Weird Problem

I have an odd problem that just started - here is my setup and specifics about the issue…

SKR 1.2 Pro
TMC 2209 Drivers
Latest Stock Firmware from V1
Standard - no dual end stops

I have been using Lightburn connected directly to the SKR via USB to cut acrylic with a diode laser and a drag knife mount for cutting vinyl.

Friday night I ran two laser cut jobs and everything was fine. Saturday morning, I made a change to my cut job in Lightburn (just removed a shape) and started the job. It moved about 1/2” and stopped - almost like it hung.

In troubleshooting, I have determined the following details:

I can move the tool head around using the TFT screen, manual commands from Lightburn, and manual commands from Repetier.

I sent a known working gcode file from a previous cut job and it did the same thing (moved a bit and then hung)

A G1 X??Y??Z?? F0 command moves normally
A G1 X??Y??Z?? F4.37 command seems to hang

I can’t tell if it’s actually hanging, or if it is just moving very slow

Any ideas?

That is 4.37mm/minute.

F0, not sure what that does, but probably not what you want.

There was a Lightburn communication bug mentioned in this post . Not a perfect match to your symptoms, but it may be related. And the post contains a reference to the Lightburn forum, which might have a fix for your specific problem.

Thanks guys - I was aware of the Lightburn issue and have a workaround for that - this issue manifested in Lightburn, but is present in other platforms.

If the F value is mm/min, then it would be moving at an almost imperceptible speed, so it may be a mismatch in units between Marlin and Lightburn then.

I will explore this possibility

In the printer world it was all mm and mm/s, but with Cnc I have found the unit standards vary a bit. I think mm/min is common for lasers since a lot of folks use diodes. So light burn has options to use either min or sec, since sec is more sane for co2 and other faster lasers.

To be honest, I prefer a single unit that I can get familiar with. mm/sec is what I use on everything. My brain has a much easier time seeing events on the seconds scale… minute scale is too much for my attention span. :stuck_out_tongue_winking_eye:


Agreed! Between the fleet of 3D printers, a CNC router, and this machine that fills in all the blanks the other machines don’t, it’s definitely a mess.

The problem was indeed the units - Lightburn was set on inches, Marlin was looking for millimeters - translation: 4mm / minute lol - not sure how it changed in Lightburn though, other than lumping it together into the “Lightburn and Marlin don’t get along so great.”

I’m switching from my 7.5 watt diode to an 80 watt CO2 in a couple weeks (chiller will be here today, mirrors / mounts / lenses / etc will be here this week, tube and power supply will be here by the 19th). Once I get the laser head, I’ll have to design and print a mount to fit it to my primo. Seriously considering trying to stuff Smoothieware on an SKR 1.4 Turbo I’m not using and see if that will play nicer with Lightburn than Marlin…

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Oh so you are going for it… co2 on an mpcnc! I am looking forward to seeing how you progress. Please keep us posted.

I have read all kinds about why mpcnc is not ideal for co2, but I can still see the logic here (beyond large format). My mpcnc, compared to my printer and other tools, is like a premium mech warrior. Has all the hard points to handle whatever load out suits your fancy for the day. Let’s see… diode etch today, router this evening, plasma in the morning, and wait there is still time in the afternoon to run the co2 through some ply.

A co2 mpcnc would really complete that picture. Is there something an mpcnc cannot do?

The way I figure, if it can handle the stress of pushing a 4 pound Makits router and 1/4" endmill through wood, it should be able to slide a 2 pound laser head around in the air. My results with the diode laser have been excellent.

I’m sure I will encounter challenges in doing this, but I have a fair amount of confidence that I will be able to solve said challenges as well. If I am way off base and the machine just can’t do it with any level of suitable results, then I will cross that bridge when I get there.

I will take pictures and keep notes - not sure if I will document the build on here as I go or if I will just collect it all and post it when I’m done - but regardless, I will post the results so everybody can learn from my success (or failure).

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Inches/min makes more sense when you are eye balling expected times. (12" +12" + 8" + 8" = 40", so at 20"/min, that should be about 2 mins each).

But I can’t think well in anything/min. Even rpms seems like a stupid unit to me.

Ok update on the CO2 Tube - It works great! I 3D printed a tool mount for the laser head and the actual tube mounts. I had to do some creative shimming to get everything aligned. I also ended up cutting the middle out of my table and making a new platform down 3/4" to allow extra clearance for material. So far it cuts great and reliably.

If I were to do it again, I would have bought tube mounts that are more adjustable - I will end up replacing these before too long as I had to do some creative cobbling to get them to be stable (that’s what I get for using somebody else’s design. More than likely, I will design my own tube mounts. I also will eventually be replacing the mirror mounts as the ones I got are what I consider “bottom of the barrel” and not as adjustable as I would like, but they are working for now.

I have not done a ton with it so far as customer orders have interfered, but it easily cuts through 1/4" plywood in a single pass. My previous laser was a 7.5 watt diode laser, so this thing is a treat! Once I get some more time, I will get into more cutting and some engraving / rastering.

What I used:
24" x 24" MPCNC Primo
80 Watt CO2 Tube and Power Supply from OMTech
Cheapest mirrors and mounts from CloudRay
SKR1.2 Pro Controller
3D Printed 80mm Tube Mounts
3D Printed Laser Head Mount (Used the “Blank Mount” as a starter)
CW3000 Passive Chiller (Will probably need to get the 5200 Active Chiller Eventually, but we’ll see)

To Do:
-Design and print or purchase better tube mounts
-Purchase better mirror mounts that are more adjustable
-Mod the laser head mount - the head screws together against the mount - it’s ok but will cause problems later I’m sure - I need to make it taller and add a properly aligned recess for the square head so it can’t twist at all, ever.

I will post a build log as a new post once I get pictures taken, but wanted to get an update thrown up here since I said I would and it’s been a hot minute.