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

[HankMcSpank]

Hi Ian,

It's *definitely* not backlash - how do I know? Well I make sure not to change direction on the leadscrew when aligning all this up ....ie to set this all up, I initially I shoot past my registration mark, then bring it back (therefore take up any backlash slack) ....then use a 'G0 Y -150' command all travelling in the same direction to move to my other registration mark.

I've since learned about the G51 'scale' command, so I reckon I can use this little procedure

Set my steps per unit to mate with the leadscrew pitch (therefore 1.5mm @ 1/2 stepping = 266.6666666666667 steps per unit)...always leave it on this (I've been adjusting the steps per unit to dial out the pcb registration mark aligment errors)
Align over my first registration mark - zero the DRO.
Use a G0 command to move the Y axis 150mm (this being the distance between my PCB registration marks)...it won't totally align
Now jog the Y axis until it does align
Enter the actual DRO reading (eg 150.7 etc) into a spreadsheet ....the spreadsheet calculates the 'scale' adjustment
enter scale value as a G51 command.
Once finsihed clear the scaling with a G50 command.

[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?

[HankMcSpank]

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!)