Forms not totally cutted

I just tried LB with cutting square on paper on K40 laser, with Arduino UNO and GRBL.

The laser is working and cutting except on each angles. (I have the same issue if I try to cut a circle. The cut is not closed)

I imported a dxf file, I also tried to draw a square, circle and polygon with LightBurn but each time it’s the same issue. Cuts are not closed.

I think that 1 parameter is not correct but, even if I search it, I don’t know which parameter is on default.

Could you help me?

Thank you by advance.

Have a read through this thread.

Using the GRBL-M3 device will probably help, as it uses constant power output, instead of power ramped by speed.

Thank you for your answer but … what is GRBL-M3?

GRBL-M3 is one of the available profile settings in LightBurn found in the ‘Device Settings’. Profile intended for pre-1.1f grbl or in this case, when one wants to have constant power output, instead of power ramped by speed.

ok, I will check tomorrow

I’m sorry but I don’t found the parameter to be modified to have a constant power output instead of power ramped by speed?

Here under the screnn I have on device setting.

I checked on GRBL 1.1 documentation and it said that M3 and M4 commands are available.

Suggestion was to try the “GRBL-M3 device profile” which can be found in the ‘Devices’ window. Click the ‘Devices’ button in the ‘Laser’ window to open this dialog.


I have only GRBL K40 in this device

I do not understand your comment. Did you try to add, setup and use the GRBL-M3 profile?

No, but now it’s ok, I found the profil.
I will try to make a new cut and let you informed of the result

With GRBL-M3, there is no fire.

With K40 (GRBL for K40), I tried to decrease the speed.
If I put more than 10 mm/s for speed, cuts are not closed.

This may help you.

You might need to change the device setting called “S value max” to match the setting of $30 in the controller.

Thank you but I already have this page (all the explanation pages for GRBL in fact.

I already checked M3 and M4 (that why I understood that M3 and M4 was implemented function in GRBL 1.1)

But I don’t know how to use this information.

OK, I will try this afternoon to modify the S value equal to the $30 parameter in GRBL

Do you mean that I have to adjust LightBurn with $30 parameter?

In my case, $30 parameter is 255.
S value max to 255?

Hello all,

it’s OK with GRBL M3 with S-value max equal to 255

Thank you all