Y Positioning Accuracy

Whilst I wait for my Mendel90 to arrive I'm designing a UV pcb exposer similar to this [www.das-labor.org]. It was going to be a standalone device but then I thought about using the M90 to do the Y positioning of the pcb; so I need to know the accuracy of positioning the M90 Y axis.

According to the M90 build manual [reprap.org] there are 80 steps to the mm (in Y).

1 step = 1/80mm = 0.0125mm (~0.5 thou)

I was surprised, given that the reprap wiki here [reprap.org], mentions an accuracy of only 0.1mm (in an unspecified axis).

My question is: Could I expect this performance in reality?
Or do you say "well, given a bit of slap in one step and a bit of tickle in the next, the actual accuracy is about 0.125mm over ten steps"?

Notes: For my purposes that 0.0125mm represents approx. 2032 dpi. A 1200x1200dpi laser can easily do what I want so in theory it ought to be OK.

... the achievable accuracy is depending more on mechanical properties, friction and stiffness/rigidity of the frame than on resolution.

My CNC-mills are specified with accuracy/repeatability of maybe 3 to 4 times coarser than the actual resolution - I have some really precise linear drives with 0.1 micron resolution ... but the absolute repeatability of a position (even without load and counter-forces) is specified around 1 to 2 microns, so 10x the accuracy! ... (mainly inaccuracies in the bearings/threads)

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Resolution and accuracy are very different things. The axes have a resolution of 12.5um but the full steps on a stepper motor only have an accuracy of about 5% and microsteps are even less precise. Then there is backlash unless you have zero friction or infinite torque, which we don't.

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[www.hydraraptor.blogspot.com]

@VDX
@nophead
Thanks for the pointers chaps. I guess a bit of suck it and see is called for as there are several other factors involved (beam focussing, laser control) which I have little gut feel for.