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

[HankMcSpank]

I recently purchased a nice little CNC on Ebay (to my eyes at least, it really is well made)...the maker quotes repeatability to 0.05mm - so I'm trying to calibrate it.

The leadscrew pitch is 1.5mm, I'm 1/2 stepping it, so I reckon that's a 'steps per unit' setting of 266.6666 steps per unit (ie 1mm)

Ok, so I whip my dial indicator out - pretty much bang on, backlash is about 0.05mm which I dial out via software - all is good so far.

The main purpose of this machine is to depanel multiple repeated pcbs on one board ...and this is where it's all going 'Pete Tong'.

On my pcb I include some crosshair registration marks of known set distance ...but when I mount this 'etched pcb' onto my machine, and jog between registration marks....my cnc DRO READOUT doesn't marry with the ('known') distance between the registration marks....the pcb is a fairly intricate design with no room for error (it has PCB tracks are near the edge of the board outline)...so I got to get to the bottom of this.

Now bearing in mind I etch my pcbs from a laser 'printed' onto 90gsm tracing paper, I'm thinking this is likely to be an error somewhere between the laser printer itself & the paper stretching while being heated. So I therefore frig my cnc macines 'steps per unit' so that my CNC marries with the registration marks exactly. But then I come back next time & it's out of whack again.

I'm of the opinion that this is now likely to be thermal expansion of my machine - basically my CNC machine sits in an unheated workshop ...if it's Baltic outside then so is my machine - if its a warmish day, then so is my machine. (the cnc machine is small and made of ali)

Now short of fitting heating to control the ambient temperature of the room & keep it constant (that ain't gonna happen), i'm not sure what my options are here? Is there a quick way of calibrating the machine to cater for these possible expansions/contractions?


Edit: The error I'm talking about is over a distance of about 150mm ....probably an error of something like 0.75mm-1.0mm over that distance. Now when I calibrate my machine with the dial indicator...that's only using a distance of about 10mm to calibrate ...so what do folks use to calibrate their machine for larger distances? (or can I safely assume if it's bang on @10mm distance, then it's going to be bang on at 150mm too?!)