Tool Wear

Tool Wear

Notes for Tool Wear-S.doc 1

TOOL WEAR

DEFINITION: TOOL WEAR

The change of shape of the tool from its original shape, during cutting, resulting fromthe gradual loss of tool material (Australian Standard, appendix B P35)

1. INTRODUCTIONS (Armarego and Brown, The Machining of Metals)

Cutting tools are subjected to an extremely severe rubbing process. They are in metal-to-metal contact, between the chip and workpiece, under conditions of very high stress at hightemperature. The situation is further aggravated (worsened) due to the existence of extremestress and temperature gradients near the surface of the tool.

During cutting, cutting tools remove the material from the component to achieve the requiredshape, dimension and surface roughness (finish). However, wears are occurring during thecutting action, and it will result in the failure of the cutting tool. When the tool wear reachcertain extent, the tool or edge change has to be replaced to guarantee the ordinary cuttingaction.

In order to continue our lecture on tool wear, we should point out the objectives of thissection of study. (If you do not have any idea about what we are going to learn in a shortlecture, you may totally miss the target.)

The Objectives:1. Study the general characteristics of tool wear2. Understanding the causes of tool wears and their consequences3. Set up the tool failure criteria and understanding the meaning of tool-life

2. TOOL WEAR PHENOMENA

Figure 1. Tool wear phenomena

As we have learned in third year course “manufacturing Science 1”, in orthogonal andoblique cutting sections, we know that the shear stress and normal stress involved in metal

Tool Wear


cutting is much higher than that used in Engineering Mechanics. Due to the high contactstress between the tool rake face and the chip, it causes severe friction at the rake face aswell as the friction between the flank and the machined surface. Hence result in all sort ofwears which can be observed at rake face and flank face

a) Crater wear (Rake Face Wear)

Crater wear: Chip flows away on the rake face, results in the severe friction between thechip and rake face, and leave a scar on the rake face which usually parallels to the majorcutting edge. The crater wear could increase the working rake angle and reduce the cuttingforce, but it will also weaken the strength of the cutting edge. The parameters used tomeasure the crater wear can be seen in the diagram. The crater depth KT is the mostcommonly used parameter in evaluating the rake face wear.

KT

KF

KBKM

Plane Ps

KF Crater front distanceKB Crater widthKM Crater centre distanceKT Crater depth

Change of the crater depth KT and width KB with cutting time

Tool Wear


Cutting time

Cra

ter a

ngle

δc

(b)

δc

(a)

Circular crater

Rake facedc

bc

lc

δc

(a) Simplified geometry of crater wear; (b) Crater angle growth(b) (after Bickel ad Krischsieder)

Variation of crater depth with time for different cutting speed V (M.C. Shaw)

From the diagram, it can be seen that the higher the speed, the shorter the time used toreach the same depth of crater wear.

from: Modern Metal Cutting, A practical Handbook, SANDVIK Coromant.

Crater Wear on the chip face can be due to abrasive and diffusion wear mechanisms. Thecrater is formed through tool material being removed from chip face either by the hardparticle grinding action or at the hottest part of the chip face through the diffusive actionbetween the chip and tool material. Hardness, hot hardness and minimum affinity betweenmaterials minimises the tendency for crater wear. Excessive crater wear changes thegeometry of the edge and can deteriorate chip formation, change cutting force directions andalso weaken the edge.

Tool Wear


b) Clearance (flank) Surface WearWear on the flank (relief) face is called Flank wear and results in the formation of a wear

land. Wear land formation is not always uniform along the major and minor cutting edges ofthe tool.

Flank wear most commonly results from abrasive wear of the cutting edge against themachined surface. Flank wear can be monitored in production by examining the tool or bytracking the change in size of the tool or machined part. Flank wear can be measured byusing the average and maximum wear land size VB and VBmax.

(a) Wear-land gowth (flank wear). (b) Simplified geometry of worn tool (flank wear)

from: Modern Metal Cutting, A practical Handbook, SANDVIK Coromant.

FLANK WEAR, as the name indicates, takes place on the flanks of the cutting edge, mainlyfrom the abrasive wear mechanism. The clearance sides: leading, trailing and nose radius orparallel land are subjected to the workpiece moving past during and after chip formation.

Tool Wear


This usually most normal type of wear and to maintain safe progressive flank wear is oftenthe ideal. In the end of flank wear will lead to poor surface texture, inaccuracy andincreasing friction as the edge change shape.

Flan

k W

ear

Steady Wear

Severe WearInitial

Wear

TimeVariation of flank wear rate with cutting time, showing the initial wear, steady wear, and

severe wear periods

Typical stage of tool wear in normal cutting situation

1. Initial (or Preliminary) wear region:Caused by micro-cracking, surface oxidation and carbon loss layer, as well as micro-roughness at the cutting tool tip in tool grinding (manufacturing). For the new cutting edge,due to the small contact area and high contact pressure, it will result in high wear rate. Theinitial wear size is VB=0.05-0.1mm normally.

2 Steady wear regionAfter the initial (or preliminary) wear (cutting edge rounding), the micro-roughnessimproved, in this region, the wear size is proportional to the cutting time. The wear rate isrelatively constant.

3 Severe (or Ultimate or catastrophic) wear:When the wear size increases to a critical value, the surface roughness of the machinedsurface decreased, cutting force and temperature increase rapidly, the wear rate increases.Then the tool loses its cutting ability. In practice, this region of wear should be avoided.

Flank wear and chipping will increase the friction, so that the total cutting force willincrease. The component surface roughness will be increased, especially when chippingoccurs.Flank wear will also affect the component dimensional accuracy. When form tool is used,flank wear will also change the shape of the component produced.

Tool Wear


Recommended wear land size for different tool material and operations.Wear (in) Tool Remarks

0.030(0.76 mm)

Carbide Roughing passes

0.010-0.015(0.25-0.38 mm)

Carbide Finishing passes

0.060 or total destruction(1.25 mm)

H.S.S. Roughing passes

0.010-0.015(0.25-0.38 mm)

H.S.S. Finishing passes

0.010-0.015(0.25-0.38 mm)

Cementedoxides

Roughing and finishing passes

c) Notch wear (Boundary Wear)

This is a special type of combined flank and rake face wear which occurs adjacent to thepoint where the major cutting edge intersects the work surface.

The gashing (or grooving, gouging) at the outer edge of the wear land is an indication of ahard or abrasive skin on the work material. Such a skin may develop during the firstmachine pass over a forging, casting or hot-rolled workpiece. It is also common inmachining of materials with high work-hardening characteristics, including many stainlesssteels and heat-resistant nickel or chromium alloys. In this case previous machiningoperation leave a thin work-hardened skin.

Wear grooves formed at free edges of chip

Notch Wear (Modern Metal Cutting, A practical Handbook, SANDVIK Coromant.)

Tool Wear


d) Chipping

Chipping of the tool, as the name implies, involves removal of relatively large discreteparticles of tool material. Tools subjected to discontinuous cutting conditions areparticularly intended to chipping.

Built-up edge formation also has a tendency to promote tool chipping. A built-up edgeis never completely stable, but it periodically breaks off. Each time some of the built-upmaterial is removed it may take with it a lump (piece) of tool edge

Chipping of the cutting edge (Modern Metal Cutting, A practical Handbook, SANDVIK

Coromant.)

e) Plastic deformationPlastic deformation takes place as a result of combined high temperatures and high pressureon the cutting edge. High speeds and feeds and hard workpiece materials mean heat andcompression. For the tool material to stand up to this and not deform plastically, high hothardness is critical. The typical bulging of the edge will lead to higher temperatures,geometry deformation, chip flow changes and so on until a critical stage is reached. The sizeof the edge rounding and cutting geometry also play a role in combating this wear type.

(Modern Metal Cutting, A practical Handbook, SANDVIK Coromant.)

f) Ultimate failure

The final result of tool wear is the complete removal of the cutting point – ultimatefailure of the tool.

Tool Wear


This may come about by temperature rise, which virtually causes the tool tip to softenuntil it flows plastically at very low shear stress. This melting process seems to start right atthe cutting edge and because material flow blunts the edge, the melting process continuesback into the tool; within a few seconds a piece of tool almost as large as the engaged depthof cut is removed.

An alternative mechanism of ultimate failure is the mechanical failure (usually abrittle fracture) of a relatively large portion of the cutting tip. This often results from aweakening of the tool by crater formation.

Ultimate failure by melting and plastic flow is most common in carbon or high-speed-steel tools, while fracture failures are most common in sintered carbide or ceramictools.

Fracture (Modern Metal Cutting, A practical Handbook, SANDVIK Coromant.)

3. CAUSES OF TOOL WEAR

a) Hard particle wear (abrasive wear)Abrasive wear is mainly caused by the impurities within the workpiece material, such ascarbon, nitride and oxide compounds, as well as the built-up fragments. This is a mechanicalwear, and it is the main cause of the tool wear at low cutting speed.

b) Adhesive wear mechanismThe simple mechanism of friction and wear proposed by Bowden and Tabor is based

on the concept of the formation of welded junctions and subsequent destruction of these.Due to the high pressure and temperature, welding occurs between the fresh surface

of the chip and rake face (chip flows on the rake face results in chemically clean surface).[Friction Welding used in twist drills, broaches and of the tool manufacturing]

Severe wear is characterised by considerable welding and tearing of the softerrubbing surface at high wear rate, and the formation of relatively large wear particles.

Adhesion wear occurs mainly at low machining temperatures on tool rake face, suchBUE.

Under mild wear conditions, the surface finish of the sliding surfaces improves

Tool Wear


c) Diffusion wear

Holm thought of wear as a process of atomic transfer at contacting asperities(Armarego’s Book).

A number of workers have considered that the mechanism of tool wear must involvechemical action and diffusion. They have demonstrated welding and preferred chemicalattack of (W) tungsten carbide in (W-Ti) tungsten-titanium carbides. They have shown thephoto-micrograph evidence of the diffusion of tool constituents into the workpiece and chip.

Result in the changes of the tool and workpiece chemical composition.

There are several ways in which the wear may be dependent on the diffusion mechanism.

(i) Gross softening of the toolDiffusion of carbon in a relatively deep surface layer of the tool may cause

softening and subsequent plastic flow of the tool. This flow may produce major changesin the tool geometry, which result in high forces and a sudden complete failure of thetool.

(ii) Diffusion of major tool constituents into the work. (chemical element loss)The tool matrix or a major strengthening constituent may be dissolved into the work

and chip surfaces as they pass the tool.In cast alloy, carbide or ceramic tools, this may be the prime wear phenomenon.With HSS tools, iron diffusion is possible, but it seems unlikely to be the predominantwear process.Diamond tool – cutting iron and steel is the typical example of diffusion wear.

(iii) Diffusion of a work-material component into the toolA constituent of the work material diffusing into the tool may alter the physicalproperties of a surface layer of the tool. For example, the diffusion of lead into the toolmay produce a thin brittle surface layer, this thin layer can be removed by fracture orchipping.

d) Oxidation wearHigh temperatures and the presence of air means oxidation for most metals.

e) Chemical wearCorrosive wear (due to chemical attack of a surface)

f) Fatigue wearFatigue wear is often a thermo-mechanical combination. Temperature fluctuations

and the loading and unloading of cutting forces can lead to cutting edge cracking andbreaking. Intermittent cutting action leads to continual generation of heat and cooling aswell as shocks of cutting edge engagement.

Tool Wear


Other forms of tool wear

Heat-electric wear can be observed at high temperature region, and it enhances the toolwear.

The high temperature results in the formation of thermal couple between theworkpiece and the tool. Due to the heat related voltage established between the workpieceand tool, it may cause electric current between the two. However, the mechanism of theheat-electric wear has not been clearly developed. Major improvement (decrease) of toolwear has been seen through experimental tests by isolated the tool and component.

Thermal Cracking and Tool Fracture in Milling

In milling, tools are subjected to cyclic thermal and mechanical loads. It may fail bymechanism not observed in continuous cutting. Two common failure mechanisms unique tomilling are thermal cracking and entry failure.

The cyclic variations in temperature in milling induce cyclic thermal stress as the surfacelayer of the tool expands and contracts. This can lead to the formation of thermal fatiguecracks near the cutting edge. In most cases such cracks are perpendicular to the cutting edgeand begin forming at the outer corner of the tool, spreading inward as cutting progresses.The growth of these cracks eventually leads to edge chipping or tool breakage.

Thermal cracking can be reduced by reducing the cutting speed or by using a tool materialgrade with a higher thermal shock resistance. In applications when coolant is supplied,adjusting the coolant volume can also reduce crack formation. An intermittent coolantsupply or insufficient coolant can promote crack formation; if a steady, copious volume ofcoolant cannot be supplied, tool-life can often be increased by switching to dry cutting.

Thermal cracking (Modern Metal Cutting, A practical Handbook, SANDVIK Coromant.)

Edge chipping is common in milling. Chipping may occur wen the tool first contacts thepart (entry failure) or, more commonly, when it exits the part (exit failure). WC toolmaterials

Entry failure most commonly occur when then outer corner of the insert strikes the part first.This more likely to occur when the cutter rake angles are positive. Entry failure is thereforemost easily prevented by switching from positive to negative rake cutters.

Tool Wear


EFFECTS OF THE TOOL WEAR ON TECHNOLOGICAL PERFORMANCEMEASURES.

Consequences of tool wear1. Decrease the dimension accuracy;2. Increase the surface roughness;3. Increase the cutting force;4. Increase the temperature;5. Likely cause vibration;6. Lower the production efficiency, component quality;7. Increase the cost.

Influence on cutting forcesCrater wear, flank wear (or wear-land formation) and chipping of the cutting edge affect theperformance of the cutting tool in various ways. The cutting forces are normally increasedby wear of the tool. Crater wear may, however, under certain circumstances, reduce forcesby effectively increasing the rake angle of the tool. Clearance-face (flank or wear-land) wearand chipping almost invariably increase the cutting forces due to increased rubbing forces.

Surface finish (roughness)The surface finish produced in a machining operation usually deteriorates as the tool wears.This is particularly true of a tool worn by chipping and generally the case for a tool withflank-land wear – although there are circumstances in which a wear land may burnish(polish) the workpiece and produce a good finish.

Dimension accuracy:Flank wear influences the plan geometry of a tool this may affect the dimensions of thecomponent produced in a machine with set cutting tool position or it may influence theshape of the components produced in an operation utilizing a form tool.(cylindrical turning could result in a tapered)

Vibration or chatter is another aspect of the cutting process which may be influenced bytool wear. A wear land increases the tendency of a tool to dynamic instability. A cuttingoperation which is quite free of vibration when the tool is sharp, may be subjected to anunacceptable chatter mode when the tool wears.

SUMMARY

The general mechanism of wear and the particular mechanics of tool wear have beenconsidered in this section.

• Different tool-workpiece material combination may result in different wear, e.g. diffusionwear is the main wear when machining steel using carbide tool together with adhesivewear and chemical wear.

• In general, to certain tool-workpiece material combination, the temperature playsdominant effect on tool wear. At the lower temperature region, mechanical wear (hardspot scratches) is the main factor, but at the high temperature region chemical wear(adhesive, diffusion and oxidation) is the main cause in tool wear

Tool Wear


• Wear process is not a simple, unique phenomenon.• Wear is a term used to describe a group of processes.• A simple sliding may change from one of these processes to another, because of quite

minor alterations in the physical conditions.• Realistic and useful mathematical models describing the wear processes are considerably

difficulty.• Tool wear is further complicated by the fact that rubbing is taking place on the freshly

formed metal surface.

Special characteristics of tool wearDue to:• Huge contact stress at the rake and flank surface• High temperature (800-1000 °C for carbide and steel combination)

Hence, generalised wear theory cannot be directly used in studying the tool wearGenerally, tool wear is a complicated and combined process.

Major factors affecting tool wear

1. Mechanical properties of the tool and workpiece materials

- Hardness- Strength/toughness- Chemical stability- Thermal diffusion conductivity- Thermal expansion- Surface inertness- Coating adhesion

2. Cutting conditions

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Tool Wear