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The material selection for a machine tool structure is one of the essential factors in determining final machine performance, with many criteria being considered, such as temporal stability, specific stiffness,homogeneity, easiness of manufacturing and cost, etc. [6].
Although there are a number of structural materials available, up to now only a few materials have been chosen to build machine tool structures. Cast iron has widely been used for many years due to inexpensiveness and good damping characteristics to minimize the influence of dynamics loads and transients. There are still many cast iron applications in precision machine tools albeit its high initial cost of fabricating patterns and moulds and the poor environment of operating foundries
[7]. Granite is another popular material used to build precision machine base and slideways because of its low thermal expansion and damping capacity. The drawback of granite is that it can absorb moisture so it should be used in a dry environment. For this reason many machine tool builders seal the granite with epoxy resin. The need for higher material damping and light weight in ultra-precision machining, combined with long-term dimensional and geometrical stability, leads to the development and usage of polymer concrete for machine tool structural elements in spite of its low strength [8].
Figure 10.2 illustrates the comparison of relative damping capacity between cast iron and polymer concrete material. The vertical scale is the amplitude of vibration, and the horizontal scale is time. This figure shows that polymer concrete has better vibration absorption capacity compared to cast iron.
The symmetry and closed loop structural configuration are widely used in precision machine tool design. Among various configurations the “T” configuration is popularly used for most of the precision turning and grinding machines. Recently, a novel tetrahedron structure proposed by the NPL in England has been applied in an internally damped space frame with all the loads carried in a closed loop. The design generates a very high stiffness coupled with exceptional dynamics stiffness [9], albeit its complexity and cost are increased in manufacturing and assembly.
10.2.1.2 The Spindle and the Feed Drive System
Spindle is a key element of the precision machine tool because the spindle motion error will have significant effects on the surface quality and accuracy of machined components. The most often used spindles in precision machine tools are aerostatic spindles and hydrostatic spindles. They both have high motion accuracy and are capable of high rotational speed. An aerostatic spindle has lower stiffness than an oil hydrostatic spindle. Aerostatic spindles are widely used in machine tools with medium and small loading capacity while hydrostatic spindles are often applied in large heavy-load precision machine tools.
Accurate linear motions are generated by the use of slideways. Similarly, aerostatic slideways and hydrostatic slideways have been frequently applied in precision machine design and are replacing contacting surface type slideways.
On the drive side, the electric AC motor and DC brushless motor for high speed spindles are frequently built into the spindle so as to reduce the inertia and friction produced by the motor spindle shaft coupling as well as the dynamic. DC and AC linear motor drives can perform a long stroke direct drive and thus eliminate the need for conversion mechanisms such as lead screws, belt drives, and rack and pinions, with potentially better performance in terms of stiffness, acceleration, speed, smoothness of motion, accuracy and repeatability, etc. [10]; however, there are only limited applications in machine tools though linear motors have been available for a long time [7]. Friction drives are very predictable and reproducible due to a prescribed level of preload at the statically determinate wheel contacts, thereby superior in machining optically smooth surfaces [11]. But there are some practical considerations that restrict the application of friction drives in machine tools. One such limitation is referred to as the thermal capacity. Therefore, it is difficult for friction drive to achieve a high speed operation.