Not Cutting 6mm plywood with 28mA

[Andrew Macvicar]

We use 6mm birch veneer plywood (not the greatest - but cheap). I have had success cutting clean shapes that drop out nicely at 15mm/s. However, it will now not cut at all (even at 5mm/s) - showing 28mA-31mA on the gauge (machine locked to 65% output). I have everything aligned. Focus produces a sharp and clean dot, perpendicular to the table. I found the #2 mirror to be dirty, so polished it clean with Simichrome fine polish which has worked fine in the past. Now I see the mirror coating is mostly burnt off. Can anyone offer advice on buying solid copper mirrors that will not have a coating to burn off, but can be polished? Are the Mo mirrors better than other coatings? Is there anything else I can try without getting a Dohickey? Any help is appreciated!

[Gene Uselman]

I would get a dohickey in any case- they are handy to test the beam along the path. Russ spent a lot of time with copper mirrors and pretty much gave up on them. I have never done anything other than lens cleaner [from eyeglass shop] with lens cleaning tissues- not sure any of these mirrors will stand up to actual polishing compound. I changed my mirrors three times I think in 5 years and normally it was just my conscience poking me that it must be time. If your air flow is good there should not be much passing by that mirror, you might want to address that. I am sure there is some difference between mirror types but I have never been bothered by it.

[sebastien laforet]

standard quality mirror are cheap enough when buying direct from china. there is no point in spending hours trying to polish mirrors to save 10$.

same for lenses. keep them clean with proper maintenance, but if they are burnt, don't bother, buy a new one.

[Andrew Macvicar]

Thanks for the quick reply - I have now done 11 passes on the 6mm plywood: still very little cut through and I am down to an absurd speed of 3mm/s. Air flow is very good ( I installed aluminum baffles and tape on the sides of the bed to increase flow over the top). I use a solid steel bed with a pin table for clean evacuation of the kerf and 1.5-2 bar air pressure at the nozzle for cutting.

I'll attach a photo of the mirror in question - maybe it's burnt out completely, but I don't have the experience to determine. The No.1 mirror looks like new but there is no 'gold' remaining on the No.2 (however still seems very reflective).

When cold, the output begins around 30mA but after a while drops to 26-27mA for some reason, but it doesn't seem to affect the cut in progress when it's actually cutting nicely.

I would like to replace the head with Russ's upgrades but I have no idea what 'series' of focus tube I have (63.5mm fl - 20mm dia lens). There are 'C'. 'E' 'T' series - but no indication on the parts.

Also - how can I get one of those Dohickeys as I believe it would be an essential diagnostic tool? This machine seems capable of much much better performance but I never seem to be able to get it dialed in like Russ.

[sebastien laforet]

I'll attach a photo of the mirror in question - maybe it's burnt out completely, but I don't have the experience to determine. The No.1 mirror looks like new but there is no 'gold' remaining on the No.2 (however still seems very reflective).

just put a piece of anything that burns (cardboard) before the faulty mirror and "pulse". check the burn mark and the time you press pulse. maybe you can decrease the pulse power to make the test more relevant (to ensure you can pulse at least 3-5s before completely burning through)

next, same AFTER the mirror. does it burn less for the same time pulsing? if yes, the mirror is at fault.

also, if mirror is defective and doesn't reflect the light, it means that its absorbs the power, so i must become hot very quickly.

[Gene Uselman]

Clear acrylic is good for this as you can see how deep the 'burn' is- probably need .5" for your machine or 2 .25" pieces glued together. It is hard to see much from pictures but if the coating is worn off the center of the first pict... it is toast.

[Andrew Macvicar]

Hello - thank you for the test advice. I just tried it and indeed the beam takes 2-3x longer to burn through the card stock on the reflected side of this mirror (No.2). I also found the mirrors to be very affordable so am ordering a few spares today.
The heat generated by the beam has discolored the anodized holder for mirror 3 (head) and it becomes misaligned regularly: I will eliminate this problem with Russ's modified version if I can get one. I'll let you know how it works when the new mirrors arrive.

[Gene Uselman]

Should be almost no heat generated if the mirror reflects instead of absorbing the beam.