I tried to post this with attachments (picture & XML) but couldn't. I posted on the Machsupport page as "Post Problem" (this can be erased or moved) so now I'm in two places.
 
While threading the carriage makes an unauthorized triangular jog at the start of the thread.
 The pass it happens on is never consistent but it always happens no matter which threading wizard I use.
 I have tried other gcode shapes and they run OK.  I use a slotted optical spindle speed sensor bypassing
the BOB and have tried denounce from 0 to 10.  I'm stuck but I have faith in you guys. Thanks, Ben
It seems like when you have one problem everything else becomes a problem while trying to solve the first. Murphy ?

What version of Mach are you using?
Post the gcode file.
Post your xml file that you are using.

RICH

To attach something, after you click the preview button, click the Additional Options, then Browse button which will provide for selecting the file you want to upload.
RICH

I'm using 3.042.038
When I attach something I get this--Sorry! There is already an attachment with the same filename as the one you tried to upload. Please rename the file and try again.

I'm using 3.042.038
When I attach something I get this--Sorry! There is already an attachment with the same filename as the one you tried to upload. Please rename the file and try again.

Yes, you need to name the file something unique, then upload it.

Dave

Thanks for your patience.

Ben,
Can you post the xml that you are using for the lathe. The one you posted didn't even have Mach configured and was for a mill.
Was the pic attached done when your actualy running the machine or emulating the program?
RICH

That pic is a actual run. Sorry about the xml, here is the correct one.

More Info
It acted like the spindle rpm pulse was noisy or eratic. I then moved the signal wire (shielded all the way) from the
BOB directly to pin 15 on the DB25 cable. I checked the "Calibrate Spindle" and there are no spikes and the rpm
only varied < 1. The phenopy happens at the start of the thread but changes sometimes pass one sometimes
another pass and I can't run a threading operation without it happining.  I'm using a OPB982 Slotted Optical Switch
and the disk has a .200" wide slot.

You are using a very old version of Mach if you are using 3.042.038, there have been fixes to  threading for the parallel port driver since them, so I suggest you try with the lockdown version.
Hood