The foregoing rule, as applied to triple threads or those of a higher number, does not always give the only distance that the carriage can be moved. To illustrate, in the preceding example the carriage movement could be equal to 0.499, or what is practically one-half inch, instead of 11/2 inch, and the tool would be properly located. The rule, however, has the merit of simplicity and can be used in most cases.
Fig. 13. Indexing Faceplate used for Multiple Thread Cutting
Special faceplates are sometimes used for multiple thread cutting, that enable work to be easily and accurately indexed. One of these is illustrated in [Fig. 13]; it consists of two parts A and B, part A being free to rotate in relation to B when bolts C are loosened. The driving pin for the lathe dog is attached to plate A. When one groove of a multiple thread is finished, bolts C are loosened and plate A is turned around an amount corresponding to the type of thread being cut. The periphery of plate A is graduated in degrees, as shown, and for a double thread it would be turned one-half revolution or 180 degrees, for a triple thread, 120 degrees, etc. This is a very good arrangement where multiple thread cutting is done frequently.
Fig. 14. Correct and Incorrect Positions of Tool for Taper Thread Cutting
Taper Threading.—When a taper thread is to be cut, the tool should be set square with axis a—a as at A, [Fig. 14], and not by the tapering surface as at B. If there is a cylindrical part, the tool can be set as indicated by the dotted lines. All taper threads should be cut by the use of taper attachments. If the tailstock is set over to get the required taper, and an ordinary bent-tail dog is used for driving, the curve of the thread will not be true, or in other words the thread will not advance at a uniform rate; this is referred to by machinists as a “drunken thread.” This error in the thread is due to the angularity between the driving dog and the faceplate, which causes the work to be rotated at a varying velocity. The pitch of a taper thread that is cut with the tailstock set over will also be slightly finer than the pitch for which the lathe is geared. The amount of these errors depends upon the angle of the taper and the distance that the center must be offset.