Metal Machining - Theory and Applications Episode 1 Part 7 ppt

Tool damage Chapter 3 considered cutting tool minimum property requirements both mechanical and thermal to avoid immediate failure.. The damages of a cutting tool are influenced by the s

h-phase) In any case, using a P-type substrate would give added life to a tool used to cut

steels once the coating wore off or if it failed To compensate for loss of toughness on changing from K- to P-type, such substrates were typically manufactured with a greater

%Co for a given grade of duty than if they were uncoated Thus began the development of special substrate compositions for coated tools By the early 1980s, substrates were being manufactured with surface layers containing from 1.5 to 3 times the amount of cobalt to

that in the bulk, and from 10 mm to 30 mm thick, on near WC-Co bulk compositions.

Toughness is maintained near the surface without reducing the hardness of the bulk Considering the high thermal expansion coefficient of cobalt (Table 3.9), the surface layer

of the substrate is better thermally matched to the coating materials and thermal strains are reduced CVD-coated tools began to find uses in interrupted turning and light milling oper-ations

Considering the thicknesses of both the coatings and modified substrate surface layers, the composition (and hence thermal and mechanical properties) of CVD-coated tools can

vary over depths of up to around 40 mm This is not insignificant relative to the size of the

stressed and heated regions during cutting Detailed understanding of the interactions between the graded surface compositions and the mechanical and thermal fields generated

in machining, leading to still further improvements in tool design, continues to develop

PVD coatings

An alternative process for manufacturing coatings is Physical Vapour Deposition (PVD)

It is similar to CVD in its productivity (in its basic form, deposition rates are also around

1 mm/hr) but requires substrates to be heated only to a few 100˚C, say 500˚C, so coatings

can be deposited without the need to guard against unfavourable changes to the substrate

In contrast to CVD, in which the metallic elements of the coating are obtained from gases

at around 10% atmospheric pressure, in PVD the metallic elements are obtained from solids in a high vacuum chamber environment There are many variants of the process but all involve establishing a large electric potential difference (of the order of kV) between the substrate and a solid source of elements to be deposited on the substrate; and creating

a glow discharge plasma between the two, typically with argon gas at low pressure Material is evaporated from the source (by some form of heating or bombardment), is ionized in the plasma and is accelerated towards and adheres to the substrate The source may have the composition of the material of the coating, or more commonly it may be a metal – for example titanium In the latter case, for example in forming a TiN coating, nitrogen gas is also admitted to the plasma The Ti ions combine with the nitrogen, to condense as TiN on the substrate

The microstructure and properties of the coating are controlled by the substrate temper-ature and the deposition rate It has been found that coatings can be grown with residual

compressive stresses in them, but thicknesses are limited to about 5 mm Coatings made by

PVD are much smoother than by CVD and can be deposited on to sharp edges Experience has shown that they are more suitable for milling operations (because of their compressive stresses) and finishing operations (because of the possibility of using sharp edged tools

(down to 10 mm to 20 mm edge radius) The range of coating types is not as wide as with

CVD TiN was the first coating type successfully to be developed by PVD This was followed by TiC and Ti(C,N); and (Ti,Al)N has also been developed There is great diffi-culty in generating Al2O3coatings with a strong, coherent microstructure Cermets as well

as cemented carbides are being coated by PVD

Tool materials 113

Coating developments and summary

Coating technologies continue to develop For example, there are intermediate processes between CVD and PVD in which coatings are formed with the chemical variability of CVD but in which the substrate needs to be heated only to, say, 800˚C Today there is a wide variety of choice in the purchase of coated tools and production engineers rely heav-ily on the advice of tool manufacturers and their own practical trials Tool manufacturers are rather secretive about their manufacturing processes; and even about what the substrate material is beneath a coating When an engineer buys a coated tool he or she rarely knows what is beneath the coating Short of cutting up a tool and examining it, the next best way

of satisfying curiosity as to what is a tool’s substrate, is to weigh it There is a strong rela-tion between density and carbide composirela-tion – and between that and tool thermal conduc-tivity – as shown in Figure 3.29

This section has concentrated on TiC, TiN and Al2O3coatings on cemented carbides

At the time of writing, there is much activity in trying to develop PCD-coated tools There are also many instances in which high speed steel tools are coated with PVD TiN, TiCN

or TiAlN Chromium nitride, boron nitride and boron carbide coatings are also under investigation TiN and TiC coatings have also been found to be useful on silicon nitride ceramic tools However, as far as this chapter is concerned, the main lesson is that Surface Engineering has enabled the substrates of cutting tools to be designed for hardness and toughness, separately from considerations of wear resistance As far as CVD-coated tools are concerned, the depth over which material composition and properties change is signif-icant relative to the distances over which stresses and temperatures penetrate the tool For PVD-coated tools, the variations of composition and properties are much more superficial

3.2.8 Tool insert geometries

At the start of Section 3.2, the stresses in a tool were considered, assuming the tool to have

a plane rake face This later led to a conclusion of the minimum wedge angle that a tool should have to avoid failure by yielding or fracture (Figure 3.27) In practice, many tools

Fig 3.29 The relation between thermal conductivity and density for coated cemented carbide and cermet substrate

materials

do not have plane rake faces This is particularly true of indexable inserts, manufactured

by sintering and first mentioned in Chapter 1

There are three main reasons for modifying cutting edge geometry: to strengthen the edge, to reduce cutting forces and to control chip flow The basic ways of achieving these are illustrated, in two dimensions, in Figure 3.30

Edge strengthening involves changing the edge shape over distances of the same order

as the feed length Figure 3.30(a) shows an edge region chamfered at an angle acover a

length T and honed to an edge radius R Recommendations in the early 1990s for edge preparation of ceramic cutting tools were typically chamfers for a length T between 0.5f and 0.75f for turning operations and 1.2f to 1.5f for milling; with ac from 15˚ to 30˚ depending on the severity of the machining operation; and edge radii ranging from 0.013

mm to 0.076 mm for finishing operations, up to 0.13 mm in more severe conditions

(Adams et al., 1991) Today, with improved grinding procedures (and perhaps better

ceramic tool toughness too), chamfer lengths for general machining are reduced to 0.1 to

0.4f for turning and 0.5f for milling; and edge radii in general machining are 0.02 to 0.03

mm, with no radiusing – only chamfering – for finishing operations

Changes to reduce cutting forces involve altering the rake face over lengths of several

times the feed (Figure 3.30(b)) The rake face beyond a land h of length between 1f and 2f

is cut-away to a depth d typically also between 1f and 2f, established over a length L from 3f to 6f The land restriction causes a reduced chip thickness.

A disadvantage of cutting away the rake as just described is that, generally, the chips become straighter, and in a continuous process (such as turning) this can lead to long unbroken chips that are difficult to dispose of In order to control the flow, the cut-away region is usually ended in a back wall (Figure 3.30(c)), so that the cut-away forms some groove shape When a chip hits the back wall, it is deflected and has a good chance of

Tool materials 115

Fig 3.30 Modifications to a square cutting edge for (a) edge strengthening, (b) cutting force reduction and (c) chip

control

breaking when its tip hits either the tool holder or the work There is a wide variety of prac-tical groove shapes They can be curved or triangular, symmetrical or unsymmetrical The

height d* of the back wall can be greater or less than the groove depth d In some cases,

the back wall is formed without a groove at all Inserts can be designed for use over a wide range of feeds by creating the groove features as a series of terraces, so that the smallest feeds involve chip contact only with the terrace nearest the cutting edge and larger feeds result in contact over several terraces Of course, the larger feed features of Figures 3.30(b) and (c) can be combined with the sub-feed strengthening features of Figure 3.30(a) Figure 3.30 takes a two-dimensional view of a cutting edge Real inserts are three dimensional – and this gives further opportunity for ingenuity in tool design Sections as

in Figure 3.30 can be varied along the cutting edge This possibility is shown in Figure 3.31(a) The rake face groove at the corner of an insert can be shaped differently from that along the edge, either to ensure that the corner is strong enough or to help guide the chip away from the corner region, or both

A different type of modification is shown in Figure 3.31(b) A curling chip is more

likely to break, when it hits an obstruction, the larger is its second moment of area, I Chips

formed over plane or smoothly varying rake faces are approximately rectangular in section

– and have a relatively small I-value If they can be corrugated, their I value is raised The

rib and pocket form of the rake face in Figure 3.31(b) can cause such corrugation, if it is designed correctly As an alternative to the rib and pocket style, the whole cutting edge may be made wavy, or bumps instead of pockets can be formed on the rake Every manu-facturer has a different way in which to achieve the same effect Interested readers should look at manufacturers’ catalogues or refer to a recent handbook (Anon, 1994)

Improved tools, combining new shapes with better surface engineering, continue to be developed Finite element modelling, introduced in Chapter 6, is starting to contribute to these better designs However, amongst its required inputs is material property information

of the sort collected in this chapter

Fig 3.31 Three-dimensional opportunities for edge strengthening and chip control, not to scale: rake profile varying

(a) smoothly along a cutting edge and (b) in a ‘rib and pocket’ manner

Adams, J H., Anschuetz, B and Whitfield, G (1991) Ceramic cutting tools In: Engineered Materials Handbook Vol 4 (Ceramics and Glasses) Metals Park, Ohio: ASM.

Anon (1994) Modern Metal Cutting – A Practical Handbook Sandviken: AB Sandvik Coromant Brookes, K J A (1992) World Directory and Handbook of Hardmetals and Hard Materials 5th edn.

East Barnet, UK: International Carbide Data.

Hoyle, G (1988) High Speed Steels London: Butterworths.

Kobayashi, S and Thomsen, E G (1959) Some observations on the shearing process in metal

cutting Trans ASME 81B, 251–262; and Eggleston, D M., Herzog, R and Thomsen, E G Trans ASME 81B, 263–279.

Komanduri, R and Samanta, S K (1989) Ceramics In: Metals Handbook 9th edn Vol 16 (Machining) Metals Park, Ohio: ASM.

Sata, T (1968) Machinability of calcium-deoxidised steels Bull Jap Soc Prec Eng 3(1), 1–8.

Santhanam, A T and Quinto, D T (1994) Surface engineering of carbide, cermet and ceramic

cutting tools In: Metals Handbook, 10th edn, Vol 5 (Surface Engineering) Metals Park, Ohio:

ASM

Santhanam, A T., Tierney, P and Hunt, J L (1990) Cemented carbides In: Metals Handbook 10th edn, Vol 2 (Properties and Selection: Nonferrous alloys and Special Purpose Materials) Metals

Park, Ohio: ASM

Trent, E M (1991) Metal Cutting, 3rd edn Oxford: Butterworth Heinemann.

References 117

Tool damage

Chapter 3 considered cutting tool minimum property requirements (both mechanical and thermal) to avoid immediate failure By failure is meant damage so large that the tool has

no useful ability to remove work material Attention is turned, in this chapter, to the mech-anisms and characteristics of lesser damages that accumulate with use, and which eventu-ally cause a tool to be replaced In reality, there is a continuous spectrum of damage severities, such that there is no sharp boundary between what is to be considered here and what might in practice be described as immediate failure There is some overlap between this chapter and the previous one

Chapters 2 and 3 have demonstrated that cutting tools must withstand much higher fric-tion and normal stresses – and usually higher temperatures too – than normal machine tool bearing surfaces There is, in most cases, no question of avoiding tool damage, but only of asking how rapidly it occurs The damages of a cutting tool are influenced by the stress and temperature at the tool surface, which in turn depend on the cutting mode – for exam-ple turning, milling or drilling; and the cutting conditions of tool and work material, cutting speed, feed rate, depth of cut and the presence or not of cutting fluid and its type

In Chapter 2, it was described in general that wear is very sensitive to small changes in sliding conditions In machining, the tool damage mode and the rate of damage are simi-larly very sensitive to changes in the cutting operation and the cutting conditions While tool damage cannot be avoided, it can often be reduced if its mode and what controls it is understood Section 4.1 describes the main modes of tool damage

The economics of machining were introduced in Chapter 1 To minimize machining cost, it is necessary not only to find the most suitable tool and work materials for an oper-ation, but also to have a prediction of tool life At the end of a tool’s life, the tool must be replaced or reground, to maintain workpiece accuracy, surface roughness or integrity Section 4.2 considers tool life criteria and life prediction

4.1 Tool damage and its classification

4.1.1 Types of tool damage

Tool damage can be classified into two groups, wear and fracture, by means of its scale and how it progresses Wear (as discussed in Chapter 2) is loss of material on an asperity

or micro-contact, or smaller scale, down to molecular or atomic removal mechanisms It usually progresses continuously Fracture, on the other hand, is damage at a larger scale than wear; and it occurs suddenly As written above, there is a continuous spectrum of damage scales from micro-wear to gross fracture

Figure 4.1 shows a typical damage pattern – in this case wear – of a carbide tool, cutting steel at a relatively high speed Crater wear on the rake face, flank wear on the flank faces and notch wear at the depth of cut (DOC) extremities are the typical wear modes Wear

measures, such as VB, KT are returned to in Section 4.2.

Damage changes, however, with change of materials, cutting mode and cutting condi-tions, as shown in Figure 4.2 Figure 4.2(a) shows crater and flank wear, with negligible notch wear, after turning a medium carbon steel with a carbide tool at high cutting speed

If the process is changed to milling, a large crater wear with a number of cracks becomes the distinctive feature of damage (Figure 4.2(b)) When turning Ni-based super alloys with ceramic tools (Figure 4.2(c)) notch wear at the DOC line is the dominant damage mode while crater and flank wear are almost negligible Figure 4.2(d) shows the result

of turning a carbon steel with a silicon nitride ceramic tool (not to be recommended!)

Large crater and flank wear develop in a very short time In the case of turning b-phase

Ti-alloys with a K-grade carbide tool, large amounts of work material are observed adhered to the tool, and part of the cutting edge is damaged by fracture or chipping (Figure 4.2(e))

Tool damage and its classification 119

Machined surface

Work surface

Depth of cut Crater

VN VC

VB

KT KM

Feed

A

A

Section A-A Chip flow

VBmax

Fig 4.1 Typical wear pattern of a carbide tool

(a) Turning a 0.45% carbon steel (b) Face milling a 0.45% carbon steel

(c) Turning Inconel 718 (d) Turning a 0.45% carbon steel

(e) Turning a Ti alloy

Fig 4.2 Typical tool damage observations – both wear and fracture: (a) Tool: cemented carbide P10, v = 150 m min –1 ,

d = 1.0 mm, f = 0.19 mm rev –1 , t = 5 min; (b) tool: cemented carbide P10, v = 400 m min –1 , d = 1.0 mm, f = 0.19

mm tooth –1 , t = 5 min; (c) tool: Al2O3/TiC ceramic tool, v = 100 m min –1 , d = 0.5 mm, f = 0.19 mm rev –1 , t = 0.5 min; (d) tool: Si3N4ceramic tool, v = 300 m min –1 , d = 1.0 mm, f = 0.19 mm rev –1 , t = 1 min; (e) tool: cemented carbide P10, v = 150 m min –1 , d = 0.5 mm, f = 0.1 mm rev –1 , t = 2 min.

4.1.2 Causes of tool damage

Chapter 2.4 outlined the general conditions leading to abrasive, adhesive and chemical wear mechanisms In the context of cutting tool damage, the importance and occurrence of these mechanisms can be classified by cutting temperature, as shown in Figure 4.3 Three causes

of damage are qualitatively identified in the figure: mechanical, thermal and adhesive Mechanical damage, which includes abrasion, chipping, early fracture and fatigue, is basi-cally independent of temperature Thermal damage, with plastic deformation, thermal diffu-sion and chemical reaction as its typical forms, increases drastically with increasing temperature (It should be noted that thermal diffusion and chemical reaction are not the direct cause of damage Rather, they cause the tool surface to be weakened so that abrasion, mechanical shock or adhesion can then more easily cause material removal.) Damage based

on adhesion is observed to have a local maximum in a certain temperature range

Mechanical damage

Whether mechanical damage is classified as wear or fracture depends on its scale Figure

4.4 illustrates the different modes, from a scale of less than 0.1 mm to around 100 mm (much greater than 100 mm becomes failure).

Abrasive wear (illustrated schematically in Figure 2.29) is typically caused by sliding

Tool damage and its classification 121

Adhesion

Thermal damage

Plastic deformation Thermal diffusion Chemical reaction

Cutting temperature

Mechanical damage

Abrasion Chipping Fracture Fatigue

Fig 4.3 Tool damage mechanisms and cutting temperature

Chipping

Micro chipping Abrasion

Fracture Attrition

Damage size ( µm)

Fig 4.4 Classification of mechanical damages

hard particles against the cutting tool The hard particles come from either the work mater-ial’s microstructure, or are broken away from the cutting edge Abrasive wear reduces the harder is the tool relative to the particles and generally depends on the distance cut (see Section 4.2.2)

Attrition wear occurs on a scale larger than abrasion Particles or grains of the tool material are mechanically weakened by micro-fracture as a result of sliding interaction with the work, before being removed by wear

Next in size comes chipping (sometimes called micro-chipping at its small-scale limit) This is caused by mechanical shock loading on a scale that leads to large fluctuations in cutting force, as opposed to the inherent local stress fluctuations that cause attrition Finally, fracture is larger than chipping, and is classified into three types: early stage, unpredictable and final stage The early stage occurs immediately after beginning a cut if the tool shape or cutting condition is improper; or if there is some kind of defect in the cutting tool or in its edge preparation Unpredictable fracture can occur at any time if the stress on the cutting edge changes suddenly, for example caused by chattering or an irreg-ularity in the workpiece hardness Final stage fracture can be observed frequently at the end of a tool’s life in milling: then fatigue due to mechanical or thermal stresses on the cutting edge is the main cause of damage

Thermal damage – plastic deformation

The plastic deformation type of thermal damage referred to in Figure 4.3 is observed when

a cutting tool at high cutting temperature cannot withstand the compressive stress on its cutting edge It therefore occurs with tools having a high temperature sensitivity of their hardness as their weakest characteristic Examples are high speed steel tools in general; and high cobalt content cemented carbide tools, or cermet tools, used in severe conditions, particularly at a high feed rate Deformation of the edge leads to generation of an improper shape and rapid material removal

Thermal damage – diffusion

Wear as a result of thermal diffusion occurs at high cutting temperatures if cutting tool and work material elements diffuse mutually into each other’s structure This is well known with cemented carbide tools and has been studied over many years, by Dawihl (1941), Trent (1952), Trigger and Chao (1956), Takeyama and Murata (1963), Gregory (1965),

Cook (1973), Uehara (1976), Narutaki and Yamane (1976), Usui et al (1978) and others.

The rates of processes controlled by diffusion are exponentially proportional to the

inverse of the absolute temperature q In the case of wear, different researchers have proposed different pre-exponential factors: Cook (1973) suggested depth wear h should increase with time t (equation 4.1(a)); earlier, Takeyama and Murata (1963) also suggested this and the further possibility of sliding distance s being a more fundamental variable (equation 4.1(b)); later Usui et al (1978), following the ideas of contact mechanics and

wear considered in Chapter 2.4, proposed wear should also increase with normal contact

stress sn(equation 4.1(c)) In all these cases, a plot of ln(wear rate) against 1/q gives a straight line, the slope of which is –C2