Last time I looked, it was about 390 - microns of backlash on the Y axis. That is almost unmentionably shit. WTF is going on? It should be pretty quick and easy to get an idea whhere that's coming from.
Manual test:
No need to get busy with the CNC system straight away. Simply turn the leadscrew by hand, backwards and forwards until you can see when the DRO starts to change. With a bit of practice, it's clear there's about 4mm of movement at a radius of 35mm. Simple maths shows that's equivalent to about 100um movement on a 5mm ballscrew (it's ~23um per mm of circumferential movement). That explains a fair bit. But given that I have no slop in the belt, that's less than a third of the measured value. Something doesn't add up. Literally.
Wait a minute...
When I turn the handwheel to move the saddle in the negative direction, the DRO actually shows an increasing reading initially, followed eventually by a decreasing reading. The DRO scale is mounted on the side of the saddle, whereas the ballscrew drives the saddle at its middle. It appears that the saddle does some sort of twist about a vertical axis but that isn't sensibly possible.
Here we go. One of the fixing screws had fallen off the Y axis DRO scale bracket and allowed the read head to wobble about as it moved. This caused it to drag against the cover and behave as discovered ("moving backwards initially, before finally moving forwards"). I took the brackets off anyway to check I wasn't missing anything and reassembled them, replacing the missing screw.
Job done.
Seemed a good idea to remove the black streaks caused by the abrasive contact while I was at it. That's better:
Now the bad (but slightly better) news. The Y axis backlash is now a consistent 100um. That compares to the X axis which manages about 35um. I guess that's down to the ball bearings, which I don't plan on messing with at this stage. It's still shit - but I guess that's what you get when you buy Chinesium ballscrews from AliExpress for peanuts.
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