The Pratt and Whitney Co. cut up chasers by the following method: In place of a hob, a milling cutter is made, having concentric rings instead of a thread. The cutters are revolved on a milling machine in the ordinary manner. The chaser is fastened in a chuck fixed on the milling machine table, and stands at an angle of 15°. It is traversed beneath the milling cutter, and thus cut up with teeth whose lengths are at a right angle to the top and bottom faces of the chaser; hence the planes of the length of the teeth are not in the same plane as that of the grooves of the thread to be cut. Thus, let a, b, c, and d, [Fig. 1330], represent the planes of the thread on the work, and e, f, g, h, will be the planes of the lengths of the chaser teeth.
The chaser, however, is given 15° of bottom rake or clearance, and this causes the sides of the chaser teeth to clear the sides of the thread.
Fig. 1331.
Now, suppose the top face a, [Fig. 1331], of the chaser to be parallel with the face of the tool steel, and to lie truly horizontal and in the same plane as the centre of the work. This clearance will cause the thread cut by the chaser to be deeper than the natural depth of the chaser teeth. Thus, in [Fig. 1331] is shown a chaser (with increased clearance to illustrate the point desired), the natural depth of whose thread is represented by the line f, but it is shown on the section of work that the thread cut by the tool will be of the depth of the line d, which is greater than the length or depth of f, as may be more clearly observed by making a line e, which, being parallel to a, is equal in length to d, but longer than f. Hence, the clearance causes the chaser under these conditions to cut a thread of the same pitch, but deeper than the grooves of the hub, and this would alter the angles of the thread. This, however, is taken into account in forming the angles of the thread upon the milling cutter, and, therefore, of the chaser, which are such that with the tool set level with the work centre, the thread cut will be of correct angle, notwithstanding the clearance given to the teeth.
Fig. 1332.
In order to enable the cutting of an inside chaser from a hub, it requires to be bent as in [Fig. 1332], in which h is the hub, r the lathe rest, and c the chaser. After the chaser is cut, it has to be straightened out, as shown in [Fig. 1318], in which is represented a washer being threaded and shown in section; c is the chaser and r the lathe rest, while p is a pin sometimes let into the lathe rest to act as a fulcrum for the back of the chaser to force it to its cut, the handle end of the chaser being pressed inwards.