Fig. 1243.
Cutting Double or other Multiple Threads in the Lathe.—In cutting a double thread the change wheels are obviously arranged for the pitch of the thread, and one thread, as a in [Fig. 251] is cut first, and the other, b, afterwards. In order to insure that b shall be exactly midway between a, the following method is pursued. Suppose the pitch of the lead screw is 4 threads per inch, and that we require to cut a double thread, whose actual pitch is 8 per inch, and apparent pitch 16 per inch, then the lead screw requires to make half a turn to one turn of the lathe spindle; or what is the same thing, the lathe spindle must make two turns to one of the lead screw, hence the gears will be two to one, and in a single-geared lathe we may put on a 36 and a 72, as in [Fig. 1243], in which the intermediate wheels are omitted, as they do not affect the case. With these wheels we cut a thread of 8 per inch and then, leaving the lead screw nut still engaged with the lead screw and the tool still in position to cut the thread already formed, we make on the change wheels a mark as at s t, and after taking off the driving gear we make a mark at space u, which is 18 teeth distant from s, or half-way around the wheel. We then pull the lathe around half a turn and put the driving gear on again with the space u engaged with the tooth t, and the lathe will cut the second thread exactly intermediate to the first one. If it were three threads that we require to cut, we should after the driving gear was taken off give the lathe one-third a revolution, and put it back again, engaging the twelfth space from s with tooth t, because one-third of 36 is 12.
It is obviously necessary, in cutting multiple threads in this way, to so select the change wheels that the driving gear contains a number of teeth that is divisible without leaving a remainder by the thread to be cut: thus, for a double thread the teeth must be divisible by two, hence a 24, 30, 34, 36, or any even number of teeth will do. For a triple thread the number of teeth in the driving gear must be divisible by 3, and so on.
But suppose the driving gear is fast upon the lathe spindle and cannot be taken off, and we may then change the position of the lead screw gear to accomplish the same object as moving the lathe spindle. Thus for a double thread we would require to remove the driving gear as before, and then pull round the lead screw so that the eighteenth tooth from t would engage with space s, which is obviously the same thing as moving the driving gear round 18 teeth.
Fig. 1244.
Fig. 1245.