tool wear&tool life

8/10/2019 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)


Side view 12 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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tool wear&tool life

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