Chapter XIX: 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.

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.

Transcriber's note:
-Applied the errata to the text
-Whole and fractional parts displayed as 3-1/2
-Made several silent typographical changes for mismatched
single/double quotes
-Italic text display as _text_
-Spelling and word usage have been retained as they appear in the
original publication, except as follows:
-Page viii: Changed "Psychologigues" to "Psychologiques".
-Page viii, 40: Changed "Enseignments" to "Enseignements".
-Page 26: Changed "the same control has" to "the same control as"
-Page 88: Changed "6 percent" to "60 percent". (The total
composition of brass should add up to 100%.)
-Page 89: Changed "are slightly taper to" to "are slightly
tapered to".
-Page 183: Changed "Rang-de-Fliers" to "Rang-du-Fliers" and
"Etaples" to "Étaples".
-Page 189: Changed "the the railway" to "the railway".

End of Project Gutenberg's Deeds of a Great Railway, by G. R. S. Darroch