Fig. 298.
Fig. 299.
In [Fig. 297] is shown Whitworth’s stocks and dies, the cap that holds the guide die a and the two chasers b, c in their seats or recesses in the stock being removed to expose the interior parts. The ends of the chasers b, c are beveled and abut against correspondingly beveled recesses in the key d, so that by operating the nut e on the end of the key the dies are caused to move longitudinally. The principles of action are more clearly shown in [Fig. 298]. The two cutting chasers b and c move in lines that would meet at d, and therefore at a point behind the centre or axis of the bolt being threaded; this has the effect of preserving their clearance. It is obvious, for example, that when these chasers cut a thread on the work it will move over toward guide a on account of the thread on the work sinking into the threads on a, and this motion would prevent the chasers b, c from cutting if they moved in a line pointing to the centre of the work. This is more clearly shown in [Fig. 299], in which the guide die a and one of the cutting dies or chasers b is shown removed from the stock, while the bolt to be threaded is shown in two positions—one when the first cut is taken, and the other when the thread is finished. For the first cut the centre of the work is at e, for the last one it is at g, and this movement would, were the line of motion as denoted by the dotted lines, prevent the chaser from cutting, because, while the line of chaser motion would remain at j, pointing to the centre of work for the first cut, it would require a line at k to point to that centre for the last one; hence, when considered with relation to the work, the line of chaser motion has been moved forward, presenting the cutting edges at an angle that would prevent their cutting. By having their motion as shown in [Fig. 299], however, the clearance of the chasers is preserved.
Fig. 300.
