Hi all,
My Shapeoko XXL lost power about 20% of the way into an engraving job. Fortunately I’d taken a screenshot of the machine position in Carbide Motion, so I restored it to exactly that position and rezero’d it. However, the job is about 0.03" out of position from the previous engraving, ruining the piece. Does anyone know what I did wrong? I’m pretty sure I reinitialized the machine as well.
What kind of precision/repeatability do you get if you repeat this test now ? (zero, power cycle the machine, home, come back to zero, check how far apart it is from previous zero)
Is it off by 0.03" in a specific direction ?
Confirm that there is enough clearance so that there is no mechanical contact between plates during homing ? That and giving the machine an overall mechanical check ? (the usual suspects: belt tension, eccentric nuts)
The mechanical homing switches are inaccurate. There are no ways to get back to an exact spot after a homing event.
So maybe a ups would be better if you have repeated power losses.
When it comes to material, just enough is never enough.
Did I hear someone say, “UPS?” 
https://community.carbide3d.com/t/communication-problems/20513?u=crookedwoodtex
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I’ve found the switches to actually be quite repeatable, certainly well within .030".
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The mechanical switches are accurate enough but are not exact. Once homed the Shapeoko is quite accurate but if you home a second time you cannot guarantee that you are exactly where you were on the first homeing cycle. In theory the proximity switches would be more accurate but I dont think you can get the same result for two homing cycles.
So be sure you dont lose power or re-home and you are good. If you power cycle or re-home all bets are off for positioning. It just the nature of the Shapeoko.
Got a source for that?
Well, now I know I suppose.
How are you zero’ing?
0.03" is not standard for switch repeatiblity error, stock switches or not.
Check out this interesting thread, where prox switches AND stock switches repeatability is being discussed, and the ballpark mentioned there is the tens of microns…not 700 hundred microns.
So something else mechanical is going on.
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+1 “Poor man’s homing”
Also its good practice to recheck your zeros on the actual piece when there’s a power interruption or skipped step. Better to trust the workflow than the machine sometimes.
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This. Despite having a newer XL with proximity switches I always zero off my stock when machining aluminum or stainless. It’s usually within 15 microns, however numerous times it’s inexplicably off by 75+ which is unacceptable for my parts.
The only way I know to combat this issue is carving pin holes on every piece. You can then go back and pin out a spoilboard to have an accurate homing.
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