Tech Talk
What are the effects of “Vibration in Machine Tools”?
The effects of vibration in machine tools are of and the process, as well as assist in trouble-shooting. A fundamental, physical relationship in mechanical structures is that the natural frequency of vibration is proportional to stiffness and inversely proportional to mass. The natural frequency of vibration is the frequency at which a structure will vibrate given an initial impulse force. This is illustrated perfectly on the strings of a guitar. Plucking a very thin makes the highest notes, light string, which is highly tensioned and very stiff. The lowest notes are made from a thick, heavy string, loosely tensioned and flexible. As a structure is made stiffer, the natural frequency increases, and as the mass of a structure increases, the natural frequency decreases. The natural frequency is the dominant, highest amplitude frequency at which a structure will vibrate. There are other harmonic frequencies of vibration, which have significant amplitude of vibration, but none as large as the natural frequency. A machine tool will generally be designed to be stable during a given band of operational frequencies.
High-speed grinding, using superabrasives, is undoubtedly the next generation grinding process. Machine tool designed have to take into account the vibrational stability required and the stiffness necessary, not only to perform the grinding operation, but also to dress the grinding wheels in the proper manner. The great majority of grinding machines today are sadly inadequate in both of these areas. There are partial solutions in the forms of new materials for machine tools, like “Granitan”, a patented, epoxy, granite material, which is used to manufacture machine tool bases, and which exhibits extremely good damping properties, coupled with good thermal and mechanical stability. Granitan is the trade name held by Studer in Switzerland, who owns the exclusive rights to manufacture the material. Increasing the stiffness of a structure by designing complex weldments is more prevalent today, as stiffness is improved and extra damping is provided in the design of the welded joints.
Thermal stability of a grinding machine is essential for super-precision grinding. Changes in the environmental temperature can drastically change the dimensional stability of the machine tool. The work environment can be air-conditioned and maintained at a constant temperature, however, there are significant heat sources in a machine tool which need to be controlled or at least directed. There is heat generated from hydraulics, fan motors on spindle drives, fans in cabinets, and fans in cutting fluid refrigeration systems, all blowing air around the machine. When running the spindle, bearings increase in temperature and grow to a stable operating temperature. The temperature of the workpieces and fixtures have to be stabilized. Even the operator gives off heat. The human body gives off approximately 100 watts of radiated heat. Imagine the effect on a new super-precision grinding machine of a whole entourage of both interested and uninterested personnel looking on and radiating 100 watts of heat each. Thermal stability has to be designed into the machine, as well as into the placement of the equipment in the workplace.
High-speed grinding and creep-feed grinding have been explored together in an array of materials from metals to ceramics. Again, the trend is higher stiffness, better vibrational stability, and precise control, but another area looms over the traditional machine design. The application of high-speed and creep-feed grinding generates swarf, in a volume far greater then ever before. Conventional machine designs cannot cope with the huge volume, nor can they cope with the high flow rates of cutting fluid, without leaking it into the atmosphere and, more commonly, all over the floor. The velocity of high-speed swarf is easy to overlook, as it was never important in the past. Grinding swarf, travelling not at 60 to 70 mph, but now at 250 to 600 mph, does an extremely fine job of eroding machine guards, workpiece fixtures, and, in fact, anything that is in its path.
For too long we have neglected the basic fundamentals of machine tool technology which will take us into the new manufacturing environment.
