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Grinding machines run at a very high R.P.M. to provide 30 m/sec linear grinding speed. Bigger (500) grinding wheels with a lower R.P.M., have a lower natural frequency. Such machines use light, but stiff structures. Machines with smaller grinding wheels (internal) have a higher R.P.M. They use heavy constructions with high stiffness, to keep the vibration frequencies low.
In machines with wide variations in frequency, such as milling machines, it is sometimes necessary to mount an additional mass on the arbor, especially if the excitation frequency coincides with the natural frequency of the spindle or arbor, and causes resonance.
Sophisticated equipment is necessary to measure the dynamic stiffness of machines. Measuring the actual static stiffness is comparatively much easier. The ratio between the dynamic and static stiffness (or deflections) is called amplification factor.
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