Metal Machining Episode 4 pot

A picture is built up of thestress and temperature conditions that a tool must survive in machining these materials.The primary shear plane shear stress is estimated from Fc cos f – FTsi

recently, these were not machinable The data come from compression testing at roomtemperature and at low strain rates of initially unworked metal The detail is presented inAppendix 4.1 Although machining generates high strain rates and temperatures, these dataare useful as a first attempt to relate the severity of machining to work material plastic flowbehaviour A more detailed approach, taking into account variations of material flow stresswith strain rate and temperature, is introduced in Chapter 6.

Work heating is also considered in Chapter 2 Temperature rises in the primary shear

zone and along the tool rake face both depend on fUworktanf/kwork Figure 3.2(a) rizes the conclusions from equation (2.14) and Figures 2.17(a) and 2.18(b) In the primary

summa-shear zone the dimensionless temperature rise DT(rC)/k depends on fUworktanf/kworkand

the shear strain gï Next to the rake face, the additional temperature rise depends on

fUworktanf/kwork and the ratio of tool to work thermal conductivity, K* Figure 3.2(b)

summarizes the typical thermal properties of the same groups of work materials whosemechanical properties are given in Figure 3.1 The values recorded are from room temper-ature to 800˚C Appendix 4.2 gives more details

Figures 3.1 and 3.2 suggest that the six groups of alloys may be reduced to three as far

as the mechanical and thermal severity of machining them is concerned Copper andaluminium alloys, although showing high work hardening rates, have relatively low shearstresses and high thermal diffusivities They are likely to create low tool stresses and lowtemperature rises in machining At the other extreme, austenitic steels, nickel and titaniumalloys have medium to high shear stresses and work hardening rates and low thermal diffu-sivities They are likely to generate large tool stresses and temperatures The body centredcubic carbon and alloy steels form an intermediate group

The behaviours of these three different groups of alloys are considered in Sections 3.1.3

to 3.1.5 of this chapter, after sections in which the machining of unalloyed metals is

Fig 3.1 Shear stress levels and work hardening severities of initially unstrained, commonly machined, aluminium,

copper, iron (b.c.c and f.c.c.), nickel and titanium alloys

described It will be seen that these groups do indeed give rise to three different levels oftool stress and temperature severity This is demonstrated by presenting representativeexperimentally measured specific cutting forces (forces per unit feed and depth of cut) andshear plane angles for these groups as a function of cutting speed Then, primary shear zone

shear stress k, average normal contact stress on the rake face (sn)avand average rake face

contact temperature (Trake)avare estimated from the cutting data A picture is built up of thestress and temperature conditions that a tool must survive in machining these materials.The primary shear plane shear stress is estimated from

(Fc cos f – FTsin f)sin f

Finally, temperatures are estimated after the manner summarized in Figure 3.2

Fig 3.2 Thermal aspects of machining: (a) a summary of heating theory and (b) thermal property ranges of Al, Cu,

Fe, Ni and Ti alloys

The machining data come mainly from results in the authors’ possession The exceptionare data on the machining of the aluminium alloy Al2024 (Section 3.1.2), which are fromresults by Kobayashi and Thomsen (1959) The data on machining elemental metals comefrom the same experiments on those metals considered by Trent in his book (Trent, 1991).

3.1.1 Machining elemental metals

Although the elemental metals copper, aluminium, iron, nickel and titanium have littlecommercial importance as far as machining is concerned (with the exception of aluminiumused for mirrors and disk substrates in information technology applications), it is interest-ing to describe how they form chips: what specific forces and shear plane angles areobserved as a function of cutting speed The behaviour of alloys of these materials can then

be contrasted with these results Figure 3.3 shows results from machining at a feed of 0.15

mm with high speed steel (for copper and aluminium) and cemented carbide (for iron,nickel and titanium) tools of 6˚ rake angle

At the lowest cutting speeds (around 30 m/min), except for titanium, the metalsmachine with very large specific forces, up to 8 GPa for iron and nickel and around 4 GPafor copper and aluminium These forces are some ten times larger than the expected shearflow stresses of these metals (Figure 3.1) and arise from the very low shear plane angles,between 5˚ and 8˚, that occur These shear plane angles give shear strains in the primaryshear zone of from 7 to 12 As cutting speed increases to 200 m/min, the shear plane anglesincrease and the specific forces are roughly halved Further increases in speed cause muchless variation in chip flow and forces The titanium material is an exception Over thewhole speed range, although decreases of specific force and increases of shear plane anglewith cutting speed do occur, its shear plane angle is larger and its specific forces are

Fig 3.3 Cutting speed dependence of specific forces and shear plane angles for some commercially pure metals (f = 0.15 mm, α = 6º)

smaller than for the other, more ductile, metals A reduction in forces and an increase inshear angle with increasing speed, up to a limit beyond which further changes do notoccur, is a common observation that will also be seen in many of the following sections.Although the forces fall with increasing speed, the process stresses remain almost

constant Figure 3.4 shows aluminium to have the smallest primary shear stress, k,

followed by copper, iron, nickel and titanium

The estimated average normal stresses (sn)av lie between 0.5k and 1.0k This would

place the maximum normal contact stresses (which are between two and three times the

average stress) in the range k to 3k This is in line with the estimates in Chapter 2, Figure

2.15

The different thermal diffusivities of the five metals result in different temperature

vari-ations with cutting speed (Figure 3.5) For copper and aluminium, with k taken to be 110

and 90 mm2/s respectively (Appendix 4.2), fUworktanf/kworkhardly rises to 1, even at thecutting speed of 300 m/min Figure 3.2 suggests that then the primary shear temperaturerise dominates the secondary (rake) heating The actual increase in temperature shown in

Fig 3.4 Process stresses, derived from the observations of Figure 3.3

Fig 3.5 Temperatures estimated from the observations of Figure 3.3

Figure 3.5 results from the combined effect of increasing fraction of heat flowing into thechip and reducing shear strain as cutting speed rises.

Iron and nickel, with k taken to be 15 and 20 mm2/s respectively, machine with

fUworktanf/kwork in the range 1 to 10 in the conditions considered In Figure 3.5, theprimary shear and average rake face temperatures are distinctly separated Over much ofthe speed range, the temperature actually falls with increasing cutting speed This unusualbehaviour results from the reduction of strain in the chip as speed increases

Finally, titanium, with k taken to be 7.5 mm2/s, machines with fUworktanf/kworkfrom 7

to 70 The rake face heating is dominant and a temperature in excess of 800˚C is estimated

at the cutting speed of 150 m/min

3.1.2 Effects of pre-strain and rake angle in machining copper

In the previous section, the machining of annealed metals by a 6˚ rake angle tool wasconsidered Both pre-strain and an increased rake angle result in reduced specific cuttingforces and reduced cutting temperatures, but have little effect on the stressses on the tool.These generalizations may be illustrated by the cutting of copper, a metal sufficiently soft(as also is aluminium) to allow machining by tools of rake angle up to around 40˚ Figure3.6 shows examples of specific forces and shear plane angles measured in turning annealedand heavily cold-worked copper at feeds in the range 0.15 to 0.2 mm, with high speed steeltools of rake angle from 6˚ to 35˚ Specific forces vary over a sixfold range at the lowestcutting speed, with shear plane angles from 8˚ to 32˚

The left panel of Figure 3.7 shows that the estimated tool contact stresses change littlewith rake angle, although they are clearly larger for the annealed than the pre-strainedmaterial The right-hand panel shows that the temperature rises are halved on changingfrom a 6˚ to 35˚ rake angle tool These observations, that tool stresses are determined by

Fig 3.6 Specific force and shear plane angle variations for annealed (•) and pre-strained (o) commercially pure copper

(f = 0.15 to 0.2 mm, α = 6º to 35º)

the material being cut and do not vary much with the cutting conditions, while tures depend strongly on both the material being cut and the cutting conditions, is a contin-uing theme that will be developed for metal alloys in the following sections.

tempera-3.1.3 Machining copper and aluminium alloys

It is often found that alloys of metals machine with larger shear plane angles and hencelower specific forces than the elemental metals themselves Sometimes a strong reason is

a lower value of the strain hardening parameter Dk/kmax, at other times the chip/tool tion (as indicated by the friction coefficient) is less; and at others again it is not at all obvi-ous why this should be so But even when the specific forces are lower, the tool contactstress can be higher In this section, examples of machining two copper and one aluminiumalloy are taken to illustrate this

fric-Figure 3.8 records the behaviours of a CuNi and a CuZn alloy The CuNi alloy, with80%Ni, might better be considered as a Ni alloy However, it machines at a higher shearplane angle at a given cutting speed than either copper or nickel, despite its strain-harden-ing characteristic being similar to or more severe than either of these (Appendix 4.1) The

CuZn alloy (an a-brass) is a well-known very easy material to machine Its shear plane

angle is twice as large as that of Cu, despite having a similar strain-hardening tic (Appendix 4.1 again) and an apparently higher friction interaction with the tool (asjudged by the relative sizes of its specific thrust and cutting forces) (Figure 3.8 describesthe machining of an annealed brass After cold-working, even higher shear plane angles,and lower specific forces are obtained.) These two examples are ones where the reason forthe easier machining of the alloys compared with the elemental metals is not obvious fromtheir room temperature, low strain rate mechanical behaviours

characteris-Figure 3.9 shows machining data for an aluminium alloy In this case the variation ofbehaviour with rake angle is shown At a rake angle and speed comparable to that shown

in Figure 3.3, the shear plane angle is five times as large and the specific cutting force ishalf as large for the alloy as for pure Al In this case both the strain-hardening and frictionfactors are less for the alloy than for pure Al

For both the copper and aluminium alloy examples, the primary shear plane shear stressand the average rake contact stresses are similar to, or slightly larger than, those for the

Fig 3.7 Average rake face contact stresses and temperatures, from the results of Figure 3.6

elemental metals Figure 3.8 shows only the values of k, but (sn)avmay be calculated to be

≈ 0.6k Figure 3.9 shows both k and (sn)av It also shows that, in this case, the estimatedrake face temperature does not change as the rake angle is reduced This is different fromthe observations recorded in Figure 3.7: perhaps the maximum temperature is limited bymelting of the aluminium alloy?

Fig 3.8 Observed and calculated machining parameters for two copper alloys (f = 0.15 mm, α = 6º)

Fig 3.9 Machining parameter variation with rake angle for Al22024-T4 alloy, at a cutting speed of 175 m/min and f

= 0.25 mm

The choice in Figure 3.9 of showing how machining parameters vary with rake anglehas been made to introduce the observation that, in this case, at a rake angle of around 35˚the thrust force passes through zero Consequently, such a high rake angle is appropriatefor machining thin walled structures, for which thrust forces might cause distortions in thefinished part.

However, the main point of this section, to be carried forward to Section 3.2 on tool

materials, is that the range of values estimated for k follows the range expected from Figure 3.1 and the estimated values of (sn)avrange from 0.5 to 1.0k This is summarized

in Table 3.4 which also contains data for the other alloy systems to be considered next

3.1.4 Machining austenitic steels and temperature resistant nickel and titanium alloys

The austenitic steels, NiCr, and Ti alloys are at the opposite extreme of severity to thealuminium and copper alloys Although their specific forces are in the same range and theirshear plane angles are higher, the tool stresses and temperatures (for a given speed and

Table 3.4 Approximate ranges of k and (σ n )avestimated from machining tests

feed) that they generate are significantly higher Figure 3.10 presents observations for twoaustenitic steels, a NiCr and a Ti alloy One of the austenitic steels (the 18Cr8Ni material)

is a common stainless steel The 18Mn5Cr material, which also contains 0.47C, is anextremly difficult to machine creep and abrasion resistant material The NiCr alloy is acommercial Inconel alloy, X750 In all cases the feed was 0.2 mm except for the Ti alloy,for which it was 0.1 mm The rake angle was 6˚ except for the NiCr alloy, for which it was0˚ Specific cutting forces are in the range 2 to 4 GPa Thrust forces are mainly between 1and 2 GPa Shear plane angles are mainly greater than 25˚ In most cases, the chip forma-tion is not steady but serrated The values shown in Figure 3.10 are average values Figure3.11 shows stresses and temperatures estimated from these The larger stresses and temper-atures are clear

3.1.5 Machining carbon and low alloy steels

Carbon and alloy steels span the range of machinability between aluminium and copperalloys on the one hand and austentic steels and temperature resistant alloys on the other.There are two aspects to this The wide range of materials’ yield stresses that can beachieved by alloying iron with carbon and small amounts of other metals, results in theirspanning the range as far as tool stressing is concerned Their intermediate thermalconductivities and diffusivities result in their spanning the range with respect to tempera-ture rise per unit feed and also cutting speed

Fig 3.11 Process stresses and temperatures derived from (and symbols as) Figure 3.10

Figure 3.12 shows typical specific force and shear plane angle variations with cuttingspeed measured in turning steel bars that have received no particular heat treatment otherthan the hot rolling process used to manufacture them At cutting speeds around 100m/min the specific forces of 2 to 3 GPa are smaller than those for pure iron (Figure 3.3),but as speed increases, the differences between the steels and pure iron reduce In the sameway as for many other alloy systems, the shear plane angles of the ferrous alloys are largerthan for the machining of pure iron.

In the hot rolled condition, steels (other than the austenitic steels considered in theprevious section) have a structure of ferrite and pearlite (or, at high carbon levels, pearliteand cementite) For equal coarsenesses of pearlite, the steels’ hardness increases with

carbon content The left panel of Figure 3.13 shows how the estimated k and (sn)avvaluesfrom the data of Figure 3.12 increase with carbon content Additional results have been

included, for the machining of a 0.13C and a 0.4C steel An increase of both k and (sn)avwith %C is clear The right panel of the figure likewise shows that the increasing carbon

Fig 3.12 Representative specific force and shear plane angle variations for hot rolled carbon and alloy steels (f = 0.15

mm, α = 6º)

Fig 3.13 Process stresses and temperatures derived from Figure 3.12

content gives rise to increasing temperatures for a given cutting speed This comes fromthe increasing shear stress levels.

This completes this brief survey of the stresses and temperatures generated by differentalloy groups in machining Tool stresses are mainly controlled by the metal beingmachined and vary little with cutting conditions (although the tool rake face area overwhich they act changes with speed and, obviously, also with feed) Temperatures, on theother hand, depend not only on the material being machined (both through stress levels andthermal properties) but also on the speeds and feeds used

3.1.6 Machining with built-up edge formation

In the previous section, data were presented mainly for cutting speeds greater than 100m/min This is because, at slightly lower cutting speeds, at the feeds considered, thosesteels machine with a built-up edge (BUE) In Chapter 2, photographs were shown of BUEformation Figure 3.14 shows, for a 0.15C steel, what changes in specific force and shearplane angle are typically associated with this In this example, the largest BUE occurred at

a cutting speed close to 25 m/min There, the specific forces passed through a minimumand the shear plane angle through a maximum Qualitatively, this may be explained by theBUE increasing the effective rake angle of the cutting tool

Built-up edge formation occurs at some low speed or other for almost all metal alloys

It offers a way of relieving the large strains (small shear plane angles) that can occur atlow speeds, but at the expense of worsening the cut surface finish For those alloys that

do show BUE formation, the cutting speed at which the BUE is largest reduces as thefeed increases Figure 3.15 gathers data for three ferrous alloys and one Ni-Cr creepresistant alloy (Nimonic 80) One definition of high speed machining is machining atspeeds above those of built-up-edge formation These are the conditions mostly focused

on in this book

Fig 3.14 Characteristics of built-up edge (BUE) formation (0.15C steel, f = 0.15 mm, α = 6º)

3.1.7 Free-cutting alloys

It is possible to make minor changes to the composition of alloys that result in majorimprovements in their machinability The data considered up to this point have not beenfor such alloys The effects of such composition changes will now be introduced, byconsidering first of all the machining of free-cutting low carbon steels

Most carbon steels contain manganese, controlled at a level of around 1%, and sulphur

as an impurity, up to a level of around 0.05% One of the non-metallic inclusions that exists

is manganese sulphide, MnS If the sulphur is increased to 0.2% to 0.3% and themanganese is also increased (typical values are 1–1.5%), the amount of MnS is increasedand becomes important It can, in some conditions, form a layer over the chip/tool contactthat can reduce chip/tool friction and hence ease chip formation Lead (Pb) can also beadded, commonly at a level of around 0.25% It can further lubricate the contact Themagnitude of the friction change has already been introduced, in Section 2.4 (Figure 2.22).The action (of MnS forming a layer in the contact area) is specific to high speed steels andcutting tools containing Ti, that is to say cemented carbides (or cermets) containing TiC ormixed TiC/TaC; and to tools coated by TiN or TiC The lubrication is only effective over

a certain contact temperature range and hence depends on the cutting speed and feed.Figure 3.16 shows a typical effect of this lubricating action The specific forces and shearplane angles observed in turning a MnS and a Pb-MnS free-cutting low carbon (0.08 to0.09C) steel are compared with those for a similar non-free-cutting steel At cutting speedsbetween 20 m/min and 75 m/min (at the feeds considered) the shear plane angles of thefree-cutting materials are double and the specific forces around half of those for the non-free cutting steel (the built-up-edge is much smaller and more stable too) As cutting speedincreases up to 200 m/min for the MnS steel and to 300 m/min for the Pb-MnS steel, thesedifferences between the free- and non-free-cutting steels become insignificant Althoughthere is clear benefit in reduced forces from the free-cutting steels, there is no reduction in

the tool normal contact stresses For all the steels in Figure 3.16, k values are estimated between 400 MPa and 450 MPa (in line with Figure 3.13) (sn)avvalues around 300 MPaare estimated for the non-free-cutting steel (also in line with Figure 3.13), but values from

350 MPa to 400 MPa are estimated for the free-cutting steels

Fig 3.15 Speed and feed dependence of built-up edge formation, after Trent (1991)

These free-cutting steels have a great commercial importance They enable small eter, intricate, parts such as spacers, screwed profiles and small electric motor spindles to

diam-be machined with a good surface finish and with less energy consumption than the alent non-free-cutting steel, in the speed range where the non-free-cutting steel wouldsuffer from the poor finish associated with built-up edge formation The free-cutting steelsare, however, less tough than their non-free-cutting equivalents and are not used in appli-cations in which the transmission of tensile stresses is critical Semi-free-cutting grades ofsteel have been developed to compromise between machinability and strength require-ments These have been developed by control of the wide variety of non-metallic inclu-sions that can be created during the deoxidation of steel melts, as considered next.Free oxygen in steel is removed from the melt most simply by adding small amounts ofaluminium, silicon or calcium, to form alumina, silica or calcium oxides Alumina is hardand abrasive and is certainly detrimental to tool life in machining The addition of siliconand calcium can result in softer inclusions It has been found that if, in addition, smallamounts of sulphur (relative to the 0.2% to 0.3% used in free-cutting steels) are added,complex layers containing calcium, manganese and sulphur can build up on the rake face

equiv-of tools Again, the tools have to contain titanium These layers have relatively smalleffects in altering specific forces and shear plane angles, but can significantly influencetool life Typical quantities of calcium are 0.002% and of sulphur 0.03 to 0.1% (with sili-con from 0.2 to 0.3%) The topics of tool wear and life are developed more fully in Chapter

4 Here, Figure 3.17 shows differences in the machining of a low alloy steel (nominally0.4C1Cr0.2Mo), produced without and with small additions of Ca and S as just described

Fig 3.16 Representative specific force and shear plane angle variations for low carbon free-machining steels turned

by a steel cutting grade of carbide tool (f = 0.1 to 0.15 mm, α = 6º)

The tool was an uncoated steel cutting grade (P-type) carbide Although differences can beseen between the specific forces and shear plane angles for these materials, the estimatedrake contact normal stresses and temperatures are estimated to be hardly different for thetwo Yet the tool wear rates, particularly the crater wear rates, are hugely different.

In Figure 3.17, there is at least some visible change in specific forces and shear planeangle brought about by controlling the deoxidation process In other cases, for example byadding a small amount of calcium but no extra sulphur, changes in tool life can beproduced with no change at all in chip form and forces A study with this conclusion, for

machining a 0.45% carbon steel, has been published by Sata et al (1968) The reader is

reminded of the comment at the start of this chapter, that stresses and temperatures definethe continuum conditions to which the cutting tool is subjected, but life (other than imme-diate failure) depends, in addition, on the work material’s microstructure and chemicalinteractions with the tool

This section has considered only free-cutting and semi-free-cutting steels Free-cuttingversions of other alloys are also manufactured The best known are leaded copper andaluminium alloys, but the purpose of the lead is different from that considered so far Up

to 1% or 2% lead causes embrittlement of chips and hence aids chip control and ability as well as reducing specific forces

as the average values recorded in the table In contrast, the temperatures that a tool mustwithstand do depend on cutting speed and feed and rake angle, and on the work material’s

Fig 3.17 Machining characterisitcs of a low alloy (•) and a semi-free-cutting low alloy (o) steel (f = 0.25 mm, α = 6º)

thermal properties: diffusivity, conductivity and heat capacity By both thermal and stressseverity criteria, the easiest metals to machine are alumimium alloys and copper alloys.The most difficult to machine are austenitic steels, nickel heat resistant alloys and titaniumalloys Ferritic and pearlitic steels lie between these extremes, with stresses and tempera-tures increasing with carbon content and hardness.

Beyond that, this section has been mainly descriptive, particularly with respect toreporting what shear plane angles have been measured for the different alloys Thisremains the main task of predictive mechanics

The next section, on tool material properties, complements this one, in describing theproperties of tool materials that influence and enable the tools to withstand the machining-generated stresses and temperatures

3.2 Tool materials

The main classes of tool materials have already been listed in Table 3.2 as carbides andcermets, high speed steels, ceramics based on alumina and silicon nitride, and the super-hard materials polycrystalline diamond and cubic boron nitride (single crystal diamondsare also used for the finishing of IT mirror and disc substrate products) Details of the vari-ous materials within these groups are given in Appendix 6 It is recommended that thedescriptive parts of Appendix 6 be read briefly, before continuing The largest amount ofspace is given to dividing the carbides and cermets into sub-groups depending on whetherthe carbides are mainly tungsten carbide (WC) or a mixture of mainly WC with titaniumand tantalum carbides (TiC/TaC) and on whether they are cemented together mainly withcobalt (Co) or a mixture of Co and nickel (Ni) In the following sections, the main purpose

is to compare the properties of these different groups, and to understand why which groupsare used in what circumstances

3.2.1 Tool mechanical property minimum requirements

The sizes of the shear stresses k or kmaxhave been considered in Section 3.1 From now

on, k or kmax will be written kwork, to distinguish work from tool properties Section 3.1 has

established that the majority of work materials are machined with a shear stress kwork

measured on the primary shear plane between 200 MPa and 800 MPa and that the average

normal contact stress on the tool face ranges between 0.5 and 1 kwork In fact, only ened steels, not considered in the previous sections, but which are increasingly machined

hard-by the superhard polycrystalline cubic boron nitride (PcBN), are likely to yield values of

kwork greater than 800 MPa In Chapter 2 it was suggested that peak normal contactstresses (at the cutting edge) may be two to three times as large as the average stress; that

is to say, in the range 1 to 3 kwork This is supported by split-tool contact stress ments (Figure 2.21) Split-tool measurements have also given tool rake face friction

measure-stresses t from 0.5 to 1 kwork, depending on rake face temperature (Figure 2.22) Theseloadings are summarized in Figure 3.18(a)

Figure 3.18(b) also shows some other possible loadings When a tool enters a cut, afinite displacement is required before the chip is fully developed Initially the contact can

look more like an indentation Then, the peak normal stress may be as large as 5kwork(this

is approximately the Vickers Hardness, or HV, value) Because the sliding of the chip over