I have just finished my home-built CO2 laser that I’ve been working on for the past year. It has a SPT TR90 tube (with the built-in red laser pointer) and a Ruida controller. I’m having great success with it… but I do have a few weird issues that keep cropping up… See the attached photos of two different runs of a coffee sign. These were both done in a single pass, with the wood in the same place on the bed, but a couple of days apart. In one of them you can see that the ends of the path’s don’t quite line up with the starts - when it completes a path it’s off in position by a millimeter or so, so there is a jagged edge like a saw-tooth wherever the loops meet. This doesn’t always happen on every cut but seems to be on at lest 50% of them. I suspected backlash or stretching of the belts but my belts are tight and I can’t feel any backlash in them… there are two belts on the Y axis (on either side of the gantry) and one on the X axis.
The other issue that I’ve found occasionally is a second shadow outline to the top-right of some cuts. This you can see in the sign on the red background… That was done in a single pass and cut through the wood just fine, but somehow a few millimeters from the cut there is a burned line in the wood as if a second, very low power pass were done a little bit offset… but I assure you it was done in a single pass… My only guess on this is that the beam is grazing the inside edge of my air-assist nozzle and somehow reflecting a little bit to create a second beam at an angle from the first one… Is this possible? This issue seems to happen only occasionally and I’ve done multiple cuts right after each other without changing anything but the sheet of wood and it just seems to happen randomly. Could it be a focus issue? Or the grazing reflection? I think all cuts were in the same area of the bed… so if it were an alignment issue that was causing the beam to graze the size seems unlikely to me because it would happen every time in that same spot (but maybe not in other spots)…
For the backlash issue, check how the pinion gears are secured. It could be a situation where there’s slip only occasionally and at certain conditions. But make sure that they’re properly secured against the flat of the stepper shaft if there is one.
The double line almost certainly has to be a misaligned beam that’s clipping off of one side. Make sure the beam travels straight down the nozzle and is perpendicular to the bed. This means testing beam dot at multiple heights. Dots should not diverge at different heights.
Thankyou. I have checked these things again and again… those must be the problems… The beam alignment seems quite plausible because I have had issues with alignment - my axis mounts for the mirror mounts don’t exactly provide adjustment in the directions I actually need… I prototyped them in 3d printed plastic and then had them printed in metal when they seemed right, but I discovered when trying with the real laser that my adjustment points that I had given myself were not really sufficient… but I thought I had managed to get everything aligned perfect anyway… At this point it would be quite a challenge to re-design the mounts.
Well I fiddled with my mirror alignment and made it significantly worse. And moving the head around the table i can see that my red-dot does move relative to the head tip - so clearly the beam is not parallel. Tracing the beam with a peice of paper I can see that the issue is from mirror 2 to mirror 3 (where mirror 1 is the fixed mirror from the tube, 2 is on the Y axis, and 3 is at the output head). I do not have any way to adjust the offset in the Y or Z direction (up/down in relation to the bed). However I believe that my measurements were nearly spot-on with that and it fixed in place perfectly… but the Y direction needs 1-2mm of shifting. I think I can address that by adding an adapter plate on the mirror holder… That is a part that is accessible without disassembling the whole machine so I will design a part in Cad now and print it as soon as my 3d printer is finished with the Van De Graaff generator pulley that it’s working on now.
Mirror 2 had enough adjustment range after I finished adjusting everything else around it. The laser arrived with all the mirror and tube positioning adjustments at their limits, so I had to work backwards from the laser head to figure out where everything should be.
Gotta say: a laser cutter makes cutting shims really easy!
Well for some silly reason I designed it with an elaborate adjustable X positioning and rough Y positioning on mirror 1, a rough X positioning adjustment on mirror 2, and no adjustment on mirror 3… The tube is easy to move in any direction… but I didn’t provide any Y adjustment at all anywhere…
Mirror 1:
The mirror can be moved horizontally by turning the screw at the front - it move along rails. It can be adjusted vertically by loosening the two screws at the bottom…
The mirror can be moved horizontally by turning the screw at the front - it move along rails. It can be adjusted vertically by loosening the two screws at the bottom…
Mirror 2: Can only be moved horizontally by loosening the screws and sliding it.
… It’s my first lase cutter… I guess I figured that any alignment error could be corrected by mirror rotation alone. I didn’t consider that if they’re way out then the beam angle means it’ll be correct in only one position.
When I started designing this, I read somewhere that all you really need to worry about is angular and that could correct anything… but if I had thought about it critically it would;t make any sense at all… Both need to be aligned - I suspect what I read was in reference to a factory built laser cutter being delivered probably indicating that the factory linear adjustment is usually solid and all you need to do is the angular…
In any case, I’ve learned that both are important and the is exactly correct and a very good piece of information.
Now, I think I have my mirrors aligned much better with the new shim plate… I still am seeing the little tag offset at the closing of some loops, but I’ll keep working at it.
