Fig. 1333.
Fig. 1334.
When an inside chaser is cut from a hub (which is the usual method) or male thread, its teeth slant the same as does the male thread on the side of the hub on which it is cut, and in an opposite direction to that of the thread on the other side of the hub. Thus, in [Fig. 1333], h is the hub, c the chaser, and r the lathe rest. The slope of the chaser-teeth is shown by the dotted line b. Now, the slant of the thread on the half circumference of the hub not shown or seen in the cut will be in an opposite direction, and in turning the chaser over from the position in which it is cut ([Fig. 1333]) to the position in which it is used ([Fig. 1334]), and applying it from a male to a female thread, we reverse the direction with relation to the work in which the chaser-teeth slant; or, in other words, whereas the teeth of the chaser should lie as shown in [Fig. 1334] at a a, they actually lie as denoted in that figure by the dotted line b b. As a consequence, the chaser has to be tilted over enough to cause the sides of the chaser-teeth to clear the sides of the thread being cut, which, as they lie at opposite angles, is sufficient to cause the female thread cut by the chaser to be perceptibly shallower than the chaser-teeth, for reasons which have been explained with reference to [Fig. 1321]. It may be noted however, that an inside chaser cannot well be used with rake, hence the tilting in this case makes the thread shallower instead of deeper.
To obviate these difficulties the hub for cutting a right-hand inside chaser should have a left-hand thread upon it, and per contra, an inside chaser for cutting a left-hand thread should be cut from a hub having a right-hand thread.
The method of starting an outside thread upon wrought iron or steel to cut it up with a chaser is as follows:—