Cartesian Coordinates Accuracy + Gears Question

[RJSakowski]

For some reason, every time I lay out a bolt pattern on my small import mill at least one set of holes is off by .008". The holes adjacent to them are off by .002" and measuring the remaining 4 possibilities on a 6 hole pattern are spot on.

Another source of error with import mills and lathes, many are set up with metric leads screws. Although the dial may say .oo1", it is actually .000984". This is due to rounding the 25.4 mm/in to 25. While this difference is negligible for a small distance, it can mount up when making larger changes.

From what you are saying though, it appears you are experiencing much larger deviation, in which case I would expect backlash to be the issue.