No VFD here, I wish!  I just have a 1hp dc motor controlled manually with the knob on the Dayton motor control box.  Mac has no control over my motor I have to turn it on with the switch, then dial up the desired speed.  I will try to run the code line by line later as suggested  and report the results, hopefully later today.

Trevor


PS, I am using the parallel port.

Think your x  stepper is loosing a step and then Mach is able to still control the steppers but every thing is out of whack. So mechanical problem leads to a software problem. I may be very, very, wrong! I would suggest that you try bringing your velocity down some. I think your step miss is x axis during the rapid before the next index, thus the the pitch is still ok. Then along with backlash , a screwy / upset program ,
it just repeats. Odd i must say. But I still want my friend to listen to the video and see if he hears what i hear.

I have had steppers miss steps, not in a long time, as i am conservative on velocity & accel settings.
Once i was milling  a circle and the stepper missed steps but the " second circle came out all right
just a 1/4" of center from the first Z pass cut circle. Mach values were wrong but it maintained control. Sort of like "the driver lost control of the car but was able to bring it back onto the road with what was in his head only the car ended up in the wrong lane". In another case the axis just went totaly out of control at  rapid setting. Post estop MACH's DRO values were nowwhere near the dro' on the mill.

12v @1.5a delived to the 195 Oz/in stepper,.....you have speed but i don't think you have much torque.
You would think that if you dry run / skim cut and all is well that you shouldn't have any problems.

RICH
 

Trevor
 You have manual spindle speed control so could you run the code and just cut air and see if slowing the speed via the pot while its moving in the Z brings on the X movement.
Hood

I had the same problem.I had it with p/p and s/s.
I had it with a g76 and g32.The only way to make it go away was to add a dwell
at the begining of each cut or increase the power to the spindle.I think as I said before
that the reference pulse is the critical thing here.Depth of cut is not the only thing that can
slow down the spindle, friction of the tool rubbing on the sides of the tread. This does not explain the random
x movement but with enough torque it seems to not start.Any one can simulate this problem by
-running the treading wizard and generate some code.
-don't load a tool or work just turn on machine and cut some air
-apply a load to the spindle ,I just lever a 2x4 up against it until a rpm change is detected
-do this while threading in air.
Watch the x axis go into stupid mode.

Rich,  I may be confused but I don't think that there is any possible way that I can be missing steps, because when the program is done running I can jog the tip of the tool to the end of the workpiece and it is at 0,0 just like before the program started.  If it is reading 0,0 then doesn't it mean that the motors haven't lost any steps.

Hood,  Keith, very good idea about cutting air and varying the spindle speed to see what happens.  I could do it by loading up the spindle with a board, or rac, or whatever, but i think I will just turn the dial on the speed indicator to see if I can create the problem.  I will go do that right now and report back, seems like a simple, but very telling idea.  Thanks

Something to try also....just out of curiosity.
Run the faulty code with an "I" setting of 0 and see what happens.
I'm wondering if the "Constant Volume" per pass math done internally might be somehow adjusting both Z and X when the spindle slows to simulate a manual compound slide set a 29 deg. or whatever you have it set at.
Just a shot, there is a lot going on "Inside".
Thanks Guys,
RC
BTW....RT, hope you're OK.
I hope you don't shoot hijackers.
RC

OK FOLKS,
Here it is and if you can manualy vary spindle speed the you can duplicate it.
The threading cycle has been changed from before in how it compensates for spindle
slow down. It's also a matter of degree of slowdown. 
 
Generate the Gcode with  M76 and have all the cuts .001" deep, 1 to  2" of thread,  and run at 100 rpm  so you can see what is happening. My dry test had 80 passes.This makes for easier monitoring.

As it should each time a new pass is made the X will move .001" for the next cut. All is good. At pass
#10  I varied the speed  70%. I am using the SS so you can't see speed change but you can see feed rate change. The DRO's "seem" to go bonkers......but i don't think so......
The program see's that thread pass as out of the norm in which it can correct, so it takes you back to starting X DRO and does an air cut down to the X value at the bad pass, so it is doing a "tapered" move. 
Maybe what it has done or is doing is recalibrating / reseting the buffer for the next pass. But it needs some good data to work with.
The next axis move will now go to the next X pass value and that value is correct, just .001 deepr than the past bad one. Let it continue for  10 passes and each one will be correct. 

I duplicated this 8 times in the 80 passes.

Now if you stop the spindle, and stop time seems a little time dependant  on how it will behave, the Z will just finish the pass ....
the thread would be history in real life .......and the threading cycles will stop. Mach has lost the index...
it has no basis on how to continue.

So either the threading cycle is ******************x'd up or the way it deals with spindle slow down has been changed.

RICH
Now I have been convinced, but the mechanical stuff still induces it.  

....... ART, Brian please clarify what the  proper interpretation of the threading cycle is........!

Hello Rich,

As i understand, the trouble is caused by the compensating for unduly changing of the spindle speed?
If so, when I can forcomm slowing down of spindlespeed then it will be OK?

Rieks

Rieks,
The compensation was deliberately built in. It was done to take care of Punny lathes, does work, but there is a limit on anything. The better your
spindle speed is to start with and maintain that speed when cutting,  and if you can minimise the mechanical influences ( backlash, uneven cutting, poor set-up etc. ) that may cause it to slow down, and if you use good machining practice, the axis components can do their job as it realtes to speed ,then you should do fine.

Threading is a "SYSTEM" of all the components. Not one thing. It's like a sterio, one part of the system not up to par and  the music you will hear has been downgraded.

RICH

  

OK so i did some more testing today, and I took video.

First here is a video of my lathe cutting a ball radius on the end of a .500 Ally rod.  I intentionally set it to cut the .500 sphere with a z center of -.500 so that it would not only round over the end of the rod, but also have to cut an extra .250 off of the end of the rod.  This isn't realted to threading at all, but it does serve to demonstrate that if nothing elsel I think that I do have pretty good repeatability of the X axis, check out the .010 nub that remains until the final pass.

http://www.youtube.com/watch?v=uuz8B3oVLNs

Next is The video of the lathe threading while cutting air as Hood had suggested.  As you will see the movements are normal at first, but then when I alter the spindle speed the funny x axis moves begin.  I also noticed that these errors can be induced with an decrease, or increase in spindle speed which makes sense though not many of us have a problem with our spindles speeding up during a cut

http://www.youtube.com/watch?v=x-rHNH8VVnI

I do also want to clarify one other thing based on my observations so far.  If a thread is to be cut in whatever number of passes, lets say 80, then it will be cut in that number of passes no matter what happens.  In other words if the problems begin and the x starts to cut a taper in the air it is in place of a programmed move.  It does not appear to me that mach is inserting some kind of extra move in order to compensate, rather it is inserting a funny move in place of a programmed move, and replacing it.  This causes the next normal move to remove more material than intended since the prior pass didn't do any cutting.  So If I have programmed 80 moves I will get 80 moves, some of them OK, and some of them goofy, but I do not end up with 160 passes of which half would be normal, and half goofy.  Let me know what you think of the videos.  Thanks

Trevor

Now if i can just come up with a way to convince the wife that I need a 2hp VFD/motor combo that can maintain +- 5 RPM I will be all set