Acceleration problem

I’ve setup my 3018 Grbl cutter with Lightburn, and having a problem with acceleration. The acceleration is very smooth in other software, running G-Code acceleration tests, and also smooth when I use the jog controls in Lightburn. However, when doing a laser etch in Lightburn, there is a clunk at the end of each pass, where it stops. I’ve tested jogging back and forth, such that it performs the same movement (you can click ‘left’ before the ‘right’ pass is finished, so that it stops and immediately comes back the other way). This is smooth, without a clunk.

In fact, if I set the etching speed to 1000 (down from the machine max of 2500), then it’s almost like there’s not acceleration at all - it just stops dead, and comes back the other way at 1000. I didn’t think the machine could override the Grbl acceleration settings, but it’s almost like there’s a minimum speed, and below that, it just stops and starts, with no consideration of acceleration.

Any suggestions on how to resolve this?

“When doing a laser etch” - Are you engraving an image, or just a vector shape?

I’ve been testing on filling a vector shape. I haven’t tried an image in LightBurn, but it also sometimes happens with images in another program (LaserGrbl). Might be an issue in the Grbl, but only seems to manifest in laser etching. Vector outline drawing seems to have no issues.

Are you in ‘Laser mode’? ($32 setting set to 1) If not, it will pause at every power change, and that happens much more often when filling shapes than when outlining them.

Yes, Laser mode is on. Here’s my settings…


For all other tests, the acceleration (120 and 121) can go over 200 with no problems at all, but because of this pausing/clunking issue, I have had to drop the acceleration right back to 50 to stop the motors from stalling during the laser etching.

Why is this set to 10000? (do you have the same value set in the S-Value max setting in LightBurn?) Not likely related, just a strange thing that stood out.

At 1000 mm/min, you’re going 16.6 mm/second. Your acceleration is set to 50 mm/sec^2 ($120 and $121), which means it will take 1/3rd of a second to stop, or get back to full speed, which isn’t long, but it should be visible.

$30 is the max spindle speed. I have an interchangeable spindle/laser on my machine. The spindle runs full speed at $10000 rpm, so by setting $30=10000, my spindle settings in the software/gcode correspond nicely with RPM.

Yes, when I run some simple acceleration tests, or jog it manually, the carriage smoothly accelerates up to full speed, and slows down again, and changes direction smoothly. It’s only when laser scanning for an etch that this doesn’t seem to happen properly, and in some cases there is no visible acceleration/deceleration at all.

I might have to take a video tonight to show what’s happening.

LightBurn is just sending GCode commands to the controller, and it’s up to the controller itself to interpret those correctly. LightBurn uses very simple GCode - G0, G1, and a few setup commands like M3, M4, M5, and that’s pretty much it. Have you run the laser with anything else before to see if it’s repeatable with other software?

Still experimenting with other software. I did have a similar issue with LaserGrbl last night - not quite the same. So yes, it might be an issue in the Grbl software on the driver board.

Thanks for the tip on the laser setting - I updated $30 last night, and just checked and I hadn’t updated the value in Lightburn, so that would have been a problem soon. :slight_smile: Might be nice if LightBurn could read the parameters automatically from the machine.

Anyway, I will follow up with the Grbl software, and see what I can get from there. Thanks for your advice.

It does, when you first configure it using the ‘Find my Laser’ tool. If you change it later, I don’t automatically update it - some people artificially keep these values different as a way of scaling the power output from LightBurn to not exceed the maximum safe power to the laser tube.

For example, if your controller is set to 1000 as the max S value, but sending 60% power fires your tube at the maximum safe amperage, you could set the S-value max parameter in LightBurn to 600 (60% of 1000) and then use 100% power in LightBurn to output that maximum safe power value.

We’ve talked about querying the settings on connection and automatically configuring based on that, but it would have to be optional for the folks who don’t have these set the same on purpose, or people whose machines don’t have properly configured travel limits, etc.

Make sense.

The acceleration problem is now resolved. $11 was set to 1.000 from the factory. Setting it back to the default of $11=0.01 has fixed the issue, and it’s running beautifully now. Thanks for your help.

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