tool wear, tool life [compatibility mode]

7/31/2019 Tool Wear, Tool Life [Compatibility Mode]

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Tool wear

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Topics

Tool wear Mechanism

Tool life equation

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Reasons for tool wear

Cutting involves high stresses, highrelative velocity between tool andchip/workpiece, and high temperatures ofup to 1000C.

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Failure Mechanism

Adhesion wear

Abrasion Wear

Diffusion wear

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Adhesion wear

Adhesion wear: small fractured pieces ofthe workpiece may get welded to the tool

surface due to high temperatures;eventually, they break off, tearing smallparts of the tool with them.

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Abrasion

Abrasion: Hard particles and microscopicvariations on the bottom surface of the

chips constantly rub against the toolsurface, causing abrasion.

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Diffusion wear

Diffusion wear: at high temperatures,some atoms in the metal crystals of thetools micro-structure will diffuse across tothe chip; the rate of diffusion is small, but

increases exponentially with the rise intemperature. This reduces the fracturestrength of the crystals, and makes thetool more likely to fracture.

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Diffusion wear

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Tool Wear

Crater wear

Flank wear

Corner wear

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Crater Wear

consists of a concavesection on the toolface formed by theaction of the chipsliding on the surface.

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Crater Wear

Crater wear affectsthe mechanics of theprocess increasingthe actual rake angleof the cutting tool and

consequently, makingcutting easier.

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At the same time, the crater wear weakensthe tool wedge and increases the

possibility for tool breakage. In general,crater wear is of a relatively small concern.

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Crater Wear

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Crater Wear

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Crater Wear Measurement

Crater wear is measured by the maximum depthof the crater, or depression formed in tool face.

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Flank Wear

occurs on the tool flank as a result offriction between the machined surface of

the workpiece and the tool flank.

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Flank Wear

Flank wear appears inthe form of so-called

wear land and ismeasured by thewidth of this wearland, VB.

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Flank Wear

Flank wear affects to the great extend themechanics of cutting. Cutting forcesincrease significantly with flank wear.

If the amount of flank wear exceeds some

critical value (VB > 0.5~0.6 mm), theexcessive cutting force may cause toolfailure.

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Flank Wear

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Flank Wear

Flank wear is measured by the average width ofthe flank wear zone

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Corner wear

occurs on the tool corner.Can be considered as a part of the wear land

and respectively flank wear since there is nodistinguished boundary between the corner wearand flank wear land.

We consider corner wear as a separate weartype because of its importance for the precisionof machining. Corner wear actually shortens thecutting tool thus increasing gradually thedimension of machined surface and introducinga significant dimensional error in machining,which can reach values of about 0.03~0.05 mm.

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Corner wear

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Tool Life

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Tool Life

Tool wear is a time dependent process. The most important wear type from the process

point of view is the flank wear, therefore theparameter which has to be controlled is thewidth of flank wear land, VB.

This parameter must not exceed an initially setsafe limit, which is about 0.4 mm for carbidecutting tools. The safe limit is referred to asallowable wear land (wear criterion), VBk.

The cutting time required for the cutting tool todevelop a flank wear land of width VBk is calledtool life, T, a fundamental parameter inmachining.

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wear curve

The general relationshipof VB versus cutting timeis shown in the figure

Although the wear curveshown is for flank wear, asimilar relationship occurfor other wear types. Thefigure shows also how todefine the tool life T for agiven wear criterion VBk.

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Effect of cutting velocity

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Tool Life Equation

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F.W. Taylors Contribution

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Tool Life Equation (Taylors Equation)

VTn

= C

Where:

V= Cutting speedT = Tool Life

n, C= Taylor constant(empirical)

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Extended Taylors Equation

The tool life also depends to a great extenton the depth of cut d and feed rate. Henceincorporating these factors in tool lifeequation:

V Tnd

mf

x=C

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Tool life Criteria

Chipping or finer cracks developing at the cutting edge

Total destruction of the cutting tool

Wear land size

Crater depth, width or other parameters

Combination of above two.

Volume of material worn off the tool Limiting value of change in component size

Limiting value of surface finished produced on thecomponent

Cutting forces and power required

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tool wear, tool life [compatibility mode]

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