According to the borehole diameter shortening and drilling tool jamming accident in Yilan,the basic reason was thought to be rapidlythickening mud cake on the wall of borehole in crack development in some rock stratum with high permeability and absorbency,but not to be rock expanding.

introduces structure characteristic,precision design calculation of drilling,machining demand and application effect of drilling die and clamp used for producing bonnet by common drill press.

A kind simple method of processing the center bore by using boring clamp;