Festi boxes.py kerf / burn correction issues for finger joints

Ok, I feel like I’m going crazy here, not to mention wasting a lot of acrylic. I’ve never had issues with wood, then again wood is a bit more spongy.

This is some quality cast acrylic that I purchased. Removed the paper on some of it and measured with calipers, came out to exactly on 1/8" (0.125", 3.175mm). Unusual as that is, I measured another sample and again came up exact.

No matter what I do with the burn value, the pieces do not fit together easily. The last three, I’ve done:
0.1 - nothing fit together at all.
0.07 - barely fit but had to press hard enough to crack.
0.06 - TIGHTER than 0.07

Am I crazy? Shouldn’t a lower value of correction make the fit looser?

I never seem to be able to get acrylic right. Even the burn test samples offered by boxes.py don’t really apply. 0.07 fit great in the test. 0.08 was barely press fit. Anything higher didn’t fit at all.

The thing is I’m extremely methodical about my steps and it seems… illogical.

Any thoughts on what I might be missing here?

I always select medium clear acrylic. I always set focus as well. The tray is level and fine, all pieces cut perfectly every time. Just do not fit in a repeatable way.

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There is a test burn for different Kerf width on here some where. It was finger joints with different kerfs. Or make up your own and try it on a piece of scrap.

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Thanks. I did try that, I noted it in my post.

Edit: By on here, did you mean the GF site, someone made?

Yes. It had like 8 different ones

I’ve used the one by boxes.py and that didn’t seem to help the situation.

It almost seems like the smaller I make the value, the tighter the fit gets. Which is backwards.
But then 0.1 will be way too loose.

Try This


Acrylic thickness extremely inconsistent (especially with cast) and has a tolerance of up to 10% on the same sheet. I’ve had kerf range from .06 to .07 on the same project and it might be what’s happening to you. Something else in your comment caught my eye; how are you using set focus on your unmasked clear acrylic? I ask because you can get a false reading with it and you have to use a little piece of masking so the GF has something to focus on. (You’re pretty savvy and might have already known this, but just in case. :slight_smile: )


Yes I’ve come to see that acrylic is a :triumph: to deal with when it comes to exacting tolerance. I’ve given up, I’m just going to make the kerf allowance rather large and try to perfect the art of acrylic welding!

The sheet is not unmasked, it comes masked with brown paper masking in both sides. Good quality cast.

As a side note, I got an 4’x8’ sheet of 1/8” for $75 and a sheet of 1/4” for $125, then paid the plastic supplier $25 to cut it all into 20 19.2”x11.8” sheets. That’s $4.38 for 1/8” and $6.88 for 1/4” so for me, it’s worth the headache of figuring out the right settings since I get so much for the money.

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Not only is acrylic uneven in thickness, but your laser beam may be slightly out of vertical. This will create a different angle on one side of the joint than the other.
0.007 inches is my go-to kerf allowance for acrylic. I just plan to use a file to adjust the angle on anything that is too tight.
Also, don’t expect acrylic to stay together when press-fit. You will need to glue it to keep things together long term.
I have found using a brush to apply water-thin solvent cement works pretty well and is often easier to control than using a dropper bottle.