I have had to relieve the diameter of tap shanks many times in order to tap deep holes or to cross a bore and the like. This morning I needed to reduce the shank of a 5/16-18 tap. The job required that all but about 3/8" of the shank diameter needed to be turned down. This meant that in order to do the turning in one chucking the end of the tap needed to be supported. The tap in question was a spiral point tap. So I used a live center for support. I put a .25 thick piece of aluminum between the point of the center and the tap. Then, with the spindle turning, I cranked the tailstock quill forward until the center and the tap points had both made pretty deep depressions in the aluminum piece. I used a coated threading insert to turn down the tap.
255 RPM and .004" feed. The material was removed in one pass. I plunged the tool into the work where the cut would not be an interrupted cut. Then fed away from the chuck by hand to remove the incomplete threads. This part of the cutting was an interrupted cut so I wanted to feel the cutting. After removing the incomplete threads the cut was reversed so that the tool was feeding towards the chuck. The operation worked quite well and the threading insert still looks good. HSS taps are hard but coated carbide is much harder. Eric- posted
5 years ago