| 摘要 |
PURPOSE:To make it possible to cut a thread having a large ridge by providing a first cutting tool and a second cutting tool with a predetermined angular space therebetween, and by shifting the positions of their cutting tips by a predetermined distance from each other in the axial direction of a workpiece so that cutting processes by them are carried out, simultaneously. CONSTITUTION:Tool holders 11, 12, 13, 14 holding cutting tools 21, 22, 23, 24, 25 on a tool bed 3 are set so that they may be extended and retracted to and from the principal axis of a lathe by means of servo-motors 31, 32, 33, 34, 35. When a thread having an outer diameter (d) is formed by simultaneous use of a roughing cutting tool 23 and a finishing cutting tool 21 which is arranged away from the former, the position of the cutting tip of the tool 23 is retracted by a suitable finishing allowance (a). Thereby both tools 21, 23 may cut one and the thread groove, simultaneously. Thereby, it is possible to cut a thread having a large ridge and a multiple thread, and thereby it is possible to manufacture screws having a high degree of accuracy with a high productivity.
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