Just did two tests, first was using the degree offset #99066=0 etc
It seemed to be working when I was cutting air but when I did the actual cut it didnt shift and just tracked the previous thread. This could be a smoothstepper thing as probably it is not implemented for the SmoothStepper

Next I did the Z start position offset and it again seemed to work fine cutting air so tried it on some stock and this time it seemed to work fine. I inspected it after it was complete and it looks very good, each start seems 120 degree out and pitch seems good for the full 100mm length.
Pic below is not very good but it will let you see hopefully.

Hood

Hi Rich,

Don't know where you got the 0.86603 constant...? Nothing close to it in my books

Yes I am cutting with a sharp V tool - don't cut large pitch threads.

Daniel

Hood,

Yep... this was the way it worked for me. I only tried a two start thread and it turned out perfectly!

Daniel

Daniel,
Check out a Machinst Handbook. Take a close look at the different forms.  Peak to Peak of the sharp thread crests
are defined as H=0.86603 x Pitch for Unified Screw Threads. If you take away some of the sharp crests ( top and bottom )  then the remaining depth of the thread is defined as 0.61343 x pitch or 17H /24 where H is as stated.     
RICH

Rich,

OK, thanks for the info Now I'll know to distinguish between the cases.

Daniel

Hmmm tried it again with just two starts and it did not work, I must be missing somthing somwhere, should the z axis move at the lead as mine is going really fast?

I am using the sherline (metric) profile would there be anything in there?

Andy

Can you do a single start thread?
With a twin start the z should move at twice the pitch, ie if you want to end up with 1mm pitch dual start then the z will move at 2mm/rev

Hood

Ahhh, the Z was going at about 245mm/rev hmmm could this be the problem.  If so how do I get it to follow the speed set by the 'F'

A

Andy,

This M76 threading code is a two start thread using a Z offset for the start of the threads and keeping end of the thread passes the same. ( first pass starts at Z=0 and second pass starts at 1/2 the pitch ) but both end at the same point.  Looking at the where the threading started on the end of the piece you can see they are 180 degrees out as expected. Looking where the threading finished for both passes you can see that they ended along the thread at the same point.
 

G00 Z0.0
G76 X0.33 Z-0.600 Q1 P0.0591 J0.004 L0 H0.005 I30 C0.01 B0.0002 T0
G00 Z.02955
G76 X0.33 Z-0.62955 Q1 P0.0591 J0.004 L0 H0.005 I30 C0.01 B0.0002 T0

Now as far as using the angle setting, you can get some intersting results but that is
a different story.

RICH

Andy,

Try what Hood has suggested. I am thinking it is a sync problem or not stable spindle speed.

Daniel