Fig. 1322.
Fig. 1323.
Fig. 1324.
The method of producing a chaser from a hob is shown in [Fig. 1320], in which h is a hob, which is a piece of steel threaded and serrated, as shown, to give cutting edges to act, as the hob rotates, upon the chaser c. If the chaser is cut while held in a constant horizontal plane, its teeth will have the same curvature as the hob, or, in other words, they will fit its circumference. Suppose that the chaser, being cut up by the hob and then hardened, is applied to a piece of work of the same diameter as the hob and held in the same vertical plane, as in [Fig. 1320], it is obvious that, there being no clearance, the teeth cannot cut. Or, suppose it be applied to a piece of work of smaller diameter, as in [Fig. 1324], it cannot cut unless its position be lowered, as in [Fig. 1322], or else it must be elevated, as in [Fig. 1323]. In either case the angle of the thread cut will be different from the angle of the sides of the chaser teeth, and the thread will be of improper depth. Thus, on referring to [Fig. 1321], it will be seen that the chaser c has a tooth depth corresponding to that on the work w along the horizontal dotted line e only, because the true depth of thread on the work is its depth measured along a radial line, as line f or g, and the chaser teeth are, at the cutting edge, of a different angle. This becomes more apparent if we suppose the chaser thickness to be extended up to the dotted line h, and compare that part of its length that lies within the two circles i j, representing the top and bottom of the thread, with the length of radial line g, that lies within these circles. If, then, the chaser be lowered, to enable it to act, it will cut a thread whose sides will be of more acute angle than are the sides of the chaser teeth or of the hob from which it was cut. The same effect is caused by using a chaser upon a larger diameter of work than that of the hob from which the chaser was cut, because the increased curvature of the chaser teeth acts to give the teeth less contact with the work, as is shown in [Fig. 1325], for the teeth cannot cut without either the lower corners a of the teeth being forced into the metal, or else the chaser being tilted to relieve them of contact. To obviate these difficulties and enable a chaser to be used upon various diameters of work, it is, while being cut up by the hob, moved continuously up and down, as denoted in [Fig. 1326], by a and b, which represent two positions of the chaser. The amount of this movement is sufficient to make the chaser teeth more straight in their lengths, and to give them a certain amount of clearance, an example of the form of chaser thus produced being shown in [Fig. 1327], applied to two different diameters of work, as denoted by the circle a and segment of a circle b, c representing the chaser.