fast89,
- Do scribe test's for reasons I mentioned. The scribe tests that i did were used to measure pitch to 0.0002" and
  you don't get that from comparison to a tap. The tolerances are noted in threading on the lathe manual, page 26.
  So for a 1/4-20 the error allowed for a 2A over .3" is 0.001". Said a different way.....one thread is 0.50", .3"/.050"=6 threads,
  thus 0.001" allowed. If 0.0008" per thread you are over by .0008 x 6= .0048" - .001"=.0038 lets just say 4X.
- Although I have a C3 board for the index never put it into use. I think Russ has used it so he may comment on it.
- How linear and accurate is your Z axis?

RICH

You may be getting tired of hearing this, but .... Can you verify the accuracy of you Z steps-per ?
Early on you said it was exact by referencing a move of .010". That would be difficult to verify, try a much longer move .. as long as you can accurately measure.
With a move of .010", the .0008 in .050 error would only show up as .00016 which would require quite q delicate indicator.
No problems ever with the C3 here, just that one got dirty from belt dust.
Russ

..and be certain that the backlash is taken out by first making a short move in the "verify" direction.

Yeah, i'm a moron. I thought my screws were .2 pitch which would be 5000 steps per inch. I have 5mm pitch screws which is 5080.01 steps per inch. Which ends up being about exactly the error i was off. Changed the steps and the first thread was perfect. The error was too small to see when jogging .010" with a .0005 indicator. Thanks to all who replied, i much appreciate it. I'll try to be a little smarter next time lol.

Has been done many times before but you were so convincing
Hood

NOW you're cookin' with GAS !
Russ
 

glad you're sorted, we'll remember this one.

Happy you found it.
We spent many hours with Art trying to get threading fixed and don't want to revisit testing. 
Hope you now understand why we are so adamant on rpm and axis accuracy.

RICH

I just want a copy of his modified screen for the Lathe, don't like any of the lathe screens, most machine tool controls have the same screen for mill and lathe.

a copy of his modified screen for the Lathe

What screen are you referring to?

RICH