The whirling vibration of the drill in the beginning of the drilling operation is experimentally investigated. Special attention is paid to how vibration develops as the drill proceeds into the workpiece. Three types of drills with different web thicknesses are used, and the characteristics and cause of the vibration are discussed. Whirling vibration, which has an elliptical orbit, is a self-exited vibration accompanied by a phase lag with respect to the revolution of the workpiece. The vibration energy is provided by the regenerative effect at the major cutting edge, while the chisel edge acts to dampen the vibration.

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