Tool Materials and

Tool Failure Mechanisms

Desirable properties of tool material

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Hot hardness– High hot hardness means higher

speeds and feed rates (higher production rates and lower costs).

Toughness and Impact strength– Tool does not chip or fracture

Thermal shock resistanceWear resistance– Tool does not have to be replaced

as oftenChemical stability and inertness– To minimize adverse reactions

Desirable properties of tool material

Development of Cutting Tool Materials

Property Carbon and low to medium alloy steels

HSS Cast Cobalt alloys

Cemented carbide

Coated carbide

Ceramics Poly -crystalline 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 Machining and grinding

Machining and grinding

Grinding Grinding Grinding Grinding Grinding and polishing

Grinding and polishing

Characteristics of Tool Materials

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

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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 cost—the 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

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No Tool Material Satisfies All These

Criterion

High alloy steelThey are either molybdenum or tungsten based but necessarily contains 4% chromium

High Speed Steel

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

Advantages of HSS

Heat treated to high hardness within the range of Rc 63–68

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

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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 538–593°C use of lower cutting speeds than those used with carbides,

ceramics

High Speed Steel

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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

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Powder metallurgy HSSUniform structure with fine carbide particles and no segregationLower in cost because of reduced material, labor, and machining

costs, compared to those made from wrought materialsNear net shapemore design flexibilityApplications : Milling cutters

Most carbide grades are made up of tungsten carbide with a cobalt binderAdvantages of WC

Hardness of softest WC is higher than hardened steel High hot hardness

Grades of WCStraight WC Co as a binder Best suited for material having abrasion as a primary tool wear e.g.

cast iron, non ferrous materials, non metals

Complex WC Comprises carbides : TiC, TaC, NbC with Co as a binder ferrous materials, non metals

Cemented Tungsten Carbide

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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 thermal deformation

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 cobalt improves the tool strength and resistance to shock

Cemented Tungsten Carbide

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 of titanium, 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.

Cemented Tungsten Carbide

C- ClassificationC1 to C4 for Cast iron

C5 to C8 for Steel

ISO- ClassificationP = Stainless Steel

M = SteelK = Cast Iron

Ceramics are primarily aluminum oxides

Inconsistent and unsatisfactory results during initial periods of development

Improvements : better control of microstructure (primarily in grain size refinement) and density, improved processing, the use of additives, the development of composite materials, 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

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 15–30 percent or more titanium carbide (TiC) and/or other alloying ingredients

Ceramics

Advantages

Increased productivity: Ceramic cutting tools are operated at higher 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

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

Best suited for precision machining with very high surface finish and to increase 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 diamond’s chemical 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

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 Rc45–68, 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

Tool Wear

Temperature in Primary and Secondary Machining Regions

Cubic Boron Nitride

Heat

Control all the mechanisms of tool failure so tool life is limited only by abrasion wear

1. Abrasive wear2. Built-up edge

• Rake surface• Flank surface

3. Thermal/mechanical cracking/chipping4. Cratering 5. Thermal deformation6. Chipping

• Mechanical• Thermal expansion

7. Notching8. Fracture

Tool Failure Mechanisms

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 the

operator 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

Comparison of Crater and Flank Wear

Crater Wear Flank WearOccurs on the rake face Occurs on the flank faceHighly sensitive to temperature Not as much sensitive to

temp as crater wear

Undesirable wear Most desirable wearUsed as failure criteria forbrittle tools such as WC andAl2O3 tools

Used as failure criteria fortough tools such as HSS

Locations of Tool Wear

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)B

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In

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Co

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Per

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Rap

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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 the amount 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-way transfer of ferrite from the steel into the tool. Carbon also diffuses into the chip.

Heat Related Tool Failure Mechanisms

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 materials—steel 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 the adhesion 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.

Heat Related Tool Failure Mechanisms

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

Heat Related Tool Failure Mechanisms

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

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 called fracture. Excessive flank wear land development, shock loading due to interrupted cutting, improper grade selection or improper insert size selection are the most frequently encountered causes of insert fracture

Mechanical Failure Mechanisms

Heat Related Tool Failure Mechanisms

Property

Carbon and low to medium alloy steels

HSS

Cast Cobalt alloys

Cemented carbide

Coated carbide

Ceramics Poly -crystalline CBN

Diamond

Hot hardness

Toughness

Wear resistance

Chipping resistance

Cutting speed

Thermal shock resistance

Total material cost