Fundementals of Machining
Dr. Oğuzhan YILMAZ (Assoc.Prof.)
Room: 319
[email protected]
Introduction
• Machining is a general term describing a group of processes that
consists of removal of material and modification of the surfaces of a
workpiece after it has been produced by various methods. Thus,
machining involves secondary and finishing operations.
• In spite of their advantages, material-removal processes have the
following disadvantages;
–
–
–
–
Waste material (although the amount may be relatively small)
Process generally takes longer than other processes
Generally require more energy than do forming and shaping operations
Have adverse effects on the surface quality an properties of the product
Introduction
• Machining consists of several major types of materials removal
processes:
– Cutting: typically involving single-point or multi-point cutting
tools, each with a clearly defined shape
– Abrasive processes: grinding and related process
– Advanced machining processes: utilizing electrical, chemical,
laser, thermal, and hydrodynamic methods to accomplish this
task.
• The machines on which these operations are performed are called
machine tools.
Introduction:
• Common cutting processes:
• Turning (wp rotating, tool
moves to the left)
• Cutting off (wp rotating, tool
moves radially)
• Slab milling (tool rotating, wp
moves)
• End milling (tool rotating, wp
moves)
Figure 1 Examples of cutting processes.
Introduction
In turning process,
Cutting tool is set a certain depth of cut (mm)
and travels to the left with a certain
velocity as the workpiece rotates.
- The feed or feed rate ( f ) (the distance the
tool travels horizontally per unit
revolution of the workpiece (mm/rev))
Figure 2 Basic principle of the turning
operations.
Introduction
• A cutting tool moves to the left along the
workpiece at a constant velocity, V, and a
depth of cut, to.
• A chip is produced ahead of the tool by
plastically deforming and shearing the
material continuously along the shear plane.
• This phenomenon can be demonstrated by
slowly scraping the surface of a stick of
butter lengthwise with a sharp knife and
observing the formation of the chip.
Figure 3. Schematic illustration of a two-dimensional cutting
processes, also called orthogonal cutting: (a) Orthogonal cutting with
a well-defined shear plane, also known as ME Merchant model. Not
that the tool shape, the depth of cut, to, and the cutting speed, V, are
all independent variables. (b) Orthogonal cutting without a welldefined shear-plane.
Mechanics of Cutting
• Major independent variables in cutting:
(a) tool material and coating
(b) tool shape, surface finish and sharpness
(c) workpiece material and condition
(d) cutting speed, feed, and depth of cut
(e) cutting fluids
(f) characteristics of the machine tool
(g) work holding and fixturing
Mechanics of Cutting
• Dependent variables in cutting (influenced by changes in the independent
variables):
(a) Type of chip produced
(b) Force and energy dissipated during cutting
(c) Temperature rise in the workpiece, the tool, and the chip
(d) Tool wear and failure
(e) Surface finish and surface integrity of the workpiece
• Chips are produced by shearing, taking place in a shear zone (shear plane) at an
angle (shear angle).
• Below shear plane, the workpiece remains undeformed; above it, the chip that is
already formed moves up the rake face of the tool.
Cutting ratio:
• Chip thickness, tc, can be determined from;
– the depth of cut, to,
– the rake angle, .
• Cutting ratio (chip thickness ratio), to/tc ;
r cos 
tan 
1 r sin 
and
to
sin 
r 
tc cos()
r < 1 since chip thickness is always greater than
the depth of cut.
Figure 3. Schematic illustration of a two-dimensional cutting processes,
also called orthogonal cutting: (a) Orthogonal cutting with a welldefined shear plane, also known as ME Merchant model. Not that the
tool shape, the depth of cut, to, and the cutting speed, V, are all
independent variables. (b) Orthogonal cutting without a well-defined
shear-plane.
Cutting ratio:
• Reciprocal of ‘ r ’is known as chipcompression ratio or chip-compression factor.
• It is a measure of how thick the chip has
become compared with depth of cut.
• Cutting ratio can be used for evaluating
cutting conditions. to is a machine setting and
a known factor.
• Cutting ratio can be calculated easily by
measuring the chip thickness with a
micrometer
Figure 3. Schematic illustration of a two-dimensional cutting processes,
also called orthogonal cutting: (a) Orthogonal cutting with a welldefined shear plane, also known as ME Merchant model. Not that the
tool shape, the depth of cut, to, and the cutting speed, V, are all
independent variables. (b) Orthogonal cutting without a well-defined
shear-plane.
Shear Strain
• Shear strain, , that the material undergoes can be expressed as;
AB AO OB
or



OC OC OC
cot(tan()
Figure 4 (a) Schematic illustration
of the basic mechanism of chip
formation in metal cutting. (b)
Velocity diagram in the cutting
zone.
Shear Strain
•
•
•
•
•
Large shear strains are associated with low shear angles or
with low or negative rake angles.
Shear strains of 5 or higher have been observed in actual
cutting operations.
Deformation in cutting generally takes place in a vey narrow
zone, d=OC is very small. Thus the rate is very high.
Shear angle influences force and power requirements, chip
thickness, and temperature.
To minimize the cutting force or the shear plane is plane of
maximum shear stress;

 
45  
2
2
: friction angle (related to µ at the tool-chip interface
(µ = tan ), α is the rake-angle (a)
General useful formula for shear angle:
0.5 < µ < 2
= 45⁰+ α -
in metal cutting. µ varies considerably along the tool-chip interface. Due to P and T
Shear Strain
•
Since the chip thickness is greater than the depth of cut, the
velocity of the chip, Vc, has to be lower than the cutting
speed, V.
Vt o Vc t c or Vc Vr
Hence
V sin 
Vc 
cos()
•
A velocity diagram also can be constructed as;
V
V
V
 s  c
cos() cos  sin 
•
Vs is the velocity at which shearing takes place in the shear
plane. Cutting ratio then;
to Vc
r 
tc V
Types of chips produced in metal cutting
The four main types of chips are:
(1) Continuous
(2) Built-up edge
(3) Serrated or segmented
(4) Dis-continious
Chip has two surfaces:
(1) One is contact with the rake of the tool and has a shiny and
burnished appearance caused by rubbing as the chip moves
up the tool face.
(2) The other is the original surface of the workpiece. It has
jagged, rough appearance, caused by the shearing
mechanism.
Types of chips produced in metal cutting
Figure 5 Basic types
of chips produced in orthogonal
metal cutting, their schematic
representation, and
photomicrographs of the cutting
zone:
(a) continuous chip with narrow,
straight, and primary shear zone;
(b) continuous chip with secondary
shear zone at the chip-tool
interface;
(c) built-up edge;
(d) segmented or non-homogeneous
chip; and
(e) discontinuous chip
Types of chips produced in metal cutting
CONTINIOUS CHIPS:
• Formed with ductile materials machined at high cutting speed and/or high rake angles
• Deformations takes place along a narrow shear zone (primary shear zone) Fig 5 (a)
• May also develop a secondary shear zone due to high friction at the tool-chip interface.
Fig 5(b)
• Produce good surface finish
• Continuous chips are not desirable (esp. for CNC) since they tend to become tangled
around the tool holder, the fixturing and the workpiece. The operation must be stopped for
clear away.
• Chip breakers can be used by changing parameters (cutting speed, feed, and doc or cutting
fluid)
Types of chips produced in metal cutting
BUILT-UP EDGE CHIPS:
• Built-up Edge (BUE) consists of layers of material from the workpiece that gradually are
deposited on the tool tip.
• As it grows larger, the BUE becomes unstable and eventually breaks apart.
• The cycles of BUE formation and destruction is repeated continuously during the cutting
operation until corrective measures are taken.
• BUE commonly observed in practice and changes the geometry of the cutting edge.
• BUE affects the surface finish. Fig. 5(c) and Fig.6 (b and c)
• Stable BUE is desirable since it reduces tool wear by protecting its rake face.
• As the cutting speed increases, the size of the BUE decreases.
Types of chips produced in metal cutting
BUILT-UP EDGE CHIPS:
• The tendency for BUE formation can be reduced by one or more of the following means:
– Increase the cutting speed
– Decrease the depth of cut
– Increase the rake angle
– Use a sharp tool
– Use an effective cutting fluid
– Use a cutting tool that has lower chemical affinity for the workpiece material.
Types of chips produced in metal cutting
BUILT-UP EDGE
(a)
(b)
(c)
 Cold-worked metals generally have less of a
tendency to form BUE than when their annealed
condition. Due to work hardening and deposition of
materials, the BUE hardness increases significantly
Figure 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
metal. (b) Surface finish in turning 5130 steel with a
built-up edge. (c) surface finish on 1018 steel in face
milling. Magnifications: 15X.
Types of chips produced in metal cutting
SERRATED CHIPS:
• Serrated chips (segmented or non-homogeneous chips) are semi-continuous chips with
large zones of low shear strain and small zones of high shear strain, hence later zone is
called shear localization.
• The chips have a saw tooth-like appearance.
DIS-CONTINIOUS CHIPS:
• They consists of segments that may be attached firmly or loosely to each other.
Discontinues chips usually form under the following conditions:
–
–
–
–
–
–
–
Brittle workpiece materials (due to less capacity to undergo high shear strain)
Workpiece materials with faults or graphite flakes structured materials, gray cast iron.
Very low or very high cutting speed
Large depth of cut
Low rake angles
Lack of effective cutting fluid
Vibration or chatter due low stiffness of tool holder and machine tool.
Types of chips produced in metal cutting
CHIP CURL
• Chips develop a curvature as they leave the workpiece surface in all cutting operations of
metallic and non-metallic materials.
• Factors affecting the chip curl are as follows:
– Distribution of stresses in the primary and secondary shear zones
– Thermal effects
– Work-hardening characteristics of the workpiece material
– The geometry of the cutting tool
– Cutting fluids
 As the depth of cut decreases, the radius of curvature decreases and chip becomes
curlier.
 Cutting fluids can make chips become more curly, thus reducing the tool-chip contact
area and concentrating the heat closer to the tip of the tool. So, tool wear increases.
Types of chips produced in metal cutting
• Continuous and long chips are
undesirable. Since they tend to become
entangled, severely interfere and
hazardous.
• Usual procedure is to break the chip
intermittently with cutting tools that have
a chip-breaker features.
• Most modern cutting tools and inserts
now have built-in chip-breaker features of
various design.
• Chips also can be broken by changing the
tool geometry to control chip flow, as in
Fig.8.
• Ideal chip size to be broken is in the shape
of either the letter C or the number 9 and
fits within a 25-mm square space.
• In milling chip breakers are not necessary
since chips are in finite size.
Figure 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 in cutting tools acting as chip
breakers
Types of chips produced in metal cutting
Controlled Contact on Tools
• If the tool-chip contact length is reduced by recessing the rake face of the tool;
– It reduces the cutting forces, energy and temperature
– Optimum length should be calculated, otherwise heat is concentrated on the
tool tip and thus increasing wear.
Figure 8 Various chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece and
breaks; (c) continuous chip moving away from workpiece; and (d) chip hits tool shank and breaks off.
PART-2
Oblique Cutting




Machining operations are in three dimensional, the cutting is oblique.
In orthogonal cutting the chip slides directly up the face of the tool.
In oblique cutting the chip is helical and at an angle i (the inclination angle)
In oblique cutting, a helical chip moves sideways and away from the cutting zone
and does not obstruct it as in orthogonal cutting.
Figure 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.
Oblique Cutting
c
Chip flow angle, the chip flows up the rake face of the tool. Measured in the plane of
the tool face.
i
e
Normal rake angle (basic geometric property of the tool). The angle between line oz
normal to the workpiece surface and line oa on the tool face.
Effective rake angle is calculated in the plane of cutting speed and chip speed planes.
c
is equal to the inclination angle (i). Then the effective rake angle is;

1
e sin (sin i cos i sin n )
i and
n
2
2
can be measured directly.
As i increases, the effective rake angle increases, the chip becomes thinner and longer
and then cutting forces decreases. See Figure 9(c).
Oblique Cutting
 All the angles should be selected properly for efficient cutting.
 These angles are produced by grinding. Inserts now commonly used.
Figure 10 (a) Schematic illustration of a right-hand cutting tool. Although these tools have
traditionally been produced from solid tool-steel bars, they have been largely replaced by carbide or
other inserts of various shapes and sizes, as shown in (b).
Cutting Forces and Power
Cutting forces are important since:
• (1) Machine tool selection (power), maintain dimensional accuracy, appropriate
tool holders and work-holding devices
• (2) Workpiece should resist these forces without excessive distortion
Cutting Forces and Power
Fc, cutting force, acts in the direction of the cutting speed, V, supplies the energy
required for cutting. The ratio of the cutting force to the cross-sectional area being
cut is referred to as the specific cutting force.
Ft, thrust force, acts in a direction normal to the cutting force.
Fc and Ft produce the resultant force, R.
R can be resolved into F (friction force) along the chip interface and a normal force ,
N, perpendicular to it.
Figure 11 Forces acting
on a cutting tool in twodimensional cutting.
Note that the resultant
force, R, must be
colinear to balance the
forces.
Cutting Forces and Power
F = R sin
and N = R cos
The resultant force is balanced by an equal and opposite force along the shear plane is
resolved into a shear force, Fs, and normal force, Fn.
Fs = Fc cos - Ft sin
and
Fn = Fc sin + Ft cos
µ, coefficient of friction at the tool-chip interface.
, friction angle. Then;
F Ft Fc tan 
 
N Fc Ft tan 
Figure 11 Forces acting
on a cutting tool in twodimensional cutting.
Note that the resultant
force, R, must be
colinear to balance the
forces.
Cutting Forces and Power
 Magnitude of the forces in actual cutting operations is generally on the order of a few
hundred Newton's, the local stresses in the cutting zone and the pressures on the tool are
very high because the contact areas are very small.
 The tool-chip contact length is typically on the order of 1mm. Thus the tool tip is
subjected to very high stresses, which lead to wear, chipping and fracture of the tool.
Cutting Forces and Power
 Thrust force in cutting is important because the toolholder, the work-holding devices,
and the machine tool must be sufficiently stiff to support that force with minimal
deflections.
For example;
If the thrust force is too high or if the machine is not sufficiently stiff, the tool will be
pushed away from the workpiece surface being machined. Thus, this reduces the depth
of cut, resulting in less dimensional accuracy in the machined part.
Ft R sin ( )
Ft Fc tan ( )
Cutting Forces and Power
 Magnitude of the cutting force Fc is always positive. Since this force supplies the
work required in cutting.
The sign of thrust force, Ft, can either be (+) or (-) depending on relative magnitudes
of
and α.
 When
> α, the sign Ft is positive (downward)
Ft R sin ( )
 When
< α, the sign Ft is negative (upward)
Ft Fc tan ( )
 Upward thrust force can be obtained by the following conditions:
(1) High rake angle
(2) Low friction at the tool-chip interface
(3) both
Cutting Forces and Power
• The power input in cutting:
Power = Fc V
• This power is dissipated mainly in the
shear zone (to shear the material) and
on the rake face of the tool.
• Power dissipated in the shear plane:
Power for shearing = Fs Vs
• Specific energy for shearing,us:
•
FsVs
us 
wtoV
w: width of cut
• The power dissipated in the friction:
Power for friction = F Vc
• Specific energy for friction, uf,
FVc
uf 
wtoV
TABLE 2 Approximate Energy Requirements in
Cutting Operations (at drive motor,
corrected for 80% efficiency; multiply by 1.25
for dull tools).
Total specific energy, ut ,
ut us u f
Measuring Cutting Forces and Power
Because of the many factors involved, reliable predictions of cutting forces and power
still is based largely on experimental data.
 Cutting forces can be measured using;
 Force transducer (typically with quartz piezoelectric sensor)
 Dynamometer
 Load cell (resistance wire strain gages) mounted on cutting tool holder.
Transducers have higher frequency and stiffness than dynamometer.
The specific energy in cutting also can be used to calculate cutting forces.
Solve Example 21.1 and Prob. 21.45
Temperatures in cutting
• In metal working processes, the energy dissipated in
cutting is converted into heat , in turn, raises
temperatures in cutting zone.
Temperature rise have the following effects:
 Excessive temperature lowers the strength,
hardness, stiffness, and wear resistance of the
cutting tool; tools also may soften and undergo
plastic deformation; thus tool shape is altered.
 Increased heat causes uneven dimensional
changes, alter dimensional accuracy and
tolerances.
 Excessive temperature rise induce thermal
damage and metallurgical changes.
Figure 12 Typical temperature
distribution the cutting zone.
Note the steep temperature
gradients within the tool and
the chip.
Temperatures in cutting
The main source of heat in machining area:
(a) The work done in shearing in the primary shear zone
(b) Energy dissipated as friction at the tool-chip interface
(c) Heat generated as the tool rubs against the machined
surface, especially dull or worn tools.
Mean temperture (in Kelvin), Tmean, in orthogonal cutting:
0.000665 Y f
T
c
3
V to
K
Yf
c
K
Tmean V a f b V
f
Flow stress in Mpa
Volumetric specific heat
in kJ/m³.K
Thermal diffusivity
(conductivity) in m²/s.
Cutting speed
feed
a=0.2 & b=0.125 for carbide tools
a= 0.5 & b=0.375 for high-speed steel
Figure 12 Typical temperature
distribution the cutting zone. Note
the steep temperature gradients
within the tool and the chip.
Temperatures Distribution
• Main sources of heat generation in machining are
concentrated in the primary shear zone and the toolchip interface.
• Presence of severe gradients and that the maximum
temperature is about halfway up to the tool-chip
interface.
Figure 12 Typical temperature
distribution the cutting zone.
Note the steep temperature
gradients within the tool and
the chip.
Temperatures Distribution
 The temperatures increases with the cutting speed and that the highest temperature is
almost 1100⁰C.
 The chips turn to dark-bluish color at such high temperatures.
 Tmean is directly related to cutting speed, thus as speed increases, temperature increases
Figure 13 Temperatures developed n turning 52100 steel: (a) flank temperature
distribution; and (b) tool-chip interface temperature distribution.
Temperatures Distribution
 The chip carries away most of the heat generated.
See Fig.14.
 90% of the energy is removed by the chip during a
typical machining operation, with the rest by the
tool and the workpiece.
 As the cutting speed increases, a larger proportion
of the total heat generated is carried away by the
chip, and less heat goes into the workpiece and tool.
( i.e HSM)
Techniques for measuring temperatures:
(1) Thermocouples embedded in the tool or the
workpiece
(2) Thermal emf (electromotive force) at the toolchip interface, which acts as a hot junction
between two different materials.
(3) Infrared radiation from cutting zone by a
radiation pyrometer.
Figure 14 Percentage of the heat
generated in cutting going into the
workpiece, tool, and chip, as a
function of cutting speed. Note that
the chip carries away most of the
heat.
PART-III
Tool Life: Wear and Failure
Tool Life: Wear and Failure
In cutting operations, cutting tools are subject to:
(a) high localized stresses at the tip of the tool
(b) high temperatures, especially along the rake face
(c) sliding of the chip along the rake face
(d) sliding of the tool along the newly cut workpiece surface
Tool
Wear
Tool wear adversely affects tool life, the quality of the machined surface and its
dimensional accuracy, and economics of cutting operations.
Wear is a gradual process (like wear on a pencil tip)
The rate of the tool wear depends on:
(1) tool and workpiece materials
(2) tool geometry
(3) process parameters
(4) cutting fluids
(5) characteristic of the machine tool
Tool Wear
Tool wear can be classified as:
- Flank wear
- Notching
- Gross fracture
-Crater wear
- Plastic deformation of the tool tip
- Nose wear
- Chipping
(a)
(d)
(b)
(e)
(c)
Figure 15 (a) Flank and crater wear in a cutting
tool. Tool moves to the left. (b) View of the rake
face of a turning tool, showing nose radius R and
crater wear pattern on the rake face of the tool. (c)
View of the flank face of a turning tool, showing
the average flank wear land VB and the depth-ofcut line (wear notch). See also Fig. 18. (d) Crater
and (e) flank wear on a carbide tool.
Flank Wear
Flank wear occurs on the relief (flank) face of the tool.
Flank wear is generally attributed to;
(a) Rubbing of the tool along the machined surface
causing adhesive and abrasive wear
(b) High temperatures, which adversely affect the toolmaterials properties.
Taylor tool life equation is established:
n
V T C
V is the cutting speed
T is the time (minutes)
n is the exponent that depends on tool and workpiece
materials and cutting conditions
C is a constant
Each combination of workpiece and tool materials and
each cutting condition have their own n and C values,
determined experimentally.
Ranges of n Values for the Taylor
Equation for Various Tool Materials
High-Speed steel
0.08-0.2
Cast Alloys
0.1-0.15
Carbides
0.2-0.5
Coated carbides
0.4-0.6
Ceramics
0.5-0.7
Table 3. n values for tool materials
Flank Wear
Cutting speed is the most important process variable associated with tool life, followed
by depth of cut and feed, f. For turning, Taylor Equation can be modified to:
V T n d x f y C
Where,
d: the depth of cut
f: feed in mm/rev
x and y must be determined experimentally for each cutting condition.
Taking n=0.15, x = 0.15 and y = 0.6 as typical values encountered in machining practice.
T C1/ nV 1/ n d x / n f y / n
Using typical values;
T C 7V 7 d 1 f 4
Tool life curves
Tool-life curves are plots of experimental data obtained by performing cutting tests on various under
different cutting conditions, such as cutting speed, feed, depth of cut, tool material and geometry,
and cutting fluids.
Figure 16 Effect of workpiece microstructure and
hardness on tool life in turning ductile cast iron.
Note the rapid decrease in tool life as the cutting
speed increases. Tool materials have been
developed that resist high temperatures such as
carbides, ceramics, and cubic boron nitride.
Figure 17 Tool-life curves for a variety
of cutting-tool materials. The negative
inverse of the slope of these curves is
the exponent n in the Taylor tool-life
equations and C is the cutting speed at
T = 1 min.
NOTEs:
-tool life decreases rapidly as the
cutting speed increases
- the condition of the workpiece
material has a strong influence on
tool life
- differences in tool life for
different workpiece-material
microstructures.
Temperature increases, flank
wear rapidly increases.
Allowable Wear Land
Cutting tools need to be replaced (or re-sharpened) when,
(1) The surface finish of the machined workpiece begins to deteriorate
(2) Cutting forces increases significantly
(3) Temperature rises significantly
For improved dimensional accuracy, tolerances, and surface finish, the allowable wear
land may be smaller than the values given in the below table.
Allowable Average Wear Land for cutting tools various
machining operations
Allowable wear land (mm)
Operation
High-speed steel tools
Carbide tools
Turning
1.5
0.4
Face milling
1.5
0.4
End milling
0.3
0.3
Drilling
0.4
0.4
Reaming
0.15
0.15
Crater Wear
Crater wear occurs on the rake face of the tool
Crater wear changes the tool-chip interface contact geometry.
The most significant factors influencing crater wear are:
(a) The temperature at the tool-chip interface
(b) The chemical affinity between the tool and workpiece materials
Additionally, the factors influencing flank wear may affect crater wear.
Crater Wear
Figure 19 Relationship between crater wear
rate and average tool-chip interface
temperature: (a) High-speed steel; (b) C-1
carbide; and (c) C-5 carbide. Note how rapidly
crater-wear rate increases as the temperature
increases
Figure 18 (a) Schematic
illustrations of types of wear
observed on various types of
cutting tools. (b) Schematic
illustrations of catastrophic tool
failures. A study of the types and
mechanisms of tool wear and
failure is essential to the
development of better tool
materials.
Crater wear
• Crater wear generally attributed to a diffusion
mechanism, which is the movement of atoms across the
tool-chip interface.
• Diffusion rate increases with increasing temperature,
crater wear increases as temperature increases.
• Applying protective coatings to tools in an effective
means of slowing the diffusion process and thus
reducing crater wear.
• Typical coatings are titanium nitride, titanium carbide,
titanium carbonitride, and aluminum oxide.
Rake
face
Crater
wear
Flank
face
chip
Figure 20.20 Cutting tool (right)
and chip (left) interface in
cutting plain-carbon steel. The
discoloration of the tool
indicates the presence of
high temperatures. Compare this
figure with Fig.20.12.
Other types of wear, chipping and Fracture
Following describe the factors involved in other types of cutting tool wear and fracture:
(1) Nose Wear: is the rounding of a sharp tool due to mechanical and thermal effects.
It dulls the tool, affects chip formation, and causes rubbing of the tool over the
workpiece, raising its temperature and possibly inducing residual stresses on the
machined surface.
(2) Notches: observed on cutting tools. The region that they occupy is the boundary
where chip is no longer in contact with the tool. Scale and oxide layers on a
workpiece surface also contribute to notch wear.
Other types of wear, chipping and Fracture
(3) Chipping: a small fragment from the cutting edge of the tool breaks away. Unlike
wear, chipping is a sudden loss of tool material and a corresponding change in its
shape. Chipping has a major detrimental effect on surface finish, surface integrity
and the dimensional accuracy of the workpiece.
- Mechanical shock: impact due to interrupted cutting
- Thermal fatigue: cyclic variations in the temperature of the tool in interrupted
cutting.
(4) Thermal cracks: usually perpendicular to the cutting edge of the tool.
It may cause chipping.
Chipping can be reduced by selecting tool materials with high-impact and thermal
shock resistance.
High positive rake angle can contribute to chipping because of the small included angle
of the tool tip.
Crater wear region can progress toward to tool tip, thus weakening the tip because of
reduced materials volume and causing chipping.
Surface finish and integrity
Figure 20.21 Surfaces produced on
steel by cutting, as observed with a
scanning electron microscope: (a)
turned surface and (b) surface
produced by shaping.
Surface finish and integrity
Figure 22 Schematic illustration of a dull tool in
orthogonal cutting (exaggerated). Note that at small depths of cut,
the positive rake angle can effectively become negative, and the
tool may simply ride over and burnish the workpiece surface.
Figure 23 Schematic illustration of feed marks in
turning (highly exaggerated). See also Fig. 20.2.