.A. Kempster, The
English Language Book Society and Hodder And Stoughton, London.
2. Jigs and Fixtures, 3rd Edition, P. H. Joshi, Tata McGraw Hill Education
private limited, New Delhi.
3. Tool Engineering and Design, G.R. Nagpal, Khanna Publishers, Delhi
4. A Text Book of Production Technology, P.C. Sharma, S. Chand &
Company Ltd. Ramnagar New Delhi.
5. Jigs and Fixture Design, Fifth Edtion, Edward G. Hoffman, Thomson
Delmar Learining, a division of Thomson Learing Inc.