Download - tool wear&tool life
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ME 557
METAL CUTTING
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TOOL LFE & TOOL WEAR
CHAPTER 4
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Process planning & cutting process
CuttingProcess
Settings:- Speed- Tool orientation- Feed/depth
Inputs:- Material- Energy- Others
Materials:- Tool- Coating- Lubricant
Equipment:- Tool geometry- Machine tool- Fixture
Outputs:- Parts- Chips- Energy- Others
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The basic wear mechanismsinvolved in tool wear:
1. Adhesive wear
2. Abrasive wear
3. Diffusion wear
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The action of one material sliding over another with surface interaction andwelding (adhesion) at localised contact areas.
Adhesion Wear
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Two body abrasive wear occurs when one surface (usually harder than the second) cuts
material away from the second, although this mechanism very often changes to three body abrasion as the wear debris then acts as an abrasive between the two surfaces.
Abrasive wear
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FORMS OF WEAR IN METALCUTTING
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Tool life ends due to:
1. Gradual wear
Creater wear
Flank wear
2. Catostrophic wear
Breaking, chipping
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Tool
Chip
Workpiece
Flank wear
Creater Wear
Face
Flank
Fig. Regions of tool wear in metal cutting
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CREATER WEAR
Under very high speed cutting conditions, creater wear is often thefactor which determines the life of cutting tool : the cratering becomesso severe that the tool edge is weakened and eventually fractures.However, when tools are used under econimical conditions, the wearof the tool on its flank, known as flank wear, is usually controllingfactor.
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FLANK WEAR
Wear on the flank of the cutting tool is caused by friction between the newly machined workpiece surface and the contactarea on the tool flank. Clearly in practice, it would be advisableto regrind the tool before the flank wear enters the last region(innext figure)where rapid breakdown occurs.
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The curve can be divided into three regions:
1. The region AB where the sharp cutting edge is quickly broken down
2. The region BC where sear progresses at a uniform rate
3. The region CD where wear occurs at a gradually increasing rate
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TOOL LIFE CRITERIA
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Figure 4.3 shows that KT is measured at the deepest point of thecreater depth. In zone B, the average wear land width is designatedwith VB, and the max. Wear land width is designated Vbmax.
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Common Criteria For HSS and CERAMIC tools:
1. VB = 0.3 mm2. VB max = 0.6 mm (if flank wear is not regularly
distributed)
Common Criteria For SINTERED CARBIDE tools:
1. VB = 0.3 mm2. VB max = 0.6 mm (if the flank is irregularly
worn)3. KT = 0.06 + 0.3f
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TOOL LIFETool life: Time of cutting during two successive grinding or indexingof the tool.
TAYLOR' s equation:
V : Cutting speedt : Tool lifen: Constantif V2 & t2 are reference cutting speed & tool life,
n
t
t
V
V
1
2
2
1
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If r V V &2 are reference cutting sped & tool life,
i.e.:r
V V 2 r t t 2 then :
n
r
r t
t
V
V
:r
t 1 (min) or 60 (sec)
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r
n
n
r
n
V C C Vt
V Vt min)1(
Cutting spped which results a toollife of 1 min
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Generalized tool life equation:
K a f t V pwmn
:
:
:
:
K
a
f
V
w
cutting speed
feed rate,a c
depth of cut
constant
For uncoated carbide tool:
13.0:
31.0:
30.0:
p
m
n
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Log t
Tool life
Log VCutting speed
Fig. Typical relationship between tool life and cuttingspeeed
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ChipTool
Built-up edge
Workpiece
Fig. Built-up edge protecting tool face
With an unstable built-up edge can increase the tool wear rate by abrading the tool faces.
A stable built-up edge protects the tool surface from wear and performs the cutting actionitself.
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The Effects of Rake Angle On Tool Life :
Cutting efficiency is relatively larger when rake angle is relativelylarger .
In an efficient cutting operation the heat generation will be
relatively low , therfore tool life will be relatively higher due to lowertemperature .
On the other hand, large rake angle reduces the tool strength. Sothere must be an optimum value for rake angle.
When the tool material is brittle , the rake angle must be set to smallvalues, even to negative values .
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A typical relatisionship between rake angle and tool life is shown in figurewhere optimum rake is approximately 14 . Experience shown that the optimumrake is roughly constant for given work and tool materials.
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Workmaterial
High-SpeedSteel,deg Carbide,deg
Cast iron,cast brass 0 0.0
Brass and Bronze 8 3.5
Soft brass andhigh-tensile steel
14 3.5
Mild Steel 27 3.5
Light Alloys 40 13.0
Recommended normal rake for roughing operations
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The Effects of Clearance Angle On Tool Life :
Experience has shown that the width ofthe flank wear land is usually the limiting
factor determining the life of the cuttingtool.
The rate of flank wear-land width is
dependent on the flank clearance.
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neo
o
ne
Cot
NB
VB
NB
VB
Cot NBVB
)()(o
VB : rate of increase of flank wear-land length
o
NB: rate of removal of tool material normal tothe cutting direction
n e
:the working normal clearance
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For small ne values, increase in ne reduces the wear rate; VB and consequentlyincreases tool life
In practice the normal clearance cannot be made too large without running the risk ofweakening the tool edge. Experience show that,
Tool Material Clearance Angle(deg)
HSS 8Carbides 5
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Tool Wear Mechanism Machining Various Hardened Steels(122 m/min., 0.12 mm/rev.)
Top view- 6 min machining
Side view - 6 min machining
4340 Steel(58HRc)
Top view- 12 min machining
Side view 12 min machining
Top view- 21 min machining
Side view 21 min machining
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52100 Steel(60-62HRc)
Top view- 6 minutesmachining time
Side view- 6 minutesmachining time
Top view- 12 minutesmachining time
Side view 12 minutesmachining time
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MACHINABILITY
The term machinability is often applied to workmaterials to describe their machining properties.
Clearly with finishing processes, tool wear and surface finish are the most important considerations;with roughing operations, tool wear and powerconsumption are important.
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Machinability may be described in terms of:
1. Tool life
creater wear
flank wear
2. Ease of metal removal
power required
Specific cutting energy
Discontinuous chip
Cutting forces
3. Workpiece quality
Surface qualityDimensional accuracy
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Tool life : Metals which can be cut without rapid tool wearare generally thought of as being quite machinable.
Tool forces and power consumption: Tool forces areimportant because the concept of machinability as the easewith which the metal is cut. Cost per part depends on
power consumption.
Chip form: There have been machinability ratings based
on the type of chip that is formed during the machiningoperation .
I dealchipsdevolopedfromavar ietyofcommonmater ials.
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I deal chips devoloped from a var iety of common mater ials.
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F actors aff ecting the machi nabil i ty of metals:
1. Tensi le strength : Increased yield strength implied higher
cutting forces during machining operations.2. Strain hardening exponent: Less strain hardness. Easy
machining.
3. Ducti l i ty4. H ardness: The lower the hardness, the higher the speed.
5. Tougness
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6. Abrasive inclusions in the mater ial7. Thermal conductivity: Metals which exhibit low
thermal conductivities will not dissipate heatfreely,therefore the tool and workpiece becomeextremely hot. This excess heat accelarates wear.
8. H eat capacity
9. Density
CONCET OF MACHINIBILITY
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CONCET OF MACHINIBILITY
CUTTING TOOL MATERIALS
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Desired properties for cutting tool materials:
1. High strength (or hardness ) at high temperatures
2. High toughness (large resistance against impact forces )
3. Low adhesion (to prevent wear and diffusion )
4. Low coefficient of friction
5. Low cost
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PRINCIPLE CUTTING TOOL MATERIAL TOOLS
1. High Carbon Steel:
heat treatable
used in hardened state
looses its hardnes over 350 C
suitable for machining soft materials;like, wood, plastic,etc
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2. High Speed Steel; HSS.
carbon steel with alloying elements;like tungsten(W),chromium(Cr), vandium(Va), molibdenum(Mo), cobalt(Co),etc.
heat treatableretains its sharpnes up to 650 C
tough material
3 C N F All
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3. Cast Non-Ferrous Alloys:
non,-ferrous alloys containing, mainly, chronium, cobalt,tungsten and small amounts of tantalum, molibdenum and
boron.
can only be produced by casting then ground for finalshape
can work at 950C without loosing its hardness
hard and non-heat treatable materials
good resistance to cratering
can resist to shock loads better than carbides
Rank midway between HSS and carbides
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4. Carbides:
Main material is tungsten carbide(WC)
Manufactured by powder metallurgy techniques
hard
Extremely high compressive strength
Retain cutting edge up to 1100 C
Rake angle must be small or negative
Main work horse of machining industry
5 Ceramic Tools:
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5. Ceramic Tools:
Aluminum oxide powder along with titanium, magnesium or chromiumoxide
Manufactured by powder metallurgy techniques
Hard
Brittle
Extremely high compressive strength
Softening point is above 1100 C
Lack of affinity against work material
High resistance to ctratering
Requires rigid and new machine tools
Use of coolant may cause thermal cracking
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6. Diamonds
Hardest material
Extremely brittle
Can not take shock loads
Extremely long tool life
Used to machine either very hard materials, like tool steels, etc., or soft
materials like, aluminum, plastics, etc.Cutting speeds may be as high as 25 m/sec
Used as dressing of grinding wheels
Expensive
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TRENDS
CNC machines are being tooled up approximately 60% coated carbides . Other 40%will be divided among ceramics and cermets .
Two types of coated cutting tool are used:1- TiN over TiC (two layer)2- Al2O3 coating with an underlayer of TiC
For steel machining approximately 80% TiN coating and 20% Al2O3 coating areused. For cast iron machining 90% Al2O3 coating and 10% TiN coating are used.
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MECHANICAL TOOL HOLDERS
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