Quote Originally Posted by Jonathan View Post
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?
I'm not using any iron to transfer...I knock up my pcb design in eagle, print onto tracing paper, expose my presensitized photo board to UV through the tracing paper, develop, etch. So the only potential for error is the laser printer itself & tracing paper stretch (due to the heat from the laser)

Re the hmm screws, not 100% sure(!) ...the maker call them 'zero backlash' screws (yeah, right!) - the advert is here ... http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=230689354532

I've set up all the axii (??!!) with my dial indicator...like I say I was getting .05mm backlash, which I dial out in software - yes it does return to zero.

I do have a digitial caliper, but then need to be sure it was totally square to the plane of travel, else I reckon I could be introducing more errors just using it to measure over such distance (then as you say there's the digital caliper error itself!)