A four-start thread for this control will require four G76 commands For these controls, you must specify one G76 command per start. With older Fanuc controls, for example, you are not allowed to specify more than one start in a G76 command. When it comes to multiple start threads, control manufacturers vary as to how much help they provide. For single-start threads, one G76 command will machine an entire thread. Fanuc, for example, uses the multiple repetitive cycle G76. Most turning center controls provide a canned cycle to help with threading. For the thread shown in the drawing, the lead is 0.5 inch. However, the pitch must be multiplied by the number of thread starts (four in this case) to determine the lead for one start. The thread shown in Figure 1 is a 4-16 thread. 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 major or minor diameter is specified along with the number of threads per inch. Many engineers define multiple start threads much like they define single start threads. The drawing on the right shows what it looks like after all four starts are machined. The drawing on the left shows what the workpiece looks like after the first start has been machined. 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.įigure 1 shows what a four-start multiple start thread looks like during machining. Multiple start threads combine the close fit of a fine thread with the quick axial motion of a coarse thread.
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