The same gauge may be used for setting internal threading tools by first facing the work quite true and then applying the gauge as in [Fig. 993].
Fig. 994.
By reason of the comparatively sharp points of thread-cutting tools, they are more readily dulled than the rounder pointed ordinary lathe tool, and by reason of their cutting edges extending along a greater length of the work, and therefore causing it to spring or bend more from the strain of the cut, they cannot be employed to take such heavy cuts as ordinary tools. Hence, in all thread cutting, it is necessary to turn the work down to the finished diameter before using the threading tool, so that the thread will be finished when it is cut to the proper depth. To test that depth on a piece of work having a United States standard, or a sharp V-thread, a gauge such as shown in [Fig. 994] may be used, consisting of a piece of sheet steel about 1⁄50 inch thick, having a single tooth formed correct for the space of the thread, so that the edge of the gauge will meet the tops of the thread when the space is cut to admit the tooth on the gauge; the most accurate method of producing such a gauge having been described in the remarks upon screw threads.
Fig. 995.
If the tool is known to be ground to the correct angle and is set properly, the gauge for depth may be dispensed with by turning the body of the work to correct diameter, and also turning a small part, as a in [Fig. 995], down to the correct diameter for the bottom of the thread, so that when the tool point meets a the thread will be cut to correct depth.