L=-._ ) ) RECOMMENDED LATHE IPRACTmCE-{contd.) when cutting standard 55 deg. threads) so that metal is mostly re- moved by the left-hand side of the tooL Side rake can be provided so that heavier cuts can be taken and the chips flow easily away. Thusona single threaded screw the terms are identical, buta notation such as i in. pitch, ! in. lead, would indicate a two start screw, and to produce this, the gearbox would require to give a saddle move- ment of ! in. for every revolution of the spindle, while the tool would be ground to produce t in. pitch section of thread.
The procedure when cutting a multiple start screw is to set the top slide parallel with lathe axis and cut the first thread in the usual manner at the correct LEAD. Cut subsequent threads by advancing the top slide each time a distance equal to the lead divided by the number of starts. This, of course, applies to aU multiple threaded screws regardless of thread section. ~ In commencing a screw cutting operation, take a light trial cut and check the number of threads per inch by measuring with a rule or screw pitch gauge. Then proceed by taking successive cuts until the full depth is reached.
Check for depth and accuracy by means of the nut to fit the screw, or by a thread gauge of the ring type. Tapered threads may be cut by means of a taper attachment or by off-setting the tailstock. In either case the tool must be set square to the work axis, and not to the tapered portion. Acme and Worm Threads. The procedure to be adopted for Acme and worm thread cutting is similar to that recommended for vee threads except for the setting angle of the compound slide. In this case the thread induded angle is 29 deg. so that a slide setting of apProx. I3 deg. from normal (i.e. 77 deg. on cross slide graduations) would be appropriate. Square Threads.
This section of thread is often used for multiple threaded screws, and the terms "pitch" and "lead" should be under- stood. Pitch is the distance from a point on one screw thread to a corresponding point on the next thread, measured parallel to theaxis. Full depth of an Acme thread is 0'5 P + o'or in. and width at bottom is 0'37°7 P - 0'0052 in. where P is the thread pitch. Lead is the distance that a screw thread advances axially in one turn. 20