Tech Talk
What are optimum machine construction criteria?
A most important part of any machine tool system is the construction of the guideways. The qualities which constitute a superior machine are rigidity, accuracy, freedom from stick-slip, good damping, and longevity. A very precise guideway system is that of a pre-loaded roller guideways. The guideways are constructed from plain, hardened steel ways, accurately machined as linear bearing surfaces for small needle roller bearings. Once assembled, the guideways are pre-loaded so that a very accurate, rigid, and precise guideway system is created. The roller guideway system is a very low friction system, and may be used for very precise machining, using encoder feedback from a stepper motor drive. One drawback is the propensity for these guideways to transmit process vibration throughout the machine system. However, pre-loaded roller bearings may be used in combination with a machine feature which helps to dampen the vibration, e.g., an epoxy granite base or a weldment base and column.
Many machines have been designed with the traditional V-flat ways. Such systems, which have more frictional drag, may require precision glass scales to verify the slideway position. The V-flat ways require a film of lubrication oil to be present between the mating surfaces. This film may cause inaccuracies, due to compression of that film under load. At various table speeds the thickness of the oil film may increase due to the hydrodynamic effect of the table riding on the oil film. Still the oil film provides a good damping coefficient. Dependent on the relative size of the V-flat system, it may have properties comparable to the pre-loaded roller guideway design. Generally, V-flat ways have superior vibrational damping.
Hydrostatic bearings are yet another alternative. Though hydrostatic bearings are extremely rigid bearing systems, they may suffer from thermal growth due to the varying temperature of the hydraulic oil. Moreover, hydrostatic systems require the strictest discipline in maintenance and cleanliness of the hydraulic oil and pumping unit.
Though there are many sizes of reciprocating grinding machines, the constituent parts of their construction are basically similar. The machine base is usually fabricated from cast iron, which provides a large mass for static stability of the machine tool. Some older machines will use the hollow structure of the casting to house the hydraulic reservoir and pumping system for the table drive. This was quickly found to be detrimental to the accuracy of the machine tool, due to thermal instability. As a move to improve accuracy and repeatability, machines have been built where the cutting fluid, which is refrigerated, is passed through the machine base casting and spindle housing to provide thermal stability to the structure. The hydraulic reservoir and pumping system on a modern machine is typically a self-contained unit, separate from the machine tool base. The need for both static and dynamic vibrational stability has prompted some machine tool builders to construct their bases from an epoxy concrete which has superior thermal stability and vibrational damping properties. In fact, not only grinding machines have been built from epoxy concrete, but also lathes and milling machines.
The column of a grinding machine has vertical ways; these ways are the guideways for the wheelhead assembly. The wheelhead assembly contains the grinding wheel spindle, mounted in precision bearings and located in the spindle bearing housing. The grinding wheel is held between flanges, which fit onto the spindle nose. The grinding wheel spindle bearings are loaded in order to take up any end thrust which might occur in the grinding process. The spindle runs true and in a plane parallel to the machine table. The infeed mechanism on the wheel head is typically a leadscrew or precision ball-screw. The infeed mechanism is able to feed accurately in increments of 0.001 mm (0.00005 in) and less, depending on the type of stepper motor and/or feedback system. Both the wheel head and the cross slide usually have a rapid traverse separate from their incremental feed capabilities. The accuracy of these slide-ways, particularly for manual operation, usually depends on glass scale calibration for exact positioning of the wheel to the workpiece.
The grinding spindle is a cantilever spindle, which means it has no outer support. The highest forces are generated through the grinding wheel on the end of the spindle. Hence, the construction of the spindle bearings, spindle nose, and vertical slideway are critical to achieve accuracy and minimize vibration and flexibility under load.
