Download ch20_with text - University of North Florida

Introduction
¾ What is machining?
ƒ Removal of material from a workpiece
• Cutting
○ Single or multipoint cutting tools. Each with a clearly defined tools shape
• Abrasive processes
Chapter 20
○ Ex: grinding
• Advanced machining processes
Fundamentals of Cutting
○ Electrical, chemical, thermal, and hydrodynamic methods, and lasers.
¾ Advantages of machining
ƒ Good dimensional accuracy
ƒ May include internal and external geometrical features, sharp
corners, and flatness
ƒ Some surface characteristics and textures can’t be produced
with other processes
ƒ Economical for very small production runs
Alexandra Schönning, Ph.D.
Mechanical Engineering
University of North Florida
Figures by
Manufacturing Engineering and Technology
Kalpakijan and Schmid
¾ Disadvantages of machining
ƒ Wastes material, require more energy, capital and labor
ƒ Time: it takes longer to remove material than it does to form it.
ƒ Typically expensive for large production runs
Page 20-1
Page 20-2
Introduction
Introduction: Chip Formation
¾ What are some cutting
operations?
¾ Cutting processes cause
ship formation ahead of
the tool
¾ Some parameters
effecting the cutting
operation
ƒ Turning
• The workpiece is rotated and
a cutting tool removes a
layer of material as it moves
in one direction
ƒ Cutting-off
ƒ Depth of cut
ƒ Feed rate = distance the
tool travels per unit
revolution of the
workpiece (for turning).
Units: mm/rev, in/rev…
• Cutting tool moves radially
inward
ƒ Slab milling
• A rotating cutting tool
removes a layer of material
from the surface of the
workpiece
ƒ End milling
Figure 20.1 Examples of cutting processes.
• Rotating cutter travels along
a certain depth in the
workpiece to produce a
cavity
Page 20-3
Figure 20.3 Schematic illustration of a twodimensional cutting process, also called
orthogonal cutting. Note that the tool shape and
its angles, depth of cut, to, and the cutting speed, V,
are all independent variables.
Variables in cutting operations
¾ Independent variables
ƒ Tool material, coatings,
and tool condition
ƒ Tool shape, surface finish,
and sharpness
ƒ Workpiece material,
condition and temperature
ƒ Cutting parameters, such
as speed, feed, and depth
of cut
ƒ Cutting fluid
ƒ Characteristics of the
machine tool, such as
stiffness and damping
ƒ Workholding and fixturing
Figure 20.2 Basic principle of the turning operations.
Page 20-4
Factors Influencing Cutting Processes
¾ Dependent variables
ƒ Type of chip produced
ƒ Force and energy
dissipated in the cutting
process
ƒ Temperature rise in the
workpiece, the chip, and
the tool
ƒ Wear and failure of the
tool
ƒ Surface finish produced on
the workpiece after
machining
Page 20-5
TABLE 20.1
Parameter
Cutting speed, depth of cut,
feed, cutting fluids
Tool angles
Continuous chip
Built-up edge chip
Discontinuous chip
Temperature rise
Tool wear
Machinability
Influence and interrelationship
Forces, power, temperature rise, tool life, type of chip, surface finish.
As above; influence on chip flow direction; resistance to tool chipping.
Good surface finish; steady cutting forces; undesirable in automated
machinery.
Poor surface finish; thin stable edge can protect tool surfaces.
Desirable for ease of chip disposal; fluctuating cutting forces; can affect
surface finish and cause vibration and chatter.
Influences tool life, particularly crater wear, and dimensional accuracy of
workpiece; may cause thermal damage to workpiece surface.
Influences surface finish, dimensional accuracy, temperature rise, forces and
power.
Related to tool life, surface finish, forces and power.
Page 20-6
1
Mechanics of Chip Formation
¾ Orthogonal cutting:
The tool edge is
perpendicular to the
movement of the tool
¾ Geometrical terms
Types of chips produced in metal cutting
ƒ Thickness of chip is
always greater than the
depth of the cut
• Cutting ratio: r = to/tc (depth
of cut/chip thickness),
always less than 1
• Chip compression ratio: 1/r,
always greater than unity
ƒ Rake angle
ƒ Relief angle
ƒ Shear angle
¾ How are the chips
formed?
¾ Why do we care what kind of chips are produced?
¾ The type of chip influences the surface finish of the
workpiece and the overall cutting operation.
¾ Types of chips produced
ƒ
ƒ
ƒ
ƒ
Continuous
Built-up edge
Serrated or segmented
Discontinuous
¾ Two sides of the chip
ƒ Shearing takes place
along a shear zone
ƒ Tool side
• Below the shear plane the
the workpiece is
undeformed
• A chip is formed above Figure 20.4 (a) Schematic illustration of the basic mechanism of chip
shear plane
formation in metal cutting. (b) Velocity diagram in the cutting zone.
• In contact with the tool face (rake face)
• Smooth surface
ƒ Other surface
• Jagged, rough appearance caused by the shearing mechanism
See also Section 20.5.3. Source: M. E. Merchant.
Page 20-7
Page 20-8
Continuous chips
¾ Usually formed with ductile
materials at high speeds and
/or rake angles
¾ Produce good surface finish
¾ Typically not desirable
Built-up edge (BUE) chips
(a)
continuous
chip with
narrow,
straight
primary
shear zone;
¾Layer of material from the workpiece is
gradually deposited on the edge of the tool.
¾BUE becomes unstable and eventually breaks up
ƒ Deposited randomly on the surface of the workpiece
¾BUE is one of the most common factors
adversely affects the finished surface.
¾Somewhat desirable: thin layer protects the
surface of the tool.
¾BUE formation is reduced by
ƒ Tangled around the tool holder,
the fixturing, and the workpiece
ƒ Operation must be stopped to
untangle/remove
¾ Problem solved by
ƒ chip breakers
ƒ changing machining parameters
(cutting speed, feed, cutting
fluids)
ƒ
ƒ
ƒ
ƒ
Decreasing the depth of the cut
Increasing the rake angle
Using a sharp tool
Using an effective cutting fluid
Page 20-9
Serrated Chips
Page 20-10
Discontinuous chips
¾Also called segmented or nonhomogeneous chips
¾Semicontinuous chips with zones of low and
high shear strain
¾Chips have a saw-tooth like appearance
¾Observed in metals with
ƒ Low thermal conductivity
ƒ Strength that decreases sharply with temperaturewith
ƒ Example: titanium
¾Segments that are firmly or loosely attached to
each other
¾Form under conditions
ƒ Brittle workpiece material
ƒ Workpiece material with hard inclusions and
impurities
ƒ Very low or very high cutting speeds
ƒ Low rake angle
ƒ Lack of an effective cutting fluid
ƒ Low stiffness of the machine tool
¾Forces vary during cutting because of the
discontinuous nature of the chip formation
Page 20-11
Page 20-12
2
Chip breakers
Examples of Chips Produced in Turning
¾ Continuous chips are
undesirable and a safety
hazard.
Figure 20.8 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.
Source: G. Boothroyd, Fundamentals of Metal Machining and Machine Tools. Copyright ©1975;
McGraw-Hill Publishing Company. Used with permission.
ƒ Become entangled and interfere
with cutting operation.
¾ Chips can be broken up with
a chip breaker
ƒ Traditionally: a piece of metal
attached to the rake of the tool –
bends the chip upward and
breaks it.
¾ Soft materials
ƒ Difficult to break using a chip
breaker
ƒ Instead:
• machine in small increments
and then pausing
• Reverse the feed by a small
increment
Figure 20.7 (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; see also Fig. 21.2.
Page 20-13
Page 20-14
Mechanics of oblique cutting
¾ Orthagonal cutting: described
previously
ƒ Tool edge is perpendicular to
the movement of the tool
ƒ Chip slides up the face of the
tool
¾ Oblique cutting
ƒ An inclination angle is used to
force the chip sideways
Cutting Forces and Power
¾ Fc: cutting force
¾ Inserts: mounted on tool
holders
ƒ In the direction of the cutting
speed
ƒ May include tools for drilling,
tapping, milling, planing,
shaping, broaching, sawing, and
filing
¾ Ft: thrust force
ƒ In the direction normal to the
cutting velocity
¾ F: friction force
¾ N: Normal force
¾ µ: coefficient of friction
¾ Shaving
ƒ Shaving: similar to shave wood
using a planer
ƒ Used to improve surface finish
and dimensional accuracy
ƒ Typically from 0.5 to 2
F
R ⋅ sin ( β )
N
R⋅ cos ( β )
Fn
Fs
µ
Figure 20.11 Forces acting on a
cutting tool in two-dimensional
cutting. Note that the resultant
force, R, must be colinear to
balance the forces.
Fc⋅ sin ( φ) + Ft⋅ cos ( φ)
Fc⋅ cos ( φ) − Ft⋅ sin ( φ)
F
N
Ft + Fc⋅ tan ( α )
Fc − Ft⋅ tan ( α )
Page 20-15
Thrust Force and Power
Page 20-16
Temperature in Cutting
¾ Thrust force
ƒ can cause deflections in the tool holder, work holding device, and the machine tool
ƒ Acts downward
¾ Power
ƒ Power=Fc*v
ƒ Dissipated mainly in the shear zone
• Power for shearing = Fs*vs
ƒ Dissipated also through friction
• Power for friction = F*vc
ƒ Cutting ratio = r = to/tc (=depth of cut / chip thickness)=vc/v
¾ Example
ƒ In an orthagonal cutting operation,
depth of cut to = 0.005 in, cutting speed v= 400
ft/min, rake angle α = 10o, and the width of cut = 0.25 in. It is observed that chip
thickness tc = 0.009 in, Fc = 125 lb, and Ft = 50 lb. Calculate what percentage of the
total energy goes into overcoming friction at the tool-chip interface.
ƒ Solution:
ƒ Percentage is: friction energy/total energy = Fvc/Fcv = F*r / Fc
ƒ r = to/tc = 5 / 9 =0.555
ƒ F=R*sin(β)
ƒ Fc = R*cos(β- α)
ƒ R = [(Ft2+Fc2)]1/2 = [(502+1252)]1/2 = 315 lb
ƒ Fc = R*cos(β- α) Æ 125 = 135*cos(β-10)
• Gives: β = 32o
¾ Why is knowledge of the
temperature rise
important?
ƒ Excessive temperature
adversely affects the
strength, hardness, and
wear resistance of the
cutting tool
ƒ Difficult to control
dimensional accuracy
ƒ Thermal damage to the
machined surface
ƒ Distortion of the machine
¾ Main source of heat
generation:
¾ What does the
temperature depend on?
ƒ
ƒ
ƒ
ƒ
Specific heat
Thermal conductivity
Depth of cut
Type of cutting fluid
¾ How is temperature
measured?
ƒ Thermocouples embedded
in the tool and/or
workpiece
ƒ Infrared radiation
ƒ Thermal emf (elctromotive
force)
ƒ Shear zone
ƒ Tool-chip interface
ƒ F=R*sin(β) Æ F=135*sin(32) = 71.5 lb
ƒ Percentage: F*r / Fc = 71.5*0.555 / 125 = 0.32 Æ 32%
Page 20-17
Page 20-18
3
Tool life: wear and failure
Tool life: wear and failure
¾ Cutting tools are subject to
ƒ
ƒ
ƒ
ƒ
¾Chipping
High localized stresses
High temperatures
Sliding of the chip along the rake of the face
Sliding of the tool along the freshly cut surface
ƒ Breaking away of small pieces from the cutting edge
of the tool
ƒ Unlike wear, which is a gradual process, chipping
results in a sudden loss of tool material and a
corresponding change in shape
¾ This effects
ƒ Tool life, quality of the machined surface, dimensional
accuracy, economics of cutting operations
¾ Types of wear
ƒ Flank wear
• On the relief face of the tool
• Due to rubbing of the tool against the machined surface and due to high
temperatures
ƒ Crater wear
• Occurs on the rake face of the tool
• Due to
○ temperature at the tool-chip interface
○ Chemical affinity between the tool and the workpiece
Page 20-19
Page 20-20
Surface finish an surface integrity
¾ Surface finish influences
ƒ Dimensional accuracy
ƒ Mechanical properties
¾ Surface integrity
ƒ Properties such as fatigue life
and corrosion resistance
ƒ Influenced by
Machinability
¾ Negative effective rake angle
ƒ Due to rounded tool edge (dull
tool)
ƒ Tool will ride over the surface
without removing any material
¾ Defined using the factors:
ƒ
ƒ
ƒ
ƒ
Surface finish and integrity of the machined part
Tool life obtained
Force and power requirements
Chip control
¾ In manufacturing plants tool life and surface roughness
are considered the most important factors in
machinability
¾ In steels:
• Temperatures generated during
processing
• Residual stresses
• Metallurgical (phase)
transformations
• Surface plastic deformation,
tearing, and cracking
ƒ Machinability has been improved by adding sulfur and lead
• Sulfur:
○ Act as stress raisers in the primary shear zone
○ Chips break up easily and are small
¾ BUE (built-up edge)
• Lead:
ƒ Has the greatest influence on
the surface finish
○ Lead melts at low temperatures
○ Low shear strength Æ acts as a liquid lubricant
Page 20-21
Page 20-22
4