Compensating for kerf?

I’ve read a couple places that you need to compensate for the kerf (the material the laser removes)… how is this done? By hand or is there easier ways to do it? Thanks!

That question is so crucial to designing boxes with finger joints and not wasting material so finger and notch fit tight. So a thin enough kerf would mean you can nest the sides. So the case maker app at Makercase when you use finger joints allows you to specify laser beam width to figure in kerf size. So designing ahead for the Glowforge means we would need more info on kerf width. Also depends on materials. I’m just assuming here taking woodworking knowledge and applying. The folks with laser experience might assist us designing in allowances for planned projects.


Thanks for the reply. I guess it’s not a one easy solution at this point. Trial and error and learning from that? :slight_smile:

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Thanks for the Makercase link marmak3261 - that’s going to come in very handy!

Material type should not factor in. But material thickness will.

The focal depth should adjust so that the focal point is centered in the material. The laser makes a narrow hourglass around that point. So thicker material means larger difference in center and edge material loss, and larger kerf (which I assume you would measure from the outside of material.

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How big of an hourglass are we typically talking? Milliradian or a percent of a degree? My trusty angle calc says that with a full degree and half inch material, we’d be looking at just about nine thousandths max divergence. So for me, anything less than a degree and I’m good. Can a staff member chime in?

Good link here. it seems some test cuts and engravings will be in order.


Oh, that’s really smart!! A simple way to get your kerf for each material!!

Interesting to see that his kerf measurements were different for 3mm acrylic compared to the Ply and Hardboard. Acrylic does a bit of a waveguide thing from what I have read, so makes some sense it would deal differently. He doesn’t specify (that I saw) if he adjusted his focus to remain centered on the material loaded.

But, as he said… very little variance.

The beam diameter of the unfocused laser, combined with the focal length of the focusing lens, will tell you the precise size of the hourglass shape. But the initial beam should be quite tiny (maximum I would say a 2mm diameter… guessing here with vague recollection of our mirror alignment tests). The focal length of their optics is likely around 50mm to 75mm, those are rather typical measures as I remember things. The other way to look at it would be that if we can cut 1/4" material, then the focal length needs to be (at a minimum) a bit more than half that so the optics do not touch the object being cut. So even with our minimum conceivable focal length, assuming an initial beam diameter of 2mm we would be looking at a kerf under 0.5mm.

Tons of hand-wavy level approximations and numbers pulled out of the nether region there. So take the last paragraph more as a guide in which to put reliable numbers and an approach on how to approximate for yourself if the team doesn’t chime in.


That maker box link is amazing thank you wow that is so freaking amazing… only problem is that things like this make me more more impatient lol…

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I was going to post a tutorial on making a box using the Makercase app the other day. But I found an issue with it. Not sure what the limitations are, but it did not work for me to make a case for a Raspberry Pi.

I cannot replicate the issues which I had the other day right now. But be careful to double check everything as you go along. Maybe load in a different browser if things start to go screwy.

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Also cross referencing to this post:

And also check on #9 superpower on the front page off How did I miss this. Somehow it knows!

Kerf actually varies with a lot of factors. Material is one: as I understand, acrylic is something of a waveguide exhibiting total internal reflection at CO2 frequencies. Wood also exhibits some reflectivity in the same manner. That produces a different kerf profile and width than you’d expect from the hourglass.

More importantly, the vaporized material isn’t perfectly contained within the beam. If the beam moves slowly, the vaporized material (and superheated air) will erode away at the walls as the hot zone moves, producing a wider kerf.

That means it varies with both materials and settings. We’re working on some techniques to compensate for this automatically - I’ll share more details of this when we get a little closer to having it ready.



Thank you guys for the input. Lots of good info.
@dan I look forward to hearing those details!!!