Chapter 8

1
Chapter 8 Metal Cutting and Machining
2
Common Machining Processes
FIGURE 8.1 Some examples of common machining
processes.
3
Orthogonal Cutting
Cutting ratio
FIGURE 8.2 Schematic illustration of a
two-dimensional cutting process, or orthogonal
cutting. (a) Orthogonal cutting with a
well-defined shear plane, also known as the
Merchant model (b) Orthogonal cutting without a
well-defined shear plane.
4
Chip Formation
Shear strain
Material removal rate
FIGURE 8.3 (a) Schematic illustration of the
basic mechanism of chip formation in cutting. (b)
Velocity diagram in the cutting zone.
5
Types of Chips
Cont. chip w. straight primary shear zone
Secondary shear zone at tool
Chip with build up
Discontinuous chip
Segmented chip
FIGURE 8.5 Shiny (burnished) surface on the tool
side of a continuous chip produced in turning.
6
Hardness in Cutting Zone
BUE Built Up Edge
FIGURE 8.6 (a) Hardness distribution in the
cutting zone for 3115 steel. Note that some
regions in the built-up edge are as much as three
times harder than the bulk workpiece. (b) Surface
finish in turning 5130 steel with a built-up
edge. (c) Surface finish on 1018 steel in face
milling. Source Courtesy of Metcut Research
Associates, Inc.
7
Chip Breakers
FIGURE 8.7 (a) Schematic illustration of the
action of a chip breaker. Note that the chip
breaker decreases the radius of curvature of the
chip. (b) Chip breaker clamped on the rake face
of a cutting tool. (c) Grooves on the rake face
of cutting tools, acting as chip breakers. Most
cutting tools now are inserts with built-in
chip-breaker features.
FIGURE 8.8 Various chips produced in turning
(a) tightly curled chip (b) chip hits workpiece
and breaks (c) continuous chip moving radially
outward from workpiece and (d) chip hits tool
shank and breaks off. Source After G. Boothroyd.
8
Oblique Cutting
FIGURE 8.9 (a) Schematic illustration of cutting
with an oblique tool. (b) Top view, showing the
inclination angle, i. (c) Types of chips produced
with different inclination angles.
Effective rake angle
9
Cutting Mechanics
FIGURE 8.56 The effect of lead angle on the
undeformed chip thickness in face milling. Note
that as the lead angle increases, the undeformed
chip thickness (and hence the thickness of the
chip) decreases, but the length of contact (and
hence the width of the chip) increases. Note that
the insert must be sufficiently large to
accommodate the increase in contact length.
FIGURE 8.57 (a) Relative position of the cutter
and the insert as it first engages the workpiece
in face milling, (b) insert positions at entry
and exit near the end of cut, and (c) examples of
exit angles of the insert, showing desirable
(positive or negative angle) and undesirable
(zero angle) positions. In all figures, the
cutter spindle is perpendicular to the page.
10
Right-Hand Cutting Tool
FIGURE 8.10 (a) Schematic illustration of a
right-hand cutting tool for turning. Although
these tools have traditionally been produced from
solid tool-steel bars, they are now replaced by
inserts of carbide or other tool materials of
various shapes and sizes, as shown in (b).
11
Cutting Forces
FIGURE 8.11 (a) Forces acting on a cutting tool
in two-dimensional cutting. Note that the
resultant forces, R, must be collinear to balance
the forces. (b) Force circle to determine various
forces acting in the cutting zone. Source After
M.E. Merchant.
Cutting force
Friction coefficient
12
FRsinß NRcosß R resultant force on
chip Fnormal force on chip ßangle between
resultant an normal force. F Friction force on
chip
Thrust force
Fc is positive Ft is pos (down) Ft is neg (up)
13
Cutting Data
FIGURE 8.12 Thrust force as a function of rake
angle and feed in orthogonal cutting of AISI 1112
cold-rolled steel. Note that at high rake angles,
the thrust force is negative. A negative thrust
force has important implications in the design of
machine tools and in controlling the stability of
the cutting process. Source After S. Kobayashi
and E.G. Thomsen.
Ft
14
Shear Force Normal Force
Shear area
FIGURE 8.13 (a) Shear force and (b) normal force
as a function of the area of the shear plane and
the rake angle for 85-15 brass. Note that the
shear stress in the shear plane is constant,
regardless of the magnitude of the normal stress,
indicating that the normal stress has no effect
on the shear flow stress of the material. Source
After S. Kobayashi and E.G. Thomsen.
15
Shear Stress on Tool Face
As shear plane area Fsshear force Fnnormal
force Fshear angle
FIGURE 8.14 Schematic illustration of the
distribution of normal and shear stresses at the
tool-chip interface (rake face). Note that,
whereas the normal stress increases continuously
toward the tip of the tool, the shear stress
reaches a maximum and remains at that value (a
phenomenon known as sticking see Section 4.4.1).
16
Shear-Angle Relationships
FIGURE 8.15 (a) Comparison of experimental and
theoretical shear-angle relationships. More
recent analytical studies have resulted in better
agreement with experimental data. (b) Relation
between the shear angle and the friction angle
for various alloys and cutting speeds. Source
After S. Kobayashi.
Merchant Eq. (8.20)
Mizuno Eqs. (8.22)-(8.23
Shaffer Eq. (8.21)
17
Power

• PowerFcV
• Total Energy per unit volume (specific energy)
• Specific energy for Friction
• Specific energy for shearing

18

• Total specific energy
• Other sources of energy
• surface energy (creating 2 new surfaces)
• Momentum change

19
Specific Energy
TABLE 8.3 Approximate Specific-Energy
Requirements in Machining Operations
20
Temperature

• Mean temperature in cutting zone

Yfflow stress of material ?cvolumetric specific
heat (in-lb/in3-F) K thermal diffusivity For a
lathe
Tool a b
Carbide 0.2 0.125
HS Steel 0.5 0.375
21
Temperatures in Cutting
FIGURE 8.1 Typical temperature distribution in
the cutting zone. Note the severe temperature
gradients within the tool and the chip, and that
the workpiece is relatively cool. Source After
G. Vieregge.
FIGURE 8.2 Temperature distribution in turning
as a function of cutting speed (a) flank
temperature (b) temperature along the tool-chip
interface. Note that the rake-face temperature is
higher than that at the flank surface. Source
After B.T. Chao and K.J. Trigger.
FIGURE 8.18 Proportion of the heat generated in
cutting transferred to the tool, workpiece, and
chip as a function of the cutting speed. Note
that most of the cutting energy is carried away
by the chip (in the form of heat), particularly
as speed increases.
22
Terminology in Turning
FIGURE 8.19 Terminology used in a turning
operation on a lathe, where f is the feed (in
mm/rev or in./rev) and d is the depth of cut.
Note that feed in turning is equivalent to the
depth of cut in orthogonal cutting (see Fig.
8.2), and the depth of cut in turning is
equivalent to the width of cut in orthogonal
cutting. See also Fig. 8.42.
23
Tool Wear
Taylor tool life equation
FIGURE 8.20 Examples of wear in cutting tools.
(a) Flank wear (b) crater wear (c) chipped
cutting edge (d) thermal cracking on rake face
(e) flank wear and built-up edge (f)
catastrophic failure (fracture). Source Courtesy
of Kennametal, Inc.
TABLE 8.4 Range of n values for various cutting
tools.
24
Effect of Workpiece on Tool Life
FIGURE 8.21 Effect of workpiece microstructure
on tool life in turning. Tool life is given in
terms of the time (in minutes) required to reach
a flank wear land of a specified dimension. (a)
Ductile cast iron (b) steels, with identical
hardness. Note in both figures the rapid decrease
in tool life as the cutting speed increases.
25
Tool-Life Curves
FIGURE 8.22 (a) Tool-life curves for a variety
of cutting-tool materials. The negative inverse
of the slope of these curves is the exponent n in
tool-life equations. (b) Relationship between
measured temperature during cutting and tool life
(flank wear). Note that high cutting temperatures
severely reduce tool life. See also Eq. (8.30).
Source After H. Takeyama and Y. Murata.
26
Tool Wear
FIGURE 8.23 Relationship between crater-wear
rate and average tool-chip interface temperature
in turning (a) high-speed-steel tool (b) C1
carbide (c) C5 carbide. Note that crater wear
increases rapidly within a narrow range of
temperature. Source After K.J. Trigger and B.T.
Chao.
TABLE 8.5 Allowable average wear lands for
cutting tools in various operations.
FIGURE 8.23 Interface of chip (left) and rake
face of cutting tool (right) and crater wear in
cutting AISI 1004 steel at 3 m/s (585 ft/min).
Discoloration of the tool indicates the presence
of high temperature (loss of temper). Note how
the crater-wear pattern coincides with the
discoloration pattern. Compare this pattern with
the temperature distribution shown in Fig. 8.16.
Source Courtesy of P.K. Wright.
27
Drills
FIGURE 8.48 Two common types of drills (a)
Chisel-point drill. The function of the pair of
margins is to provide a bearing surface for the
drill against walls of the hole as it penetrates
into the workpiece. Drills with four margins
(double-margin) are available for improved drill
guidance and accuracy. Drills with chip-breaker
features are also available. (b) Crankshaft
drills. These drills have good centering ability,
and because chips tend to break up easily, they
are suitable for producing deep holes.
FIGURE 8.49 Various types of drills and drilling
operations.
28
Material Removal Rate (vol/time)

• Drilling

29
Face Milling
FIGURE 8.54 Face-milling operation showing (a)
action of an insert in face milling (b) climb
milling (c) conventional milling (d) dimensions
in face milling.
FIGURE 8.55 Terminology for a face-milling
cutter.
30
Material removal rate slab milling
Vspeed of cutter edge (in/min) D cutter
dia Nrotational speed (rpm) tcchip thickness
(undeformed) f feed per tooth (dist/tooth) ddept
h of cut vlinear speed (feed rate) n of
teeth llength of work piece lcdist from 1st
contact to mid
31
Economics
Cpcost per piece Cmmachining cost Cscost to
set up Cl cost to load Cttooling cost
Tmmachine time per piece Lmlabor cost Bmburden
rate (overhead)
32
Tltime to lad and unload, change speeds, feed
rates etc. Lmlabor cost Bmburden rate (overhead)
33
Ni parts per insert Nf parts per face Tctime
to change insert Titime to index
insert Didepreciation in
34
Tptime t produce 1 part
Tmtime to perform an operation. E.g. For
turning L length of cut N rpm D work piece dia
35
From taylor tool-life
Ni parts per insert Nf parts per face Tctime
to change insert Titime to index
insert Didepreciation in
m number of faces used
Np parts per insert considering tool life and
of faces on an insert used.
36
Opt tool life
Reduces to
37
Opt tool life
38
Acoustic Emission and Wear
FIGURE 8.25 Relationship between mean flank
wear, maximum crater wear, and acoustic emission
(noise generated during cutting) as a function of
machining time. This technique has been developed
as a means for continuously and indirectly
monitoring wear rate in various cutting processes
without interrupting the operation. Source After
M.S. Lan and D.A. Dornfeld.
39
Surface Finish
FIGURE 8.26 Range of surface roughnesses
obtained in various machining processes. Note the
wide range within each group, especially in
turning and boring. (See also Fig. 9.27).
40
Surfaces in Machining
FIGURE 8.27 Surfaces produced on steel in
machining, as observed with a scanning electron
microscope (a) turned surface, and (b) surface
produced by shaping. Source J.T. Black and S.
Ramalingam.
FIGURE 8.28 Schematic illustration of a dull
tool in orthogonal cutting (exaggerated). Note
that at small depths of cut, the rake angle can
effectively become negative. In such cases, the
tool may simply ride over the workpiece surface,
burnishing it, instead of cutting.
41
Inclusions in Free-Machining Steels
FIGURE 8.29 Photomicrographs showing various
types of inclusions in low-carbon, resulfurized
free-machining steels. (a) Manganese-sulfide
inclusions in AISI 1215 steel. (b)
Manganese-sulfide inclusions and glassy
manganese-silicate-type oxide (dark) in AISI 1215
steel. (c) Manganese sulfide with lead particles
as tails in AISI 12L14 steel. Source Courtesy of
Ispat Inland Inc.
42
Hardness of Cutting Tools
FIGURE 8.30 Hardness of various cutting-tool
materials as a function of temperature (hot
hardness). The wide range in each group of tool
materials results from the variety of
compositions and treatments available for that
group.
43
Tool Materials
TABLE 8.6 Typical range of properties of various
tool materials.
44
Properties of Tungsten-Carbide Tools
FIGURE 8.31 Effect of cobalt content in
tungsten-carbide tools on mechanical properties.
Note that hardness is directly related to
compressive strength (see Section 2.6.8) and
hence, inversely to wear see Eq. (4.6).
45
Inserts
FIGURE 8.32 Methods of mounting inserts on
toolholders (a) clamping, and (b) wing lockpins.
(c) Examples of inserts mounted using threadless
lockpins, which are secured with side screws.
Source Courtesy of Valenite.
46
Insert Strength
FIGURE 8.33 Relative edge strength and tendency
for chipping and breaking of inserts with various
shapes. Strength refers to that of the cutting
edge shown by the included angles. Source
Courtesy of Kennametal, Inc.
FIGURE 8.34 Edge preparations for inserts to
improve edge strength. Source Courtesy of
Kennametal, Inc.
47
Historical Tool Improvement
FIGURE 8.35 Relative time required to machine
with various cutting-tool materials, with
indication of the year the tool materials were
introduced. Note that, within one century,
machining time has been reduced by two orders of
magnitude. Source After Sandvik Coromant.
48
Coated Tools
FIGURE 8.36 Wear patterns on high-speed-steel
uncoated and titanium-nitride-coated cutting
tools. Note that flank wear is lower for the
coated tool.
FIGURE 8.37 Multiphase coatings on a
tungsten-carbide substrate. Three alternating
layers of aluminum oxide are separated by very
thin layers of titanium nitride. Inserts with as
many as 13 layers of coatings have been made.
Coating thicknesses are typically in the range of
2 to 10 µm. Source Courtesy of Kennametal, Inc.
49
Properties of Cutting Tool Materials
FIGURE 8.38 Ranges of properties for various
groups of cutting-tool materials. (See also
Tables 8.1 through 8.5.)
FIGURE 8.39 Construction of polycrystalline
cubic-boron-nitride or diamond layer on a
tungsten-carbide insert.
50
Characteristics of Machining
TABLE 8.7 General characteristics of machining
processes.
51
Lathe Operations
FIGURE 8.40 Variety of machining operations that
can be performed on a lathe.
52
Tool Angles
FIGURE 8.41 Designations and symbols for a
right-hand cutting tool. The designation right
hand means that the tool travels from right to
left, as shown in Fig. 8.19.
TABLE 8.8 General recommendations for tool
angles in turning.
53
Turning Operations
FIGURE 8.42 (a) Schematic illustration of a
turning operation, showing depth of cut, d, and
feed, f. Cutting speed is the surface speed of
the workpiece at the tool tip. (b) Forces acting
on a cutting tool in turning. Fc is the cutting
force Ft is the thrust or feed force (in the
direction of feed) and Fr is the radial force
that tends to push the tool away from the
workpiece being machined. Compare this figure
with Fig. 8.11 for a two-dimensional cutting
operation.
54
Cutting Speeds for Turning
FIGURE 8.43 The range of applicable cutting
speeds and feeds for a variety of cutting-tool
materials.
TABLE 8.9 Approximate Ranges of Recommended
Cutting Speeds for Turning Operations
55
Lathe
FIGURE 8.44 General view of a typical lathe,
showing various major components. Source
Courtesy of Heidenreich Harbeck.
56
CNC Lathe
FIGURE 8.45 (a) A computer-numerical-control
lathe, with two turrets these machines have
higher power and spindle speed than other lathes
in order to take advantage of advanced cutting
tools with enhanced properties (b) a typical
turret equipped with ten cutting tools, some of
which are powered.
57
Typical CNC Parts
FIGURE 8.46 Typical parts made on
computer-numerical-control machine tools.
58
Typical Production Rates
TABLE 8.10 Typical production rates for various
cutting operations.
59
Boring Mill
FIGURE 8.47 Schematic illustration of the
components of a vertical boring mill.
60
Drills
FIGURE 8.48 Two common types of drills (a)
Chisel-point drill. The function of the pair of
margins is to provide a bearing surface for the
drill against walls of the hole as it penetrates
into the workpiece. Drills with four margins
(double-margin) are available for improved drill
guidance and accuracy. Drills with chip-breaker
features are also available. (b) Crankshaft
drills. These drills have good centering ability,
and because chips tend to break up easily, they
are suitable for producing deep holes.
FIGURE 8.49 Various types of drills and drilling
operations.
61
Speeds and Feeds in Drilling
TABLE 8.11 General recommendations for speeds
and feeds in drilling.
62
Reamers and Taps
FIGURE 8.50 Terminology for a helical reamer.
FIGURE 8.51 (a) Terminology for a tap (b)
illustration of tapping of steel nuts in high
production.
63
Typical Machined Parts
FIGURE 8.52 Typical parts and shapes produced by
the machining processes described in Section 8.10.
64
Conventional and Climb Milling
FIGURE 8.53 (a) Illustration showing the
difference between conventional milling and climb
milling. (b) Slab-milling operation, showing
depth of cut, d feed per tooth, f chip depth of
cut, tc and workpiece speed, v. (c) Schematic
illustration of cutter travel distance, lc, to
reach full depth of cut.
65
Face Milling
FIGURE 8.54 Face-milling operation showing (a)
action of an insert in face milling (b) climb
milling (c) conventional milling (d) dimensions
in face milling.
FIGURE 8.55 Terminology for a face-milling
cutter.
66
Milling Operations
TABLE 8.12 Approximate range of recommended
cutting speeds for milling operations.
FIGURE 8.58 Cutters for (a) straddle milling
(b) form milling (c) slotting and (d) slitting
operations.
67
Milling Machines
FIGURE 8.59 (a) Schematic illustration of a
horizontal-spindle column-and-knee-type milling
machine. (b) Schematic illustration of a
vertical-spindle column-and-knee-type milling
machine. Source After G. Boothroyd.
68
Broaching
FIGURE 8.60 (a) Typical parts finished by
internal broaching. (b) Parts finished by surface
broaching. The heavy lines indicate broached
surfaces (c) a vertical broaching machine.
Source (a) and (b) Courtesy of General Broach
and Engineering Company, (c) Courtesy of Ty
Miles, Inc.
69
Broaches
FIGURE 8.61 (a) Cutting action of a broach,
showing various features. (b) Terminology for a
broach.
FIGURE 8.62 Terminology for a pull-type internal
broach, typically used for enlarging long holes.
70
Saws and Saw Teeth
FIGURE 8.63 (a) Terminology for saw teeth. (b)
Types of saw teeth, staggered to provide
clearance for the saw blade to prevent binding
during sawing.
FIGURE 8.64 (a) High-speed-steel teeth welded on
a steel blade. (b) Carbide inserts brazed to
blade teeth.
71
Gear Manufacture
FIGURE 8.65 (a) Schematic illustration of gear
generating with a pinion-shaped gear cutter. (b)
Schematic illustration of gear generating in a
gear shaper, using a pinion-shaped cutter note
that the cutter reciprocates vertically. (c) Gear
generating with a rack-shaped cutter. (d) Three
views of gear cutting with a hob. Source After
E.P. DeGarmo.
72
Machining Centers
FIGURE 8.66 A horizontal-spindle machining
center, equipped with an automatic tool changer.
Tool magazines in such machines can store as many
as 200 cutting tools, each with its own holder.
Source Courtesy of Cincinnati Machine.
FIGURE 8.67 Schematic illustration of a computer
numerical-controlled turning center. Note that
the machine has two spindle heads and three
turret heads, making the machine tool very
flexible in its capabilities. Source Courtesy of
Hitachi Seiki Co., Ltd.
73
Reconfigurable Machines
FIGURE 8.68 Schematic illustration of a
reconfigurable modular machining center, capable
of accommodating workpieces of different shapes
and sizes, and requiring different machining
operations on their various surfaces. Source
After Y. Koren.
74
Reconfigurable Machining Center
FIGURE 8.69 Schematic illustration of assembly
of different components of a reconfigurable
machining center. Source After Y. Koren.
75
Machining of Bearing Races
FIGURE 8.70 Sequences involved in machining
outer bearing races on a turning center.
76
Hexapod
FIGURE 8.71 (a) A hexapod machine tool, showing
its major components. (b) Closeup view of the
cutting tool and its head in a hexapod machining
center. Source National Institute of Standards
and Technology.
77
Chatter Vibration
FIGURE 8.72 Chatter marks (right of center of
photograph) on the surface of a turned part.
Source Courtesy of General Electric Company.
FIGURE 8.73 Relative damping capacity of (a)
gray cast iron and (b) epoxy-granite composite
material. The vertical scale is the amplitude of
vibration and the horizontal scale is time.
FIGURE 8.74 Damping of vibrations as a function
of the number of components on a lathe. Joints
dissipate energy thus, the greater the number of
joints, the higher the damping. Source After J.
Peters.
78
Machining Economics
FIGURE 8.75 Qualitative plots showing (a) cost
per piece, and (b) time per piece in machining.
Note that there is an optimum cutting speed for
both cost and time, respectively. The range
between the two optimum speeds is known as the
high-efficiency machining range.
79
Case Study Ping Golf Putters
FIGURE 8.76 (a) The Ping Anser golf putter (b)
CAD model of rough machining of the putter outer
surface (c) rough machining on a vertical
machining center (d) machining of the lettering
in a vertical machining center the operation was
paused to take the photo, as normally the cutting
zone is flooded with a coolant Source Courtesy
of Ping Golf, Inc.