Not able to finish a single circle


I have a very annoying problem with my laser, when working with vectors, I am not able to just finish a simple circle.
The result gives a circle where the end point does not touch the beginning point, so I get about a half millimeter not cut.
It is very annoying, because I have to recut every single round manually after. When I engrave a raster image, I do not see this problem.

I did tighten the belts, clean the rails, change gcode generating and sending software, change image, change GRBL version etc …

I think it only happens on X axis, but I am not sure.

Here is a simple example to understand, I modified GCODE to keep the laser ON, so that I see the travels, and I see that the errors happens when the laser start a new shape :

Thanks in advance for your help, I tested all I know and I really do not know what to do.

In lightburn (picture is vectorized and elements are correct) :

The result on a paper with horizontal lines (this one is with another software + older version of GRBL) :

Another finer cut, in case it helps :

This bit specifically:

… makes it look like your laser is missing steps. Those lines aren’t falling short so much as not landing in the same place at all. It does look like it’s slipping along the X axis, so I would check that the belt tension is correct along X - make sure it’s not too loose or too tight, and that there’s no play in the head when it switches direction.

This could also be the paper moving, or that your acceleration or speed settings are too high, and the steppers are skipping.

You’ve said “my laser”, “CNC diode”, and “GRBL” - is this a self-built machine, or something purchased? Did you configure the settings, or are you using whatever it shipped with?

Hello, and thanks for this reply.

Yes I can understand it is missing steps, but I do not understand why. You are right, about the top of the image (do not mind the 2 straight diagonal lines below, it is another test), it seems like either the start or the end is not at the right place, therefore steps are missing.
I did try to lower the speed and it does not change anything, but I did not know that a belt could be too tight, the vendor told me to tighten then at much as possible. I will the try to make them a bit looser.

The head is not moving at all (I blocked it carefully), I already checked, and I use scotch tape to avoir that the paper moves.

I bought the laser on Aliexpress, it is not self-built. I did use the settings from the vendor :
$0=10 (step pulse, usec)
$1=255 (step idle delay, msec)
$2=0 (step port invert mask:00000000)
$3=1 (dir port invert mask:00000001)
$4=0 (step enable invert, bool)
$5=0 (limit pins invert, bool)
$6=0 (probe pin invert, bool)
$10=15 (status report mask:00001111)
$11=0.080 (junction deviation, mm)
$12=0.002 (arc tolerance, mm)
$13=0 (report inches, bool)
$20=0 (soft limits, bool)
$21=1 (hard limits, bool)
$22=0 (homing cycle, bool)
$23=0 (homing dir invert mask:00000000)
$24=25.000 (homing feed, mm/min)
$25=500.000 (homing seek, mm/min)
$26=250 (homing debounce, msec)
$27=1.000 (homing pull-off, mm)
$100=100.000 (x, step/mm)
$101=100.000 (y, step/mm)
$102=100.000 (z, step/mm)
$120=200.000 (x accel, mm/sec^2)
$121=200.000 (y accel, mm/sec^2)
$122=200.000 (z accel, mm/sec^2)
$130=200.000 (x max travel, mm)
$131=200.000 (y max travel, mm)
$132=200.000 (z max travel, mm)

And here is the supplementary config that I did not know how to set in lightburn :

I hope it helps

For the purpose of this topic, self-assembled is similar to self-built - I was basically asking if you purchased a machine that was ‘ready to run’, or had assembled it yourself.

If the belts are too tight they can restrict the movement of the stepper motors. They should be snug, not slack, but not ‘guitar string tight’ either.

The engraving speed is just the speed setting in your layer. Your image didn’t show the cuts window, but you can configure LightBurn to show mm/min or mm/sec, depending on your preference. The travel speed is normally set by the $110 and $111 parameters in the controller itself. I just covered that for another user in this topic: Slower blank movements, is it possible?

Pixel size is ‘Interval’ in LightBurn. The default is 0.1mm. The working size is the page size, and the ‘weak power’ value can be configured in LightBurn by enabling the ‘Laser Fire’ button in the device settings, and then adjusting the power value in the Move window.


I did assemble it myself, right.

I did some tests yesterday evening, on X axis, from too loose where I hear the belt shift a tooth … to hyper tight, and not any configuration removes the problem :frowning:

Thanks for the explanation, I did try to set $110 and $111 to 200mm/min, same for the engraving speed … it is veeeeery slow … but the problem is still present.

I will search for the other parameters, but I am pretty sure it does not have impact, as with the vendor software (Laseraxe CNCC, worst software ever), I get the same problem.

I know you already helped a lot, would you have any other idea please ?

Thanks for your time.

It will be mechanical - make sure that there is no play or movement in the rollers or carriages as they roll along the rails, make sure things like set-screws that hold pinions or coupling shafts are snug and not slipping. That’s about the best I can tell you.


I did again a lot of mechanical tests and I cannot manage to make it work, anyway I think you are totally right.
Thanks again for the help.

One last question : I did not find where to change the “Interval” setting from 0.1mm to 0.2mm, I only found “Line Interval” in raster images, which is not the same. Can you please tell me where it is ?


That is the same value. It’s called Line Interval in LightBurn because it controls how big a step the laser takes between lines when drawing an image or a filled shape.

Ok thanks a lot for your help

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