274 Mr Clark's Description of' a 



The first thing to be executed is the tap, Fig. 4, Plate III. 

 L is a steel rod, on which is cast a brass cylinder H, and which 

 must afterwards be turned quite cylindrically, and screwed 

 with a comb-screwing tool of the pitch required. Into the 

 cylinder is to be cut five or seven dovetail grooves running 

 parallel to its sides, and the dovetails pointing to the centre, 

 as exhibited in the section Fig. 5, and into these grooves are 

 to be fitted pieces of steel, K K, Fig. 4, precisely the length 

 of the cylinder, and which are kept in their places by a screw- 

 ed nut (G I, Fig. 4.) at each end. The steel rods must now 

 be cut conformably to the screw in the brass ; then taken out, 

 and hardened with as low a heat as merely to bring them to 

 a spring temper. The screw is next to be worked entirely off 

 the body of the brass cylinder, and the steel cutters restored, 

 each to its proper groove, and prevented from shifting by the 

 nuts G and I. 



This tap is intended merely to mark the brass bushes in the 

 frame represented by Fig. 1, the construction of which will 

 easily be understood by a little attention to the drawing, of 

 which Fig. 2 is a top view, or transverse section. Spaces must 

 be cut into the brass bushes A A, following the obliquity of 

 the marks made by the tap ; and into these spaces are to be 

 inserted cutters made of plate steel. Fig. 6. 



The piece intended to be screwed must be turned as nearly 

 as possible into a perfect cylinder ; and to reduce any inequa- 

 lities that may be left by the turning tool, there must be cast 

 on the cylinder a piece of lead eight or ten inches long, which 

 is to be sawn longitudinally through the middle. The metal 

 is then to be laid on an even surface or table, and ground in 

 the direction of its length by one of the lead sections, until it 

 become smooth, and all its inequalities disappear. 



The cutting now commences. The first tier of cutters must 

 be brought nearly to a sharp edge, so as to make rather a deep 

 impression, observing that they must be made to cut and not, 

 toj^orce, as in this case they will throw up what is termed a 

 false thread ; and, if the diameter of the intended screw be 

 small, may lengthen the rod, and thus not only enlarge the 

 thread, but probably produce a screw of unequal pitch. 



The second set of cutters will likely finish the screwing 



