Some idea of the infinitely small fractions representing the tolerances allowed may be gathered by taking a sheet of ordinary thin note-paper, the thickness of which is about 3/1000 parts of an inch, or ·003 inch; try to imagine this sheet of thin note-paper made ten times thinner still, and you will find it reduced to a minimum of tissue thickness, 3/10000 parts of an inch or ·0003 inch; remember now that on good artillery depends not only the lives, maybe of your nearest and dearest, but the honour of the nation too; and you have firstly some idea of the imperceptible fractions of an inch to which a shell-gauging instrument is made; secondly the reason for the proportionate degree of accuracy demanded in the manufacture of artillery ammunition itself. [Illustration] [Illustration: 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. 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.