Transcript Fundamentals of Machining

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Zach Ratzlaff Moises Narvaez Weston Dooley Todd Miner
Fundamentals of Cutting
Cutting-Tool Materials and Cutting Fluids
Fundamentals of Machining
Mechanics of Cutting
Cutting Forces and Power
Temperatures in Cutting
Common Machining Operations
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The cutting process, and how chips
are produced
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Factors that influence the cutting
process.
• Cutting speed, Depth of cut,
feed rate, and cutting fluids.
• Tool Angle
• Continuous chip
• Built-up edge chip
• Discontinuous Chip
• Temperature rise
• Tool wear
• Machinability
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(a)
(d)
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(b)
(c)
(e)
(f)
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Chip breakers
(a) Schematic
illustration of the
action of a chip
breaker. 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.
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Cutting With an Oblique Tool
The majority of machining operations are done with an 3D
shaped cutting tool this is called oblique cutting.
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Cutting Forces and Power
Data on cutting forces is essential
so that:
• Machine tools can be properly
designed.
• To ensure that the work piece is
capable of withstanding the
forces without excessive
distortion.
• Power requirements must be
taken into account when
selecting machinery.
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Cutting Force, Thrust Force and
Power.
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Temperatures in cutting
As in all metal working where plastic
deformation is involved, the energy
dissipated in cutting is converted into
heat which, in turn raises the
temperature in the cutting zone.
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Effects of temperature rise
• Excessive temperature lowers the
strength, hardness, stiffness, and
wear resistance of cutting tools.
• Increased heat causes uneven
dimensional changes in the part.
• Excessive temperature rise can
cause thermal damage to the
surface of the part.
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Temperature Distribution
Typical temperature
distribution over the
cutting zone.
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Heat distribution during machining.
Percentage of the heat generated
in cutting going into the work piece,
tool, and chip, as a function of
cutting speed.
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TOOL LIFE: Wear and Failure
Cutting tools are subjected to many
factors that determine the wear of the
tool. Some of the most important are:
• High localized stresses at the tip of the
tool.
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TOOL LIFE: Wear and Failure
• High temperatures.
• Sliding of chips along the rake face.
• Sliding of tool along cut work piece.
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TOOL LIFE: Wear and Failure
Wear is a gradual process, and it also
depends on:
• Tool and workpiece materials.
• Tool geometry.
• Process parameters.
• Cutting Fluids.
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TOOL LIFE: Wear and Failure
•
•
•
•
•
•
Tool wear and changes in tool
geometry manifest as:
Flank wear.
Crater wear.
Nose wear.
Notching.
Chipping or gross fracture.
Plastic deformation of the tool tip.
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TOOL LIFE: Wear and Failure
• FLANK WEAR: Occurs on the relief
face of the tool (flank) due to rubbing
of the tool on the machined surface,
causing adhesive and /or abrasive
wear, and high temperatures.
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TOOL LIFE: Wear and Failure
• CRATER WEAR: It is attributed to the
diffusion of atoms across the tool-chip
interface. Diffusion rates increase
with temperature; thus, crater wear
increases with increasing
temperature.
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TOOL LIFE: Wear and Failure
The location of the
maximum depth
of crater wear
coincides with
the location of
the maximum
temperature at
the tool-chip
interface.
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TOOL LIFE: Wear and failure
• NOSE WEAR: Rounding of a sharp
tool due to mechanical and thermal
effects. Affects chip formation and
causes rubbing of the tool over the
workpiece increasing the
temperature.
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TOOL LIFE: Wear and Failure
• NOTCHING: A groove or notch develops
in a region that undergoes workhardening. This region develops a thin
work-hardened layer that can originate a
groove.
Oxide layers on a workpiece also
contribute to notch wear because these
are hard and abrasive. To prevent this,
the depth of the cut must be grater than
oxide layer thickness.
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TOOL LIFE: Wear and Failure
• CHIPPING: Sudden loss of material due to
mall fragments of the cutting edge of
the tool breaking away. It occurs
typically on brittle tool materials such as
ceramics.
Chipping also occurs in a region where a
small crack or defect already exists.
The two main causes of chipping are
mechanical shock and thermal fatigue.
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TOOL LIFE: Wear and Failure
• PLASTIC DEFORMATION: May occur when
the tool undergoes stresses higher than
the yield strength of the tool material.
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TOOL LIFE: Wear and failure
(b)
(a)
(d)
(c)
(e)
(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-of-cut line (wear notch). See also Fig.
20.18. (d) Crater and (e) flank wear on a carbide tool.
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TOOL LIFE: Wear and failure
TOOL-LIFE CURVES: Plots of experimental
data obtained from cutting tests under
different cutting conditions such as
cutting speed, feed, depth of cut, and
tool material and geometry.
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TOOL LIFE: Wear and failure
The tool-life curves are derived from
the approximation:
VT  C
n
Where
• V is the cutting speed,
• T is the time needed to develop a
certain flank wear land,
• C and n are tool material
constant
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TOOL LIFE: Wear and failure
Notice the rapid decrease in tool life as the
cutting speed increases. Several tool
materials have been developed that resist
high temperatures such as carbides,
ceramics, and cubic boron nitride
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TOOL LIFE: Wear and failure
Tool-life curves for a
variety of cuttingtool 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.
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TOOL LIFE: Wear and failure
ALLOWABLE WEAR LAND: In order to have
good dimensional accuracy, surface
finish, and to keep within the allowed
tolerances, cutting tools need to be
replaced or resharpened when:
• The surface finish of workpiece begins to
deteriorate.
• Cutting forces increase.
• Temperature rises significantly.
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TOOL LIFE: Wear and failure
The following table shows the average
allowable wear for various machining
operations. Notice that allowable wear for
ceramic tools is about 50% higher.
TABLE 20.4 Allowable Average Wear Land (VB)
for Cutting Tools in Various Operations
Allowable wear land (mm)
Operation
High-speed Steels
Carbides
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
Note: 1 mm = 0.040 in.
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TOOL LIFE: Wear and failure
TOOL-CONDITION MONITORING: Computer
controlled machine tools require precise
and reliable cutting tools that are able
to perform repeatedly.
• Direct methods: Involve optical
measurement of wear and changes on
the tool profile. Requires to stop
operations.
Example: Use of a tool’s maker
microscope.
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TOOL LIFE: Wear and failure
• Indirect methods: Determine the tool
condition by measuring process
parameters such as cutting forces,
power, temperature rise, vibration,
workpiece surface finish.
Example: Acoustic Emission technique
which analyzes acoustic emissions that
result vibrations and stresses.
Example 2: Tool-cycle time.
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TOOL LIFE: Wear and failure
SURFACE FINISH AND INTEGRITY:
• Surface Finish: refers to the geometric
characteristics of the surface.
Factors affecting surface finish are:
-A dull tool with a large tip radius will rub
over the machined surface causing
residual surface stresses, tearing and
cracking.
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TOOL LIFE: Wear and failure
-Vibration and Chatter may cause
variations of the dimensions of the cut,
and chipping and premature failure of
brittle cutting tools.
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TOOL LIFE: Wear and failure
MACHINABILITY: Good machinability
indicates good surface finish and
surface integrity. The machinability of
a material is defined by:
• Surface finish and integrity
• Tool life
• Force and power required
• Level of difficulty on chip control
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TOOL LIFE: Wear and failure
Machinability of Ferrous Metals:
• Low Carbon steels: Have a wide range
of machinability depending on ductility
and hardness.
• Free-machining steels: Contain sulfur
and phosphorous allowing a decrease
on size of chips and an increase in
machinability.
• Leaded Steels: Pb is insoluble in Fe, Cu
and Al. Works as a solid lubricant.
Consider that Lead is toxic pollutant.
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TOOL LIFE: Wear and failure
• Alloy Steels: Machinability can not be
generalized because of the wide variety
of composition and hardness.
Machinability of Nonferrous Metals:
• Aluminum: Easy to machine, although
the softer grades tend to form build up
edge resulting on poor surface finish.
Possible dimensional tolerance problems
due to thermal expansion.
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TOOL LIFE: Wear and failure
• Copper: Difficult to machine when Cu is
in wrought condition. Cast Cu alloys are
easy to machine as well as Brasses,
especially if these contain lead.
• Beryllium: Easy to machine, but be
aware that fine particles produced while
machining are toxic - requires machining
in controlled environment.
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TOOL LIFE: Wear and failure
Machinability of Thermo Plastics: These
materials have low thermal conductivity
and elastic modulus, and are thermally
softening. Therefore, require sharp tools
with positive rake angles and small
depths of cuts and feeds.
Machinability of Ceramics: These materials
have improve machinability due to the
development of machinable ceramics
and nanoceramics.
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Introduction
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•
•
•
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Carbon and Medium-Alloy Steels
High-Speed Steels
Cast-Cobalt Alloys
Carbides
Coated Tools
•
•
•
•
•
Alumina-based ceramics
Cubic boron nitride
Silicon-nitride-based ceramics
Diamond
Whisker-reinforced materials & nanomaterials
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Introduction
Cutting tools are subjected to:
• High Temperatures
• High Contact Stresses
• Rubbing along tool-chip interface
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Choosing a Cutting Tool
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Hot Hardness
Toughness and impact strength
Thermal shock resistance
Wear resistance
Chemical stability and inertness
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Hot Hardness
• Hardness of
various
cutting-tool
materials as a
function of
temperature
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High Speed Steels (HSS)
• Good wear resistance
• Relatively inexpensive
Suitable for:
• High positive rake tools (small angles)
• Interrupted cuts
• Tools subjected to vibration and chatter
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Cast-Cobalt Alloys
• Higher hot hardness than HSS
• Cuts almost twice as quick as HSS
Main use:
• Remove large amounts of materials
as quick as possible (roughing cuts)
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Carbides
• Most cost effective,
versatile tool used in
manufacturing
• Two major types of
carbides (Tungsten and
Titanium)
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Types of Carbides
Tungsten Carbides
• Manufactured using powder-metallurgy
• Used to cut steels, cast iron, and abrasive
non ferrous metals
Titanium Carbides
• Higher wear resistance than Tungsten
Carbides but is not as tough
• Cuts at higher speeds than Tungsten
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Carbide Inserts
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Edge Strength
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Multi Phase Coatings
• Reduces abrasion and chemical reactivity
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Machining Time
• In less than 100 years the time to machine parts
has reduced by 2 orders of magnitude
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Ceramic Tool Materials
• Ceramic tool materials were
introduced in the early 1950’s
• A very effective cutting tool
Types:
Alumina based Ceramics
Cubic Boron Nitride
Silicon Nitride
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Alumina-Based Ceramics
• These ceramic tools have some good
properties which make it good for
cutting
• Very High Abrasion Resistance
• Hot Hardness
• Chemically more stable than high
speed steels and carbides
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Cermets
• Good chemical stability and
resistance to edge build up
• Brittle
• High cost
• Mostly aluminum oxide
• Performance between a ceramic
and a carbide
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Properties for Groups of Tool
Materials
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Cubic Boron Nitride (CBN)
• Hardest material
presently available
other than
Diamond
• Very high wear
resistance and has
a good cutting
edge strength
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Cubic Boron Nitride (CBN)
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Silicon Nitride Based Ceramics
• Consists of Silicon Nitride with
additions of Aluminum oxide and
titanium carbide.
• Have good hardness
• Good thermal shock resistance
• Example: Sialon
(silicon,aluminum,oxygen and
nitrogen)
• Good for machining cast irons and
nickel based super alloys
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Sialon Applications
• seals and bearings.
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Diamond
• Hardest of all known materials
• Desirable cutting tool properties
Low Friction
High Wear resistance
Sharp Edge (able to maintain)
Good Surface Finish
Good Dimensional Accuracy
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Diamond Edge Saw Blade
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Diamond Tip Drill bits
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Diamond Polishing
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Whisker Reinforced Tool Materials
• High fracture toughness
• Resistance to thermal shock
• Cutting edge strength
• Creep resistance
Whiskers are used as reinforcing fibers in
composite cutting tool materials.
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WG-600 Whisker Reinforced
Ceramic Cutting Tool
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Cutting Fluids
• Cutting fluids have been extensively
used in machining operations
Reduce Friction and wear
Reduce force and energy
consumption
Cool the cutting zone
Flush away chips
Protect the Machined surface from
environmental corrosion
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Cutting Fluids
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Cutting Fluids
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Considerations for Selecting cutting
fluids
• Need for a lubricant or Coolant, or
both.
• Levels of temperatures expected
• Forces encountered
• Cutting speed
The need for a cutting depends on
severity
of the operation:
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Machining Processes
1.
2.
3.
4.
5.
6.
7.
8.
Sawing
Turning
Milling
Drilling
Gear cutting
Thread cutting
Tapping
Internal broaching
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Increasing
Severity
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Types of Cutting Fluids
1.
2.
3.
4.
Oils - often called straight oils, includes
mineral, animal, vegetable,
compounded, and synthetic oils.
Emulsions- often called soluble oils,
mixtures of oil and water and additives.
Semi-synthetics- chemical emulsions
containing little mineral oil, reduced size of
oil particles
Synthetics- chemicals with additives
diluted in water and contain no oil.
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Methods of Cutting fluids
1.
2.
3.
4.
Flooding- Most common method. Flow
rates depend on application.
Mist- Supplies fluid to inaccessible areas.
Similar to using an aerosol can (spray paint
or hairspray)
High Pressure Systems- use specialized
nozzles that aim powerful jet of fluid
towards the cutting zone.
Through the cutting tool system- an
effective method. A narrow passage can
be produced in the cutting tool, where it
can be applied under high pressure
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Application of Cutting Fluids
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Effects of Cutting Fluids
• The effect on the work piece and
machining tools
• Biological Considerations
• The Environment
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Near Dry Machining
• Economic and environmental concerns
have caused a trend to eliminate
metalworking fluids.
• Near dry machining Benefits
Relieve Environmental impact of using
cutting fluids
Reduce Cost
Improved Surface Quality
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Cryogenic Machining
• Most recent development
• Uses nitrogen and carbon dioxide as
coolant in machining (-200 C)
• Liquid nitrogen injected into the
cutting zone.
• Allows higher cutting speeds, tool life
enhancement and machinibilty
increase.
• Nitrogen simply evaporates, no
environmental impact
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References
• http://www.Haniblecarbide.com
• http://www.crucibleservice.com/
• http://www.azom.com/details.asp?Ar
ticleID=268&head=Sialons#_Cutting_T
ools
• http://www.manufacturingcenter.co
m/tooling/archives/0304/0304westec
_pages.asp
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