Ok, did as you said.

Put a .001'' dial indicator on X and set it to zero. Set Jog Cycle to .0010'', first tap nothing, second tap dial indicator moved. DRO reads +0.0024'' so double that .0048''.

Does that sound right Hood?

Ok just to clarify. you jogged in one direction onto dial and then zeroed DRO in Mach before reversing?
If so then thats correct and your backlash is half of what you read in the DRO rather than double as you are in Dia mode in Mach I think.
So it sounds like backlash is not an issue.


If you could cut a dia of say 1 inch and measure then tell it to cut 0.9 inch and again measure and see what the difference is.
Hood

Ok

I jogged onto the indicator so that it started moving a few places, then I stopped and zero'd the DRO. Then I jogged in reverse and stopped the minute the dial moved, in this case one tap with .001'' set and the DRO reads .0016''. So even less.

I am in DIA mode so thats .0008''.

I have some '' I can turn to say .900'' and then .800''

Cheers

Thats fine, just make sure you command to the 0.9 and measure what it cuts and then command to 0.8 and again measure.
Hood

OK so I measured the OD of the stock and it came out at .995''

First pass was set to cut .900'', that measured .899'' to .900''

Second pass set to cut .800'', that measured .800'' to .8005''

Third pass set to cut .700'', that measured .698'' to .701''

Fourth pass set to cut .600'', that measured .600'' to .6005''

Fifth pass set to cut .500'', that measured .4995'' to .500''

All cuts based on the original diameter of .995'', so later cuts had lots of fresh air passes.

So it seems to be working. But I have not changed any settings.

I took shallower cuts of .005'' at a time, before I had been taking .010'' and gone from .995'' to .500'' in one cycle. Could that have been too much?

Just did it in one pass and got .485'' instead of .500''

So why is it .015'' out when cut in a single pass, I used the shallower cuts too.

That sounds fairly good, the shallower cuts may be helping as you may have flex somewhere, cant really say what would be a good DOC for your lathe as I dont know how rigid it is but 5 thou does sound very shallow, I have 2 lathes here, the big one can take 10mm DOC if I want but usually stick to 2 or 3 mm DOC. The wee lathe which is based on a Myford ML10  I will take maybe 1mm DOC and it is fine.
Hood

Definitely sounds like flex somewhere.
Hood

Hi Hood
Do you have any idea why its so far out when cut in a single pass?

With shallow cuts too.

Something is either loose or flexing with the extra cutting forces. You will  have to put the clock on various points and push/pull and see if you can find it.

Hood