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.

Far from falling within the category of those employers on the portals of whose workshops may have been inscribed the fatal words "too late," Mr. Cooke was early in the field as a purchaser of the hob thread-milling machine.

Delivered with the machine was a hob cutter, but strangely enough instead of being spiral this cutter was straight fluted. The occasion of a visit to the Works of a certain Government Tool Inspector was productive of an amusing little comedy. Noticing the backing-off lathe above referred to, and being in urgent need of a supply of spiral hob cutters, he expressed his intention of commandeering the lathe for the exclusive manufacture of these particular hobs for Government purposes, heedless of the fact that the lathe was and had been continuously employed for locomotive purposes. Being under the impression too that only one firm of expert tool makers in the country was capable of cutting spiral hobs, great was his astonishment and delight on discovering that Crewe was not only equally capable of doing so, but previously had been performing this very class of work; the net result of this little episode being that the lathe in question was immediately requisitioned for a continuous supply of spiral hob cutters which were to be sent to shell manufacturing firms, and a second backing-off lathe was speedily forthcoming, in order that the Company's own locomotive requirements should be in no way impeded.

THE END

PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES, ENGLAND.

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