Abstract
Non-coated normal drill generates long pitch helix type chip at the deeper drilling position than 7-8 times of drill diameter. TiN-coated normal drill has long tool life and lowers cutting resistance. However, it generates less long pitch helix type chip. TiN-coating decreases the long pitch helix type chip. Yet, it has been found that a TiN-coated twist drill which has narrow face of small rake angle shows good characteristics. The results are as follows : (1) The TiN-coated twist drill which has narrow face of 5-degree rake angle generates the long pitch helix type chip three times as much as the non-coated normal twist drill in 100 mm wet drilling. (2) And it generates the long pitch helix type chip which is connected with the conical helix type chip in 60 mm dry drilling. (3) The TiN-coated twist drill which has narrow face of 0 or 5-degree rake angle generates fragile chip that seems to be connected segment type chips with small area. These chips are exhausted out of the drilling hole very well. In case of these twist drills all chips are exhausted out of the drilling hole very well, the segment type chip occurs by fracture of this fragile chip after exhausting out of the drilling hole.
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