Edinburgh New Philosophical Journal (1834 Edinburgh New Philosophical Journal edinburghnewphil18edin Year: 1834 - 1835 318 Mr Robison on making Screw Taps and Dies. tap fairly into the work. The rest of the thread, from this bare portion to within a few threads of the upper end of the tap, should have the angle turned off, beginning by removing nearly the whole of it, and taking less and less as the other end is ap- proached, where five or six turns of the thread should be left entire. When this has been done, then one side of the tap should be filed or ground away, giving nearly the form of
Edinburgh New Philosophical Journal (1834 Edinburgh New Philosophical Journal edinburghnewphil18edin Year: 1834 - 1835 318 Mr Robison on making Screw Taps and Dies. tap fairly into the work. The rest of the thread, from this bare portion to within a few threads of the upper end of the tap, should have the angle turned off, beginning by removing nearly the whole of it, and taking less and less as the other end is ap- proached, where five or six turns of the thread should be left entire. When this has been done, then one side of the tap should be filed or ground away, giving nearly the form of a half-round opening bit. 2<i, Of the Dies.—It appears to me, that these should be made so as to have only one or two cutting points, instead of the multiplicitypresented in the constructions usually adopted. With this view, I form one of the dies so as to act merely as a guide, and the other die to cut by one or two threads only. The figure is a representation of dies made in this way. The die A has the thread filed away at a o, so as to admit the bolt which is to be screwed, to apply itself solidity to the cavity; while in the die B, so much of the thread is cut out as to leave only about two turns. The die b may be whetted when its cutting edge becomes blunt, by being ground in the direction of the dotted line d; and as the tap is likewise susceptible of being kept in a sharp cutting state, it must be the fault of the workmen if as clean threads be not produced by these means, as can be made by the chasing tool in the turning lathe, and with the advantage of not being liable to be irregular in pitch or diameter. As few operations recur more frequently in practical mechanics, than the making of screws, any thing which facilitates this opera- tion, or which enables workmen to produce better work at the same expense of time and trouble, is deserving of examination. I hope, therefore, that such members of the Society of Arts as may have the means of testing the accuracy of
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