In tapping, chips jamming in the tap's flute or twining around the tap cause tool breakage very frequently. It has been an obstacle to automation of tapping. In order to resolve this problem, a chip-breaking tool has been developed, which forms grooves axially on the inner wall of hole before tapping. In this paper described are design of the tool and its effectiveness. The tool was composed of a cylindrical body and three V-groove cutting edges. The diameter of body was equal to that of tap drills. The major diameter of V-groove cutting edges was slightly larger than that of taps. Each V-groove cutting edge had a chip pocket designed after observing the chip formation in similar V-groove cutting. To investigate the effectiveness of the tool, tapping experiments for M3 and M6 threads were performed with and without using the tool. The work material was structural steel SS400 and the taps used were the conventional spiral fluted taps made of high speed steel. By using a machining center the tapping operations were automatically repeated till when the tap was broken without any human assistance for removing the chips jamming in the tap's flute or twining around the tap. To shorten the experimental time, the cutting velocity for tapping was set at 30m/min which was much higher than conventional, and besides, the thread was long as two and a half times of diameter. It has been ascertained by experiments that by using the newly developed tool, the tool life of tap lasts much longer and the tapping torque becomes smaller and stable, and moreover, the surface roughness becomes smoother.

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