Metal cutting

MACHINING & METAL CUTTING
Group A-1-1
BME 4TH YEAR, JADAVPUR
UNIVERSITY

WHAT IS MACHINING?
 Machining is a manufacturing process in which excess
material is removed from parent material in the form of
chips using suitably shaped tools called cutting tools
in order to achieve desired size shape surface finish
surface integrity of the intended product.
• Machining is a finishing process which follows other
manufacturing processes such as casting, forging,
rolling extrusion etc.
• Shearing and cutting are not machining as they do not
involve chip formation.

MACHINE TOOLS
It is a machine in which machining can
be done, e.g., Lathe, Shaping m/c,
Drilling machine
 All machines are not machine tools but
all machine tools are machines.
 Forging m/c, moulding m/c, die casting
m/c etc. are machines but not machine
tools.

CUTTING TOOLS
These are tools used for metal cutting
They involve certain geometry, angles planes
clearances etc.
 Broadly they can be classified into two groups- 1)
Single point cutting tools and 2) Multi point cutting
tools
 A single point cutting tool contains only one main
cutting edge
 A double point cutting tool contains two cutting edges
and multi-point cutting tool contains more than two
main cutting edges
 Cutting tools can also be classified as Right Hand &
Left Hand cutting tool

Lathe tool (turning/facing) is a single
point tool.
Milling cutters are multi-point cuttersDrill bits are the example of double point
cutting tools

Right Handed Cutting Tool Left Handed Cutting Tool
A right handed cutting
tool, the common
one, can remove
material while moving
leftward

DIFFERENCES BETWEEN SINGLE AND MULTI
POINT CUTTING TOOL
Single Point Cutting Tool Multi Point Cutting Tool
While machining with single point cutting
tool, only one cutting edge continuously
engage in material removal action.
While machining with multi point cutting tool,
more than two cutting edges simultaneously
engage in material removal action.
Usually, these cutting tools have only one
wedge shaped main cutting edge. However,
insert based single point cutting tools may
have multiple cutting edges present on a
single tool, out of them only one will partake
in cutting action at a pass. In the next pass,
the previous cutting edge can be replaced
by a new sharp one. So in any case, more
than one cutting edge will not engage in
cutting action at a time.
Multi point cutting tools may have only few
(about 4) to hundreds of cutting edges.
However, the number of cutting edges
engage in cutting action at a time depends
on various factors, such as depth of cut,
infeed, width, etc. By the by, all cutting
edges successively engage in cutting action
in a single pass.
Chip load per tooth is usually high. Due to presence of multiple teeth, chip load
per tooth reduces.
Since one cutting edge continuously
remains in contact with the workpiece, so
rate of rise in tool temperature is high.
Due to successive engagement of teeth,
some amount of heat gets dissipated from
the teeth when these are not in contact with
the workpiece. Consequently rate of rise in
tool temperature is low.

Single Point Cutting Tool Multi Point Cutting Tool
In case of unplanned breakage of the
cutting edge, the entire process needs to be
paused until the tool is replaced by a new
one.
In case of breakage of one tooth, the other
tooth can continue cutting action without
much problem.
Design and fabrication of single point cutting
tools are comparatively easy.
Design and fabrication of multi point cutting
tools are quite difficult.
Usually single point cutting tools are given
low feed rate, so Material Removal Rate
(MRR) and thus productivity are
comparatively low.
Higher feed rate can be provided, which
increases MRR and productivity. So
machining operation with multi point cutting
tool is more economic.
Turning tool, also known as Single Point
Turning Tool (SPTT), is the perfect example
of a single point tool. Apart from SPTT,
shaping, planing, slotting, boring tools are
also single point tool.
Milling cutters, hobs, broaching tools,
grinding wheels, etc. are examples of multi
point tools.
DIFFERENCES BETWEEN SINGLE AND MULTI
POINT CUTTING TOOL

METAL CUTTING WITH SINGLE &
MULTI POINT CUTTING TOOLS

TYPES OF CUTTING
 1) Orthogonal cutting: It is a type of metal
cutting in which the cutting edge of wedge
shape cutting tool is perpendicular to the
direction of tool motion.
 2) Oblique cutting: another type of cutting
in which the cutting edge of wedge shape
cutting tool make a angle except right angle to
the direction of tool motion.

DIFFERENCES BETWEEN ORTHOGONAL AND OBLIQUE
CUTTING
S. No.
Orthogonal Cutting Oblique Cutting
1.
The cutting edge is perpendicular to
the chip velocity
The cutting edge is not perpendicular to
the chip velocity
2. In orthogonal cutting only two
components
of force considered cutting force and
thrust force which can be represent by
2D
coordinate system.
In oblique cutting three component of
force
are considered, cutting force, thrust force
and radial force which cannot represent
by 2D coordinate. It used 3D coordinate
to represent the forces acting during
cutting, so it is known as 3D cutting.
3.
This tool has lesser cutting life
compare
to oblique cutting.
This tool has higher cutting life.
4.
The shear force act per unit area is
high
which increase the heat developed per
unit area.
The shear force per unit area is low,
which
decreases heat develop per unit area
hence increases tool life.

3 types of chips are produced during machining-
1) Continuous chips 2) Continuous chips with BUE & 3) Discontinuous
chips
TYPES OF CHIPS

TYPES OF CHIPS
 1) Discontinuous chip:
 Discontinuous chips is formed by a series of rupture occurring
approximately perpendicular to the tool place face’ each chip
element passing off along the tool face the chip element’ in the form
of small segment may adhere loosely to each other and becomes
slightly longer.
 Since the chips break up into small segments the friction between
the tool and the chips reduces’ resulting in better surface finish.
These chips are convenient to collect’ handle and dispose off.
Discontinuous chips tends to be formed when one or more or the
following conditions exist:
 1. Brittle material , such as cast iron and bronze.
 2. large chip thickness
 3. low cutting speed
 4. small rake angle

TYPES OF CHIPS
 2: Continuous Chips:
 Continuous chips are formed by the continuous plastic
deformation of metal without fracture in front of the cutting edge
of the tool and is formed by the smooth flow of the chip up the
tool face. The chips obtained have same thickness throughout.
This type of chip is the most desirable. Since it is stable cutting,
resulting in generally good surface finish. On the other hand
these chips are difficult to handle and dispose off.
 Continuous chips tend to be formed when the following condition
exist:
 1. ductile material
 2. high cutting speed
 3. small chip thickness
 4. large rake angle
 5. minimum friction of chip on tool face

TYPES OF CHIPS
 3: Continuous Chip with Built up Edge:
 This type of chip is very similar to the continuous chip with the
difference that it has a built up edge adjacent to tool face
 While machining ductile material in high local temperature and
extreme pressure in the cutting and high friction in the tool chip
interference, the work material may adhere to the cutting edge of the
tool due to welding.
 Successive layers of work material are then added to the built up
edge.
 When this edge becomes larger and unstable , it breaks up and
part of it is carried up the face of the tool along with the chip while
the remaining is left over the surface being machined, which
contributes to the roughness of the surface. So BUE is
undesirable.
 If speed is increased, lubricant is applied we can get rid of BUE.

SINGLE POINT CUTTING TOOL GEOMETRY

18
Cutting-Tool Terms
 Front, End, Relief
(Clearance)
◦ Allows end of cutting tool
to enter work
 Side Relief (Side)
◦ Permits side of tool to
advance into work

19
Cutting-Tool Terms
 Side Cutting Edge Angle
◦ Angle cutting edge meets work
 Positive
 Negative - protects point at start and end of cut
 Nose Radius
◦ Strengthens finishing point of tool
◦ Improves surface finish on work 𝐇 𝐦𝐚𝐱 =
𝐟 𝟐
𝐑 𝐧𝐨𝐬𝐞
 Too large – chatter, increased cutting force,
more power;
 too small – weakens point

20
Side Rake
 Large as possible to allow
chips to escape.
 Too large- Weakens the tool.
 Amount determined (5° − 15°)
◦ Type and grade of cutting tool
◦ Type of material being cut, (for
softer material-higher)
◦ Feed per revolution
 Angle of keenness
◦ Formed by side rake and side
clearance

21
Back Rake
 Angle formed between top face of tool
and top of tool shank
◦ Positive
 Top face slopes downward
away from point
◦ Negative
 Top face slopes upward
away from point
◦ Neutral

Tool Nomenclature Systems
 Tool in Hand System-(w.r.t tool different
planes are defined)
1)ISO System : ORS/ NRS - Orthogonal/ Normal
Rake System
 Machine Reference System-(w.r.t machine
different tools are defined)
2) ASA System – American Standards Association

TOOL SIGNATURE- ASA
 The ASA system consists of seven elements to denote
a single point cutting tool. They are always written in
the following order. Back rake angle, Side rake angle,
End relief angle, Side relief angle, End cutting edge
angle, Side cutting edge angle, and nose radius (𝛄 𝐲 −
𝛄 𝐱 , −𝛂 𝐲 , −𝛂 𝐱 , −Ø 𝐞 − Ø 𝐬 − 𝐫)
 For example, tool signature 0, 10, 6, 6, 10, 12, 1
means
Back rake angle = 0° Side rake angle = 10°
End relief angle = 6° Side relief angle = 6°
End cutting edge angle = 10° Side cutting edge angle
= 12°

 The ORS system comprises seven parameters to
describe a tool. The main elements of ORS
designated in the following order Angle of inclination,
Normal rake angle, Side relief angle, End relief angle,
End cutting edge angle, Approach angle and Nose
radius. (𝛌 − 𝛄 𝟎 − 𝛂 𝟎 − 𝛂 𝟎
′
− 𝛟 𝟏 − 𝛟 − 𝐫)
 Example: Tool signature 5, 10, 6, 6, 5, 90, 1
Angle of inclination = 5° End relief angle = 6°
End cutting edge angle = 5° Approach angle = 90°
Nose radius =1mm
Normal rake angle = 10° Side relief angle = 6°
TOOL SIGNATURE- ORS

∅ = 𝐏𝐫𝐢𝐧𝐜𝐢𝐩𝐥𝐞
𝐂𝐮𝐭𝐭𝐢𝐧𝐠 𝐄𝐝𝐠𝐞 𝐚𝐧𝐠𝐥𝐞

MECHANISM OF CHIP
FORMATION IN MACHINING
Machining is a process of gradual removal of excess
material from the preformed blanks in the form of
chips. The form of the chips is an important index of
machining because it directly or indirectly indicates :
• Nature and behaviour of the work material under
machining condition
• Specific energy requirement (amount of energy
required to remove unit volume of work material) in
machining work
• Nature and degree of interaction at the chip-tool
interfaces.

The form of machined chips depend mainly upon :
• Work material
• Material and geometry of the cutting tool
• Levels of cutting velocity and feed and also to some
extent on depth of cut
• Machining environment or cutting fluid that affects
temperature and friction at the chip-tool and work-tool
interfaces.
MECHANISM OF CHIP FORMATION
IN MACHINING

IN MACHINING
Whenever and wherever the value
of the shear stress reaches or
exceeds the shear strength of that
work material in the deformation
region, yielding or slip takes place
resulting shear deformation in that
region and the plane of maximum
shear stress.
But the forces causing the shear stresses in the region of the chip quickly
diminishes and finally disappears while that region moves along the tool
rake surface towards and then goes beyond the point of chip-tool
engagement. As a result the slip or shear stops propagating long before
total separation takes place. In the mean time the succeeding portion of
the chip starts undergoing compression followed by yielding and shear.
This phenomenon repeats rapidly resulting in formation and removal of
chips in thin layer by layer.

IN MACHINING

IN MACHINING
The chip thickness (𝑎2) usually
becomes larger than the uncut
chip thickness (𝑎1). The reason
can be attributed to
• compression of the chip ahead
of the tool
• frictional resistance to chip flow
• lamellar sliding according to
Piispannen
ζ =
𝑎2
𝑎1
> 1.00 (since 𝑎2> 𝑎1) ζ =chip reduction ratio

IN MACHINING
𝐴𝐶 = 𝑎2 = 𝑂𝐴 cos 𝛽𝑜 − 𝛾𝑜
AB = 𝑎1 = 𝑂𝐴 sin𝛽𝑜
𝜁 = cos 𝛽𝑜 − 𝛾𝑜 / sin𝛽𝑜
tan 𝛽0 = (cos 𝛾0/(𝜁 − sin 𝛾_0))

MERCHANT CIRCLE DIAGRAM:
-
-:
For any Machining operation, power is required and
mathematically –
(POWER), P= 𝐶𝑈𝑇𝑇𝐼𝑁𝐺 𝑉𝐸𝐿𝑂𝐶𝐼𝑇𝑌 X (CUTTING FORCE)
Here if we have the cutting velocity(VC), we require the
force(Fc).
Model proposed by ‘Merchant’ in 1944. It involves
assumptions as follows -

ASSUMPTIONS:
• Orthogonal cutting.
• Continuous chip without BUE.
• Deformation takes place in a
very thin zone, considered as
single Plane- ‘Shear Plane’.
• Cutting tool is perfectly Sharp
and no contact of Chip and
Flank Surface of tool.
• Constant cutting speed.
• Constant friction coefficient at
Chip-Tool interface.
• 2-dimensional deformation of
chip.

• Workpiece must be rigid and
perfectly elastic.
• Chip is considered rigid under
equilibrium of two collinear equal
and
opposite forces
say – ‘R’ and ‘R΄’. Where -
R΄ = force acting on chip at Shear
Plane. &
R = force acting on chip at
chip-tool interface.
combining those two force triangles we
can get the “Merchant Circle Diagram”

Since, the force vectors in the previous Diagram are cyclic in
nature.
Hence by geometry:
𝑂𝑒||𝑎𝑏
From the fig:
= 𝑂𝑎
= be
= bf + fe
= bf + Cd
And Cd= fe
Henc
e:
⇒ 𝐹 = 𝐹𝑡 𝐶𝑜𝑠𝛼 + 𝐹𝑐 sin 𝛼
F
Similarl
y, N=𝐹𝑐 cos 𝛼 + 𝐹𝑡 sin 𝛼
In matrix form:
𝐹
𝑁
=
sin 𝛼 cos 𝛼
cos 𝛼 −sin 𝛼
X – Y PLANE
𝐹𝑐
𝐹𝑡

Y – Z PLANE
From the fig. :-
𝐹𝑠 = 𝑂𝑃
= ON- NP
= 𝐹𝑐 cos ∅ − 𝐹𝑡 sin ∅
Similarly,
𝐹𝑠 = 𝐹𝑐 cos ∅ − 𝐹𝑡 sin ∅
𝐹 𝑁 = 𝐹𝑐 sin ∅ + 𝐹𝑡 cos ∅
In matrix form:
𝐹 𝑁
𝐹𝑠
=
sin ∅ cos ∅
cos ∅ −sin ∅
Other relations that can be obtained from
geometry:
𝑅 = 𝐹𝑠 sec(∅ + 𝜂 − 𝛼)
𝐹𝑐 = 𝑅 cos(𝜂 − 𝛼)

Theoretical Determination Of Cutting
Force Important for estimating cutting force only from known
physical parameters without the help of dynamometer.
Now,
FC = R cos (ɳ − ɣo)
Putting, R = FS sec (β+ɳ − ɣo)
And, Fs = 𝜏 𝑆 AS
We get,
For orthogonal cutting, we may write, a1 b = s t
So,
FC =
τS a1 b cos (ɳ − ɣo)
sinβ cos(β+ɳ−ɣo)
FC =
τS s t cos (ɳ − ɣo)
sinβcos(β+ɳ−ɣo)

Problems Involved In Determination Of Cutting Forces:
(1) Determination of under τS cutting conditions.
(2) Finding a suitable relation of sinβ in terms of
cos (ɳ − ɣo)
cos(β+ɳ−ɣo)
, i.e.,
relation between β, ɳ, ɣo.
Merchant’s First Solution
Based on the principal of minimum
energy.
Now, Power Consumption in Metal Cutting, U = Cutting Force X
Velocity
U = FC V
So,
Assumptions,
(1) Shear stress is a true property of the material. Therefore, τS =
constant.
(2) Total energy in the system has been consumed in deformation of
material.
(3) At the chip-tool interface, friction force is invariant, i.e., ɳ =
constant. Hence, β is the only variable on which power
U =
V

During chip formation, β would take such a value that least amount of energy is
conserved. Thus, power required is minimum.Therefore,
𝜕𝑈
𝜕β
= 0
So,
𝜕{𝑐𝑜𝑠𝑒𝑐 β . 𝑠𝑒𝑐 (β+ɳ−ɣo)}
𝜕β
= 0
Solving this we get,
As, cutting force is, FC =
,
Putting 2β =
π
2
- ɳ + ɣo , and solving we get,
Or if we put β + ɳ + ɣo =
π
2
- β , then we get cutting force as ,
2β =
π
2
-
ɳ + ɣo
FC = 2 τS a1
b cotβ
FC =
2 τS a1 b cos (ɳ − ɣo)
1 − sin(ɳ − ɣo)

Merchant’s Second Solution
Here it is assumed that shear stress is linearly dependent on
normal stress.
Therefore, τS = τO + KσN
Where,
τS = Shear stress in direction of shear
τO = Initial Shear stress
K = Slope
σN = Normal stress
Now, τS = τO + K
FN
AS
Putting , FN = FS tan (β+ɳ − ɣ) and solving we get,
Now cutting force is, FC =
Putting, τS =
τo
1 – K tan (β+ɳ−ɣ)
in the expression of cutting force and then using it in
the expression of Power Consumed, we get, U =
τO a1 b cos (ɳ − ɣo)
sinβ cos(β+ɳ−ɣo) {1−K tan(β+ɳ−ɣo)}
V
τS =
τo
1 – K tan (β+ɳ−ɣ)

Now, for minimum energy consideration,
𝜕𝑈
𝜕β
= 0
Putting here,
U =
τO a1 b cos (ɳ − ɣo)
sinβ cos(β+ɳ−ɣo) {1−K tan(β+ɳ−ɣo)}
V and
solving we get,
cot (2β + ɳ − ɣo) = K
So, cot Cm = K
where, Cm = 2β + ɳ − ɣo
Cm is called Machining Constant
• Usually Cm < π/2, depending upon work material. In general Cm is 70O-
80O
• Cm is the property of material unaffected by cutting condition, but
microstructure and grain size have effect on Cm . Cold working
increases the value of Cm.
• The value of cutting force obtained from above solution is in well
agreement with results obtained for machining of Mild Steel, Copper,

Force analysis of
DYNAMOMETER
The device through which the forces during
metal cutting are measured are called
dynamometer.
A dynamometer consists of number of strain
gauges arranged in a particular manner.
Strain gauges measures the strain produced,
with the help of which force acting on the
component can be calculated.
For every strain gauge, Gauge factor is
Gauge factor(GF) =
𝑑𝑅/𝑅
𝐸

Uniaxially Loaded Members
Simplest method of measuring
unidirectional forces
 Ɛ1=Ɛ4 (longitudinal strain) =
𝐹
𝐴𝐸
 Ϭ= poisson’s ratio
 Ɛ2=Ɛ3 (lateral strain) =
−𝐹
𝐴𝐸
ϭ

Cantilever beam
 Gauges need to be mounted such that
bending moment present at various
location of beam can be used to
calculate force.
 Cantilever beam is of dimension l x b x
h

 Ft is measured by gauges 1,2,3,4; whreas
Fc will be measured by gauges 1’,2’,3’,4’
 Ɛ1=Ɛ2=Ɛ3=Ɛ4= (6Ft l)/(Ebh2)
 Ɛ’1=Ɛ’2=Ɛ’3=Ɛ’4= (6Fcl)/(Ebh2)

Ring Type Dynamometer
 This type of dynamometer provides a high
ratio of sensitiveness to stiffness, and at the
same time it has adequate stability against
buckling.
 Gauges 1,2,3,4 at 90 from vertical measure
Ft and auges 1’,2’,3’,4’ at angle ϴ= 39.60
from vertical measure Fc.
 Due to Ft , ring is compressed. The strain in
gauge at ϴ= 900
Ɛ90 = ± 1.09 Ft r/Ebt2

 Due to Fc , strain in gauge at ϴ =39.60
Ɛ90 =±2.31 Fc r/Ebt2

 Often the production rate is more important than
cost/piece
 Max production rate => minimum time /piece
𝑡
= 𝑇𝑠
(setting+idle time)/piece
+ 𝑇 𝑚
machining time/piece
+ 𝑇𝑐𝑡 ×
𝑇 𝑚
𝑇
Tool changing time
Putting the values as earlier:-
𝑡 =
𝜋𝐷𝐿
1000𝑉𝑠
+ 𝑇𝑠 + 𝑇𝑐𝑡 ×
𝜋𝐷𝐿
1000𝑘
𝑉
1
𝑛−1
𝑠
1
𝑚−1
ECONOMICS OF METAL
CUTTING

For minimum time
𝜕𝑡
𝜕𝑉
= 0 &
𝜕𝑡
𝜕𝑠
= 0
𝑉 =
𝑘
𝑇𝑐𝑡
1
𝑛
− 1 𝑠
1
𝑚
& 𝑠 =
𝑘
𝑇𝑐𝑡
1
𝑚
− 1 𝑉
1
𝑛
 Optimum speed for min time is always greater than
that for min cost
ECONOMICS OF METAL
CUTTING

NUMERICAL CONTROL (NC)
Programmable automation in which the mechanical actions of a
‘machine tool’ are controlled by a program containing coded
alphanumeric data that represents relative positions between a
work head (e.g., cutting tool) and a work part.

NC Coordinate Systems
For flat and prismatic (block-like) parts:
 Milling and drilling operations
 Conventional Cartesian coordinate system
 Rotational axes about each linear axis
For rotational parts:
 Turning operations
 Only x- and z-axes

Motion Control Systems
Point-to-Point systems
 Also called position systems
 System moves to a location and performs an
operation at that location (e.g., drilling)
 Also applicable in robotics
Continuous path systems
 Also called contouring systems in machining
 System performs an operation during movement
(e.g., milling and turning)

Interpolation Methods
1. Linear interpolation
◦ Straight line between two points in
space
2. Circular interpolation
◦ Circular arc defined by starting point,
end point, center or radius, and
direction
3. Helical interpolation
◦ Circular plus linear motion
4. Parabolic and cubic interpolation
◦ Free form curves using higher order
equations

Absolute vs. Incremental
Positioning
Absolute positioning:
Locations are defines w.r.t. origin of the axis system
Move is: x = 40, y = 50
Incremental positioning:
Next Work head Position is defined relative to present
location
Move is: x = 20, y = 30.

Computer Numerical Control
(CNC)
 Storage of more than one part program
 Various forms of program input
 Program editing at the machine tool
 Fixed cycles and programming subroutines
 Interpolation
 Acceleration and deceleration computations
 Communications interface
 Diagnostics

DNC
 Direct numerical control (DNC) – control of multiple
machine tools by a single (mainframe) computer
through direct connection and in real time
◦ 1960s technology
◦ Two way communication
 Distributed numerical control (DNC) – network
consisting of central computer connected to machine
tool MCUs, which are CNC
◦ Present technology
◦ Two way communication

NC Application Characteristics
(Machining)
 Batch and High Volume production
 Repeat and/or Repetitive orders
 Complex part geometries
 Mundane operations
 Many separate operations on one part

Cost-Benefits of NC
Costs
 High investment cost
 High maintenance effort
 Need for skilled programmers
 High utilization required
Benefits
 Cycle time reduction
 Nonproductive time reduction
 Greater accuracy and repeatability
 Lower scrap rates
 Reduced parts inventory and floor space
 Operator skill-level reduced

NC Part Programming
In Manual part programming , the programmer
prepares the NC Code using a low level language that
can be understood by MCU. The coding system is
based on binary numbers.
Creating Instructions for NC
 Bit - 0 or 1 = absence or presence of hole in the tape
 Character - row of bits across the tape
 Word - sequence of characters (e.g., y-axis position)
 Block - collection of words to form one complete
instruction
 Part program - sequence of instructions (blocks)

Types of Words
N - sequence number prefix
G - preparatory words
◦ Example: G00 = PTP rapid traverse move
X, Y, Z - prefixes for x, y, and z-axes
F - feed rate prefix
S - spindle speed
T - tool selection
M - miscellaneous command
◦ Example: M07 = turn cutting fluid on

Example: Word Address
Format
N001 G00 X07000 Y03000 M03
N002 Y06000

OPEN LOOP CLOSED LOOP AND
ADAPTIVE CONTROL
CNC systems require motor drives to control both
the position and the velocity of the machine axes.
Each axis must be driven separately and follow the
command signal generated by the NC control.
There are two ways to activate the servo drives: the
open-loop system and the closed-loop system.

OPEN LOOP CONTROL
Open Loop - Programmed
instructions are fed into the
controller through an input device.
These instructions are then
converted to electrical pulses
(signals) by the controller and sent
to the servo amplifier to energize
the servo motors. The cumulative
number of electrical pulses
determines the distance each servo
drive will move, and the pulse
frequency determines the velocity.

OPEN LOOP CONTROL
 The primary drawback of the open-loop system is
that there is no feedback system to check whether the
program position and velocity has been achieved. If
the system performance is affected by load,
temperature, humidity, or lubrication then the actual
output could deviate from the desired output.
 For these reasons, the open-loop system is generally
used in point-to-point systems where the accuracy
requirements are not critical. Very few, if any,
continuous-path systems utilize open-loop control.

CLOSED LOOP CONTROL
Closed Loop - The closed-loop
system has a feedback subsystem to
monitor the actual output and
correct any discrepancy from the
programmed input. The feedback
system could be either analog or
digital. The analog systems
measure the variation of physical
variables such as position and
velocity in terms of voltage levels.
Digital systems monitor output
variations by means of electrical
pulses

CLOSED LOOP CONTROL
 Closed-loop systems are very powerful and accurate because
they are capable of monitoring operating conditions through
feedback subsystems and automatically compensating for any
variations in real-time.
 Most modern closed-loop CNC systems are able to provide
very close resolution of 0.0001 of an inch. Closed-looped
systems would, naturally, require more control devices and
circuitry in order for them to implement both position and
velocity control. This, obviously, makes them more complex
and more expensive than the open-loop system. A
closed/open comparison is shown below.

THERMAL ASPECTS OF METAL
CUTTING
 During machining of metals, considerable heat is
generated through conversion of mechanical
energy.
 The three distinct sources of heat in metal cutting
are given below: • The shear zone, 1,where
the primary plastic or shear
deformation takes place
• The chip-tool interface, 2,
where secondary plastic
deformation due to friction
between the heated chip
and tool takes place.
• The work- tool interface, 3,
at flanks where frictional
rubbing occurs.

 The heat generated is shared by the chip, cutting
tool and the blank. The apportionment of sharing
that heat depends upon the configuration, size and
thermal conductivity of the tool – work material and
the cutting condition.

 The rate of energy consumption during orthogonal
cutting is given by
Wc = Fc Vc where,
Fc = Cutting force , N
Vc = cutting speed , m/min
 When a material is deformed clastically, the energy
used is stored in the material as strain energy and
no heat is generated. However, when a material is
deformed plastically almost all the energy used is
converted into heat. In metal cutting , the material
is subjected to extremely high strains and elastic
deformation forms a very small proportion of the
total deformation, hence all the energy is assumed
to be converted into heat.
 Thus Q = Fc Vc / J where J is mechanical
equivalent of heat

 The cutting energy is converted into heat in two
principal regions of plastic deformation:
 The shear zone or primary deformation zone AB
 Secondary deformation zone BC
Temperature distribution in work
and chip during orthogonal
cutting.

EFFECTS OF HIGH CUTTING
TEMPERATURE
On tool
Rapid tool wear , which
reduces tool life
Cutting edges plastically
deform and tool may
loose its hot hardness
Thermal flaking and
fracturing of cutting edges
may take place due to
thermal shock
Built up edge formation
On work
Dimension inaccuracy of
work duet to thermal
distortion and expansion
and contraction during
and after machining.
Surface damage by
oxidation, rapid corrosion,
burning etc.
Tensile residual stresses
and microcracks at the
surface and sub surfaces.However, often the high cutting temperature helps in reducing the
magnitude of the cutting forces and cutting power consumption to some
extent by softening or reducing the shear strength of the work material
ahead the cutting edge. To attain or enhance such benefit the work
material ahead the cutting zone is often additionally heated externally. This
technique is known as Hot Machining and is beneficially applicable for the
work materials which are very hard and hardenable like high manganese

DETERMINATION OF CUTTING TEMPERATURE
The magnitude of the cutting temperature need to be
known or evaluated to facilitate
 Assessment of machinability which is judged
mainly by cutting forces and temperature and tool
life
 Design and selection of cutting tools
 Evaluate the role of variation of the different
machining parameters on cutting temperature
 Proper selection and application of cutting fluid
 Analysis of temperature distribution in the chip, tool
and job.

Cutting temperature can be determined by two ways :
 Analytically – using mathematical models
(equations) if available or can be developed. This
method is simple, quick and inexpensive but less
accurate and precise.
 Experimentally – this method is more accurate,
precise and reliable.
 The temperatures which are of major interests are:
𝜃𝑠 : average shear zone temperature
θi : average (and maximum) temperature at the chip-
tool interface
θf : temperature at the work-tool interface (tool flanks)
𝜃 𝑎 : average cutting temperature

(a) Average shear zone temperature, 𝜃𝑠
 The cutting energy per unit time, i.e., 𝑃𝑧 ∙ 𝑉𝑐 gets used to
cause primary shear and to overcome friction at the rake face
as,
𝑃𝑧 ∙ 𝑉𝑐 = 𝑃𝑠 ∙ 𝑉𝑠 + 𝐹 ∙ 𝑉𝑓
where, 𝑉𝑠 = slip velocity along the shear plane and
𝑉𝑓 = average chip – velocity So, 𝑃𝑠 ∙ 𝑉𝑠 = 𝑃𝑧 ∙ 𝑉𝑐 – 𝐹 ∙ 𝑉𝑓
Equating amount of heat received by the chip in one minute
from the shear zone and the heat contained by that chip, it
appears,
𝐴𝑞1(𝑃𝑧 ∙ 𝑉𝑐 − 𝐹 ∙ 𝑉𝑓)
𝐽
= 𝐶𝑣 𝑎1 𝑏1 𝑉𝑐(𝜃𝑠 − 𝜃 𝑎)
where, 𝐴 = fraction (of shear energy that is converted into heat)
𝑞1 = fraction (of heat that goes to the chip from the shear zone)
𝐽 = mechanical equivalent of heat of the chip / work material
𝐶𝑣 = volume specific heat of the chip
𝜃 𝑎 = ambient temperature
𝑎1 𝑏1 = cross sectional area of uncut chip

𝜃𝑠 = 𝜃 𝑎 +
𝐴𝑞1(𝑃𝑧 ∙ 𝑉𝑐 − 𝐹 ∙ 𝑉𝑓)
𝐽𝐶𝑣 𝑎1 𝑏1 𝑉𝑐
Generally A varies from 0.95 to 1.0 and q from 0.7 to 0.9 in
machining like turning.
(b)Average chip – tool interface temperature, 𝜽𝒊
 By dimensional analysis it can be found to be proportional as:
𝜃 𝑎 ∝ 𝑈𝑐
𝑉𝑐 𝑎1
𝑘𝜌𝐶
Where,
𝑈𝑐=specific energy,
𝑉𝑐 =cutting speed
𝑎1=uncut chip thickness
𝑘=thermal conductivity of material
𝜌=density 𝐶=specific heat

Experimental methods of determination of
cutting temperature
Tool work thermocouple technique
• In machining like turning,
the tool and the job
constitute the two
dissimilar metals and the
cutting zone functions as
the hot junction. Then the
average cutting
temperature is evaluated
from the mV after
thorough calibration for
establishing the exact
relation between mV and
the cutting temperature.

Infrared Photographic Technique
This modern and powerful method is based on taking
infra-red photograph of the hot surfaces of the tool,
chip, and/or job and get temperature distribution at
those surfaces. Proper calibration is to be done before
that. This way the temperature profiles can be
recorded in PC as indicated in figure. The fringe
pattern readily changes with the change in any
machining parameter which affect cutting temperature.

Role of variation of the various machining
parameters on cutting temperature
 The magnitude of cutting
temperature is more or
less governed or
influenced by all the
machining parameters like
:
Work material :
- specific energy
requirement
- ductility
-thermal properties (λ, cv)
Process parameters :
- cutting velocity (VC)
- feed (so)
- depth of cut (t)
Cutting tool material :
- thermal properties
- wear resistance
- chemical stability
tool geometry : - rake
angle(γ)
- cutting edge angle (φ)
- clearance angle (α)
- nose radius (r)
Cutting fluid :
- thermal and lubricating
properties
- method of application

ToolWearAnd ToolLife Of SinglePoint
Cutting Tool

Toolwear
Wear is loss of material on an asperity or
micro-contact, or smaller scale, down to
molecular or atomic removal mechanisms. It
usually progresses continuously. Tool wear
describes the gradual failure of cutting tools
due to regular operation. It is a term often
associated with tipped tools, tool bits, or drill
bit that are used with machine tools.

Typesof tool wear
• Flank wear
• Crater wear
• Nose wear

Flankwear
Flank wear occurs on the tool flank as a result of
friction between the machined surface of the
workpiece and the tool flank. Flank wear appears
in the form of so-called wear land and is
measured by the width of this wear land, VB,
Flank wear affects to the great extend the
mechanics of cutting. Cutting forces increase
significantly with flank wear. If the amount of
flank wear exceeds some critical value (VB >
0.5~0.6 mm), the excessive cutting force may
cause tool failure.

Crater wear
 Crater wear consists of a concave
section on the tool face formed by the
action of the chip sliding on the
surface. Crater wear affects the
mechanics of the process increasing
the actual rake angle of the
cutting tool and consequently, making
cutting easier. At the same time, the
crater wear weakens the tool wedge
and increases the possibility for tool
breakage. In general, crater wear is of

Nosewear
Nose wear occurs on the tool corner. Can be
considered as a part of the wear land and
respectively flank wear since there is no
distinguished boundary between the corner wear
and flank wear land. We consider nose wear as a
separate wear type because of its importance for
the precision of machining. Nose wear actually
shortens the cutting tool thus increasing gradually
the dimension of machined surface and
introducing a significant dimensional error in
machining, which can reach values of about
0.03~0.05 mm.

Effects of ToolWear
Some General effects of tool wear include:
• Increased cutting forces
• Increased cutting temperatures
• Poor surface finish
• Decreased accuracy of finished part
• May lead to tool breakage
• Causes change in tool geometry

Wear Control
Therate of tool wear strongly depends on the cutting
temperature, therefore , any measures which could be
applied to reduce the cutting temperature would reduce the
tool wear aswell. Thefigure shows the process parameters
that influence the rate of tool wear:

Reduction in tool wear can be accomplished by using
lubricants and coolants while machining. These
reduce friction and temperature, thus reducing the
tool wear.
Additional measures to reduce the tool wear include
the application of advanced cutting tool materials,
such as coated carbides, ceramics, etc.

Tool Life Of Single Point
Cutting Tool

Definition
(a) In R & D : Actual machining time (period) by which a
fresh cutting tool (or point) satisfactorily works after which
it needs replacement or reconditioning. The modern tools
hardly fail prematurely or abruptly by mechanical breakage
or rapid plastic deformation. Those fail mostly by wearing
process which systematically grows slowly with machining
time. In that case, tool life means the span of actual
machining time by which a fresh tool can work before
attaining the specified limit of tool wear. Mostly tool life is
decided by the machining time till flank wear, VB reaches
0.3 mm or crater wear, KT reaches
0.15 mm.
(b) In industries or shop floor : The length of time of
satisfactory service or amount of acceptable output
provided by a fresh tool prior to it is required to replace or
recondition.

Assessmentof tool life
There are three ways of assessment of tool life
1. No. of pieces of work machined – This
is used commonly when the tool
operates continuously .
2. Total volume of material removed –This is
used commonly when the tool is used for
high stock removal.
3. Total length of cut

Modes Of Failure OfTool
1 Premature Failure
(a)Fracture failure - Cutting force becomes
excessiveand/or dynamic, leading to
brittle fracture.
(b)Thermal failure - Cutting temperature is
too high for the toolmaterial.
2 Gradual Wear-Gradual failure

Factors Affecting ToolLife
Cutting speed
Feed and depth of cut
Tool geometry
Tool material
Work material
Nature of cutting
Rigidity of machine tool and work
Use of cutting fluids

Depth of cut
Toosmall
• Lossof chip control
• Vibration
• Excessiveheat
• Uneconomical
Toodeep
• High power consumption
• Insert breakage
• Increased cutting force

FeedRate
Toolight
• Stringers
• Rapid flank wear
• Build-up edge
• Uneconomical
Tooheavy
• Lossof chip control
• Poor surface finish
• Crater wear/plastic deformation
• High power consumption

ToolGeometry
Rake angle-If it is increased in positive direction , the cutting
force and amount of heat generated are reduced. This increases
the life of the tool. But if it is increased too much , cutting
edge is weakened and capacity to conduct heat also decreases.
Relief angle-These are provided on the cutting tool to prevent
rubbing of tool flank with machined work surface. Thus it
reduces the amount of heat generated and increases tool life.
Cutting edge angles- these angles affect tool wear. Up to a
certain value of these angles , higher speed without an adverse
affect on tool life can be used.

Toolmaterial
Hardness
Cutting tool material must be 1 to 1/2 times harder than
the material it is being used to machine.
Wear Resistance
• Able to maintain sharpened edge throughout the
cutting operation
• Same as abrasive resistance
Shock Resistance
• Able to take the cutting loads and forces
Shape and Configuration
• Must be available for use in different sizes and shapes.

Properties of cutting fluid..
 It should posses good lubricating properties
to reduce the frictional force
 Non-corrosive to the work or the machine.
 Natural so as not to react chemically.
 It should not very easily flammable.
 It should be stable in use and storage.
 High heat conductivity.
 Harmless to the skin of the operators.
 Low priced to minimum production cost.

Principles of cutting fluid action:

Method of applications of
cutting fluid:
 Flood under gravity:
High discharge, used in
rough machining
 Mist application:
Spray form is discharged, used in finish machining.
 Drop by drop under gravity
 In the form of liquid jet

•Z-Z method:
This is a centrifugal process
through grinding wheel or
pores. Used in high speed
machining application.
•Using more than one
nozzle:
More than one nozzle may be
used in some cutting
operations to direct the cutting
fluid at the cutting fluid closest
to the cutting zone.

•High pressure system:
In operations like deep hole drilling, pressurized fluid is
often sent through axial or inner spiral holes of the drill. For
effective cooling and lubrication in high speed machining of
ductile metals having wide and plastic chip-tool contact,
cutting fluid may be pushed at high pressure to the chip-
tool interface through holes in the cutting tool.

Types of cutting fluid & their application:
 Air blast or compressed air:
•Water:
•Soluble Oil:

•Chemical fluids:
•Solid or semi-solid lubricant:

Rigidity of machine tools:
The service conditions or the operating properties of machine
tools are often determined by the degree of rigidity of their
individual units. Rigidity is defined as the degree of deformation
undergone by a member for an external load
 Static compliance of a machine tool:
The rigidity of an element of a machine tool is determined by a
progressive application of loads, when the relation between
deflection and load is
The relationship is a straight line such that

Consider the case of a centre lathe, when a shaft is being turned
between centers with a single point cutting tool as shown below
Equivalent stiffness of different elements of a centre
lathe:
The deflection pattern for the case ie as given below

From the above figure:
 =deflection at the cutting point due to flexibilities of the headstock
and tailstock
= deflection at the headstock
= deflection at the tailstock
Again,
And,
Thus,

Now,
Hence,
=Deflection of the machine tool at the cutting point
=Deflection at the saddle
So,
Therefore, Compliance of the machine tool:
Very exacting demands are made on modern machine tools
because of advancement in the field of automation. Rigidity is
particularly important for ensuring the adequate accuracy of items
produced on the machine tools. In fact, it is the resilience of the
whole machine element system which controls the offered

Vibration in machine tools:
 Sources of vibration:
•Effects of vibration:
(1) On machine tool:

(2) On cutting conditions:
(3) On tool-life:

Machine tool chatter:
Elimination of vibration:

Introduction to Gearbox
• Gearbox often referred as transmission is a unit that uses gears and
gear
trains to provide speed and torque conversions from a rotating power
source to another device. Gearboxes are employed to convert input
from a
high speed power sources to low speed(E.g. Lift, Cranes
and Crushing Machine) or into a many of speeds(Lathe, Milling
Machine
and Automobiles).
• A gearbox that converts a high speed input into a single output it is
called
a single stage gearbox. It usually usually has two gears and shafts.
• A gearbox that converts a high speed input into a number of different
speed output it is called a multi-speed gear box. Multi speed gear box
has
more than two gears and shafts. A multi speed gearbox reduces the
speed

Working of a principle gear box
• In a gear box, the counter shaft is mashed to the clutch with a
use of a
couple of gear. So the counter shaft is always in running
condition. When
the counter shaft is bring in contact with the main shaft by use of
meshing gears, the main shaft start to rotate according to the gear
ratio.
• When want to change the gear ratio, simply press the clutch
pedal which disconnect the counter shaft with engine and change
connect the main shaft with counter shaft by another gear ratio by
use of gearshift lever.
• In an gear box, the gear teeth and other moving metal must not
touch.
They must be continuously separated by a thin film of lubricant.
This
prevents excessive wear and early failure. Therefore a gearbox

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