Calibrating my new machine - "never the same twice" woes

[Jonathan]

Thermal expansion will be negligible. The PCB will change dimensions slightly when you hit it with the iron to transfer the design.

Standard 10mm dial indicator is 0.01mm resolution, so if you calibrate from the full range of that it should be maximum 150/10*0.01=0.15mm out over 150mm. It sounds like your error is more than that which implies it is an error in the screw pitch. What screws are you using, ballscrews/ACME?

Have you got a digital caliper? Standard ones measure up to 150mm so you can use them to check over that distance. The resolution is 0.01mm, but the instructions state the accuracy is worse however it's significantly better than about 0.15mm over 150mm from a dial indicator. Just be careful with either of them that you mount them carefully, if the indicator is not precisely parallel to the axis then the reading will be out.

I take it it does move back to 0,0 precisely?