Multiple start threads combine the close fit of a fine thread with the quick axial motion of a coarse thread. Any time a design engineer needs a fine fit combined with fast motion, it is likely that he or she will use a multiple start thread.
Figure 1 shows what a four-start multiple start thread looks like during machining. The drawing on the left shows what the workpiece looks like after the first start has been machined. The drawing on the right shows what it looks like after all four starts are machined.
Many engineers define multiple start threads much like they define single start threads. The major or minor diameter is specified along with the number of threads per inch. While this will allow you to calculate the pitch of the thread, you must know the number of starts on the thread in order to determine the lead for each start.
The thread shown in Figure 1 is a 4-16 thread. The pitch is 0.0625 (one divided by 16). However, the pitch must be multiplied by the number of thread starts (four in this case) to determine the lead for one start. For the thread shown in the drawing, the lead is 0.5 inch.
Most turning center controls provide a canned cycle to help with threading. Fanuc, for example, uses the multiple repetitive cycle G76. For single-start threads, one G76 command will machine an entire thread.
When it comes to multiple start threads, control manufacturers vary as to how much help they provide. With older Fanuc controls, for example, you are not allowed to specify more than one start in a G76 command. For these controls, you must specify one G76 command per start. A four-start thread for this control will require four G76 commands
The trick is to completely machine one start and then move the threading tool in Z by the pitch for the thread (the distance from one start to the next). For our example thread, this means moving 0.125 inch in Z between starts. Then the next start is machined with another G76 command. This is repeated for each thread start. In essence, we’re commanding the machine to cross-thread each start in a controlled manner.
Unfortunately, it may not be possible to move the threading tool in the Z axis in some applications. There may be an obstruction in the way. For this reason, some controls allow you to specify the angular position of the start within the G76 command. While you must still specify multiple G76 commands, you will not need to move the threading tool in Z during the operation.
Unfortunately, most controls don’t have a special command for multiple start threads. To overcome this problem, you must completely machine each start as described above. Then command one or more spring passes over each start. The spring passes will be of zero depth and can be commanded by a one-pass canned cycle (G92 on a Fanuc control).
Be careful not to exceed the maximum feed rate. Maximum feed rate is a parameter setting—most machine tool builders set maximum feed rate to about half the machine’s rapid rate. Unfortunately, many machines will not generate an alarm if maximum feed rate is exceeded. The machine will simply move as fast as it can–and the thread’s lead and pitch will not be correct.