Just to put your minds at rest, here is a little info for you to chew over, it might put things straight.
What you are making is a 9tpi thread multiplied by four but at the same depth as a 36 tpi one, all running parallel to each other. So the multistart will travel forwards four times faster than a single cut 36 tpi thread.
This pic explains it a lot better than trying to tell you, it is based on a standard four lead screwthread. See how far forwards it moves for each revolution.
It is for that reason, chasers shown above cannot be used for cutting multistart threads. They will only cut or dress a single thread.
Darren, it can be an absolute pig to do.
The main problem, the smaller the TPI, the faster the cut over a given length is done. Also, this lathe is designed for a min TPI of 12, by making it do 9TPI would grossly overload the gear train when driven by the spindle, and so has a tendency to smash the gears or holding bolts (I have had it happen to me), so you have to modify the lathe so that the drive comes from the leadscrew, usually by fitting a handle onto the end of the leadscrew and turning it by hand. This solves two problems, the first being the stress on the gear train, and the second being, you don't have to have the reactions of a s**thouse rat to to disengage the half nuts before the cutter hits the chuck when doing it under power.
These are columns I made for one of my engines, they are based on a 2.5 TPI, two start thread, using a specially ground cutting tool. Doing these on my large lathe under power, smashed the heads off two 3/8" diameter gear train holding bolts like they were lollipop sticks. So it does happen, if anyone doesn't believe what I am telling them. They are only just over 1/4" diameter, so it wasn't the cutting itself that caused the breakage, it was the stress in the geartrain trying to move the saddle across at high speed to do the cutting.
Bogs
