The whirling vibration of the drill when beginning the drilling of a workpiece with a pilot hole, is theoretically examined. It becomes clear that the whirling is a regenerative chatter vibration which is caused by the regenerative effect at the major cutting edge and the frictional force occurring due to the interference between the flank surface of the major cutting edge and the workpiece surface. As the amplitude and the vibration speed of the drill are restricted by the characteristic of the frictional force, distinctive vibration wave forms are observed. It is also found that the frequency of the whirling vibration which is decreased by the frictional force, is considerably lower than the natural frequency of the system, and that the odd number polygon is produced by the regenerative effect.

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