Acrylic inlay question (making tight piece fit)

Anyone know if slightly heating/warming the outside cast acrylic would help with fitting a tight inlay piece?


Doubtful. You’re not causing it to compress it’s molecular state (heat would actually expand), you’re just making it more liquid, and that liquid would have to go somewhere… in this case, a bulbous deformed surface.

You could sand the edges down with fine grit sandpaper.


If it is the piece going in freezing it is the way to go. Your best bet is to learn from your mistakes and try again.

Also, make a kerf kompensation file for testing kerf.


Acrylic has a relatively high thermal expansion, so I would guess warming the outside piece or cooling the inside piece would work.


I’m on try #5 of a complex inlay with some very small, intricate pieces and at some point it goes from learning exercises to just wanting to get one done lol. I was oh so close and was hoping to salvage this one. :roll_eyes: I have found the kerf adjustments isn’t absolutely consistent across all the pieces and I’m trying to dial it in. I really want to go as glueless as possible and I’m almost there. (I’m a bit of a perfectionist to my own detriment. ) I can’t really sand the pieces because they are too intricate/fragile. I learned that the very narrow areas get a little hotter during cutting compared to the wider areas and I think this is causing the very slight kerf inconsistency I’m seeing. Some of the pieces are .005 kerf and the others are better at .006 and one or two pieces at even .007. I’m going to up the cutting speed to lessen this difference at all.

^^ Thanks plamercr, that’s actually exactly the info I was looking for and just didn’t articulate it well. I will experiment to see if it will let me get the ever so slightly tight pieces in without cracking the piece. It’s just too fragile for any man-handling to force it.


Great question, great responses… thanks all, this is very useful


wouldn’t warming the outside piece just make the opening smaller as the outside piece expands? i would think you’d want to freeze both pieces.

No I don’t think so. Coopers heat metal bands to put them around barrels. The inside expands as well as the outside.


metal becomes more flexible when heated so it will stretch. it would shrink back up as it cooled. acrylic may become more flexible, but that means likely to deform as well as stretch.

Pretty much all materials expand when you heat them because the atoms and molecules vibrate more, so the average bond length is longer. It is nothing to do with whether they go soft, which is a phase change.

I made an acrylic door about 5ft tall and it had clearance to the aluminium door frame in the winter but now it is about 2mm too big in the summer. It hasn’t gone soft, it is just about 10C warmer.

If you expand a material in all directions then everything gets bigger, including any holes. It is just like scaling up a picture of it. If you consider the material around the perimeter of the hole, it all expands, so the circumference of the holes gets bigger and therefore the diameter by the same percentage.


if the material expands into the hole, then doesn’t the hole get smaller? that’s the point i was making. while the outside would expand for the exterior dimensions, if it expands, it would also expand into the hole.

and acrylic would become more “pliable” (maybe a better word than soft) when heated, wouldn’t it?

The hole should expand along with the rest of the material


Acrylic only becomes pliable at its glass transition temperature where it has phase change from a glass to a plastic. It is around 105C for pure PMMA. Then at 165C it melts and become a liquid. At a higher temperature still it vaporises to a gas.

No the material doesn’t expand into the hole. If the sheet is warmed uniformly then every dimension expands by the same percentage, including both inside and outside dimensions, or indeed the distance between any two points on the surface.

So for simplicity imagine a 100mm square with a 50mm hole in the centre expanding 1%. It will now be 101mm square with a 50.5mm hole. The walls of the hole will go from 25mm each side to 25.25mm each side.

25.25 + 50.5 + 22.25 = 101.

So the walls of the hole have got thicker but their outside edges have moved away twice as much, meaning their inside edges also move apart.


beat ya :slight_smile:


got it. thanks.

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The new Glowforge movie: Glueless in Seattle

Different kerfs for different colored material and different sheets of the same material. It happens. I have been curious about this. I wonder if the ramp up and down around intricate shapes is a factor in this. I don’t know what they have programmed for curves. Straight cuts seem to have a smaller kerf on average in my experience.

I have found that the profile of the kerf is NOT negligible when doing tight fits. The kerf is wider at the top and narrow at the bottom so I have found that sometimes, especially in thicker acrylic, I could flip the cut of the inlay piece in the design to get even a tighter fit. Heating the matrix and freezing the inlay works. Machinists do this for setting arbors and such.


I’ve done a bunch of testing on cutting cardboard and (less) acrylic, and can report that yes, once you get into the curvy parts the speed number is mostly advisory. There’s a limit to how rapidly a big hunk of metal like the GF head can accelerate/decelerate, and for the time being there’s nothing in the software that looks at actual speed and adjusts power accordingly. So when the head is moving around sharp curves (especially when both X and Y are changing direction) you get deeper cuts/more kerf.

One thing you might try (although it would be a learning curve) is to engrave the outline at full depth. Then the head is moving at constant speed. You will still get kerf variations from heat buildup, but it might be worth trying.


First, that is a killer inlay! :sunglasses:
Because of the slope of the kerf, Marion’s idea of reversing the cut and flipping the piece would come close to the kerf angles matching I think.


Heating the matrix and cooling the insert will make it go in, but be aware you are causing significant internal stress to the acrylic! In metal this is less of a problem, but with a brittle plastic you might run into trouble with cracking, especially if the shape of the hole is complex with a lot of sharp angles to form stress risers.

I would consider annealing the entire piece when you are done to try and equalize the stress.


Thought this was a brillant idea! I think we all are familiar with freezing or heating something up to achieve a shink-fit, but the annealing idea to lessen the internal stresses I thought was very clever!

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