Hi from Glasgow

[bartdietrich]

Hi all,

I'm a hobbyist based in Glasgow. I know my way around electronics, 3D printers and programming but am new to machining. A couple of years ago I bought a cheap used desktop CNC mill from eBay intending to use it to route/drill printed circuit boards. This seemed like a good idea especially for double-sided boards where layer alignment can be an issue. For two years I haven't done anything with it, usually blaming lack of time and the fact that I'm quite good at aligning layers on double-sided boards for the usual etching techniques. What with the virus going around and me being stuck at home, I don't really have the lack of time excuse anymore so I dug up the mill and decided to give it a go.

It's a cheap Chinese job I think (see photo, I bought it second-hand from someone in the UK so can't be certain but it looks like some I've seen online). I extended the x and y axes by 10 cm so as to be able to do the largest PCBs I would ever normally do. The mill basically works. I'm getting usable results (ok but not amazing): I realise I can't expect too much for £120. It's got a few issues which I'm trying to iron out one at a time as I'm familiarising myself with it. The latest is that the z-axis rods have a bit of play inside their bushings so that as the tool touches the PCB surface and begins pressing down, it slides forward on the copper surface by a tiny distance before penetrating it while the motor carriage repositions itself as much as the play between the z-rods and bushings will allow. I've ordered some Igus bushings: hopefully they'll be a tighter fit and sort that particular problem.

The most annoying issue however is the centring of the tool with the spindle axis. Whenever I change tools I have to fiddle with the two grub screws that attach the chuck to the motor shaft to get the tool aligned with the motor axis again. I have, to some extent, figured out a system to it, but it still involves a lot of loosening one grub screw, moving the tool/chuck a bit, tightening the grub screw, and spinning the chuck by hand to see if the tool is centred. Given enough time, I can get it centred pretty well but it's extremely annoying and time-consuming. Something tells me it shouldn't be like that.

Is there a repeatable and sure-fire way of centring the tool with the motor axis? I think the motor shaft itself is ok: there doesn't seem to be any wobble when I spin it and it doesn't have any sideways play. Have I managed to deform the chuck itself (I did crash the machine a few times)? Or can someone point me to where I can buy a replacement chuck of better quality? I've read about ER11 collets, would this be the way forward here? My mill is quite small (and so is the motor) so any replacement chuck would have to be fairly small as well. The current chuck is about 3 cm long, fits on a 3 mm diameter motor shaft (measured as 2.91 mm) and takes collets for 3.175 mm tools.

Apologies if this is something really obvious or stupid, but it's doing my head in! Any suggestions or pointers would be very welcome!

Thanks,
Bart