Dr. M.

Safa Bodur
 Overview of Machining Technology
 Theory of Chip Formation in Metal Machining
 Force Relationships and the Merchant Equation
 Power and Energy Relationships in Machining
 Cutting Temperature

2
 A family of shaping operations,
the common feature of which is
removal of material from a
starting workpart so the
remaining part has the desired
geometry
 Machining – material removal by
a sharp cutting tool, e.g., turning,
milling, drilling
 Abrasive processes – material
removal by hard, abrasive
particles, e.g., grinding
 Nontraditional processes -
various energy forms other than
sharp cutting tool to remove
material
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  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).
 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

 Good surface finish

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 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

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 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

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 Most important machining operations:
 Turning
 Drilling
 Milling

 Other machining operations:

 Shaping and planing
 Broaching
 Sawing

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 Single point cutting tool removes material from a rotating
workpiece to form a cylindrical shape
 The speed motion in turning is provided by the rotating workpart,
and the feed motion is achieved by the cutting toolmoving slowly
in a direction parallel to the axis of rotation of theworkpiece
 Used to create a round hole, usually by means of a rotating tool
(drill bit) with two cutting edges
 Drilling is used to create a round hole. It is accomplished by a
rotating tool. The tool is fed in a direction parallel to its axis of
rotation into the workpart to form the round hole

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 Rotating multiple-cutting-edge tool is moved across work to cut
a plane or straight surface
 Two forms: peripheral milling and face milling

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 Single-Point Tools  Multiple Cutting Edge Tools
 One dominant cutting edge  More than one cutting edge
 Point is usually rounded to  Motion relative to work
form a nose radius achieved by rotating
 Turning uses single point  Drilling and milling use
tools rotating multiple cutting
edge tools

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 Three dimensions of a
machining process:
 Cutting speed v – primary motion
 Feed f – secondary motion
 Depth of cut d – penetration of tool
below original work surface

 For certain operations, material removal rate can be computed

as
MRR = v f d
where v = cutting speed; f = feed; d = depth of cut

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 In production, several roughing cuts are usually taken on the part, followed
by one or two finishing cuts
 Roughing - removes large amounts of material from starting workpart
 Creates shape close to desired geometry, but leaves some material for finish
cutting
 High feeds and depths, low speeds

 Finishing - completes part geometry

 Final dimensions, tolerances, and finish
 Low feeds and depths, high cutting speeds

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 Roughing Finishing
Objective of rough pass is to remove Objective of finish pass is to improve
bulk amount of excess material from surface finish, dimensional accuracy
workpiece in every pass. and tolerance.
Higher feed rate and depth of cut are Very low feed rate and depth of cut
utilized. are utilized.
Material removal rate (MRR) is high. MRR is comparatively low.
Surface roughness after rough pass is Surface roughness after finish pass is
more; thus surface finish is poor. low; and thus surface finish is good.
It cannot provide high dimensional It can provide high dimensional
accuracy and close tolerance. accuracy and close tolerance.
An old cutter can be utilized for Sharp cutter is highly desired to
roughing pass. achieve good finish.
It is performed prior to finish pass. It can be performed only after rough
pass. 17
 A power-driven machine that performs a machining operation,
including grinding
 Functions in machining:
 Holds workpart
 Positions tool relative to work
 Provides power at speed, feed, and depth that have been set

 The term is also applied to machines that perform metal

forming operations

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 orthogonal cutting uses a wedge-shaped tool in which the
cutting edge is perpendicular to the direction of cutting speed.
 As the tool is forced into the material, the chip is formed by
shear deformation along a plane called the shear plane, which
is oriented at an angle f with the surface of the work

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 The tool in orthogonal cutting has only two elements of
geometry: (1) rake angle and (2) clearance angle.
 As indicated previously, the rake angle a determines the
direction that the chip flows as it is formed from the workpart;
and the clearance angle provides a small clearance between
the tool flank and the newly generated work surface.

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 to
r 
tc
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

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 Based on the geometric parameters of the orthogonal model,
the shear plane angle  can be determined as:
r cos 
tan  
1  r sin 
where r = chip ratio, and  = rake angle

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 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 machining can be computed from the following
equation, based on the preceding parallel plate model:

where  = shear strain,  = shear plane angle, and  = rake angle

of cutting tool
 In a turning operation, spindle speed is set to provide a cutting
speed of 1.8 m/s. The feed and depth of cut of cut are 0.30 mm
and 2.6 mm, respectively. The tool rake angle is 8. After the cut,
the deformed chip thickness is measured to be 0.49 mm.
Determine (a) shear plane angle, (b) shear strain, and (c)
material removal rate. Use the orthogonal cutting model as an
approximation of the turning process.
to r cos 
r  tan  
tc 1  r sin 

RMR = v f d

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 In a turning operation, spindle speed is set to provide a cutting
speed of 1.8 m/s. The feed and depth of cut of cut are 0.30 mm
and 2.6 mm, respectively. The tool rake angle is 8. After the cut,
the deformed chip thickness is measured to be 0.49 mm.
Determine (a) shear plane angle, (b) shear strain, and (c)
material removal rate. Use the orthogonal cutting model as an
approximation of the turning process.
(a) r = to/tc = 0.30/0.49 = 0.612
 = tan-1(0.612 cos 8/(1 – 0.612 sin 8)) = tan-1(0.6628) = 33.6
(b)  = cot 33.6 + tan (33.6 - 8) = 1.509 + 0.478 = 1.987
(c) RMR = (1.8 m/s x 103 mm/m)(0.3)(2.6) = 1404 mm3/s

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 Discontinuous chip
 Continuous chip
 Continuous chip with Built-up Edge (BUE)
 Serrated chip

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 Consist of segments that may be firmly
or loosely attached to each other
 These chips occur when machining
hard brittle materials such as cast iron.
 Brittle failure takes place along the
shear plane before any tangible plastic
flow occurs
 form in brittle materials at low rake
angles (large depths of cut).
 Hard inclusions act as sites of cracks

 Low cutting speeds

 Large feed and depth of cut

 High tool-chip friction

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 Ductile work materials

 Small feeds and depths

 Sharp cutting edge

 Low tool-chip friction

 Continuous chips are usually formed at

high rake angles and/or high cutting
speeds.
 A good surface finish is generally
produced. not always desirable,
particularly in automated machine tools,
tend to get tangled around the tool
operation has to be stopped to clear away
the chips.

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 consists of layers of material from the workpiece
that are gradually deposited on the tool.
 then becomes unstable and eventually breaks up

 material is carried away on the tool side of the

chip
 the rest is deposited randomly on the workpiece
surface.
 BUE results in poor surface finish

 reduced by increasing the rake angle and

therefore decreasing the depth of cut.
 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
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 Segmented chips or non-homogeneous chips

 Semi continuous chips with zones low and

high shear strain
 Low thermal conductivity and strength metals
exhibit this behavior
 Cyclical chip forms with alternating high
shear strain then low shear strain
 Associated with difficult-to-machine metals at
high cutting speeds

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 Friction force F and Normal force to friction N
 Shear force Fs and Normal force to shear Fn

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 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

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 Coefficient of friction between tool and chip:

F

N

Friction angle related to coefficient

of friction as follows:

  tan 

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Shear stress acting along the shear plane:
Fs
S
As
where As = area of the shear plane
t ow
As 
sin 
Shear stress = shear strength of work material during cutting

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 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

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Equations can be derived to relate the forces that cannot be
measured to the forces that can be measured:
F = Fc sin + Ft cos
N = Fc cos - Ft sin
Fs = Fc cos - Ft sin
Fn = Fc sin + Ft cos
Based on these calculated force, shear stress and coefficient of
friction can be determined

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 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
 
  45  
2 2
Derived by Eugene Merchant
 Based on orthogonal cutting, but
validity extends to 3-D machining
 To increase shear plane angle
 Increase the rake angle
 Reduce the friction angle (or
coefficient of friction)

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 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
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 A machining operation requires power
 The power to perform machining can be computed from:
Pc = Fc v
where Pc = cutting power; Fc = cutting force; and v = cutting
speed
In U.S. customary units, power is traditional expressed as
horsepower (dividing ft-lb/min by 33,000)
Fcv
HPc 
33,000
where HPc = cutting horsepower, hp
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 Gross power to operate the machine tool Pg or HPg is given by

Pc HPc
Pg  or
HPg 
E E
where E = mechanical efficiency of machine tool
Typical E for machine tools  90%
 Useful to convert power into power per unit volume rate of
metal cut
 Called unit power, Pu or unit horsepower, HPu

Pc HPc
PU = HPu =
RMR or RMR
where RMR = material removal rate
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 Unit power is also known as the specific energy U

Pc Fc v
U = Pu = =
RMR vt ow
 Units for specific energy are typically N-m/mm3 or
J/mm3 (in-lb/in3)

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 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

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 High cutting temperatures
 Reduce tool life
 Produce hot chips that pose safety hazards to the machine
operator
 Can cause inaccuracies in part dimensions due to thermal
expansion of work material

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 Analytical method derived by Nathan Cook from dimensional
analysis using experimental data for various work materials
0.333
0.4U  vt o 
T   
C  K 
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

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 Pc Fc v
U = Pu = =
RMR vt ow

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 0.333
0.4U  vt o 
T   
C  K 

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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
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 Turning and Related Operations
 Drilling and Related Operations
 Milling
 Machining Centers and Turning Centers
 Other Machining Operations
 High Speed Machining

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 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

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 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

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 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

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Close-up view of a turning
operation on steel using a
titanium nitride coated
carbide cutting insert (photo
courtesy of Kennametal Inc.)
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 Facing Contour Turning
 Tool is fed radially inward  Instead of feeding tool
parallel to axis of rotation,
tool follows a contour that is
other than straight, thus
creating a contoured shape

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 Chamfering Cutoff
 Cutting edge cuts an angle  Tool is fed radially into
on the corner of the rotating work at some
cylinder, forming a location to cut off end of
"chamfer" part

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

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 1. Bed: It is the main body of the machine. All main components are bolted on it.
It is usually made by cast iron due to its high compressive strength and high
lubrication quality. It is made by casting process and bolted on floor space.
 2. Tool post: It is bolted on the carriage. It is used to hold the tool at correct
position. Tool holder mounted on it.
 3. Chuck: Chuck is used to hold the workspace. It is bolted on the spindle which
rotates the chuck and work piece. It is four jaw and three jaw according to the
requirement of machine.
 4. Head stock: Head stock is the main body parts which are placed at left side of
bed. It is serve as holding device for the gear chain, spindle, driving pulley etc.
It is also made by cast iron. 66
 5. Tail stock: Tail stock situated on bed. It is placed at right hand side of the bed.
The main function of tail stock to support the job when required. It is also used
to perform drilling operation.
 6. Lead screw: Lead screw is situated at the bottom side of bed which is used to
move the carriage automatically during thread cutting.
 7. Carriage: It is situated between the head stock and tail stock. It is used to
hold and move the tool post on the bed vertically and horizontally. It slides on
the guide ways. Carriage is made by cast iron.
 8. Guide ways: Guide ways take care of movement of tail stock and carriage on
bed.
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 9. Spindle: It is the main part of lathe which holds and rotates the chuck.
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 Holding the work between centers
 Chuck
 Collet
 Face plate

69
 The chuck, is available in several designs, with three or four
jaws to grasp the cylindrical workpart on its outside diameter.
 The jaws are often designed so they can also grasp the inside
diameter of a tubular part.

70
 Holding the work between centers refers to the use of two
centers, one in the headstock and the other in the tailstock
 This method is appropriate for parts with large length-to-
diameter ratios. At the headstock center, a device called a dog
is attached to the outside of the work and is used to drive the
rotation from the spindle.
 The tailstock center has a cone-shaped point which is inserted
into a tapered hole in the end of the work. The tailstock center
is either a ‘‘live’’ center or a ‘‘dead’’ center.

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 A live center rotates in a bearing in the tailstock, so that there
is no relative rotation between the work and the live center,
hence, no friction between the center and the workpiece.
 In contrast, a dead center is fixed to the tailstock, so that it does
not rotate; instead, the workpiece rotates about it. Because of
friction and the heat build up that results, this setup is normally
used at lower rotational speeds. The live center can be used at
higher speeds.

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 A collet consists of a tubular bushing with longitudinal slits
running over half its length and equally spaced around its
circumference.
 The inside diameter of the collet is used to hold cylindrical work
such as bar stock.
 Owing to the slits, one end of the collet can be squeezed to
reduce its diameter and provide a secure grasping pressure
against the work.
 Because there is a limit to the reduction obtainable in a collet of
any given diameter, these workholding devices must be made in
various sizes to match the particular workpart size in the
operation.

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 is a workholding device that
fastens to the lathe spindle
and is used to grasp parts
with irregular shapes.
 Because of their irregular
shape, these parts cannot be
held by other workholding
methods.

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 Difference between boring and
turning:
 Boring is performed on the inside
diameter of an existing hole
 Turning is performed on the outside
diameter of an existing cylinder
 In effect, boring is internal turning
operation
 Boring machines
 Horizontal or vertical - refers to the
orientation of the axis of rotation of
machine spindle

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 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

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 Through-holes - drill exits opposite side of work
 Blind-holes – does not exit work opposite side

through-hole blind hole.

77
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through-hole blind hole. 79
 80

through-hole blind hole.

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

 The tool is called a reamer, and it

usually has straight flutes.

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

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 Counterboring
 Provides a stepped hole, in which a
larger diameter follows smaller
diameter partially into the hole
 A counter bored hole is used to seat
bolt heads into a hole so the heads
do not protrude above the surface

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 (d) Countersinking. This is similar to counterboring, except that
the step in the hole is cone-shaped for flat head screws and
bolts.
 (e) Centering. Also called center drilling, this operation drills a
starting hole to accurately establish its location for subsequent
drilling. The tool is called a center drill.
 (f) Spot facing. Spot facing is similar to milling. It is used to
provide a flat machined surface on the workpart in a localized
area.

84
 Upright drill press stands on
the floor

 Bench drill similar but

smaller and mounted on a
table or bench

85
 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

86
  Peripheral milling
 Cutter axis parallel to surface being
machined
 Cutting edges on outside periphery of
cutter
peripheral milling

 Face milling
 Cutter axis perpendicular to surface being
milled
 Cutting edges on both the end and outside
periphery of the cutter

87
face milling.
88
 Slab milling: Basic form of peripheral milling
in which the cutter width extends beyond the
workpiece on both sides

 Slotting: Width of cutter is less than workpiece

width, creating a slot in the work

 Side milling: in which the cutter machines the

side of the workpiece;

89
 Conventional face milling: Cutter overhangs work on
both sides

 End milling: Cutter diameter is less than work width, so a

slot is cut into part

 Profile milling: Form of end milling in which the outside

periphery of a flat part is cut

90
 Pocket milling: Another form of end milling used to mill
shallow pockets into flat parts

 surface contouring: Ball-nose cutter fed back and forth

across work along a curvilinear path at close intervals to
create a three dimensional surface form

91
 Climb milling (down milling)

Conventional Milling (Up milling) •Cutting starts with the thickest location of the chip

•Max. Chip thickness at the end of the cut •Downward force component holds the workpiece in place
(good for slender parts)
•Contamination, scale does not effect the tool life
•Results high impact loading
•Dominant method
•-Not suitable for workpieces that have surface scale such
•Smoother cutting process as hot-worked metals, forgings, and castings, causes
•-tendency to chatter excessive tool wear

•-tendency to be pulled upward, need proper clamping •Good for finishing cuts on aluminum parts 92
(a) Schematic illustration of a column-and-knee-type milling machine.
(b) Schematic illustration of a vertical-spindle column-and-knee-type
milling machine.
93
 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

94
 Highly automated machine tool that can perform turning,
milling, and drilling operations
 General configuration of a turning center
 Can position a cylindrical workpart at a specified angle so a
rotating cutting tool (e.g., milling cutter) can machine features
into outside surface of part
 Conventional turning center cannot stop workpart at a defined
angular position and does not include rotating tool spindles

95
 Similar operations
 Both use a single point cutting tool moved linearly relative to
the workpart
 A straight, flat surface is created in both operations
 Interrupted cutting
 Subjects tool to impact loading when entering work

 Low cutting speeds due to start-and-stop motion

 Typical tooling: single point high speed steel tools

96
Figure 22.30 Components of a shaper.

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Figure 22.31 Open side planer.

98
 Moves a multiple tooth cutting tool
linearly relative to work in direction of
tool axis
 The broach's rows of teeth
progressively increase in size.
 Each tooth removes the excess
material gradually and the desired
shape is complete only after the final
broach tooth has passed through the
material.
 Advantages:
 Good surface finish
 Close tolerances
 Variety of work shapes possible
 Cutting tool called a broach
 Owing to complicated and often
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custom-shaped geometry, tooling is
expensive
 Performed on internal surface of a hole
 A starting hole must be present in the part to insert broach at
beginning of stroke

100
 Cuts narrow slit in work by a tool consisting of a series of
narrowly spaced teeth
 Tool called a saw blade
 Typical functions:
 Separate a workpart into two pieces
 Cut off unwanted portions of part

101
Figure 22.35 (a) power hacksaw –linear reciprocating motion
of hacksaw blade against work.

102
Figure 22.35 (b) bandsaw
(vertical) – linear continuous
motion of bandsaw blade,
which is in the form of an
endless flexible loop with
teeth on one edge.

103
Figure 22.35 (c) circular saw – rotating saw blade provides
continuous motion of tool past workpart.

104
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 Cutting at speeds significantly higher than those used in
conventional machining operations
 Persistent trend throughout history of machining is higher and
higher cutting speeds
 At present there is a renewed interest in HSM due to potential
for faster production rates, shorter lead times, and reduced
costs

106
 Conventional vs. high speed machining

Indexable tools (face mills)

Work material Conventional speed High speed

m/min ft/min m/min ft/min

Aluminum 600+ 2000+ 3600+ 12,000+
Cast iron, soft 360 1200 1200 4000
Cast iron, ductile 250 800 900 3000
Steel, alloy 210 700 360 1200
Source: Kennametal Inc.

107
 hp/rpm ratio = ratio of horsepower to maximum spindle speed
 Conventional machine tools usually have a higher hp/rpm ratio
than those equipped for HSM
 Dividing line between conventional machining and HSM is
around 0.005 hp/rpm
 Thus, HSM includes 15 hp spindles that can rotate at 30,000 rpm
(0.0005 hp/rpm)

108
 Emphasis on:
 Higher production rates
 Shorter lead times
 Rather than functions of spindle speed

 Important non-cutting factors:

 Rapid traverse speeds
 Automatic tool changes

109
 Special bearings designed for high rpm
 High feed rate capability (e.g., 50 m/min)
 CNC motion controls with “look-ahead” features to avoid
“undershooting” or “overshooting” tool path
 Balanced cutting tools, toolholders, and spindles to minimize
vibration
 Coolant delivery systems that provide higher pressures than
conventional machining
 Chip control and removal systems to cope with much larger
metal removal rates

110
 Aircraft industry, machining of large airframe components from
large aluminum blocks
 Much metal removal, mostly by milling

 Multiple machining operations on aluminum to produce

automotive, computer, and medical components
 Quick tool changes and tool path control important

 Die and mold industry

 Fabricating complex geometries from hard materials

111
 Good machinability indicates:
 Good surface finish and Integrity
 Long tool life
 Low cutting force and power (Low strength, high brittleness, limited
strain-hardening)
 Easy collection of chips ( does not interfere with cutting operation )
 Tool Life
 Tool Materials
 Tool Geometry
 Cutting Fluids

113
 Two principal aspects:
 Tool material
 Tool geometry

114
 Fracture failure
 Cutting force becomes excessive and/or dynamic, leading to brittle
fracture
 Temperature failure
 Cutting temperature is too high for the tool material

 Gradual wear
 Gradual wearing of the cutting tool

115
 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)

116
117
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).

118
119
120
121
 Relationship is credited to F. W. Taylor

vT n  C

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

122
 vT n  C

Tool material n C (m/min) C (ft/min)

High speed steel:

Non-steel work 0.125 120 350
Steel work 0.125 70 200
Cemented carbide
Non-steel work 0.25 900 2700
Steel work 0.25 500 1500
Ceramic
Steel work 0.6 3000 10,000
123
 Complete failure of cutting edge
 Visual inspection of flank wear (or crater wear) by the machine
operator
 Fingernail test across cutting edge
 Changes in sound emitted from operation
 Chips become ribbon-like, stringy, and difficult to dispose of
 Degradation of surface finish
 Increased power
 Workpiece count
 Cumulative cutting time

124
 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

125
 Typical hot hardness
relationships for selected tool
materials.
 Plain carbon steel shows a rapid
loss of hardness as temperature
increases.
 High speed steel is substantially
better, while cemented carbides
and ceramics are significantly
harder at elevated temperatures.

126
 Highly alloyed tool steel capable of maintaining hardness at
elevated temperatures better than high carbon and low alloy
steels
 One of the most important cutting tool materials
 Especially suited to applications involving complicated tool
geometries, such as drills, taps, milling cutters, and broaches
 Two basic types (AISI)
 Tungsten-type, designated T- grades
 Molybdenum-type, designated M-grades

 Typical alloying ingredients (Tungsten and/or Molybdenum,

Chromium and Vanadium, Carbon, of course, Cobalt in some
grades)
 Typical composition (Grade T1):
 18% W, 4% Cr, 1% V, and 0.9% C
127
 Class of hard tool material based on tungsten carbide (WC)
using powder metallurgy techniques with cobalt (Co) as the
binder
 Two basic types:
 Non-steel cutting grades - only WC-Co
 Steel cutting grades - TiC and TaC added to WC-Co

128
 High compressive strength but low-to-moderate tensile
strength
 High hardness (90 to 95 HRA)
 Good hot hardness
 Good wear resistance
 High thermal conductivity
 High elastic modulus - 600 x 103 MPa (90 x 106 lb/in2)
 Toughness lower than high speed steel

129
 Used for nonferrous metals and gray cast iron
 Properties determined by grain size and cobalt content
 As grain size increases, hardness and hot hardness decrease, but
toughness increases
 As cobalt content increases, toughness improves at the expense of
hardness and wear resistance

130
 Used for low carbon, stainless, and other alloy steels
 TiC and/or TaC are substituted for some of the WC
 Composition increases crater wear resistance for steel cutting
 But adversely affects flank wear resistance for non-steel cutting
applications

131
 Combinations of TiC, TiN, and titanium carbonitride (TiCN),
with nickel and/or molybdenum as binders.
 Some chemistries are more complex
 Applications: high speed finishing and semifinishing of steels,
stainless steels, and cast irons
 Higher speeds and lower feeds than steel-cutting carbide grades
 Better finish achieved, often eliminating need for grinding

132
 Cemented carbide insert coated with one or more thin layers of
wear resistant materials, such as TiC, TiN, and/orAl2O3
 Coating applied by chemical vapor deposition or physical
vapor deposition
 Coating thickness = 2.5 - 13 m (0.0001 to 0.0005 in)
 Applications: cast irons and steels in turning and milling
operations
 Best applied at high speeds where dynamic force and thermal
shock are minimal

133
 TiC High resistance to flank wear
 TiN Low coefficient of friction
Good adhesion to substrate
 Al2O3 Resistance to high temperature

High chemical inertness

Low thermal conductivity
 TiCN Harder and tougher than TiN

134
Photomicrograph
of cross section of
multiple coatings
on cemented
carbide tool (photo
courtesy of
Kennametal Inc.)

135
 Primarily fine-grained Al2O3, pressed and sintered at high
pressures and temperatures into insert form with no binder
 Applications: high speed turning of cast iron and steel
 Not recommended for heavy interrupted cuts (e.g. rough
milling) due to low toughness
 Al2O3 also widely used as an abrasive in grinding

136
 Sintered polycrystalline diamond (SPD) - fabricated by
sintering very fine-grained diamond crystals under high
temperatures and pressures into desired shape with little or no
binder
 Usually applied as coating (0.5 mm thick) on WC-Co insert
 Applications: high speed machining of nonferrous metals and
abrasive nonmetals such as fiberglass, graphite, and wood
 Not for steel cutting

137
 Next to diamond, cubic boron nitride (CBN) is hardest material
known
 Fabrication into cutting tool inserts same as SPD: coatings on
WC-Co inserts
 Applications: machining steel and nickel-based alloys
 SPD and cBN tools are expensive

138
 Two categories:
 Single point tools
 Used for turning, boring, shaping, and planing

 Multiple cutting edge tools

 Used for drilling, reaming, tapping, milling, broaching, and sawing

139
Figure 23.7 (a) Seven
elements of single-point
tool geometry; and (b) the
tool signature convention
that defines the seven
elements.

140
 Figure 23.9 Three ways of holding and presenting the cutting
edge for a single-point tool: (a) solid tool, typical of HSS; (b)
brazed insert, one way of holding a cemented carbide insert;
and (c) mechanically clamped insert, used for cemented
carbides, ceramics, and other very hard tool materials.

141
 Figure 23.10 Common insert shapes: (a) round, (b) square, (c)
rhombus with two 80 point angles, (d) hexagon with three 80
point angles, (e) triangle (equilateral), (f) rhombus with two 55
point angles, (g) rhombus with two 35 point angles. Also
shown are typical features of the geometry.

142
 By far the most common cutting tools for hole-making
 Usually made of high speed steel

143
 Rotation and feeding of drill bit result in relative motion
between cutting edges and workpiece to form the chips
 Cutting speed varies along cutting edges as a function of distance
from axis of rotation
 Relative velocity at drill point is zero, so no cutting takes place
 A large thrust force is required to drive the drill forward into hole

144
 Chip removal
 Flutes must provide sufficient clearance to allow chips to be
extracted from bottom of hole during the cutting operation
 Friction makes matters worse
 Rubbing between outside diameter of drill bit and newly formed
hole
 Delivery of cutting fluid to drill point to reduce friction and heat is
difficult because chips are flowing in opposite direction

145
 Principal types:
Plain milling cutter Face milling cutter

End milling cutter

146
 Used for peripheral or slab milling

Figure 23.13 Tool

geometry elements of
an 18-tooth plain milling
cutter

147
 Teeth cut on side and periphery of the cutter

Figure 23.14 Tool geometry elements of a four-tooth face

milling cutter: (a) side view and (b) bottom view.
148
 Looks like a drill bit but designed for primary cutting with its
peripheral teeth
 Applications:
 Face milling
 Profile milling and pocketing
 Cutting slots
 Engraving
 Surface contouring
 Die sinking

149
 Any liquid or gas applied directly to machining operation to
improve cutting performance
 Two main problems addressed by cutting fluids:
 Heat generation at shear and friction zones
 Friction at tool-chip and tool-work interfaces

 Other functions and benefits:

 Wash away chips (e.g., grinding and milling)
 Reduce temperature of workpart for easier handling
 Improve dimensional stability of workpart

150
 Cutting fluids can be classified according to function:
 Coolants - designed to reduce effects of heat in machining
 Lubricants - designed to reduce tool-chip and tool-work
friction

151
 Water used as base in coolant-type cutting fluids
 Most effective at high cutting speeds where heat generation
and high temperatures are problems
 Most effective on tool materials that are most susceptible to
temperature failures (e.g., HSS)

152
 Usually oil-based fluids
 Most effective at lower cutting speeds
 Also reduce temperature in the operation

153
 Tramp oil (machine oil, hydraulic fluid, etc.)
 Garbage (cigarette butts, food, etc.)
 Small chips
 Molds, fungi, and bacteria

154
 Replace cutting fluid at regular and frequent intervals
 Use filtration system to continuously or periodically clean the
fluid
 Dry machining

155
 Advantages:
 Prolong cutting fluid life between changes
 Reduce fluid disposal cost
 Cleaner fluids reduce health hazards
 Lower machine tool maintenance
 Longer tool life

156
 No cutting fluid is used
 Avoids problems of cutting fluid contamination, disposal, and
filtration
 Problems with dry machining:
 Overheating of tool
 Operating at lower cutting speeds and production rates to prolong
tool life
 Absence of chip removal benefits of cutting fluids in grinding and
milling

157