Topic 2 machining 160214

BDA 3052 Manufacturing
Technology
Material Removal Process (Metal
Machining Process)
1.1 Theory of Metal Cutting
overview, theory of chip formation, force &
merchant equation, power & energy, cutting
temperature
1.2 Machining Operations and Machine Tools
turning, drilling, milling, machine centers,
cutting tool technology

Technology
Material Removal Processes
Introduction
 It is shaping operations, it remove material from a
starting workpart so the remaining part has the
desired geometry
 Divided into three main groups:
1) Machining – material removal by a sharp
cutting tool, e.g., turning, milling, drilling
2) Abrasive processes – material removal by hard,
abrasive particles, e.g., grinding
3) Nontraditional processes - various energy
forms other than sharp cutting tool to remove
material

Technology
Why Machining is Important
 Variety of work materials can be machined
 Most frequently used to cut metals
 Variety of part shapes and special geometric
features possible, such as:
 Screw threads
 Accurate round holes
 Very straight edges and surfaces
 Good dimensional accuracy and surface finish

Technology
Disadvantages with Machining
 Wasteful of material
 Chips generated in machining are wasted
material, at least in the unit operation
 Time consuming
 A machining operation generally takes more
time to shape a given part than alternative
shaping processes, such as casting, powder
metallurgy, or forming

Technology
Machining in Manufacturing Sequence
 Generally performed after other manufacturing
processes, such as casting, forging, and bar
drawing
 Other processes create the general shape of
the starting workpart
 Machining provides the final shape,
dimensions, finish, and special geometric
details that other processes cannot create

Technology
Machining
Nomenclature of single point tool

Technology
Seven elements of single-point tool geometry; and
(b) the tool signature convention that defines the
seven elements.

Technology
Simplified 2-D model of machining that
describes the mechanics of machining
fairly accurately
Figure 21.6 Orthogonal cutting: (a) as a three-dimensional process.
Orthogonal Cutting Model

Technology
Orthogonal and Oblique cutting

Assumptions in orthogonal cutting
(Merchant theory)
 01.The tool is perfectly sharp and no contact along clearance face.
 02. The shear surface is a plane extending upward from the
cutting edge.
 03.The cutting edge is a straight line, extending perpendicular to
the direction of motion and generates a plane surface as the work
moves past it.
 04.The chip does not flow to either side.
 05.The depth of cut is constant.
 06.Width of the tool is greater than the work piece.
 07.The work moves relative to the tool with uniform velocity.
 08.A continuous chip is produced with no built up edge.
Technology

Assumptions in orthogonal cutting
(Merchant theory)
 09. Chip is assume to shear continuously
across plane AB on which the shear stress
reaches the value of shear flow stress.
 10.Width of chip is remains equal to the width
of the work piece. i.e. Plane strain conditions
exist.
Technology

Difference between orthogonal and
oblique cutting
Orthogonal cutting Oblique cutting
01.The cutting edge of the tool is perpendicular to the
direction of the tool travel.
The cutting edge is inclined at angle with the normal
direction of the tool travel.
02. The cutting edge clears the width of the work piece
on either ends.
The cutting edge may or may not clear the width of the
workpiece.
03. The chip flows over the tool. The chip coils in tight. The chip flows on the tool face making an angle with
the normal cutting edge. The chip flows side ways in a
long curl.
04. Only two components of the cutting force acting on
the tool.
Three components of the forces acting on the tool.
05.Maximum chip thickness occurs at the middle. Maximum chip thickness may not occur at middle.
06. For the given feed rate and DOC, the force which
act or shears the metal acts on a smaller area and
therefore, the heat developed per unit area due to
friction along the tool work interface is less and the tool
life is less.
It acts on larger area and thus tool life is more.

Technology
 Cutting action involves shear deformation of
work material to form a chip
 As chip is removed, new surface is exposed
(a) A cross-sectional view of the machining process, (b) tool with
negative rake angle; compare with positive rake angle in (a).
Machining

Technology
Cutting Process in Turning

Technology
Relationship between chip thickness,
rake angle and shear plane angle
)cos(
sin
)cos(
sin









s
s
l
l
r

Technology
Chip Thickness Ratio
where r = chip thickness ratio; to = thickness of the
chip prior to chip formation; and tc = chip thickness
after separation
 Chip thickness after cut always greater than
before, so chip ratio always less than 1.0
c
o
t
t
r 

Technology
Example of Problem
In a machining operation that approximates orthogonal
cutting, the cutting tool has a rake angle = 10. The chip
Thickness before the cut to = 0.50 mm and the chip
thickness after the cut tc = 1.125 mm. Calculate the shear
plane angle and the shear strain in the operation.
Answer :  = 25.4
 = 2.386

Technology
Example of Problem 2
In an orthogonal cutting operation, the tool has a rake
angle = 15. The chip thickness before the cut = 0.30
mm and the cut yields a deformed chip thickness = 0.65
mm. Calculate (a) the shear plane angle and (b) the
shear strain for the operation.

Technology
Shear strain during chip formation: (a) chip formation depicted as a series of
parallel plates sliding relative to each other, (b) one of the plates isolated
to show shear strain, and (c) shear strain triangle used to derive strain
equation.
Shear Strain in Chip Formation

Technology
Shear Strain
Shear strain in machining can be computed
from the following equation, based on the
preceding parallel plate model:
 = tan( - ) + cot 
where  = shear strain,  = shear plane angle, and  =
rake angle of cutting tool
BD
DCAD
BD
AC 


Technology
More realistic view of chip formation, showing shear zone rather
than shear plane. Also shown is the secondary shear zone resulting
from tool-chip friction.
Chip Formation

Technology
Four Basic Types of Chip in Machining
1. Discontinuous chip
2. Continuous chip
3. Continuous chip with Built-up Edge (BUE)
4. Serrated chip

Technology
 Brittle work
materials
 Low cutting
speeds
 Large feed and
depth of cut
 High tool-chip
friction
1. Discontinuous Chip

Technology
 Ductile work
materials
 High cutting
speeds
 Small feeds and
depths
 Sharp cutting edge
 Low tool-chip
friction
2. Continuous Chip

Technology
 Ductile materials
 Low-to-medium
cutting speeds
 Tool-chip friction
causes portions of
chip to adhere to
rake face
 BUE forms, then
breaks off, cyclically
Continuous with BUE

Technology
 Semicontinuous -
saw-tooth
appearance
 Cyclical chip forms
with alternating high
shear strain then low
shear strain
 Associated with
difficult-to-machine
metals at high cutting
speeds
Serrated Chip
Figure 21.9 (d) serrated.

Technology
 Higher shear plane angle means smaller shear
plane which means lower shear force, cutting
forces, power, and temperature
Effect of shear plane angle  : (a) higher  with a resulting lower shear
plane area; (b) smaller  with a corresponding larger shear plane
area. Note that the rake angle is larger in (a), which tends to increase
shear angle according to the Merchant equation
Effect of Higher Shear Plane Angle

Technology
 Friction force F and Normal force to friction N
 Shear force Fs and Normal force to shear Fn
Forces in metal cutting: (a)
forces acting on the chip in
orthogonal cutting
Forces Acting on Chip

Technology
Resultant Forces
 Vector addition of F and N = resultant R
 Vector addition of Fs and Fn = resultant R'
 Forces acting on the chip must be in balance:
 R‘’ must be equal in magnitude to R
 R’ must be opposite in direction to R
 R’ must be collinear with R

Technology
Cutting Forces

Technology
Coefficient of Friction
Coefficient of friction between tool and chip:
Friction angle related to coefficient of friction
as follows:
N
F

 tan
-(1)
-(2)

Technology
Shear Stress
Shear stress acting along the shear plane:
sin
wt
A o
s 
where As = area of the shear plane
Shear stress = shear strength of work material during
cutting
s
s
A
F
S  -(3)
-(4)

Technology
 F, N, Fs, and Fn cannot be directly measured
 Forces acting on the tool that can be measured:
 Cutting force Fc and Thrust force Ft
Forces in metal
cutting: (b) forces
acting on the tool that
can be measured
Cutting Force and Thrust Force

Technology
Merchant’s circle diagram

Technology
Forces in Metal Cutting
 Equations can be derived to relate the forces that
cannot be measured to the forces that can be
measured:
F = Fc sin  + Ft cos  (5)
N = Fc cos  - Ft sin  (6)
Fs = Fc cos  - Ft sin  (7)
Fn = Fc sin  + Ft cos  (8)
 Based on th ese calculated force, shear stress
and coefficient of friction can be determined

Technology
Example of Problem 3
Cutting force = 1559 N
Thrust force = 1271 N
Width of cutting = 3 mm
Rake angle = 10
Shear plane angle = 25.4
Original Thickness = 0.5 mm
Determine the shear strength of the work
material.
shear stress, S / shear strength,  = 247 N/mm2

Technology
Forces in Metal Cutting
 From equation 3, the force diagram (Merchant ‘s Circle
Diagram), can be used to derived the following
equations:
)cos(
)cos(
)cos(sin
)cos(









 so
c
FwSt
F
)cos(
)sin(
)cos(sin
)sin(









 so
t
FwSt
F

Technology
The Merchant Equation
 From equation 3, 4 and 7, Merchant Equation for shear
stress can be expressed as,
)sin/(
sincos



o
tc
t
FF 


Technology
The Merchant Equation
 Of all the possible angles at which shear
deformation can occur, the work material will
select a shear plane angle  that minimizes
energy, given by
 Derived by Eugene Merchant
 Based on orthogonal cutting, but validity extends
to 3-D machining
22
45

 

Technology
What the Merchant Equation Tells Us
 To increase shear plane angle
 Increase the rake angle
 Reduce the friction angle (or coefficient of
friction)
22
45

 

Technology
Power and Energy Relationships
 A machining operation requires power
 The power to perform machining can be
computed from:
Pc = Fc 
where Pc = cutting power (Nm/s); Fc = cutting force
(N); and  = cutting speed (m/min)

Technology
 In U.S. customary units, power is traditional
expressed as horsepower
HPc = Fc/33,000
where HPc = cutting horsepower, hp

Technology
 Gross power to operate the machine tool Pg or
HPg is given by
or
where E = mechanical efficiency of machine tool
 Typical E for machine tools  90%
E
P
P c
g 
E
HP
HP c
g 

Technology
Unit Power in Machining
 Useful to convert power into power per unit
volume rate of metal cut
 Called unit power, Pu or unit horsepower, HPu
or
where MRR = material removal rate (mm3/s)
MRR
P
P c
U 
MRR
HP
HP c
u 

Technology
Specific Energy in Machining
Unit power is also known as the specific energy U
Units for specific energy are typically N-m/mm3 or
J/mm3
wt
F
wvt
vF
MRR
P
PU
o
c
o
cc
u 

Technology
Cutting Temperature
 Approximately 98% of the energy in machining is
converted into heat
 This can cause temperatures to be very high at
the tool-chip
 The remaining energy (about 2%) is retained as
elastic energy in the chip

Technology
Cutting Temperature is Important
High cutting temperatures
1. Reduce tool life
2. Produce hot chips that pose safety hazards to
the machine operator
3. Can cause inaccuracies in part dimensions due
to thermal expansion of work material

Technology
Cutting Temperature
 Analytical method derived by Nathan Cook
from dimensional analysis using
experimental data for various work materials
where T = temperature rise at tool-chip interface; U =
specific energy; v = cutting speed; to = chip thickness
before cut; C = volumetric specific heat of work
material; K = thermal diffusivity of work material
3330
40
.
.







K
vt
C
U
T o


Technology
Example Problem 6
Cutting speed = 100 m/min
Chip original thickness = 0.5 mm
Thermal diffusivity = 50 mm2/s
Specific Energy = 1.038
Volumetric specific heat work material = 3 x10-3 J/mm3
Find the mean temperature rise at the tool-chip
Interface.

Technology
Cutting Temperature
 Experimental methods can be used to measure
temperatures in machining
 Most frequently used technique is the tool-chip
thermocouple
 Using this method, Ken Trigger determined the
speed-temperature relationship to be of the form:
T = K vm
where T = measured tool-chip interface
temperature, and v = cutting speed
K and m depend on the cutting conditions and
work material

Machining
A material removal process in which a sharp cutting
tool is used to mechanically cut away material so
that the desired part geometry remains
 Most common application: to shape metal parts
 Most versatile of all manufacturing processes in
its capability to produce a diversity of part
geometries and geometric features with high
precision and accuracy
 Casting can also produce a variety of shapes,
but it lacks the precision and accuracy of
machining

 Rotational - cylindrical or disk-like shape
 Nonrotational (also called prismatic) -
block-like or plate-like
Machined parts are classified as: (a) rotational, or (b) nonrotational,
shown here by block and flat parts.
Classification of Machined Parts

Machining Operations and Part
Geometry
Each machining operation produces a
characteristic part geometry due to two
factors:
1. Relative motions between tool and workpart
• Generating – part geometry determined
by feed trajectory of cutting tool
2. Shape of the cutting tool
• Forming – part geometry is created by
the shape of the cutting tool

Figure 22.2 Generating shape: (a) straight turning, (b) taper turning, (c)
contour turning, (d) plain milling, (e) profile milling.
Generating Shape

Forming to create shape: (a) form turning, (b) drilling, and (c)
broaching.
Forming to Create Shape

Combination of forming and generating to create shape: (a) thread
cutting on a lathe, and (b) slot milling.
Forming and Generating

Turning
Single point cutting tool removes material from a
rotating workpiece to generate a cylinder
 Performed on a machine tool called a lathe
 Variations of turning performed on a lathe:
 Facing
 Contour turning
 Chamfering
 Cutoff
 Threading

Turning Operation
Close-up view of a
turning operation on
steel using a titanium
nitride coated carbide
cutting insert (photo
courtesy of Kennametal
Inc.)

Tool is fed
radially inward
Facing

 Instead of feeding tool
parallel to axis of
rotation, tool follows a
contour that is other
than straight, thus
creating a contoured
shape
Contour Turning

 Cutting edge cuts an angle on the corner of
the cylinder, forming a "chamfer"
Chamfering

 Tool is fed radially into rotating work at some
location to cut off end of part
Cutoff

 Pointed form tool is fed linearly across surface
of rotating workpart parallel to axis of rotation
at a large feed rate, thus creating threads
Threading

The rotational speed in turning related to the desired cutting
speed at the surface of the cylindrical workpiece by the
equation:
N = rotational speed, rev/min;  = cutting speed, m/min,
And Do = original diameter of the part, m.
Cutting Conditions in Turning - 1


D
N 

The change in diameter is determined by the depth of cut,
d:
Do – Df = 2d
Do = original diameter, mm; Df = final diameter, mm
d = depth of cut

The feed in turning is generally expressed in mm/rev. This
feed can be converted to linear travel rate in mm/min by the
formula:
fr = Nf
fr = feed rate, mm/min; f = feed mm/rev

The time to machine from one end of a cylindrical workpart
to the other is given by:
Tm = L/fr
Tm = time of actual machining, minutes; and L = length of
the cylindrical workpart, mm

The volumetric rate of material removal rate can be most
conveniently determined by the following equation:
MRR = vfd
MRR = material removal rate, mm3/min, f = feed, mm

A cylindrical workpart 200 mm in diameter
and 700 mm long is to be turned in an
engine lathe. Cutting conditions are as
follows: cutting speed is 2.30 m/s, feed is
0.32 mm/rev, and depth of cut is 1.80
mm.
Determine (a) cutting time, and (b)
metal removal rate.
Cutting Conditions in Turning
Problem 1

A work materials are to be turned to final size of
175 mm length having diameter of 60 mm.
Total length of the work material is 300 mm. A
single point tool having a certain degree of rake
angle is used. The work material rotates at 1400
RPM. The feed is 0.35 mm / revolution.
Final size of the work material is 51 mm.
Calculate:
1. Cutting velocity,
2. Time taken to machine to the length of 55 mm,
3. Total Material removal rate to get 51 mm diameter.
Cutting Conditions in Turning
Problem 2

Milling
Machining operation in which work is fed past a
rotating tool with multiple cutting edges
 Axis of tool rotation is perpendicular to feed
 Creates a planar surface
 Other geometries possible either by cutter
path or shape
 Other factors and terms:
 Interrupted cutting operation
 Cutting tool called a milling cutter, cutting
edges called "teeth"
 Machine tool called a milling machine

Peripheral Milling vs. Face Milling
 Peripheral milling
 Cutter axis parallel to surface being machined
 Cutting edges on outside periphery of cutter
 Face milling
 Cutter axis perpendicular to surface being
milled
 Cutting edges on both the end and outside
periphery of the cutter

METHODS OF MILLING-1
1) Up milling is also referred to as conventional
milling. The direction of the cutter rotation
opposes the feed motion. For example, if the
cutter rotates clockwise , the workpiece is fed
to the right in up milling.

METHODS OF MILLING-2
2) Down milling is also referred to as climb
milling. The direction of cutter rotation is
same as the feed motion. For example, if the
cutter rotates counterclockwise , the
workpiece is fed to the right in down milling

 Basic form of peripheral milling in which the
cutter width extends beyond the workpiece
on both sides
Slab Milling

 Width of cutter is less than workpiece width,
creating a slot in the work
Slotting

Cutter overhangs work
on both sides
Conventional Face Milling

Form of end milling in
which the outside
periphery of a flat
part is cut
Profile Milling

 Another form
of end milling
used to mill
shallow
pockets into
flat parts
Pocket Milling

 Ball-nose cutter
fed back and forth
across work along
a curvilinear path
at close intervals
to create a three
dimensional
surface form
Surface Contouring

 Cutter diameter is
less than work
width, so a slot is
cut into part
End Milling

Machining Centers
Highly automated machine tool can perform
multiple machining operations under CNC
control in one setup with minimal human
attention
 Typical operations are milling and drilling
 Three, four, or five axes
 Other features:
 Automatic tool-changing
 Pallet shuttles
 Automatic workpart positioning

High speed face
milling using
indexable inserts
(photo courtesy
of Kennametal
Inc.).
Milling Operation

The cutting speed is determined at the
outside diameter of a milling cutter.
N = rotational speed, rev/min;  = cutting speed,
m/min,
And Do = outside diameter of a milling cutter,mm.
Cutting Conditions in Milling - 1
D
N




The feed, f in milling is usually given as a feed per cutter
tooth; called the chip load, it represents the size of the chip
formed by each cutting edge.
fr = Nnt f
fr = feed rate, mm/min; N = spindle speed, rev/min; nt =
number of teeth on the cutter; f = chip load in mm/tooth

The material removal rate,
MRR = wdfr
w = width; d = depth of cut; fr = feed rate, mm/min;

Approach distance A, to reach full cutter depth given by:
d = depth of cut, mm, and D = diameter of the milling cutter,
mm
)( dDdA 

The time to mill the workiece Tm is
therefore;
r
m
f
AL
T



Face Milling – cutter is centered

Where A and O are each to half the cutter
diameter;
A = O = D/2
D= cutter diameter, mm

Face Milling – cutter is offset

Where A and O are each to half the cutter
diameter;
w= width of the cut, mm
)( wDwOA 

The time to mill the workiece in face
milling,Tm is therefore;
r
m
f
AL
T
2


A peripheral milling operation is performed
on the top surface of a rectangular
workpart which is 400 mm long by 60 mm
wide. The milling cutter, which is 80 mm in
diameter and has five teeth, overhangs the
width of the part on both sides. The cutting
speed is 70 m/min, the chip load is 0.25
mm/tooth, and the depth of cut is 5.0 mm.
Determine (a) the time to make one pass
across the surface, and (b) the maximum
material removal rate during the cut.
Cutting Conditions in Milling
Problem 1

A face milling operation is performed to finish
the top surface of a steel rectangular work
piece 350 mm long by 55 mm wide. The milling
cutter has four teeth (cemented carbide inserts)
and a 85 mm diameter. Cutting conditions are:
v = 600 m/min, f = 0.35 mm / tooth, and d =
3.5 mm.
Determine:
a) the time to make one pass across the surface.
b) the metal removal rate during the cut.
Cutting Conditions in Milling
Problem 2

 Creates a round
hole in a workpart
 Compare to boring
which can only
enlarge an existing
hole
 Cutting tool called
a drill or drill bit
 Machine tool: drill
press
Drilling

Through-holes - drill exits opposite side of work
Blind-holes – does not exit work opposite side
Two hole types: (a) through-hole, and (b) blind hole.
Through Holes vs. Blind Holes

 Used to slightly
enlarge a hole,
provide better
tolerance on
diameter, and
improve surface
finish
Reaming

 Used to provide
internal screw
threads on an
existing hole
 Tool called a tap
Tapping

 Provides a stepped
hole, in which a
larger diameter
follows smaller
diameter partially
into the hole
Counterboring

Letting N represent the spindle rev/min,
 = cutting speed, m/min; D = the drill
diameter,mm.
Cutting Conditions in Drilling - 1
D
N




Feed can be converted to feed rate using the the same
equation as for turning:
fr = Nf
fr = feed rate, mm/min; N = spindle speed, rev/min; f = feed
in drilling, mm/rev

The time to drill through holes;
Tm= machining time, min; t = work thickness,
mm; fr = feed rate, mm/min
r
m
f
At
T



The allowance is given by;
A = approach allowance, mm;  = drill point angle







2
90tan5.0

DA

The time to drill blind holes;
Tm= machining time, min; d = hole depth,
mm; fr = feed rate, mm/min
r
m
f
d
T 

A drilling operation is to be performed with a
12.7 mm diameter twist drill in a steel
workpart. The hole is a blind hole at a depth of
60 mm and the point angle is 118. The cutting
speed is 25 m/min and the feed is 0.30
mm/rev.
Determine
(a) the cutting time to complete the drilling
operation, and
(b) metal removal rate during the operation,
after the drill bit reaches full diameter.
Cutting Conditions in Drilling
Problem 1

CUTTING TOOL TECHNOLOGY
1. Tool Life
2. Tool Materials
3. Tool Geometry
4. Cutting Fluids

Cutting Tool Technology
Two principal aspects:
1. Tool material
2. Tool geometry

Three Modes of Tool Failure
1. Fracture failure
 Cutting force becomes excessive and/or
dynamic, leading to brittle fracture
2. Temperature failure
 Cutting temperature is too high for the tool
material
3. Gradual wear
 Gradual wearing of the cutting tool

Preferred Mode: Gradual Wear
 Fracture and temperature failures are premature
failures
 Gradual wear is preferred because it leads to the
longest possible use of the tool
 Gradual wear occurs at two locations on a tool:
 Crater wear – occurs on top rake face
 Flank wear – occurs on flank (side of tool)

Figure 23.1 Diagram of worn cutting tool, showing the principal
locations and types of wear that occur.
Tool Wear

Figure 23.2 Crater wear,
(above), and flank wear (right) on
a cemented carbide tool, as seen
through a toolmaker's
microscope (photos by K. C.
Keefe, Manufacturing Technology
Lab, Lehigh University).

Taylor Tool Life Equation
Relationship is credited to F. W. Taylor
CvT n

where v = cutting speed; T = tool life; and n and C
are parameters that depend on feed, depth of cut,
work material, tooling material, and the tool life
criterion used
 n is the slope of the plot
 C is the intercept on the speed axis at one minute
tool life

Tool Life Criteria in Production
1. Complete failure of cutting edge
2. Visual inspection of flank wear (or crater
wear) by the machine operator
3. Fingernail test across cutting edge
4. Changes in sound emitted from operation
5. Chips become ribbon-like, stringy, and
difficult to dispose of
6. Degradation of surface finish
7. Increased power
8. Workpiece count
9. Cumulative cutting time

Tool Materials
 Tool failure modes identify the important
properties that a tool material should possess:
 Toughness - to avoid fracture failure
 Hot hardness - ability to retain hardness at
high temperatures
 Wear resistance - hardness is the most
important property to resist abrasive wear

Topic 2 machining 160214

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Topic 2 machining 160214