Newbie looking at 6040 (China CNC) for polypropylene 15mm sheet

[Zeeflyboy]

Can't see the all the way round the parts on the photos but is it wider the whole way round (ie suggesting G-code isn't setup up right with cutter size or stock to leave) or does it flip from overhang to underhang suggesting the zero has moved?

3D probe is great, but not cheap and isn't perfect - If I set mine up without dialing it in with an indicator every time then the runout can normally be expected to be around 0.03mm... can reliably dial it into around 0.005mm within a few minutes of tweaking but if you don't do that every time then you are already starting with a centre error of 0.03mm and I find the repeatability to be around 0.005-0.01mm so you're probably anything up to 0.04mm off centre.

If you spend the time to dial it in each time (which adds perhaps up to 10 minutes to each time you need to find centre or edge of something) then you could reasonably expect to be within 0.01mm or so.

It does come with constraints though - the probe body and length of the probe may preclude you from being able to probe stuff in a vice if it stands too tall... worth checking if that would be an issue with your intended probe. Some are massively long as they are really intended for mills.

They can also get unreliable over time due to oxidation of the contacts.

I do love my probe, but you can achieve a lot of what it does for much less with a short length of tool steel and an alligator clip when using conductive materials (or even just a conductive fence used for part location).

saw a very cool tip regarding super glue for holding down small parts the other day btw since you mention the faff:

lay down a strip of masking tape (I'd probably use decently wide stuff) on the bed and use a blade/whatever to squeegee it down flat. Do the same on the stock you want to fix down and leave a few cm sticking out beyond the sides so that it's easy to line up with the tape on the bed. Good idea to have some sort of fence you can line up with to ensure you're aligned to your X and Y if that's going to be critical for the amount of spare stock you have.

You then run a bead of superglue along the tape on the part, spray some activator on the tape on the bed and then press the part down firmly.

What you end up with is effectively double sided tape but with a firmer hold (carpet tape etc tends to have some elastic give in it) and the ease of removal of masking tape. Having used double sided tape in the past it can be a pig to pull up off the bed and can leave a lot of residue on the part, superglue on it's own is good but again can be a pain to clean up afterwards.

I think you are better off getting a jig sorted for this particular job, but something to try on low volume/one offs in the future. Given your semi-prodiuction like goals on this part it might be worth considering making an actual fixture plate to do several at a time rather than just soft jaws in a vice.