Mr Robison on making' Screw Taps and Dies. 317 



would doubtless suggest numerous improvements in the detail, 

 especially as concerns the size of the dials and the motion of the 

 pencils, which particulars will depend upon the nature of the 

 investigations we may desire to make. It is, however, hoped, 

 thai the foregoing drawings will make the nature and constiuc- 

 tion of the machine perfectly intelligible. 



Deso'iptiun and Drawings of an Improved Metliod of Making 

 Screw Taps and Dies.^ By John Roeisox, Esq. Sec. R. S. E. 

 and M. S. A. In a Letter to the Secretary. 



Dear Sir, — In reply to your inquiries regarding the method 

 of making Screw Taps and Dies, which I mentioned at a meet- 

 ing of the Society of Arts about two years ago, I now beg leave 

 to offer the following detailed description of them. 



\st. Of the Tap. — I propose that this should be made half 

 round, as it will be found that a tap formed in this way will cut 

 a full clear thread, (even if it may be of a sharp pitch,) with- 

 out making up any part of it by the burr, as is almost univer- 

 sally the case, when blunt-edged or grooved taps are used. 



It has sometimes been objected to me by persons who had 

 not seen half-round taps in use, that, from their containing less 

 substance than the common forms do, they must be very liable 

 to be broken by the strain required to turn them in the work. 

 It is proved, however, by experience, that the strain in their case 

 is so much smaller than usual, that there is even less chance of 

 breaking them than the stouter ones. Workmen are aware 

 that a half-round opening bit makes a better hole, and cuts 

 faster than a five-sided one, and yet that it requires less force 

 to use it. 



The method I recommend for constructing such taps, is to 

 begin by making the screwed part cylindric, or of equal diame- 

 ter up to the point; then to turn off five or six turns of the 

 thread from the point, leaving just a faint trace of the bottom 

 of the thread. This portion of the tap then serves as a gauge for 

 the size of the hole to be tapped, and as a guide for entering the 



" Read before the Society of Arts, 14th November 1829. 



vol.. XVIII. NO. XXXVI. ATKII. 183-5. V 



