CNC

The calibration of your machine is off? (Has this always been the case, or just recently, and how old is the machine?) Tooling is worn or mismeasured? There's an error in the software? You need to tell us more if you want a useful response.

Another useful response could be: "don't worry about it, program it to 0.020" less", but, I suspect that sooner or later that will change. It is, imo, better to find out why now, and be prepared to deal with future fluctuations in that value, or prevent them.

It's a recent thing. Last week it happened maybe every third set-up. But now it's every set-up. Tooling and turret are all on center and was rechecked Fri.

Is this a constant difference on any length or on a specific case.

The conversion factor between inches and mm is sometimes incorrectly assumed.

On the Z axis it may be because of working to the centre or tip of a ball nose tool.

Or an inaccurate limit switch or homing procedure.

It's constant on Z axis,using a WNMG 433 insert. It's between .018 and .020 off.

are you turning on the tool nose rad. compensation? If not, that is the root cause.

You didn't say, does this machine have a probe? are you using the probe?