DocumentL3

Tool: geometry, wear and life

Tool Geometry

L3: Tool geometry, wear and life

Back rake angle: angle between the face of the tool and a line parallel to the base of the shank in a plane parallel to the side cutting edge. Side rake angle: angle by which the face of the tool is inclined side ways

Functions of Rake angle: It allows the chip to flow in convenient

direction. It reduces the cutting force required to shear

the metal and consequently helps to increase the tool life and reduce the power consumption.

It improves the surface finish A tool with negative rake will withstand far more loading than a tool with positive rake.

A tool with a positive rake angle reduces cutting forces by allowing the chips to flow more freely across the rake surface

Tool Geometry


Positive rake : Reduces compression, the forces, and the friction,

yielding a thinner, less deformed and cooler chip. Reduces the strength of the tool section, and heat

conduction capacity. Used when cutting tough, alloyed materials that tend to

work-harden, such as certain stainless steels Negative Rake: Provides greater strength at the cutting edge and better

heat conductivity Causes high compression, tool force, and friction,

resulting in highly deformed, hot chip Used on carbide, ceramic, polycrystalline diamond, and

polycrystalline cubic boron nitride cutting tools. Negative rakes are recommended on tool which does not

possess good toughness useful in making intermittent cuts and in absorbing the

impact during the initial engagement of the tool

Tool Geometry


Relief angles minimize physical interference or rubbing contact

with machined surface and the work piece. Reduces tool breakage and to increase tool life. If the relief angle is too large, the cutting tool may

chip or break. If the angle is too small, the tool will rub against the

workpiece and generate excessive heat hard and strong materials : Small relief angles Larger feed will require greater side relief angle

Side relief angle: prevents the side flank of the tool

from rubbing against the work when longitudinal feed is given.

End relief angle: prevents the axillary flank from rubbing against the machined surface .

A minimum relief angle is given to provide maximum support to the tool cutting edge by increasing the lip angle. The front clearance angle should be increased for large diameter works.

Tool Geometry


Side cutting edge angle: The function of end cutting edge angle is to

prevent the trailing front cutting edge of the tool from rubbing against the work.

The side cutting edge angle of the tool has practically no effect on the value of the cutting force or power consumed for a given depth of cut and feed.

increases tool life as, for the same depth of cut; the cutting force is distributed on a wider surface.

It dissipates heat quickly for having wider cutting edge.

Large side cutting edge angles are lightly to cause the tool to chatter. End cutting edge angle: A large end cutting edge angle unnecessarily weakens the tool.

Tool Geometry


L2: Forces in machining

Nose radius: The function of nose radius is as follows: Greater nose radius clears up the feed marks caused by the previous shearing action and provides better surface finish. All finish turning tool have greater nose radius than rough turning tools. It increases the strength of the cutting edge, tends to minimize the wear taking place in a sharp pointed tool with consequent increase in tool life. Accumulation heat is less than that in a pointed tool

Tool Geometry

Tool Wear


The life of tool comes to an end due to: Gradual or progressive wearing away of certain regions of the face or flank of the

tool Failures bringing the life of the tool to a premature end

Tool Wear


The life of tool comes to an end due to: Gradual or progressive wearing away

of certain regions of the face or flank of the tool

Failures bringing the life of the tool to a premature end

Tool Wear

Crater wear: consists of a concave section on the tool face formed by the action of the chip sliding on the surface. Flank wear: occurs on the tool flank as a result of friction between the machined surface of the workpiece and the tool flank Corner wear: occurs on the tool corner.


Tool Wear

Types of tool wear according to standard ISO 3685:1993. The major cutting edge is considered to be divided in to four regions Region C is the curved part of the cutting edge at the tool corner; Region B is the remaining straight part of the cutting edge in zone C; Region A is the quarter of the worn cutting edge length b farthest away from the tool corner; Region N extends beyond the area of mutual contact between the tool workpiece for approximately 12 mm along the major cutting edge. The wear is of notch type.


Tool Wear

Tool life Criteria High Speed Steel 1. Catastrophic failure, or 2. VB=0.3 mm if the flank is regularly worn

in zone B, or 3. VB max =0.6mm if the flank is irregularly

worn, scratched, chipped of badly grooved in zone B

Sintered-Carbied Tools 1. VB=0.3 mm or 2. VB max =0.6mm if the flank is irregularly

worn, 3. KT=0.06+0.3f, where is the feed


Tool Wear

Mechanism of Tool wear Tool wear occurs at microscopic and atomic level Types of wear are governed by different mechanism because of difference in

temperature, sliding velocity, work material, process parameters and stresses Flank wear leads to dimensional inaccuracy; crater wear leads to weakening of

tool

Four mechanisms of tool wear ( Holmberg and Mathews, 1994) Adhesive Abrasive Chemical Instability

Diffusion Solution electrochemical

Delamination


Tool Wear

Adhesive Tool wear Caused by formation of welded asperity between chip and the tool face fracture of the junctions by the shearing


relative motion between the two bodies

fragments of softer workpiece adhere to the harder tool

adhered material separates and tears small fragments of

the tool material.

Tool Wear

Abrasive Tool wear caused by hard particles in the work material Tool and workpiece contain carbides, oxides, and nitrides with hard

microstructures causing abrasion wear


tool shears away small particles from the work material.

work material also removes small particles from tool material at a smaller rate

hard particles are caught between tool and workpiece, Causing additional

abrasion wear

Tool Wear

Diffusion Tool wear diffusion of atoms of the tool material into the workpiece Requirements for diffusion wear: metallurgical bonding of the two surfaces ,

temperature for rapid diffusion , and some solubility of the tool material phases in the work material


temperature of the tool and workpiece increase at the contact

zones

The atoms in the two migrate to the opposite material where the concentration

of the same atom is less.

diffusion of the tool materials into the chip gradually leads to a weakened

cutting edge and eventual chipping or breakage of the tool.

Tool Wear

Solution wear : wear takes place when the wear rate is controlled by the dissolution rate rather than by convective Electrochemical wear is due to a thermoelectric emission generated at the chip-tool junction causing electric currents to circulate resulting in the passage of ions from the tool surface to the workpiece material. Delamination wear is due to plastic deformation of the surface leading to subsurface crack nucleation propagation and the liberation of tiny flakes from the tool surface. This has been observed when high speed tools soften due to annealing during machining

wear of the cutting tool determined by electrochemical instability


Tool Wear

Flank wear is caused mainly adhesive and abrasive wear, some diffusion wear also exists low cutting speeds Crater wear Crater wear is caused primarily by the dissolution of tool material by diffusion

or solution wear since it occurs in the region of maximum temperature rise

The temperature is greatest near the midpoint of the tool-chip contact length, where the greatest amount of crater wear occurs due to intensive diffusion.

Crater wear can be minimized by selecting a tool material that has the least affinity to the workpiece material in terms of diffusion.

L3: Tool geometry, wear and life L3: Tool geometry, wear and life

Tool Wear Evaluation

Tool wear evaluation is to find relationship between the amount of flank (rake) wear and the cutting time, or the overall length of the cutting path. Wear curves: (a) normal wear curve, (b) evolution of flank wear land VBB as a function of cutting time for different cutting speeds



Taylors Equation for Tool life Taylor presented the following equation: VcT

n =C where Vc is the cutting speed (m/min), T is the tool life (min) taken to develop a certain flank wear (VBB), n is an exponent that depends on the cutting parameters and C is a constant.



Expanded Taylors Tool Life Formula Taylor tool life formula, was obtained using high-carbon and high-

speed steels as tool materials. With the further development of carbides and other tool materials, it

was found that the cutting feed and the depth of cut are also significant.

The Taylors tool life formula was modified to accommodate these changes as:

VcT

n fa db = C where d is the depth of cut (mm) and f is the feed (mm/rev). The exponents a and b are to be determined experimentally for each combination of the cutting conditions.



Tool wear and tool geometry


Tool Materials

Essential properties of tool material 1. Harder then work-piece at normal and elevated temperature

as well 2. Tough enough to withstand shock 3. Wear resistive 4. Low coefficient of friction 5. High Thermal conductive and specific heat 6. Chemical stability or inertness


Tool Materials

Hardness: Hardness of tool and work piece material declines with temperature Softening of work-piece is compensated by strength attained due to

plastic deformation In place of static hardness (room temp without strain) Modified hardness

ratio is used ( considering hot hardness and high strain)


Tool Materials

Hardness: In place of static hardness (room temp without strain) Modified hardness

ratio is used ( considering hot hardness and high strain)


Tool Materials

Toughness Toll should be tough enough to withstand shock load, chipping and fracturing, vibration, misalignments, runouts etc.


Tool Materials

Cutting tool materials Carbon & medium alloy steels High speed steels Cast-cobalt alloys Carbides Coated tools Alumina-based ceramics Silicon-nitride-base ceramics Cermates Cubic boron nitride Diamond Whisker-reinforced materials


Carbon & medium alloy steels Oldest of tool materials Used for drills taps, broaches , reamers Inexpensive ,easily shaped ,sharpened No sufficient hardness and wear

resistance Limited to low cutting speed operation

HSS Highly alloyed steels Retains hardness and strength at

elevated temperatures Two types of HSS : M type and T type

Tool Materials


HSS Highly alloyed steels Retains hardness and strength at elevated temperatures Two types of HSS : M type and T type M-series - Contains 10% molybdenum, chromium, vanadium, tungsten, cobalt T-series - 12 % - 18 % tungsten, chromium, vanadium & cobalt Molybdenum, when added to low alloy steels, improves high

temperature strengths and hardness. Tungsten increases hardness particularly at elevated temperatures due

to stable carbides, refines grain size Vanadium increases strength, hardness, creep resistance and impact

resistance due to formation of hard vanadium carbides Chromium improves hardenability, strength and wear resistance,

sharply increases corrosion resistance at high concentrations

Tool Materials

Cutting tool materials Cast-cobalt alloys Commonly known as stellite tools Composition ranges 38% - 53 % cobalt 30%- 33% chromium

10%-20%tungsten Good wear resistance ( higher hardness) Less tough than high-speed steels and sensitive to impact forces Less suitable than high-speed steels for interrupted cutting

operations Used for deep continuous roughing cuts relatively high feeds &

speeds


Tool Materials Cutting tool materials

Carbide A carbide tool material consists of carbide particales (carbides of tungsten,

titanium, tantalum or some combination of these) bound together in a cobalt matrix by sintering

Sintering is a process of forming objects from a metal powder by heating the powder at a temperature below its melting point.

Two groups of carbides used for machining operations Tungsten carbide Titanium carbide Tungsten carbide Composite material consisting of tungsten-carbide particles bonded together in a

cobalt matrix Alternate name is cemented carbides Amount of cobalt present affects properties of carbide tools As cobalt content

increases strength hardness & wear resistance increases


Tool Materials Cutting tool materials Titanium carbide Titanium carbide has higher wear resistance than tungsten carbide Nickel-Molybdenum alloy as matrix TiC suitable for machining hard materials

Steels & cast irons Speeds higher than those for tungsten carbide

Carbide tools are generally in form of inserts which are clamped or brazed to shank


(a) Clamping and (b) Wing lockpins. (c) Examples of inserts attached to toolholders with threadless lockpins, which are secured with side screws (d) brazed

Tool Materials Cutting tool materials Coated tools Tool materials ( in general carbides ) are coated with material like TiN, TiC, Al2O3 due to: (a) provide increased surface hardness (b) increase resistance (abrasive and adhesive wear, flank or crater wear) (c) reduce friction coefficients to ease chip sliding, reduce cutting forces, prevent adhesion to the contact surfaces, reduce heat generated due to chip sliding etc., (d) reduce the portion of the thermal energy that flows into the tool, (e) increase corrosion and oxidation resistance (f) improved the surface quality of finished parts.


Multiple coating

Tool Materials Cutting tool materials Coated tools Tool materials ( in general carbides ) are coated with material like TiN, TiC, Al2O3 due to: (a) provide increased surface hardness (b) increase resistance (abrasive and adhesive wear, flank or crater wear) (c) reduce friction coefficients to ease chip sliding, reduce cutting forces, prevent adhesion to the contact surfaces, reduce heat generated due to chip sliding etc., (d) reduce the portion of the thermal energy that flows into the tool, (e) increase corrosion and oxidation resistance (f) improved the surface quality of finished parts.


Bottom layer TiC: bond well with substrate Top layer: wear resistive and low thermal conductive Intermittent: compatible with top and bottom

Tool Materials

Cutting tool materials Ceramics Primarily fine-grained Al2O3, pressed and sintered at high pressures and temperatures into

insert form with no binder.

Ceramic cutting tools are harder and more heat-resistant than carbides, but more brittle.

Applications: high speed turning of cast iron and steel (Mild steels can be cut at speeds up to

1500 fp

Applications: high speed finishing and semifinishing of steels, stainless steels, and cast irons

The alumina-based ceramics are used for high speed semi- and final-finishing of ferrous and some

non-ferrous materials.

The silicon nitride-based ceramics are generally used for rougher and heavier machining of cast

iron and the superalloys.


Tool Materials Cutting tool materials Cermets

Bonded material containing ceramics and metals, widely used in jet engines and

nuclear reactors. Cermets behave much like metals but have the great heat resistance

of ceramics.

Tungsten carbide, titanium carbide, zirconium bromide, and aluminum oxide are

among the ceramics used; iron, cobalt, nickel, and chromium are among the metals.

Properties:

Higher speeds and lower feeds than steel-cutting carbide grades.

Better finish achieved, often eliminating need for grinding.

Applications: high speed finishing and semifinishing of steels, stainless steels, and cast

irons


Tool Materials Cutting tool materials Cubic boron nitride (CBN) Next to diamond, cubic boron nitride (CBN) is hardest material known. Made by bonding ( 0.5-1.0 mm ( 0.02-0.04-in) Layer of poly crystalline cubic boron

nitride to a carbide substrate by sintering under pressure While carbide provides shock resistance CBN layer provides high resistance and cutting

edge strength Cubic boron nitride tools are made in small sizes without substrate Applications: machining very hard ferrous materials like steel and also nickel-based

alloys


Fig : (a) Construction of a polycrystalline cubic boron nitride or a diamond layer on a tungsten-carbide insert. (b) Inserts with polycrystalline cubic boron nitride tips (top row) and solid polycrystalline CBN inserts (bottom row).

Tool Materials

Cutting tool materials Diamond Hardest known substance Low friction, high wear resistance Ability to maintain sharp cutting edge Single crystal diamond of various carats used for special applications Machining copperfront precision optical mirrors for ( SDI) Diamond is brittle , tool shape & sharpness is important Low rake angle used for string cutting edge Polycrystalline diamond Sintered polycrystalline diamond (SPD) - fabricated by sintering very fine-grained

diamond crystals under high temperatures and pressures into desired shape with little or no binder.

Polycrystalline diamond cutting tools can outlast regular carbide by a factor of 100! Applications: high speed machining of nonferrous metals and abrasive nonmetals such

as fiberglass, graphite, and some plastics.


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