Machining Processes and Metrology

Machining Processes and Metrology (MEE2006)
Faculty: Dr. B.Venkateshwarlu, Associate Professor,
School of Mechanical Engineering
VIT University, Vellore

Text Book
1. Serope Kalpakjian; Steven R. Schmid (2013),
Manufacturing Engineering and Technology, 6th
Edition,
Publisher: Prentice Hall, ISBN-10 0-13-608168-1,
ISBN- 13 978-0-13-608168-5..
Reference Books
1. P.N.Rao (2013), Manufacturing Technology, McGraw Hill
Education, New Delhi.
2. R.K. Rajput (2015), A Textbook of Manufacturing
Technology, Laxmi publications, New Delhi.
3. P.C. Sharma (2000), Text book of Production Technology,
S.Chand & Company Ltd, New Delhi.
4. O.P. Khanna & M. Lal (2006), A Text book of Production
Technology, Dhanpat Rai Publications, New
Delhi.

Mode of Evaluation :
Digital Assignments /Surprise
Test/ Seminars/CAT/FAT

Mechanics of metal cutting - cutting tool
materials, temperature, wear, and tool life
considerations, geometry and chip formation,
surface finish and machinability, optimization.
Module 1
Metal Cutting

Machining
Why machining is required?

Removal of material to get:
•required shape,
•dimensional accuracy,
•surface finish, etc.
Machining
Importance of Machining

1. Types of chips in metal cutting
2. Mechanism of chip formation
3. Orthogonal & Oblique cutting
4. Machining Forces
5. Merchant’s Circle Diagram
6. Thermal Aspects of metal cutting
7. Machinability
8. Cutting Tool materials
9. Tool wear & Tool life calculations

UNIT - 1
THEORY OF METAL CUTTING
1. Types of chips in metal cutting
2. Mechanism of chip formation
3. Orthogonal & Oblique cutting
4. Machining Forces
5. Merchant’s Circle Diagram
6. Thermal Aspects of metal cutting
7. Machinability
8. Cutting Tool materials
9. Tool wear & Tool life calculations

Types of Chips
3 Types of Chips:
1.Continuous Chips
2.Discontinuous Chips
3.Continuous Chips with Built-Up-Edge
(BUE)

Work piece
Cutting tip of the tool
(Single point cutting tool)
Too
l
Types of Chips in metal cutting
Finished surface / work piece
Continuous
Chips

Continuous Chips
Produced during machining of more ductile
materials
Reason: Large plastic deformations possible
and longer continuous chips
Features: Chips remain in contact with the
tool face for longer period, result in more
frictional heat

Advantages: Results in
1. Stable cutting
2. Good surface finish
Disadvantage:
Difficult to handle and dispose off
because of chip curl around the work piece

1. Continuous Chips
2. Discontinuous Chips
3. Continuous Chips with Built Up Edges

Tool
Work piece
Discontinuous Chip (Segmental chips)

Discontinuous Chips
Produced during machining of more brittle
materials like grey cast iron, bronze and
brass
Reason: lack the ductility necessary for
appreciable plastic deformation of chips
Small fragments of discontinuous chips

• Advantages: Results in
1.Better surface finish because of less
friction between tool and chips
2.No chip curl like in continuous chips
3.Chips are convenient to collect, handle
and dispose off
Disadvantage:
Low cutting speeds are desirable

Tool
Work pieceTool
Chip
Finished Work piece

Continuous Chips with BUE
Tool
Work piece

Continuous Chips with BUE
Produced during machining of more ductile
materials
Conditions:
• High local temperature and extreme pressure
• High friction in tool- chip interface
These conditions cause work material to
adhere or weld to cutting edge of tool forming
the BUE

• Disadvantage: Results in
• Vibrations
• Poor surface finish
Use coolant to reduce or eliminate the BUE

Mechanism of Chip formation
• Ideal Chip formation
• Realistic chip formation
• Chip formation mechanism

Work piece
Cutting tip of
the tool
Too
l
to = Uncut Chip Thickness
tc = Cut Chip Thickness

Work piece
to
tcto = Uncut Chip thickness
Chip

Work piece
to
tc = Cut chip thickness
Chip

Work piece
to
If to = tc Ideal case
Chip

Work piece
to
If to = tc Ideal case
But always to < tc Real case
Chip
Why?

Mechanism of Chip formation
to = tc
to < tc
• Ideal Chip formation
• Realistic chip formation
• Chip formation mechanism
Chip thickness ratio, r = to/tc
Value of chip thickness ratio is always less than 1

Work piece
Shear Zone
Friction Zone

Work piece
Unaligned
compression
Shear Force
When exceeds
the shear
strength
Breakage of
material from the
parent metal
CHIPS
Fracture

Work
piece
Tool
Positive
Rake
angle
Tool
Zero
Rake
angle
Direction of tool movement
Movie-1 Movie-2 Movie-3
Movie-4
Tool
Negative
Rake
angle
Rake angle

Positive and Negative rake angles

The recommended rake angle depends on tool material, material to be cut,
speed and depth of cutting.

Force on a cutting tool with positive Rake angle

Positive rake angles:
•Make the tool more sharp and pointed and reduces strength of the tool, as
the small included angle in the tip may cause it to chip away.
•Used in the cutting of soft materials
•Reduce cutting forces and power requirements.
•HSS tools are typically given a positive rake angle
•Helps in the formation of continuous chips in ductile materials.
•Can help avoid the formation of a built-up edge.
•They used in machining of low-strength ferrous and non-ferrous metal.
•Positive rake angle is not preferred to high-speed operation.

Force on a cutting tool with negative Rake angle

Negative rake angles:
•Make the tool more blunt, increasing the strength of the cutting edge.
•Used to cut high strength materials
•Cemented carbide tools are normally given negative rake angle.
•Increase the cutting forces.
•It can be used in the high-speed cutting operation.
•Higher cutting force during machining, this also increases the power
consumption.
•Increase vibration, friction and temperature at cutting edge.

Neutral Rake angle
• Simplest and easiest to manufacture
• It causes a massive crater wear when compared to other types
• Neutral rake angle obstructs the movement of chip flow and
causes build-up edge chip formation.

TOOL
ORTHOGONAL
CUTTING
90
0
In orthogonal cutting, cutting edge of the tool is perpendicular to the direction
of motion.

TOOL
OBLIQUE CUTTING
In oblique cutting the cutting edge make an angle with the direction of motion.

Orthogonal cutting Oblique cutting

Machining Forces
Fc Cutting Force
Measured forces
Ft Tangential Force
Ff Friction Force
Calculated forces
(With the help of equations)
N Normal Force
Fs Shear Force
Fn Normal Shear Force

Work piece
Chip
Tool
Fc
Ft
Ff
N
Fs
Fn
Fc Cutting Force
Ft Tangential Force
Ff Friction Force
N Normal Force
Fs Shear Force

Fc
Ft
TOOL
R
Ff
N
Fs
Fn
Shear plane
Φ
ζ
Uncut Chip thickness
CutChipthickness Fc Cutting Force
Ft Tangential Force
Ff Friction Force
N Normal Force
Fs Shear Force
Tool Rake angle

To = Uncut Chip thickness
Tc = Cut Chip thickness
= Tool Rake angle
Fc = Cutting Force
Ft = Tangential Force
Ff = Ft Cos + Fc Sin
N = Fc Cos - Ft Sin
Fs = Fc Cos Φ - Ft Sin Φ
Fn = Fc Sin Φ + Ft Cos Φ
Rc Cos
tan Φ =
1 – Rc Sin
To
Rc = Cutting Ratio =
Tc
Coefficient of Friction
µ = Ff
N
Friction angle
ζ = tan (µ)
-1

MERCHANT’S CIRCLE DIAGRAM
(MCD)

What is MCD?
Fc Cutting Force
Ft Tangential Force
Ff Friction Force
N Normal Force
Fs Shear Force
Measured Forces
Calculated Forces
Withthehelp
ofequations
G
raphicalM
ethod
Ff = Ft Cos + Fc Sin
N = Fc Cos - Ft Sin
Fs = Fc Cos Φ - Ft Sin Φ
Fn = Fc Sin Φ + Ft Cos Φ

Fc
Ft
TOOL
R
Merchant’s Circle

Fc
Ft
TOOL
Merchant’s Circle
R
Ff

Fc
Ft
TOOL
Merchant’s Circle
R
Ff
N

Fc
Ft
TOOL
Merchant’s Circle
R
Ff
N
to

Shear Plane
Fc
Ft
TOOL
Merchant’s Circle
R
Ff
N
to

Shear Plane
Fc
Ft
TOOL
Merchant’s Circle
R
Ff
N
to
Fs

Shear Plane
Fc
Ft
TOOL
Merchant’s Circle
R
Ff
N
to
Fs
Fn

Shear Plane
Fc
Ft
TOOL
Merchant’s Circle
R
Ff
N
to
Fs
Fn
Φ
ζ

The resulting diagram is pictured below

Φ =
Π
4
1
2
ζ
Shear plane angle determined by Merchant
ζ = Friction angle
= Rake angle
Where

Procedure for construction of
Merchant circle diagram
• Merchant's Force Circle With Drafting (Optional)
• Merchant's Force Circle is a method for calculating the various forces involved in the cutting process. This
will first be explained with vector diagrams, these in turn will be followed by a few formulas.
• The procedure to construct a merchants force circle diagram (using drafting techniques/instruments) is,
• 1. Set up x-y axis labeled with forces, and the origin in the centre of the page. The scale should be enough
to include both the measured forces. The cutting force (Fc) is drawn horizontally, and the tangential force (Ft)
is drawn vertically. (These forces will all be in the lower left hand quadrant) (Note: square graph paper and
equal x & y scales are essential)
• 2. Draw in the resultant (R) of Fc and Ft.
• 3. Locate the centre of R, and draw a circle that encloses vector R. If done correctly, the heads and tails of
all 3 vectors will lie on this circle.
• 4. Draw in the cutting tool in the upper right hand quadrant, taking care to draw the correct rake angle (a)
from the vertical axis.
• 5. Extend the line that is the cutting face of the tool (at the same rake angle) through the circle. This now
gives the friction vector (F).
• 6. A line can now be drawn from the head of the friction vector, to the head of the resultant vector (R). This
gives the normal vector (N). Also add a friction angle (t) between vectors R and N. As a side note recall that
any vector can be broken down into components. Therefore, mathematically, R = Fc + Ft = F + N.
• 7. We next use the chip thickness, compared to the cut depth to find the shear force. To do this, the chip is
drawn on before and after cut. Before drawing, select some magnification factor (e.g., 200 times) to multiply
both values by. Draw a feed thickness line (t1) parallel to the horizontal axis. Next draw a chip thickness line
parallel to the tool cutting face.
• 8. Draw a vector from the origin (tool point) towards the intersection of the two chip lines, stopping at the
circle. The result will be a shear force vector (Fs). Also measure the shear force angle between Fs and Fc.
• 9. Finally add the shear force normal (Fn) from the head of Fs to the head of R.
• 10. Use a scale and protractor to measure off all distances (forces) and angles.

1. High Carbon Steel
2. High Speed Steel (HSS)
3. Cast Alloys
4. Cemented carbides (cermet) (sintered carbide)
5. Coated Carbides
6. Ceramics
7. Diamonds
CUTTING TOOL MATERIALS

1. High Carbon Steel
This material is one of the earliest cutting materials used in machining. It is however now virtually superseded by other materials used in
engineering because it starts to temper at about 220oC . This softening process continues as the temperature rises. As a result cutting
using this material for tools is limited to speeds up to 0.15 m/s for machining mild steel with lots of coolant.
2. High Speed Steel (HSS)
This range of metals contain about 7% carbon, 4% chromium plus additions of tungsten, vanadium, molybdenum and cobalt. These
metals maintain their hardness at temperature up to about 600o, but soften rapidly at higher temperatures. These materials are suitable
for cutting mild steel at speeds up maximum rates of 0.8 m/s to 1.8 m/s.
3. Cast Alloys
These cutting tools are made of various nonferous metals in a cobalt base. They can withstand cutting temperatures of up to 760oC and
are capable of cutting speeds about 60% higher than HSS.
4. Cemented carbides (cermet) (sintered carbide)
This material usually consists of tungsten carbide or a mixture of tungsten carbide, titanium, or tantalum carbide in powder form, sintered
in a matrix of cobalt or nickel. As this material is expensive and has low rupture strength it is normally made in the form of tips which are
brazed or clamped on a steel shank. The clamped tips are generally used as throw away inserts.
5. Coated Carbides
The cutting system is based on providing a thin layer of high wear-resistant titanium carbide fused to a conventional tough grade carbide
insert, thus achieving a tool combining the wear resistance of one material with the wear resistance of another. These systems provide a
longer wear resistance and a higher cutting speed compared to conventional carbides.
6. Ceramics
Ceramics are made by powder metallurgy from aluminium oxide with additions of titanium oxide and magnesium oxide to improve cutting
properties. These have a very high hot resistance and wear resistance and can cut at very high speed. However they are brittle and have
little resistance to to shock. Their use is therefore limited to tips used for continuous high speed cutting on vibration-free machines.
7. Diamonds
Diamonds have limited application due to the high cost and the small size of the of the stones. They are used on very hard materials to
produce a fine finish and on soft materials. especially those inclined to clog other cutting materials. They are generally used at very high
cutting speed with low feed and light cuts. Due to the brittleness of the diamonds the machine has to be designed to be vibration
free. The tools last for 10 (up to 400) times longer than carbide based tools.

Machinability
“It is the easiness with which a given
work piece can be machined.”
“It indicates how the work piece responds to
the cutting process”

Machinability
“Good machinability means the work piece is
cut with
• Good surface finish
• Long tool life
• Low power requirements
• Low cutting forces
• Low cost

Machinability
Machinability depends upon
1. The properties of the w/p and the tool
2. Machining parameters such as speed, feed & depth of cut
3. Condition of the machine tool
4. Expertise of the operator etc………
5. …..
6. ………..

• Tool wear (Failure of cutting tools)
Tool wear & tool life calculations
 Categories of Tool wear
 Regions of Tool wear
 Reasons of Tool wear
• Tool life calculations
 Taylors Tool life equation

Failure of cutting tools
• Categories of tool failure…….
» Sudden mechanical failure due to excess shocks
» Plastic deformation due to excess stresses
» Gradual wear

• Regions of Tool wear
– CRATER Wear
– FLANK Wear
Work piece
Tool
Chips

• Reasons of tool wear
» Abrasive wear
» Adhesive wear
» Diffusion wear
» Corrosion wear
» Fracture wear

» Abrasive wear
It is due to the presence of hard particles scattered
inside the work piece

» Adhesive wear
It is due to the sticking up of work piece material upon
the tool surface or vice-versa

» Diffusion wear
It is due to the diffusion between the atoms of tool and
work piece material

» Corrosion wear
It is due to chemical attacks at the tool-w/p interface

» Fracture wear
It is due to sudden fracture of the tool

• Tool wear
Tool wear & tool life calculations
 Categories of Tool wear
 Regions of Tool wear
 Reasons of Tool wear
• Tool life calculations
 Taylors Tool life equation

Tool Life
– It is defined as the useful life of a tool in
between two successive re-conditionings.

Tool Life
• Taylor’s tool life equation
He experimentally found that
At HIGH cutting speed
Production rate
increases
LESS tool life
At LOW cutting speed
Production rate
decreases
MORE tool life

Tool Life
• Taylor’s tool life equation
At slow cutting speeds High tool life
At high cutting speeds Less tool life
Therefore his findings were

Tool Life
Therefore there exists a relationship
between the cutting speed & tool life
ie; there exists an optimum value for the
cutting speed & tool life

Tool Life
Where
V = Cutting Speed in rpm
T = Tool life expressed in minutes
n = Index Value
C = a constant
ie;
V1 T1 = V2 T2 = V3 T3 ……. = C
n n n
Taylor’s Tool life equation V T = C
n

The following values may be taken for “n”
n=0.1 to 0.15 for HSS tools
n=0.2 to 0.4 for Carbide tools
n=0.4 to 0.6 for Ceramic tools

Taylor’s tool life equation
Problem 1
If in turning of a steel rod by a given cutting tool at a given machining condition
under a given environment, the tool life decreases from 80 min to 20 min, due to
increase in cutting velocity (Vc) from 60 m/min to 120 m/min, then at what
cutting velocity the life of that tool under the same condition and environment
will be 40 min.?
Answer: 84.8528 m/min
Problem 2:
A tool has a life of 9 min when cutting at 250 m/min. Calculate the cutting speed
for the same tool to have a tool life of 160 min. Take Taylor tool life parameter
n=0.22.
(Ans: 132.73 m/min)

Problem 2
A lathe spindle running at 4000 rpm is cutting
Mild Steel work piece with a H.S.S. tool.
If the spindle speed is reduced to 3000 rpm,
calculate the percentage change in tool life.
(For the given work piece – tool combination,
assume n = 0.125)

CUTTING FLUIDS IN MACHINING
A. Functions of cutting fluid
a) Primary Functions
i. Lubricating the entire cutting process
ii. Cooling the work piece & tool
iii. Washing away the chips
b) Secondary Functions
i. Improved tool life
ii. Improved surface finish
iii. Reduced work piece deformation
B. Various methods to apply cutting fluid
a) Flood application
b) Jet application
c) Mist application (Spray application)
FLOOD application
JET application
MIST (spray)
application
Coolant
pipe
Air jet

MIST (spray) application
Coolant pipe
Air jet

• Thermal aspects of metal cutting
- causes, effects, assessment & control
Refer page No. 34, 35, 36 & 37
Manufacturing Technology- Metal cutting and Machine tools
By P.N.RAO

UNIT - 1
THEORY OF METAL CUTTING
Mechanism of chip formation - Orthogonal & Oblique
cutting - Machining forces - Merchant’s Circle Diagram
- Thermal aspects of metal machining - Cutting fluids -
Machinability - Cutting tool materials - Tool wear and
Tool life calculations.
Completed

Machining Processes and Metrology

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