Cutting is different to the design

LightBurn Hardware Compatibility Ortur Lasers
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I am cutting out some slots in 3.1mm ply to assemble a box. However, the slots keep coming out larger than the design on Lightburn. I want the slot 3.15, but the slots keep coming out at between 3.3mm and 3.4mm.
I have checked the belts and eccentric nuts, they all seem fine and the laser head slides from left to right freely and gantry front to back freely.

Is there anything else i can try? Everything i cut is always bigger than the design and it is really irritating.

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A couple things to consider:

  1. Is the difference in size of the slots due to kerf (the width of the cut due to the laser) or does your laser not cut to dimensional accuracy. Check this with both inside and outside cuts.
  2. If you can determine that the laser is not dimensionally accurate you can calibrate X and Y axes by changing GRBL configuration $100 and $101. You can also calibrate them through LightBurn’s Machine Settings->Calibrate Axis function.
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First point, how does the kerf work? It is currently off. When i cut the hole is 3.3mm and the piece is 3.1mm.
Second, when i tried to calibrate via lightburn, it got worse. Nothing i do sets my machine up right. Nothing is square, nothing is round. I am at a loss.

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The kerf setting in the cut layer is kerf compensation. It’s meant to account for kerf but does not control kerf size. Kerf size is whatever kerf size your machine will create for that given material. Keep in mind that the machine moves by default with the presumption that you’re cutting along the lines of the design. If your laser cut was infinitely thin then parts cut out would match the design dimensions unless there was a calibration issue. However, if the cut is not infinitely thin which I guarantee is the case then that kerf will eat into the dimensions of the part.

First, confirm whether or not you have a kerf issue or a calibration issue. I’d suggest rolling back the calibration change you made to begin.

  1. engrave the largest square you can of known dimension onto something that can take a clean engraving with the finest line you can generate.
  2. measure the square very carefully aligning against the center portion of the burn line.
  3. Compare the known dimensions to the measured dimensions.
  4. If measurements are off then use Calibrate axis to account for the difference

If measurements are on then you’re likely just dealing with an issue of kerf.

In that case, you can use several different strategies to identify kerf size. The simplest would be to cut a square in the material that you’re trying to work with. The larger the cut the less opportunity for error to be introduced. Then measure the hole vs the cutout. Take the difference in length of the hole vs the cutout and divide by 2. That will be the size of the kerf. If you wanted to do a kerf correction in LightBurn you would take kerf size and divide again by 2 since you only want to move out by half the kerf size. Similarly, you could outset the part by the same half kerf amount to do this manually.

In your example of 3.3 mm hole vs 3.1 mm piece your kerf size would be (3.3 - 3.1)/2 = .1 mm. If using kerf offset you’d want to use .1mm / 2 = .05 mm. It’s likely that with such a small part there’s likely to be a bit of error so I suggest you test this on a larger piece.

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