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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 was considered. Both pre-strain and an increased rake angle result in reduced specific cutting forces 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°. Figure 3.6 shows examples of specific forces and shear plane angles measured in turning annealed and heavily cold-worked copper at feeds in the range 0.15 to 0.2 mm, with high speed steel tools of rake angle from 6° to 35°. Specific forces vary over a sixfold range at the lowest cutting speed, with shear plane angles from 8° to 32°.
The left panel of Figure 3.7 shows that the estimated tool contact stresses change little with rake angle, although they are clearly larger for the annealed than the pre-strained material. The right-hand panel shows that the temperature rises are halved on changing from a 6° to 35° rake angle tool. These observations, that tool stresses are determined bythe material being cut and do not vary much with the cutting conditions, while temperatures depend strongly on both the material being cut and the cutting conditions, is a continuing theme that will be developed for metal alloys in the following sections.
3.1.3 Machining copper and aluminium alloys
It is often found that alloys of metals machine with larger shear plane angles and hence lower 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 friction (as indicated by the friction coefficient) is less; and at others again it is not at all obvious why this should be so. But even when the specific forces are lower, the tool contact stress can be higher. In this section, examples of machining two copper and one aluminium alloy are taken to illustrate this.
Figure 3.8 records the behaviours of a CuNi and a CuZn alloy. The CuNi alloy, with 80%Ni, might better be considered as a Ni alloy. However, it machines at a higher shear plane angle at a given cutting speed than either copper or nickel, despite its strain-hardening 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 characteristic (Appendix 4.1 again) and an apparently higher friction interaction with the tool (as judged by the relative sizes of its specific thrust and cutting forces). (Figure 3.8 describes the 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 for the easier machining of the alloys compared with the elemental metals is not obvious from their room temperature, low strain rate mechanical behaviours.
Figure 3.9 shows machining data for an aluminium alloy. In this case the variation of behaviour 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 is half as large for the alloy as for pure Al. In this case both the strain-hardening and friction factors are less for the alloy than for pure Al.
For both the copper and aluminium alloy examples, the primary shear plane shear stress and the average rake contact stresses are similar to, or slightly larger than, those for theelemental metals. Figure 3.8 shows only the values of k, but (sn)av may be calculated to be ≈ 0.6k. Figure 3.9 shows both k and (sn)av. It also shows that, in this case, the estimated rake 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?
The choice in Figure 3.9 of showing how machining parameters vary with rake angle has 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 appropriate for machining thin walled structures, for which thrust forces might cause distortions in the finished 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)av range 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 the aluminium and copper alloys. Although their specific forces are in the same range and their shear plane angles are higher, the tool stresses and temperatures (for a given speed and feed) that they generate are significantly higher. Figure 3.10 presents observations for two austenitic 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 an extremly difficult to machine creep and abrasion resistant material. The NiCr alloy is a commercial 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 was 0°. Specific cutting forces are in the range 2 to 4 GPa. Thrust forces are mainly between 1 and 2 GPa. Shear plane angles are mainly greater than 25°. In most cases, the chip formation is not steady but serrated. The values shown in Figure 3.10 are average values. Figure 3.11 shows stresses and temperatures estimated from these. The larger stresses and temperatures are clear.
3.1.5 Machining carbon and low alloy steels
Carbon and alloy steels span the range of machinability between aluminium and copper alloys 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 be achieved by alloying iron with carbon and small amounts of other metals, results in their spanning the range as far as tool stressing is concerned. Their intermediate thermal conductivities and diffusivities result in their spanning the range with respect to temperature rise per unit feed and also cutting speed.
Figure 3.12 shows typical specific force and shear plane angle variations with cutting speed measured in turning steel bars that have received no particular heat treatment other than the hot rolling process used to manufacture them. At cutting speeds around 100 m/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 same way as for many other alloy systems, the shear plane angles of the ferrous alloys are larger than for the machining of pure iron. In the hot rolled condition, steels (other than the austenitic steels considered in the previous section) have a structure of ferrite and pearlite (or, at high carbon levels, pearlite and 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)av values from 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)av with %C is clear. The right panel of the figure likewise shows that the increasing carbon content gives rise to increasing temperatures for a given cutting speed. This comes from the increasing shear stress levels.
This completes this brief survey of the stresses and temperatures generated by different alloy groups in machining. Tool stresses are mainly controlled by the metal being machined and vary little with cutting conditions (although the tool rake face area over which they act changes with speed and, obviously, also with feed). Temperatures, on the other hand, depend not only on the material being machined (both through stress levels and thermal 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 100 m/min. This is because, at slightly lower cutting speeds, at the feeds considered, those steels machine with a built-up edge (BUE). In Chapter 2, photographs were shown of BUE formation. Figure 3.14 shows, for a 0.15C steel, what changes in specific force and shear plane 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 minimum and the shear plane angle through a maximum. Qualitatively, this may be explained by the BUE 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 at low 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 the feed increases. Figure 3.15 gathers data for three ferrous alloys and one Ni-Cr creep resistant alloy (Nimonic 80). One definition of high speed machining is machining at speeds above those of built-up-edge formation. These are the conditions mostly focused on in this book.
3.1.7 Summary
Section 3.1 mentioned the variety of specific forces and shear plane angles that are commonly observed in machining aluminium, copper, ferrous, nickel and titanium alloys. It has sought to establish that the average contact stresses that a tool must withstand depend mainly on the material being machined, through the level of that material’s shear flow stress and hardly at all on the cutting speed and feed nor on the tool rake angle. Table 3.4 lists the range of these stresses. Peak contact stresses may be two to three times as large as the average values recorded in the table. In contrast, the temperatures that a tool must withstand do depend on cutting speed and feed and rake angle, and on the work material’s 96 Work and tool materials 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 stress severity 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 titanium alloys. Ferritic and pearlitic steels lie between these extremes, with stresses and temperatures increasing with carbon content and hardness. Beyond that, this section has been mainly descriptive, particularly with respect to reporting what shear plane angles have been measured for the different alloys. This remains the main task of predictive mechanics. The next section, on tool material properties, complements this one, in describing the properties of tool materials that influence and enable the tools to withstand the machininggenerated stresses and temperature .