填料箱蓋多頭鉆床設計-多孔鉆專機含11張CAD圖

填料箱蓋多頭鉆床設計-多孔鉆專機含11張CAD圖,填料,多頭,鉆床,設計,多孔,專機,11,十一,cad 3 Work and tool materials In Chapter 2, the emphasis is on the mechanical, thermal and friction conditions of chip formation. The different work and tool materials of interest are introduced only as exam- ples. In this chapter, the materials become the main interest. Table 3.1 summarizes some of the main applications of machining, by industrial sector and work material group, while Table 3.2 gives an overview of the classes of tool materials that are used. In Section 3.1 data will be presented of typical specific forces, tool stresses and temperatures generated when machining the various work groups listed in Table 3.1. In Section 3.2 the properties of the tools that resist those stresses and temperatures will be described. A metal’s machinability is its ease of achieving a required production of machined components relative to the cost. It has many aspects, such as energy (or power) consumption, chip form, surface integrity and finish, and tool life. Low energy consumption, short (broken) chips, smooth finish and long tool life are usually aspects of good machinability. Some of these aspects are directly related to the continuum mechanical and thermal conditions of the Table 3.1 Some machining activities by work material alloy and industrial sector Alloy system General engineering Auto-motive Aerospace Process engineering Information technology Carbon and Structures Power train, Power train, Structures Printer alloy steels fasteners, steering, control and spindles and power train, suspension, landing mechanisms hydraulics hydraulics gear fasteners Stainless For corrosion – Turbine For corrosion – steels resistance blades resistance Aluminium Structures Engine block Airframe For corrosion Scanning and pistons spars, skins resistance mirrors, disc substrates Copper – – – For corrosion – resistance Nickel – – Turbine Heat – blades and exchangers, discs and corrosion resistance Titanium – – Compressor/ Corrosion – airframe resistance Table 3.2 Recommended tool and work material combinations Soft non- ferrous (Al, Cu) Carbon/ low alloy steels Hardened tool and die steels Cast iron Nickel -based alloys Titanium alloys High speed steel O/? O/? x ?/x ?/x ?/x Carbide (inc. coated) O √/O ? √/O √ O Cermet ?/x √ x ? x x Ceramic x √/O O √ √/O x cBN ?/x x √ √/O O O PCD √ x x x x √ √ good; O all right in some conditions; ? possible but not advisable; x to be avoided. Table 3.3 Mechanical, thermal and materials factors affecting machinability Main tools for study Process variables Machinability attribute Cutting speed and feed Chip form Tool shape Tool forces Applied mechanics Work mechanical and thermal properties Power consumption and thermal analysis Tool thermal properties Tool stresses and temperatures Tool failure properties Tool failure Chip/tool friction laws Surface integrity and finish Materials engineering Work/tool wear interactions Tool wear and life 3.1 Work material characteristics in machining machining process. In principle, they may be predicted by mechanical and thermal analy- sis (but at the current time some are beyond prediction). Other aspects, principally tool life, depend not only on the continuum surface stresses and temperatures that are generated but also on microstructural, mechanical and chemical interactions between the chip and the tool. Table 3.3 summarizes these relations and the principal disciplines by which they may be studied (perhaps chip/tool friction laws should come under both the applied mechanics and materials engineering headings?). This chapter is mainly concerned with the work material’s mechanical and thermal properties, and tool thermal and failure properties, which affect machinability. Tool wear and life are so important that a separate chapter, Chapter 4, is devoted to these subjects. According to the analysis in Chapter 2, cutting and thrust forces per unit feed and depth of cut, and tool stresses, are expected to increase in proportion to the shear stress on the primary shear plane, other things being equal. This was sometimes written k and some- times kmax. Forces also increase the smaller is the shear plane angle and hence the larger is the strain in the chip. The shear plane angle, however, reduces the larger is the strain harden- ing in the primary shear region, measured by Dk/kmax (equation (2.7)). Thus, kmax and Dk/kmax are likely to be indicators of a material’s machinability, at least as far as tool forces and stresses and power consumption are concerned. Figure 3.1 gathers information on the typical values of these quantities for six different groups of work materials that are impor- tant in machining practice. The data for steels exclude quench hardened materials as, until 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 recently, these were not machinable. The data come from compression testing at room temperature and at low strain rates of initially unworked metal. The detail is presented in Appendix 4.1. Although machining generates high strain rates and temperatures, these data are useful as a first attempt to relate the severity of machining to work material plastic flow behaviour. A more detailed approach, taking into account variations of material flow stress with 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) summa- rizes the conclusions from equation (2.14) and Figures 2.17(a) and 2.18(b). In the primary shear zone the dimensionless temperature rise DT(rC)/k depends on fUworktanf/kwork and the shear strain g. 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 whose mechanical 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 and aluminium alloys, although showing high work hardening rates, have relatively low shear stresses and high thermal diffusivities. They are likely to create low tool stresses and low temperature rises in machining. At the other extreme, austenitic steels, nickel and titanium alloys 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 centred cubic 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. 3.2 Thermal aspects of machining: (a) a summary of heating theory and (b) thermal property ranges of Al, Cu, Fe, Ni and Ti alloys described. It will be seen that these groups do indeed give rise to three different levels of tool stress and temperature severity. This is demonstrated by presenting representative experimentally measured specific cutting forces (forces per unit feed and depth of cut) and shear 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)av and average rake face contact temperature (Trake)av are estimated from the cutting data. A picture is built up of the stress and temperature conditions that a tool must survive in machining these materials. The primary shear plane shear stress is estimated from (Fc cos f – FT sin f)sin f k = ——————————— (3.1) fd The average normal contact stress on the tool rake face is estimated from the measured normal component of force on the rake face, the depth of cut and the chip/tool contact length lc: Fc cos a – FT sin a (sn)av = ———————— (3.2) lcd lc is taken, from the mean value data of Figure 2.9(a), to be cos(f – a) lc = 1.75f ————— [m + tan(f – a)] (3.3) sin f Finally, temperatures are estimated after the manner summarized in Figure 3.2. The machining data come mainly from results in the authors’ possession. The exception are data on the machining of the aluminium alloy Al2024 (Section 3.1.2), which are from results by Kobayashi and Thomsen (1959). The data on machining elemental metals come from 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 little commercial importance as far as machining is concerned (with the exception of aluminium used 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 are observed 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 metals machine with very large specific forces, up to 8 GPa for iron and nickel and around 4 GPa for copper and aluminium. These forces are some ten times larger than the expected shear flow 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 primary shear zone of from 7 to 12. As cutting speed increases to 200 m/min, the shear plane angles increase and the specific forces are roughly halved. Further increases in speed cause much less variation in chip flow and forces. The titanium material is an exception. Over the whole speed range, although decreases of specific force and increases of shear plane angle with 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) 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 smaller than for the other, more ductile, metals. A reduction in forces and an increase in shear angle with increasing speed, up to a limit beyond which further changes do not occur, 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/kwork hardly rises to 1, even at the cutting speed of 300 m/min. Figure 3.2 suggests that then the primary shear temperature rise dominates the secondary (rake) heating. The actual increase in temperature shown in Figure 3.5 results from the combined effect of increasing fraction of heat flowing into the chip 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, the primary shear and average rake face temperatures are distinctly separated. Over much of the speed range, the temperature actually falls with increasing cutting speed. This unusual behaviour 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/kwork from 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.