3.1. Effect of tool vibration on the critical depth of cut Groove cutting experiments were carried out in order to investigate the effect of tool vibration on the brittle–ductile transition. The workpiece of fused silica was inclined by a small angle onthemounting plate so that the depth of cut can vary continuously in one revolution of the spindle. A singlecrystal diamond cutting tool with a rake angle of 08 and a nose radius of 0.8 mm was used. The maximum vibration speed of the cutting tool is 45 m min1. The feed rate is chosen to be large enough so that the grooves are generated without interference.