The tapping of nuts has hitherto been worked by using several kinds of tapping machines as follows : (a) The hand tapping machines of vertical type, by which the revolution of the tap spindle reverses, the nuts being fed by hand ; (b) The automatic tapping machine of vertical type same as above ; (c) The multi-spindle tapping machine, by which the taps with pretty long shank are fed by hand or automatically without reversing the revolution and when each shank of the taps is filled up by threaded nuts, the operator take them off together with the tap ; (d) The automatic tapping machine, by which a machine tap with bent shank, so called bent tap, is used. The automatic tapping machine using a bent tap should be most preferable for the mass production of nuts of high grade, if the accuracy of threaded nuts and the durability of bent tap are high. So, experimental researches were carried out by the last automatic tapping machine (d) to obtain the threaded nuts of high grade and prolong the life of the bent tap and the results are very satisfactory.
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