Transcript Chapter 22

Group Two (The Calipers)
We’re “DEAD ON!”
 Anthony Menicucci  Chris King
 Keith Jansen
 Nathan Burns
 Bill Fan
“Fundamental Of Machining”
and
“Cutting-Tool Materials and Cutting
Fluids”
Above: Lathe cutting tool
Side: Nano Drill-Bit
= 1 um2
Types of Machining
Straight Turning and Cutting Off
Slab Milling
End Milling
Types Of Chips produced
•Continuous
•Discontinuous
•Serrated or Segmented
•Built Up Edge
Continuous and Discontinuous Chips
Chip Breakers
Chip Breakers
can be used to
prevent
discontinuous
chips from
forming.
Serrated or Segmented
(a)
(d)
(b)
(e)
(c)
(f)
Serrated chips, looked at under a microscope,
resemble a saw tooth pattern because of the
semi-continuity of the chip as in Figure (a).
Chip
(a)
BUE
Built Up Edge
Built Up edges
affect the
performance of a
cutting tool
because of their
hardness and the
perceived dulling
of the tool.
Hardness of a BUE
Chip
230
Oblique cutting
Most machining operations involve oblique cutting.
Why, on the most basic level, is this this so?
Other forms of oblique cutting
Quick Review
Types of Machining
Turning and Cutting off
Slab Milling
End Milling
Chip Formation
Continuous
Discontinuous
Built Up Edge
Oblique Cutting
21.4 Temperatures in Cutting
 As in all metalworking processes where plastic
deformation is involved, the energy dissipated in
cutting is converted into heat which in turn, raises
the temperature in the cutting zone.
Major 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.
Major Effects
 Increased heat causes uneven dimensional changes
in the part being machined, making it difficult to
control its dimensional accuracy and tolerances.
Major Effects
 Excessive temperature rise can induce
thermal damage and metallurgical
changes in the machined surface,
adversely affecting its properties.
Temperature Distribution
 The maximum temperature is about
halfway up the tool-chip interface.
Techniques for Measuring
Temperature
Traditional
Infrared
Thermometer
 Thermocouples embedded in the tool.
 Thermal emf (electormotive force) at the toolchip interface, which acts as a hot junction
between two different materials.
 Infrared radiation from the cutting zone may be
monitored with a radiation pyrometer.
21.5 Tool Life: Wear and Failure
 Tool wear is a major
consideration in all
machining operations.
Tool wear adversely
affects tool life, quality of
the machined surface
and its dimensional
accuracy, and cutting
operations
Tool Wear
 Crater Wear
 Tool-chip interface
 Predominant at high speed
 Mitigated by efficient use of
carbides
Flank wear
 Tool-work piece inter
 Predominant at low speeds
Tool Wear
 (a) Crater Wear
 (b) Flank wear on a carbide tool
Flank Wear
Tool Life Curves
 Effect of work piece microstructure on tool life in turning. Tool
life is given in terms of the time(min) required to reach a flank
wear land of a specified dimension. (a) ductile cast iron. (b)
Steels, with identical hardness. Note the rapid decrease in tool
life as the cutting speed increases.
Tool Life Curves
 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 equation. (b) Relationship between measured temperature during
cutting and tool life (flank wear). Note that high cutting temperatures
severely reduce tool life.
Extended Taylor’s Equation
Crater Wear
Relationship between crater wear rate and average tool chip interface temperature.
(a) High speed steel, (b) Carbide, ( c ) C5 carbide
Other Types of Wear, Chipping and
Facture
 (a) Schematic illustration
of types of wear
observed on various
cutting tools.
 (b) Catastrophic tool
failures.
Chapter 21 Sections 6 & 7
21.6 Surface Finish and Integrity
21.7 Machinability
21.6 Surface Finish and
Integrity
 Surface Finish describes the geometric
features of a surface.
 Surface integrity pertains to the material
properties.
Building exterior suffers from unsightly damage of mold infestation due to
high humidity. The strong oxidation effect of photocatalyst effectively
removes mold and protects the surface integrity
Effects of tool-tip profile
 Built-up edge has the greatest influence
on surface finish.
(a)
(b)
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. Source: J. T. Black and S. Ramalingam.
Effect of tool-tip profile
 Ceramic and diamond tools generally
produce better surface finish than other
tools because of their much lower
tendency to form a BUE.
Dull Tools
 Large radius along its edge.
 If tip radius of the tool is large in relation to
the depth of cut, the cool simply will rub
over the machined surface.
 May cause surface damage, such as
tearing and cracking
Vibration and chatter
 Vibration and chatter will affect the dimension of
the workpiece surface finish adversely.
 Vibrating tool periodically changes the dimensions
of the cut.
 Excessive chatter also can cause chipping and
premature failure of the more brittle cutting tools.
Factors influencing surface
integrity
 Temperatures generated during
processing and possible metallurgical
transformations.
 Surface residual stresses.
 Severe plastic deformation and strain
hardening of the machined surfaces,
tearing, and cracking.
Finish Machining and Rough
Machining
 In finish machining, it is important to
consider the surface finish to be produced
 In rough machining, the main purpose is to
remove a large amount of material at a
high rate. Surface finish is not a primary
consideration.
Finish and Rough Machining
21.7 Machinability
 Machinability can be defined in terms of
four factors:
 1. Surface finish and surface integrity of the
machined part.
 2. Tool life.
 3. Force and power required.
 4. The level of difficulty in chip control.
Machinability
 Good Machinability indicates good surface
finish and integrity, long tool life, and low
force, and low power requirement.
 Tool life and surface roughness are
considered to be the most important
factors.
Machinability Ratings (index)
 Standard material: AISI 1112 steel, with a
rating of 100.
 This means; for a tool life of 60min, this
steel should be machined at a cutting
speed of 100ft/min (30m/min).
 Some examples; 3140 steel at 55; freecutting brass at 300; 2011 wrought
aluminum at 200.
Machinability of ferrous metals
 Machinability of steels, alloy steels,
stainless steels, and cast iron.
 21.7.1 Pg: 638.
 Effects of various elements in steels.
 Presence of aluminum and silicon in steel is
harmful.
 Carbon and manganese have various effects
depending on their composition.
Machinability of nonferrous
metals
 Examples of nonferrous metals are:
 Aluminum, Copper, Magnesium, Titanium,
and Zirconium.
 21.7.2 Pg: 640
Machinability of miscellaneous
materials
 Thermoplastics
 Thermosetting plastics
 Polymer-matrix composites
 Metal-matrix and ceramic matrix
composites
 Graphite and Ceramics
 Wood
 21.7.3 Pg: 641
Thermally assisted machining (hot
machining)
 Metals and alloys that are difficult to
machine at room temperature can be
machined more easily at elevated
temperatures.
 A source of heat (such as a torch, induced
coil, electric current, laser-beam, electronbeam, and plasma arc) is focused onto an
area just ahead of the cutting tool.
Hot machining
 General advantages;




Reduced cutting force
Increased tool life
Higher material-removal rates
Reduced tendency for vibration and chatter.
Chapter 22
Cutting-Tool Materials and Cutting Fluids
Cutting Tool Properties
 Hot hardness
 High hot hardness means higher speeds and feed
rates (higher production rates and lower costs).
 Toughness and Impact strength
 Tool does not chip or fracture
 Thermal shock resistance
 Wear resistance
 Tool does not have to be replaced as often
 Chemical stability and inertness
 To minimize adverse reactions
Hot hardness
Cutting Tool Materials
 High-speed steels
 Cast-cobalt alloys
 Carbides
 Coated tools
 Ceramics
 Diamond
High-speed steels
 Molybdenum (M-series)





Contains up to 10% Molybdenum
High abrasion resistance
Low distortion during heat treating
Low cost
95% of all high-speed steels are M-series
 Tungsten (T-series)
 Improved strength and hot hardness
 More expensive than M-series
Cast-cobalt alloys
 38 to 53% cobalt
 High hardness
 Not as tough as high-speed
steels
 Used for deep continuous
roughing cuts
 High feed rates
Carbides
 Tungsten carbides (WC)
 Bonded together in a cobalt
matrix
 Sintered
 6 to 16% cobalt
 Have largely replaced HSS tools
 Used for cutting steels and cast
irons
Carbides
 Titanium carbides (TiC)
 Bonded in a Nickelmolybdenum matrix
 Higher wear resistance
than WC
 Not as tough as WC
 Used for cutting steels
and cast irons
 Suitable for higher
speeds than WC
Inserts
 Cuts down on tool
changing time
 Increases number of
cutting points
 Have advanced chip
breaker features
 Come in various
shapes and sizes
Inserts
 Insert shape determines strength
 Various ways if holding insert in place
Coated tools
 Coating properties
 Lower friction
 Higher adhesion
 Higher resistance to
wear and cracking
 Acting a a diffusion
barrier
 Higher hot hardness
and impact resistance
Coating materials
 Titanium-nitride
 Low friction and high
hardness
 Titanium carbide
 Improves wear
resistance on WC
 Ceramics
 Diamond
 Increases tool life ten
fold compared to other
coatings
22.6: Alumina Based Ceramics
 Ceramic Tools
 Primarily made of aluminum oxide
 Introduced in the early 1950s
 After being cold pressed into insert shapes
under high pressure and sintered at high
temperature, they are known as “white
ceramics”.
22.6: Alumina Based Ceramics
 Characteristics
 Chemically stable
 Not very tough
 High abrasion resistance
 Hot hardness
 Poor tensile strength
22.6 Alumina Based Ceramics
 Cermets (Black Ceramics)
 Consist of ceramic materials in a metallic matrix
 Typically: 70% Aluminum Oxide, 30% Titanium
Carbide
 Not widely used due to high cost
Cermets tip
cutting blade.
22.7: Cubic Born Nitride
 Cubic Boron Nitride
 cBN: 2nd hardest material available
 Formerly known as Borazon
 Polycrystalline cubic boron nitride is bonded to a carbide
substrate by sintering under high pressure and temp.
 High thermal conductivity
 Excellent wear resistance
CBn coated cutting inserts
22.8: Silicon-Nitride-Based
Ceramics
 Consist of silicon nitride, aluminum oxide, yttrium
oxide, and titanium carbide.
 Characteristics:
 Tough
 Hot hardness
 Good thermal shock resistance
Sialon sink roll
bearing
22.9: Diamond
 Characteristics:
 Hardest material known today
 High wear resistance
 Low friction
 Maintains a sharp cutting edge
 Produces a very accurate cut and good surface
finish.
 Most effective in light uninterrupted finishing cuts.
22.9: Diamond
 Synthetic diamonds are preferred because natural
diamonds have flaws which at times make them
unpredictable.
Image of a
synthetic Russian
diamond
22.9: Diamond
 Polycrystalline Diamond Tools
 Known as compacts
 May be used as dies for fine wire drawing
 Between .5 and 1 mm of diamond are bonded to a
carbide substrate. (similar to cBN tools)
Image of a
compact
22.10: Whisker-Reinforced Tool
Materials
 Due to the high reactivity of silicon carbide with ferrrous
metals, SiC tools are unsuitable for machining irons and
steels.
 Whisker-reinforced cutting tools include:
 silicon-nitride based tools with silicon-carbide whiskers
 Aluminum-oxide based tools reinforced with 25- 40 % siliconcarbide whiskers
Image of whiskerreinforced ceramics
22.11: Tool Costs and
Reconditioning of Tools
 Factors that affect tool cost:
 Material
 Size
 Shape
 Chip-breaker features
 Quality
22.11: Tool Costs and...
Tooling costs account for approximately 2 to 4 %
of manufacturing costs.
• Typical costs of a
typical .5 inch insert:
• Uncoated carbides:
– $2-10
• Cubic Boron Nitride:
– $60-90
• Diamond-coated carb.:
– $50-60
• Ceramics
– $8-12
• Coated carbides:
– $6-10
• Diamond-tipped insert:
– $90-100
22.11: Tool Costs and...
 Reconditioning tools allows for longer use.
 Resharpening
 Recoating of coated tools
 Tools will only be reconditioned if it is economical
 Often times tools will be recycled
Equipment used in the
reconditioning of cutting tools
22.12: Cutting Fluids
 Purpose of cutting fluids




Reduce friction and wear
Cool cutting zone
Flush chips away from the cutting zone
Protect the machined surface from environmental corrosion
 These factors improve tool life and help make a
better more efficient cut.
Image of cutting
fluid and its
container
22.12: Cutting Fluids
 Lubricants reduce friction
 Coolants effectively reduce high temperatures of tools/
work pieces
 At times, using a cutting fluid may cause the material to
become “curly”, which concentrates the heat closer to
the tip. This is detrimental because it decreases the
tool’s life.
 It is these defects that have turned machinists to “neardry machining” (22.12).
22.12: Cutting Fluids
 Types of cutting fluids:
 Oils: mineral, animal, vegetable, compounded, and synthetic
oils. Only used in operations where temp rise is insignificant.
 Emulsions: mixture of oil and water and additives. Good for
operations where temperature rise is significant.
 Semisynthetics: chemical emulsions containing little mineral
oil diluted in water with additives that reduce size of particles.
 Synthetics: chemicals with additives, diluted in water, without
oil.
Image of a “synthetic”
cutting fluid.
22.12 Cutting Fluids
 Methods of Application
 Flooding: most common; rates of up to 225 L/min for multitooth cutters; poor visibility; 100- 2000 Psi.
 Mist: most effective w/ water based fluids; requires venting
but is popular because of good visibility; similar to using an
aerolsol can; 10- 80 Psi.
 High-pressure systems: use a powerful jet/nozzle to target
the hot area; 800- 5000 Psi; can be used as a chip-breaker to
clear debris away.
 Through the cutting tool system: passages are made in
the tool/ tool handle that allow for a direct route for the
coolant to the hot area.
22.12 Cutting Fluids
 Special Considerations for use of cutting fluids
 Machines need to be washed after fluids have been
used.
 Used cutting fluids may undergo chemical changes.
Settling skimming centrifuging and filtering help to avoid
any bad effects they may cause.
 Cutting fluids containing:
 Sulfur should not be used on Nickel based alloys.
 Chlorine should not be used with Titanium
22.12: Near-dry and Dry Machining
 Introduced in 1990’s to minimize use of metalworking
fluids.
 In these processes, chips are removed from the cutting
zone by application of pressurized air.
 Dry machining:
 Used for turning, milling, and gear cutting on steels, steel alloys,
and cast irons.
 Near-dry cutting:
 The application of a mist of a mixture of water and cutting fluid
(vegetable oil) inserted through the spindle of the machine tool,
85 psi.
22.12: Near-dry and Dry Machining
 Cryogenic Machining:
 Cryogenic gases such as nitrogen and carbon dioxide are used
as a coolant. They are shot through a small nozzle at
temperatures around -200 C; good for tool life, good for the
environment.
Image of a liquid
nitrogen cooling
system.