topic 3 material removal process cutting technology-2
TRANSCRIPT
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CUTTING TOOL TECHNOLOGY
1. Tool Life
2. Tool Materials
3. Tool Geometry
4. Cutting Fluids
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Cutting Tool Technology
Two principal aspects:
1. Tool materialdeveloping materials that can
withstand forces, temperatures, wearing
2. Tool geometry
optimizing the geometry of thecutting tool for the material and for the given
operation
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1- Tool LifeThree Modes of Tool Failure
1. Fracture failure
Cutting force becomes excessive and/or
dynamic, leading to brittle fracture
2. Temperature failure
Cutting temperature is too high for the tool
material
3. Gradual wear
Gradual wearing of the cutting tool
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Preferred Mode: Gradual Wear
Fracture and temperature failures are premature
failures
Gradual wear is preferred because it leads to the
longest possible use of the tool Gradual wear occurs at two locations on a tool:
Crater wearoccurs on top rake face (cause
by chip sliding on the surface)
Flank wear
occurs on flank (side of tool)(cause by sliding on the workpiece surface)
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Figure 23.1 Diagram of worn cutting tool, showing the principal
locations and types of wear that occur.
Tool Wear
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Figure 23.2 Crater wear, (above), and flank wear (right) on a cementedcarbide tool, as seen through a toolmaker's microscope (photos by K. C.Keefe, Manufacturing Technology Lab, Lehigh University).
Tool Wear
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FLANK WEAR and BUE
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CRATER WEAR
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FRACTURE OF TOOL
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1. Abrasioncause by particles
2. Adhesion force and high pressure,
temperature, welding occurs.3. Diffussionexchange atoms effect
hardness
4. Chemical reactionshigh
temperatures, oxidation
5. Plastic deformationcutting edge
becoming soft
The mechanism that cause wear at thetool chip and tool work
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Figure 23.3 Tool wear as a function of cutting time. Flank
wear (FW) is used here as the measure of tool wear.
Crater wear follows a similar growth curve.
Tool Wear vs. Time
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BUILT UP EDE
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Figure 23.4 Effect of cutting speed on tool flank wear (FW) for three
cutting speeds, using a tool life criterion of 0.50 mm flank wear.
Effect of Cutting Speed
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Figure 23.5 Natural log-log plot of cutting speed Vs tool life.
Tool Life vs. Cutting Speed
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Taylor Tool Life Equation
Relationship is credited to F. W. Taylor
CvTn
where v= cutting speed; T= tool life; and nand C
are parameters that depend on feed, depth of cut,
work material, tooling material, and the tool life
criterion used
nis the slope of the plot
Cis the intercept on the speed axis at one minute
tool life
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Tool Life Criteria in Production
1. Complete failure of cutting edge
2. Visual inspection of flank wear (or craterwear) by the machine operator
3. Fingernail test across cutting edge4. Changes in sound emitted from operation
5. Chips become ribbon-like, stringy, anddifficult to dispose of
6. Degradation of surface finish7. Increased power
8. Workpiece count
9. Cumulative cutting time
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2- Tool Materials
Tool failure modes identify the important
properties that a tool material should possess:
Toughness - to avoid fracture failure
Hot hardness - ability to retain hardness athigh temperatures
Wear resistance - hardness is the most
important property to resist abrasive wear
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Figure 23.6 Typical hot hardness relationships for selected toolmaterials. Plain carbon steel shows a rapid loss of hardness astemperature increases. High speed steel is substantially better, while
cemented carbides and ceramics are significantly harder at elevatedtemperatures.
Hot Hardness
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Typical Values of nand C
Tool material n C (m/min) C (ft/min)
High speed steel:
Non-steel work 0.125 120 350
Steel work 0.125 70 200
Cemented carbide
Non-steel work 0.25 900 2700Steel work 0.25 500 1500
Ceramic
Steel work 0.6 3000 10,000
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High Speed Steel (HSS)
Highly alloyed tool steel capable of maintaining
hardness at elevated temperatures better than
high carbon and low alloy steels
One of the most important cutting tool
materials
Especially suited to applications involving
complicated tool geometries, such as drills,
taps, milling cutters, and broaches
Two basic types (AISI)
1. Tungsten-type, designated T- grades
2. Molybdenum-type, designated M-grades
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High Speed Steel Composition
Typical alloying ingredients:
Tungsten and/or Molybdenum
Chromium and Vanadium Carbon, of course
Cobalt in some grades
Typical composition (Grade T1):
18% W, 4% Cr, 1% V, and 0.9% C
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Cemented Carbides
Class of hard tool material based on tungsten
carbide (WC) using powder metallurgy techniques
with cobalt (Co) as the binder
Two basic types:
1. Non-steel cutting grades - only WC-Co
2. Steel cutting grades - TiC and TaC added to
WC-Co
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Cemented Carbides GeneralProperties
High compressive strength but low-to-moderate
tensile strength
High hardness (90 to 95 HRA)
Good hot hardness
Good wear resistance
High thermal conductivity
High elastic modulus - 600 x 103 MPa (90 x 106
lb/in2)
Toughness lower than high speed steel
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Non-steel Cutting Carbide Grades
Used for nonferrous metals and gray cast iron
Properties determined by grain size and cobalt
content
As grain size increases, hardness and hothardness decrease, but toughness increases
As cobalt content increases, toughness
improves at the expense of hardness and wear
resistance
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Steel Cutting Carbide Grades
Used for low carbon, stainless, andother alloy steels
TiC and/or TaC are substituted for some
of the WC Composition increases crater wear
resistance for steel cutting
But adversely affects flank wear
resistance for non-steel cuttingapplications
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Cermets
Combinations of TiC, TiN, and titanium carbonitride
(TiCN), with nickel and/or molybdenum as
binders.
Some chemistries are more complex Applications: high speed finishing and semi-
finishing of steels, stainless steels, and cast irons
Higher speeds and lower feeds than
steel-cutting carbide grades Better finish achieved, often eliminating need
for grinding
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Coated Carbides
Cemented carbide insert coated with one or
more thin layers of wear resistant materials,
such as TiC, TiN, and/orAl2O3
Coating applied by chemical vapordeposition or physical vapor deposition
Coating thickness = 2.5 - 13 m (0.0001 to
0.0005 in)
Applications: cast irons and steels in turningand milling operations
Best applied at high speeds where dynamic
force and thermal shock are minimal
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Coated Carbide Tool
Photomicrographof cross section ofmultiple coatingson cemented
carbide tool (photocourtesy ofKennametal Inc.)
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Ceramics
Primarily fine-grained Al2O3, pressed and sintered at
high pressures and temperatures into insert form
with no binder
Applications: high speed turning of cast iron andsteel
Not recommended for heavy interrupted cuts (e.g.
rough milling) due to low toughness
Al2O3 also widely used as an abrasive in grinding
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Synthetic Diamonds
Sintered polycrystalline diamond (SPD) -
fabricated by sintering very fine-grained
diamond crystals under high temperatures
and pressures into desired shape with littleor no binder
Usually applied as coating (0.5 mm thick)
on WC-Co insert
Applications: high speed machining ofnonferrous metals and abrasive nonmetals
such as fiberglass, graphite, and wood
Not for steel cutting
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Cubic Boron Nitride
Next to diamond, cubic boron nitride (cBN) is
hardest material known
Fabrication into cutting tool inserts same as SPD:
coatings on WC-Co inserts Applications: machining steel and nickel-based
alloys
SPD and cBN tools are expensive
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Tool Geometry
Two categories:
Single point tools
Used for turning, boring, shaping, and
planing Multiple cutting edge tools
Used for drilling, reaming, tapping, milling,
broaching, and sawing
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Figure 23.7 (a) Sevenelements of single-pointtool geometry; and (b) the
tool signature conventionthat defines the sevenelements.
Single-Point Tool Geometry
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Figure 23.9 Three ways of holding and presenting the cutting edgefor a single-point tool: (a) solid tool, typical of HSS; (b) brazed
insert, one way of holding a cemented carbide insert; and (c)
mechanically clamped insert, used for cemented carbides,
ceramics, and other very hard tool materials.
Holding the Tool
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Figure 23.10 Common insert shapes: (a) round, (b) square, (c)rhombus with two 80 point angles, (d) hexagon with three 80point angles, (e) triangle (equilateral), (f) rhombus with two 55point angles, (g) rhombus with two 35 point angles. Also shownare typical features of the geometry.
Common Insert Shapes
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A collection of metal cutting
inserts made of various materials
(photo courtesy of Kennametal
Inc.).
Common Insert Shapes
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By far the most common cutting tools forhole-making
Usually made of high speed steel
Figure 23.12 Standard geometry of a twist drill.
Twist Drills
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Twist Drill Operation
Rotation and feeding of drill bit result in relative
motion between cutting edges and workpiece to
form the chips
Cutting speed varies along cutting edges as afunction of distance from axis of rotation
Relative velocity at drill point is zero, so no
cutting takes place
A large thrust force is required to drive the drillforward into hole
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Twist Drill Operation - Problems
Chip removal
Flutes must provide sufficient clearance to
allow chips to be extracted from bottom of hole
during the cutting operation Friction makes matters worse
Rubbing between outside diameter of drill bit
and newly formed hole
Delivery of cutting fluid to drill point to reducefriction and heat is difficult because chips are
flowing in opposite direction
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Milling Cutters
Principal types:
Plain milling cutter
Face milling cutter
End milling cutter
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Used for peripheral or slab milling
Figure 23.13 Toolgeometry elements ofan 18-tooth plain millingcutter
Plain Milling Cutter
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Teeth cut on side and periphery of the cutter
Figure 23.14 Tool geometry elements of a four-tooth facemilling cutter: (a) side view and (b) bottom view.
Face Milling Cutter
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End Milling Cutter
Looks like a drill bit but designed for primary
cutting with its peripheral teeth
Applications:
Face milling Profile milling and pocketing
Cutting slots
Engraving
Surface contouring
Die sinking
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Cutting Fluids
Any liquid or gas applied directly to machining
operation to improve cutting performance
Two main problems addressed by cutting fluids:
1. Heat generation at shear and friction zones2. Friction at tool-chip and tool-work interfaces
Other functions and benefits:
Wash away chips (e.g., grinding and milling)
Reduce temperature of workpart for easier
handling
Improve dimensional stability of workpart
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Cutting Fluid Functions
Cutting fluids can be classified according tofunction:
Coolants - designed to reduce effects of
heat in machining Lubricants - designed to reduce tool-chip
and tool-work friction
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Coolants
Water used as base in coolant-type cutting fluids
Most effective at high cutting speeds where heat
generation and high temperatures are problems
Most effective on tool materials that are mostsusceptible to temperature failures (e.g., HSS)
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Lubricants
Usually oil-based fluids
Most effective at lower cuttingspeeds
Also reduce temperature in theoperation
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Cutting Fluid Contamination
Tramp oil (machine oil, hydraulic fluid, etc.)
Garbage (cigarette butts, food, etc.)
Small chips
Molds, fungi, and bacteria
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Dealing with Cutting Fluid
Contamination
Replace cutting fluid at regular and frequent
intervals
Use filtration system to continuously or
periodically clean the fluid Dry machining
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Cutting Fluid Filtration
Advantages:
Prolong cutting fluid life between changes
Reduce fluid disposal cost
Cleaner fluids reduce health hazards
Lower machine tool maintenance
Longer tool life
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Dry Machining
No cutting fluid is used
Avoids problems of cutting fluid contamination,
disposal, and filtration
Problems with dry machining: Overheating of tool
Operating at lower cutting speeds and
production rates to prolong tool life
Absence of chip removal benefits of cuttingfluids in grinding and milling
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CUTTING FLUIDS and FUNCTIONS
To cool the tool, work materials & conduct the heat
generated in cutting away from the cutting zone,
To decrease the adhesion between chip and tool,
provide lower friction and wear and avoid built up
edge,
Surface roughness produced by using coolant is
superior,
To wash away the chips,
Bright surface is maintained and acts as a coating,
To decrease the tool wear and increase the tool life,
It improves machinabilityease of machining,
Prevent expansion of work materials.
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CLASSIFICATION OF CUTTINGFLUID
Solids, liquids and gases.
Graphite, stick waxes and Modisulphide.
Liquids and gases
Water base and mineral oil base.
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LIQUID CUTTING FLUIDS
Water solutions, oil base cuttingfluids, mineral oils, straight fatty oil,mineral lard oil etc.
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SELECTION OF CUTTING FLUID
a). Viscosity- If the viscosity is too low, the oil
may be too volute and will break smoke.
Viscosity is too high, consumption is too high.
Heavy oil will keep dirt and chips insuspension for longer time,
b). Cost of cutting fluid,
c). Chemical action, d). Easily available.