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 what I have stated above, 

 will think it right to do so, and to communicate to the Society the 

 result of their trials. In all the cases which have come within my 

 own knowledge, the trial has been very successful. I have had a 

 most favourable report from the persons concerned in erecting the 



