TrevorH,
I used your Gcode that you posted to cut a 1/2-13 thread in 6061 on my Punny little lathe. ( 400 rpm spindle speed)The posted pic is of the G32. Also cut one useing M76. The results were the same. In the pic you can see a difference of 2 thou at the thread beginning and that is due to the actual diameter dfference as i didn't start off with a finished stock diameter.
The spindle did slow down. Never experienced an X runaway. During the spring cuts there were a few times when more than a hair shaving was taken off. I have about .002" backlash in the X and a little under the same in the Z. So during the spring cut is it's possible for me to get more than just .0001" cut. i can adjust / take out more of the backlash but I leave it as is for now.
The flank cutting ( your code had 15 deg infeed ) worked as it should, just a nice forward curl. Note there are no
steps in the backside of the threads. Your's has them ( that's backlash and i can duplicate it by increasing mine).
I believe your problem is mechanical. Note no waviness in my threads. Software dosen't do that. Mach reacts rather
quickly, but not instantaneously to a slow down, when that accurs the next pass will try and fix it. Yours almost looks like a gouge. On the next cut if it happens again, because of some other / prior pass, then again it tries to fiix, but
then who knows what it's fxing ( just my thought). You may be better off not taking as deep a first pass cut ( .014").
This way you get a v started which can help guide the tool. Just some thoughts for you.
Now i will admit that there MAY be some quirky thing from the software point of view, but not convinced yet.
Or maybe i just haven't stepped in poop lately ( i do have the knack of finding it).
RICH