Fig. 10. End View of Lathe Headstock
Cutting a Left-hand Thread.—The only difference between cutting left-hand and right-hand threads in the lathe is in the movement of the tool with relation to the work. When cutting a right-hand thread, the tool moves from right to left, but this movement is reversed for left-hand threads because the thread winds around in the opposite direction. To make the carriage travel from left to right, the lead-screw is rotated backwards by means of reversing gears a and b ([Fig. 10]) located in the headstock. Either of these gears can be engaged with the spindle gear by changing the position of lever R. When gear a is in engagement, as shown, the drive from the spindle to gear c is through gears a and b, but when lever R is raised thus shifting b into mesh, the drive is direct and the direction of rotation is reversed. The thread is cut by starting the tool at a, [Fig. 8], instead of at the end.
Fig. 11. End of Square Thread Tool, and Graphic Method of Determining Helix Angle of Thread
Cutting a Square Thread.—The form of tool used for cutting a square thread is shown in [Fig. 11]. The width w is made equal to one-half the pitch of the thread to be cut and the end E is at an angle with the shank, which corresponds to the inclination x—y of the threads. This angle A depends upon the diameter of the screw and the lead of the thread; it can be determined graphically by drawing a line a—b equal in length to the circumference of the screw to be cut, and a line b—c, at right angles, equal in length to the lead of the thread. The angle α between lines a—b and a—c will be the required angle A. (See end view of thread tool). It is not necessary to have this angle accurate, ordinarily, as it is simply to prevent the tool from binding against the sides of the thread. The end of a square thread tool is shown in section to the right, to illustrate its position with relation to the threads. The sides e and e1 are ground to slope inward, as shown, to provide additional clearance.
When cutting multiple threads, which, owing to their increased lead, incline considerably with the axis of the screw, the angles for each side of the tool can be determined independently as follows: Draw line a—b equal in length to the circumference of the thread, as before, to obtain the required angle f of the rear or following side e1; the angle l of the opposite or leading side is found by making a—b equal to the circumference at the root of the thread. The tool illustrated is for cutting right-hand threads; if it were intended for a left-hand thread, the end, of course, would incline in the opposite direction. The square thread is cut so that the depth d is equal to the width. When threading a nut for a square thread screw, it is the usual practice to use a tool having a width slightly greater than one-half the pitch, to provide clearance for the screw, and the width of a tool for threading square-thread taps to be used for tapping nuts is made slightly less than one-half the pitch.