Layer settings not changing on same image file

Using 5500mW diode on Sainsmart 3018 to burn an image from GUMP & Da Big Gimpin. I set the layer to 600mm/min @60% power and it was too dark. I didn’t change anything but flipping the wood blank over and set the layer to 900mm/min@60% power and press start. The output didn’t seem to change. It didn’t appear to speed up, and you can see the 2nd (partial) looks no different. It seems if I shut down LB and reconnect, it makes the change.

Could this be something in my settings or a G code line I need to use?

Post your firmware settings, type $$ into the console, then hit ‘Enter/Return’. Copy and paste here for review. You may be asking the system to travel faster than the limits set.


600 mm/min is only 10 mm/sec, which is really slow, so you’ll likely need to turn the power down. What DPI was the image processed at, and did you enable pass-through in the layer settings in LightBurn?

I processed it at 200 DPI through Da Big Gimpin and 100x100 mm. Pass thru is on. Seems like if I go much faster, it suddenly stops burning at all.

Is 200dpi too high? I know I was using 600 daily with horrible results. LOL

You’re running an 8-bit GRBL controller that requires 800 motor steps to move 1mm. 8-bit GRBL can pulse a motor at a maximum of 30000 times per second, so your maximum speed is 37.5 mm/sec or about 2250 mm/minute. It’s possible that if you get too close to that the motors will stall, because you’re spinning lead-screws, not moving belts, and steppers don’t do well at higher RPM.

Shorter version: You have a CNC machine with a laser attached, but it still moves like a CNC machine - they aren’t built to be fast.

Did you shrink the image at all after bringing it into LightBurn? If you did, that would have the effect of increasing the DPI, since LightBurn isn’t allowed to rescale / dither it if you have ‘pass-through’ enabled.

The good news is I have an actual laser machine with a 3500mW and a frame coming for the 5500mW. Also, a K40 on its way here. I’m trying to get a head start on Christmas. Yeah, I’m ready to get back to making sawdust with this one.

Thanks guys.

Good to know about shrinking an image. Yep I did shrinking it.

So if it started as 4" x 4", and you shrank it to 2" x 2", you doubled the DPI at the same time. This is one (among many) reasons we recommend just using LightBurn’s built-in image processing features until you get a feel for how things work.

You can load a grayscale image, set the DPI, power, and speed, and it’ll look decent. If you select the image, then right-click and choose ‘Show properties’, drop the ‘Gamma’ to about 0.8, and set Enhance Radius to 5 and Enhance Amount to 200, that will sharpen the edges a little and bring up the middle tones, no script or other software required.

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