toolmaterialsandtoolwear-100930105217-phpapp01

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Tool Materials

and

Tool Failure Mechanisms


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Desirable properties of tool material

SAM, VJTI


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Development of Cutting Tool Materials


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Property Carbonand low

to

mediu

m alloy

steels

HSS Cast

Cobalt

alloys

Cemente

d carbide

Coated

carbide

Ceramics Poly -

crystallin

e

CBN

Diamond

Depth of

cut

Light to

medium

Light to

heavy

Light to

heavy

Light to

heavy

Light to

heavy

Light to

heavy

Light to

heavy

Very light

for single

crystal

Finish

Obtainable

Rough Rough Rough Good Good Very good Very good excellent

Method of

processing

Wrought Wrought,

cast, HIP,

sintering

Cast, HIP

and

sintering

Cold

pressing

and

sintering

CVD Cold

pressing

and

sintering

High

pressure

and high

temp.

sintering

High

pressure

and high

temp

sintering

Fabrication Machini

ng andgrinding

Machinin

g andgrinding

Grinding Grinding Grinding Grinding Grinding

andpolishing

Grinding

andpolishing

Characteristics of Tool Materials


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Hardness and condition of the workpiece material

Operations to be performed

Amount of stock to be removed

Accuracy and finish requirements

Type, capability, and condition of the machine tool to be used

Rigidity of the tool and workpiece

Factors affecting selection of Tool Materials

SAM, VJTI


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Production requirements influencing the speeds and feeds

selected

Operating conditions such as cutting forces and temperatures

Tool cost per part machined, including initial tool cost, grinding

cost, tool life, frequency of regrinding or replacement, and

labor costthe most economical tool is not necessarily the one

providing the longest life, or the one having the lowest initial

cost

Factors affecting selection of Tool Materials

SAM, VJTI

No Tool Material

Satisfies All These

Criterion


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High alloy steel

They are either molybdenum or tungsten based but

necessarily contains 4% chromium

High Speed Steel

SAM, VJTI

M = Molybdenum

T = Tungsten

M >40 = Super HSS materials; capable of treating to high hardness


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Advantages of HSS

Heat treated to high hardness within the range of Rc 6368

M40 series of HSSs is normally capable of being hardened to Rc70, but

a maximum of Rc68 is recommended to avoid brittleness

HSSs also possess a high level of wear resistance

HSS tools possess an adequate degree of impact toughness and are

more capable of taking the shock loading of interrupted cuts than

carbide tools

When HSSs are in the annealed state they can be fabricated, hot

worked, machined, ground, and the like, to produce the cutting tool

shape

Toughness in HSSs can be increased by adjusting the chemistry to a

lower carbon level

High Speed Steel

SAM, VJTI


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Limitations of HSS Tendency of the carbide to agglomerate in the centers of large

ingots Improved properties and grindability are important advantages of

powdered metal HSSs

hardness of these materials falls off rapidly when machining

temperatures exceed about 538593C

use of lower cutting speeds than those used with carbides,

ceramics

High Speed Steel

SAM, VJTI


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Applications of HSS Most drills, reamers, taps, thread chasers, end mills, and gear

Cutting tools are made from HSSs HSS tools are usually preferred for operations performed at low

cutting speeds and on older, less

Rigid, low-horsepower machine tools

High Speed Steel

SAM, VJTI

Powder metallurgy HSS Uniform structure with fine carbide particles and no segregation

Lower in cost because of reduced material, labor, and machining

costs, compared to those made from wrought materials

Near net shape more design flexibility

Applications : Milling cutters


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Most carbide grades are made up of tungsten carbide with a

cobalt binder

Advantages of WC

Hardness of softest WC is higher than hardened steel

High hot hardness

Grades of WC

Straight WC

Co as a binder

Best suited for material having abrasion as a primary tool wear e.g.

cast iron, non ferrous materials, non metalsComplex WC

Comprises carbides : TiC, TaC, NbC with Co as a binder

ferrous materials, non metals

Cemented Tungsten Carbide

SAM, VJTI


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Tungsten carbide is extremely hard and offers the excellent

resistance to abrasion wear

The most significant benefit of TiC is a reduction in the

tendency of the tool to fail by cratering.

The most significant contribution of TaC is that it increases the

hot hardness of the tool, which in turn reduces thermaldeformation

Effect of Co as a binder

Co is more sensitive to heat, abrasion and welding

The more cobalt present, the softer the tool, making it more

sensitive to thermal deformation, abrasive wear and chip

welding

Cobalt is stronger than carbide. Therefore, more cobaltimproves the tool strength and resistance to shock



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Classification system

ISO classification number ranges from 05 to 50 : e.g. P20, K35, M40;05 is most wear resistance whereas 50 is most fracture resistance

Coated carbide tools is the most significant advance in cutting tool

materials since the development of WC tooling

Various single and multiple coatings of carbides and nitrides oftitanium, hafnium, and zirconium and coatings of oxides of

aluminum and zirconium, as well as improved substrates better

suited for coating, have been developed to increase the range of

applications for coated carbide inserts.


C- Classification

C1 to C4 for Cast iron

C5 to C8 for Steel

ISO- Classification

P = Stainless Steel

M = Steel

K = Cast Iron


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Ceramics are primarily aluminum oxides

Inconsistent and unsatisfactory results during initial periodsof development

Improvements : better control of microstructure (primarily

in grain size refinement) and density, improved processing,

the use of additives, the development of compositematerials, and better grinding and edge preparation

methods. Tools made from these materials are now stronger,

more uniform, and higher in quality

Types of ceramics

Plain ceramics, which are highly pure (99 percent or more)

and contain only minor amounts of secondary oxides

(produced by powder metallurgy)

Ceramics


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Types of ceramics

Plain ceramics, which are highly pure (99 percent or more)and contain only minor amounts of secondary oxides

(produced by powder metallurgy)

Composite ceramics : are Al203-based materials containing

1530 percent or more titanium carbide (TiC) and/or other

alloying ingredients

Ceramics


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Advantages

Increased productivity: Ceramic cutting tools are operated athigher cutting speeds than tungsten carbide tools

Good hot hardness, low coefficient of friction, high wear

resistance, chemical inertness, and low coefficient of thermal

conductivity

Most of the heat generated during cutting is carried away in

the chips, resulting in less heat buildup in the workpiece,

insert and toolholder

Better size control by less tool wear

Machining of many hard materials

Ceramics


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Limitations

Brittle than carbides

Less mechanical and thermal shock resistance

Less interchangeability with the carbide tool holders

Applications

High speed machining of steel and cast iron requiring

continuous machining

Most suitable for machining of chemically active materials

Face milling and turning applications

Ceramics


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Best suited for precision machining with very high surface finish and toincrease productivity by reducing downtimes

Diamond is the cubic crystalline form of carbon that is produced in

various sizes under high heat and pressure. Natural, mined single-crystal

stones of the industrial type used for cutting tools are cut (sawed,

cleaved, or lapped) to produce the cutting-edge geometry required for

the application.

Advantages

Hardest material known. Indentation hardness is five times than carbide.

Extreme hardness and abrasion resistance can result retaining their

cutting edges virtually unchanged throughout most of their useful lives

Because of the diamondschemical inertness, low coefficient of friction,

and smoothness, chips do not adhere to its surface or form built-up

edges when nonferrous and nonmetallic materials are machined.

Single crystal and polycrystalline diamonds


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Super abrasive crystal that is second in hardness and abrasion

resistance only to diamond

CBN crystals are used most commonly in super abrasive wheels for

precision grinding of steels and super alloys

Advantages

Greater heat resistance than diamond tools

High level of chemical inertness

Compacted CBN tools are suitable, unlike diamond tools, for the high

speed machining of tool and alloy steels with hardness to Rc70, steel

forgings and Ni-hard or chilled cast irons with hardness from R c4568,

surface-hardened parts, and nickel or cobalt-based super alloys

They have also been used successfully for machining powdered metals,

plastics, and graphite.

Cubic Boron Nitride


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Tool Wear


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Temperature in Primary and Secondary Machining

Regions


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Cubic Boron Nitride

Heat

Control all the mechanisms

of tool failure so tool life is

limited only by abrasion

wear


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1. Abrasive wear

2. Built-up edge Rake surface

Flank surface

3. Thermal/mechanical cracking/chipping

4. Cratering5. Thermal deformation

6. Chipping

Mechanical

Thermal expansion

7. Notching

8. Fracture

Tool Failure Mechanisms


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Comparison of Catastrophic and Progressive Failure

Catastrophic Failure Progressive Wear

Caused by dynamic changes

Intermittent cutting

Ramping

Sudden changes in tool load

In-homogeneity (hard particles or

voids) in the raw material

Micro-cracks in tool during HT Temp gradient due to non-uniform

coolant flow

Caused by gradual wear of the tool

due to

Adhesion,

Abrasion

Diffusion.

Undesirable since

Tool is lost for ever

Damage the part or injure theoperator

Unpredictable and hence

corrective action is not possible

Desirable since

The tool can be reused by

regrinding or indexing/

Changing the bit

Predictable and hence corrective

action is possible

Closed loop control system used to

prevent tool failure

Time bound regrinding is suggested

approach


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Comparison of Crater and Flank Wear

Crater Wear Flank WearOccurs on the rake face Occurs on the flank face

Highly sensitive to temperature Not as much sensitive to

temp as crater wear

Undesirable wear Most desirable wear

Used as failure criteria for

brittle tools such as WC and

Al2O3 tools

Used as failure criteria for

tough tools such as HSS


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Locations of Tool Wear


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Abrasive wear occurs as a result of the

interaction between the workpiece and

the cutting edge.

The width of the wear land is

determined by the amount of contact

between the cutting edge and the

workpiece.

Abrasive Wear (Abrasion)

Brea

kInPeriod

ConstantPeriod

R

apidfailure


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Cratering (Chemical Wear)

The chemical properties of the tool-material and the affinity of the tool-material to the workpiece material determine the development of the

crater wear mechanism

Hardness of the tool-material does not have much affect on the process.

The metallurgical relationship between the materials determines theamount of crater wear.

Tungsten carbide and steel have an affinity to each other

The mechanism is very temperature-dependent, making it greatest at

high cutting speeds. Atomic interchange takes place with a two-waytransfer of ferrite from the steel into the tool. Carbon also diffuses into

the chip.

Heat Related Tool Failure Mechanisms


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Built-up Edge (Adhesion)

It occurs mainly at low machining temperatures on the chip face of

the tool. It can take place with long chipping and short-chipping

workpiece materialssteel and aluminum.

This mechanism often leads to the formation of a built-up edge

between the chip and edge.

It is common for the build-up edge to shear off and then to reform.

At certain temperature ranges, affinity between tool and workpiece

material and the load from cutting forces combine to create theadhesion wear mechanism.

Machining work-hardening materials, such as austenitic stainless

steel, this wear mechanism can lead to rapid build-up at the depth of

cut line resulting in notching as the failure mode.



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Built-up Edge (Adhesion)

Increased surface speeds, proper application of coolant, and tool

coatings are effective control actions for built-up edge

Thermal Cracking (Fatigue wear)

Thermal cracking is a result of thermo mechanical actions

Temperature fluctuations plus the loading and unloading of cutting

forces lead to cracking and breaking of the cutting edge

Carbide and ceramics are relatively poor conductors of heat which

leads to fatigue wear

Thermal Deformation

As the cutting edge loses its hot hardness the forces created by the feed

rate cause the cutting edge to deform



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Chipping (Mechanical)

Small chipping of tool material

Cutting force should be less than shearing force. Chipping is

larger on flank surface than on a face

Mechanical Failure Mechanisms

Rake Surface Flank Surface


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Insert Fracture

When the edge strength of an insert is exceeded by the forces

of the cutting process the inevitable result is the catastrophic

failure calledfracture.

Excessive flank wear land development, shock loading due tointerrupted cutting, improper grade selection or improper

insert size selection are the most frequently encountered

causes of insert fracture

Mechanical Failure Mechanisms


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Property

Carbon and

low to

medium

alloy steels

H

S

S

Cast

Cobalt

alloys

Cemented

carbide

Coated

carbide

Ceramics Poly -

crystalline

CBN

Diamond

Hot

hardness

Toughness

Wear

resistanceChipping

resistance

Cutting

speed

Thermal

shock

resistance

Total material

cost

Documents

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