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畢業(yè)設(shè)計(jì)翻譯原文 題目名稱： 金屬機(jī)械加工原理及應(yīng)用 院系名稱： 機(jī)電學(xué)院 班 級(jí)： 機(jī)自071班 學(xué) 號(hào)： 200700314109 學(xué)生姓名： 馬昆鵬 指導(dǎo)教師： 胡 敏 2011 年03月 - 1 - Strain gauge dynamometers A common type of dynamometer uses strain gauges to sense elastic strains caused by cutting forces. Figure 5.7 shows a basic elastic beam type dynamometer with gauges bonded to its surface. It also shows an example of a wire-type gauge and a Wheatstone bridge and amplifier system usually used to measure strain changes in the gauges. The main cutting force FC will cause the beam to bend, so that the gauge on the top surface will be placed in tension, that on the bottom surface will be placed in compression, and those gauges on the side surfaces (atthe neutral axis) will experience no strain. Likewise, a feed force will strain the side-face gauges but not those at the top or bottom. The arrangement shown in Figure 5.7 is not sensitive to force along the axis of the beam as this causes equal strain changes in all gauges. The fractional resistance change of a strain gauge (△R/R) is related to its fractional length change or direct strain (△L/L) by its gauge factor Ks: Ks=(△R/R)/( △L/L) (5.3) For wire strain gauges, Ks is typically from 1.75 to 3.5. Strains down to 10–6 may be detected with a bridge circuit. The upper limit of strain is around 2 × 10–3, determined by the elastic limit of the beam. Fig. 5.7 A strain gauged cantilever dynamometer with its bridge circuit A disadvantage of the simple cantilever dynamometer is that the gauges’ strains depend basically on the moment applied to the section at which they are positioned. Fig. 5.8 Octagonal ring and parallel beam dynamometer designs: (a) Octagonal ring type tool dynanometer; (b) parallel beam type tool dynanometer They therefore depend on the gauges’ distance from where the load is applied, as well as on the size of the load. Better designs, less sensitive to where the load is applied, are the octagonal ring and parallel beam designs shown in Figure 5.8. Supporting the load on well-separated thin sections results in the sum of the strains in the gauges being unchanged when the point of application of the load is changed, even though the strains are redistributed between the sections. Fig. 5.9 The loading of a ring by radial and tangential forces It is possible to connect the strain gauges in a bridge circuit so that the output is not sensitive to where the force is applied.The choice of parallel beams or octagonal rings is a matter of manufacturing choice. For both, it is important, as a matter of convenience, to minimize cross-sensitivity between the different orthogonal components of electrical output and mechanical input. Fig. 5.10 The principle of piezoelectric dynamometry Fig. 5.11 A piezoelectric tool dynamometer For the parallel beam design, this is achieved by manufacturing the two sets of beams perpendicular to each other. For the octagonal ring design, it is important to choose a particular shape of octagon. When a circular ring (Figure 5.9) is loaded radially there is zero strain at the positions B and B′, ± 39.9? from the point of application of the radial load; likewise when the ring is loaded tangentially, there is zero strain at A and A′, ± 90? from the load. Gauges placed at A and A′ will respond only to radial loads; and at B and B′ only to tangential loads. The strains will depend on the loads and the ring dimensions (radius R, thickness t and width b) and Young’s modulus E as The manufacture of the ring outer surface as an octagon rather than a cylinder is just a practical matter. The need to generate detectable strain imposes a maximum allowable stiffness on a dynamometer. This, in turn, with the mass of the dynamometer depending on its size or on the mass supported on it, imposes a maximum natural frequency. Simple beam dynamometers, suitable for measuring forces in turning from 10 N to 10 kN, can be designed with natural frequencies of a few kHz. The ring and the strut types of dynamometer tend to have lower values, of several hundred Hz (Shaw, 1984, Chapter 7). These frequencies can be increased tenfold if semiconductor strain gauges (Ks from 100 to 200) are used instead of wire gauges. However, semiconductor gauges have much larger drift problems than wire gauges. They are used only in very special cases (an example will be given in Section 5.2.2). An alternative is to use piezoelectric force sensors. Piezoelectric dynamometers For certain materials, such as single crystals of quartz, Rochelle salt and barium titanate,a separation of charge takes place when they are subjected to mechanical force. This is the piezoelectric effect. Figure 5.10 shows the principle of how it is used to create a three-axis force dynamometer. Each force component is detected by a separate crystal oriented relative to the force in its piezoelectric sensitive direction. Quartz is usually chosen as the piezoelectric material because of its good dynamic (low loss) mechanical properties. Its piezoelectric constant is only ≈ 2 × 10–12 coulombs per Newton. A charge amplifier is therefore necessary to create a useful output. Because the electrical impedance of quartz is high, the amplifier must itself have high input impedance: 105 MW is not unusual. Figure 5.11 shows the piezoelectric equivalent of the dynamometers of Figure 5.8. The stiffness is basically that of the crystals themselves. Commercial machining dynamometers are available with natural frequencies from 2 kHz to 5 kHz, depending on size. 5.2.2 Rake face stress distributions In addition to overall force measurements, the stresses acting on cutting tools are important, as has been indicated in earlier chapters. Too large stresses cause tool failure, and friction stresses strongly influence chip formation. Fig. 5.10 The principle of piezoelectric dynamometry The possibility of using photoelastic studies as well as split-tool methods to determine tool stresses has already been introduced in Chapter 2 (Section 2.4). The main method for measuring the chip/tool contact stresses is the split-tool method (Figure 2.21), although even this is limited – by tool failure – to studying not-too-hard work materials cut by not-too-brittle tools. Figure 5.12 shows a practical arrangement of a strain-gauged split-tool dynamometer. The part B of the tool (tool 1 in Figure 2.21) Fig. 5.11 A piezoelectric tool dynamometer Fig. 5.12 A split-tool dynamometer arrangement has its contact length varied by grinding away its rake face. It is necessary to measure the forces on both parts B and A, to check that the sum of the forces is no different from machining with an unsplit tool. It is found that if extrusion into the gap between the two tool elements (g, in Figure 2.21) is to be prevented, with the surfaces of tools A and B (1 and 2 in Figure 2.21) at the same level, the gap should be less than 5 mm wide (although other designs have used values up to 20 mm and a downward step from ‘tool 1’ to ‘tool 2’). The greatest dynamometer stiffness is required. This is an instance when semiconductor strain gauges are used. Piezoelectric designs also exist. Split-tool dynamometry is one of the most difficult machining experiments to attempt and should not be entered into lightly. The limitation of the method – tool failure, which prevents measurements in many practical conditions that could be used to verify finite element predicted contact stresses and also to measure friction stresses directly – leaves a major gap in machining experimental methods. 5.3 Temperatures in machining There are two goals of temperature measurement in machining. The more ambitious is quantitatively to measure the temperature distribution throughout the cutting region. However, it is very difficult, because of the high temperature, commonly over 700?C even or cutting a plain carbon steel at cutting speeds of 100 m/min, and the small volume over hich the temperature is high. The less ambitious goal is to measure the average temperature at the chip/tool contact. Thermocouple methods can be used for both (the next ection concentrates on these); but thermal radiation detection methods can also be used Section 5.3.2 summarizes these). (It is possible in special cases to deduce temperature fields from the microstructural changes they cause in tools – see Trent, 1991 – but this will not covered here.) 5.3.1 Thermocouple methods Figure 5.13 shows an elementary thermocouple circuit. Two materials A and B are connected at two junctions at different temperatures T1 and T2. The electro-motive force(EMF) generated in the circuit depends on A and B and the difference in the temperatures T1and T2. A third material, C, inserted at one of the junctions in such a way that there is no temperature difference across it, does not alter the EMF (this is the law of intermediate metals). In common thermocouple instrument applications,A and B are standard materials, with a well characterized EMF dependence on temperature difference. One junction, usually the colder one, is held at a known temperature and the other is placed in a region where the temperature is to be deduced from measurement of the EMF generated. Standard material combinations are copper-constantan (60%Cu,40%Ni), chromel (10%Cr,90%Ni)–alumel (2%Al,90%Ni-Si-Mn) and platinum–rhodium. In metal machining applications, it is possible to embed such a standard thermocouple combination in a tool but it is difficult to make it small enough not to disturb the temperature distribution to be measured. One alternative is to embed a single standard material, such as a wire, in the tool, to make a junction with the tool material or with the chip material at the tool/chip interface. By moving the junction from place to place, a view of the temperature distribution can be built up. Another alternative is to use the tool and Fig. 5.13 An elementary thermocouple circuit (above) with an intermediate metal variant (below) work materials as A and B, with their junction at the chip/tool interface. By this means, the average contact temperature can be deduced. This application is considered first, with its difficulties stemming from the presence of intermediate metals across which there may be some temperature drop.Fx