Manufacture of a Hob-cutter, in "Relieving" or "Backing-off" Lathe.
[Appendix C
[APPENDIX C]
THE THREAD-MILLER, AND THE "BACKING-OFF" LATHE, AS APPLIED TO SHELL-MANUFACTURE
The action of a thread-milling machine is as follows:—The cutter fixed on a mandrel, and the shell in the milling machine, revolve in opposite directions to one another. The teeth of the cutter, contrary to expectation, are simply a series of parallel rings cut the correct pitch of the thread required; consequently all that is necessary is that the shell shall travel longitudinally the distance of one pitch of the thread during one complete revolution, and the job is done.
Particular interest centres, perhaps, in the little thread-cutter itself, and in the amount of ingenuity evinced in the method of its manufacture.
In the first place a rotary form-milling cutter regulated by a dividing head is run across the eventuating hob thread-cutter in a series of spiral lengths. The profile of this form-milling cutter forms both a rounded clearance for the cutting edge of the hob cutting teeth and a "gash" or groove necessary for the clearance of the chasing tool when cutting the teeth.
The reason for these spiral gashes is that each separate tooth shall come into play individually and consecutively instead of collectively and simultaneously, the life of the teeth being in this manner very materially prolonged.
We notice then, that instead of adhering to a uniform circumference, the teeth, or series of teeth, are what is known as "backed off" from the cutting edge, a clearance being in this wise imparted, the true form of the teeth being thereby maintained in subsequent grinding operations.