Study of a tapping method for large size threads (1st report) ― Consideration for the initial thrust force at tapping

In this paper we discuss the cutting behavior in the chamfer of a tap for a large size threads with a geometrical analysis and experiments. We investigated three kinds of the taps an ordinary tap, a spiral fluted tap and a spiral pointed tap using the imitated cutting edges manufactured and analyzed the behavior in the chamfer of a tap by measuring cutting resistance. The results are as follows. (1) The connecting cut-length of a tap can be expressed by a function using factors, pitch ; P, the number of flutes ; N, flank angle ; δ, chamfer angle ; θ. The maximum connecting cut-length ; Z1 ( max) can be obtained by the equation as follows : Z1(max)=P{(1+tanδ)tanθ}/N.(2) The connecting cut-length of a tap ; Z1 changes by a percentage of thread engagement ; K, that is, the size of a tap hole and its periodical change is shorter as the chamfer angle ; θ smaller. (3) Carrying out the analysis and the experiment for the tap where P=1-6, N= 4, it is clarified that the initial thrust force of a tap with large size threads where K=60-100% shows less than 100 N and in a spiral fluted tap it shows the minus thrust force.