Hi Guys,
About a month ago I decided to try to cut a double start thread on my Emco lathe. I chucked a 15mm dia aluminum bar stock and gave it a go... and it worked perfect right on the first attempt.
I think that in order to cut multi start thread it's necessary to first understand the nomenclature and how it works. Pitch, P, is defined as the distance between two adjacent crests (for metric threads). Lead, is the distance the screw advances when it's turned one revolution. For a single start thread, lead and pitch are equal.
Thus, when you cut a (multi start) thread what you have to consider is the LEAD and NOT the PITCH. So if you want to cut a 1mm pitch double start thread, you have to program a feed of 2mm/Rev for each of the starts. Obviously, the two starts are 1mm apart, so after cutting the first lead you have to move the tool 1mm in Z and start the move from there for the second lead. Note that the depth of thread has to be the same as if you were cutting a single start thread with the same pitch (which is 1mm for this example).
Like was noted in the previous posts here, the end of cut (in Z direction) has to be adjusted for each lead so that that they end in the same point along Z.
Hope it helps
BTW, Andy, both the threads on your picture above look messy. The single start thread looks awful - like mine used to be with the DC spindle motor that was not rotating steadily in the same RPM. Turned out my belt was worn out unevenly along the perimeter and was causing fluctuations in the spindle speed.
Daniel