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.
This clearance is obtainable by means of the "backing-off" or "relieving" mechanism of a lathe especially designed for the purpose. A geared shaft running parallel with the bed of the lathe causes to revolve a cam. This cam, by virtue of the eccentricity of its face, imparts a transverse movement to the lathe rest through the intermediary of a transverse spindle on which is fixed a second cam also eccentrically faced.
The revolving face of the one bearing against the stationary face of the other causes the latter to draw the lathe-rest inwards against the compression of a spring; no sooner have the cam edges passed one another than the spring relaxes, allowing the lathe-rest to jump back again to its former position.
It is during the inward movement of the lathe-rest that the form-chasing tool cuts the required clearance behind the hob miller's teeth, finishing into the cross-cut spiral gash; the outward movement has the effect of pushing the chasing tool in the nick of time clear of the succeeding row of teeth, ready to repeat a similar operation. The eccentric profile of the cam faces, needless to say, is responsible for the curved relief, or clearance, behind the edge of the hob miller's teeth, and the cams can be changed at will according to the curve required. The delaying action of the lathe-rest necessary to coincide with the spiral gash is imparted by means of a slide set diagonally to the requisite angle of the spiral; as this was a series of rings and not a screw thread, it was necessary, when cutting the teeth, to disengage the leading screw from the backing-off gear to enable the teeth to be cut while the saddle of the lathe was in a stationary position.