These methods of cutting left-hand threads with right-handed ones are mentioned simply as curiosities of thread cutting, and not as being of any practical value.
Fig. 308.
Fig. 309.
To proceed, then: to avoid these difficulties it is usual to thread the dies with a hob or master tap of a diameter equal to twice the depth of the thread, larger than the size of bolt the dies are to thread. In this case the dies fit to the bolt at the first cut, as shown in [Fig. 308], c, d being the cutting edges. The relation of the circle of the thread in the dies to that of the work during the final cut is shown in [Fig. 309].
There is yet another objection to tapping the dies with a hob of the diameter of the bolt to be threaded, in that the teeth fit perfectly to the thread of the bolt when the latter is threaded to the proper diameter, producing a great deal of friction, and being difficult to make cut, especially when the cutting edges have become slightly dulled from use.
Referring now to taking a cut up the bolt or work as well as down, it will be noted that supposing the dies to have a right-hand thread, and to be rotating from left to right, they will be passing down the bolt and the edges c, d ([Fig. 308]) will be the cutting ones. But when the dies are rotated from right to left to bring them to the end of the bolt again, c, d will be rubbed by the thread, which tends to abrade them and thus destroy their sharpness.