Acrylic inlay question (making tight piece fit)

What a great post. I never really stopped to ponder yonder hole.


This is exactly what I’m finding to. And watching it cut I could see that the narrow parts got hotter than wider areas and I wondered if this heating made a difference when it came to need.

Such good info here, thanks everyone. My initial gut reaction was that heating would expand the material outward making the hole slightly larger. I had nothing to really base it on though so it’s a good to know my thinking wasn’t far off base. I think cooling the inlays will help too.

The biggest issue I face is inconsistent kerf. I have several version of this I want to make so this inlay method isn’t going to be feasible in the long run, but it has been a good learning experience. I’m going to have to work on a different method to achieve consistent results though.

Glueless in California


Would you be interested in methods to get similar looking results?

For instance, engraving the orange acrylic and then doing a color-fill with white acrylic paint would probably look pretty similar (from one direction at least).

If you could position the design accurately enough (which seems to be a big “if”) you could engrave and color-fill both sides.


Or, cover the backside with masking tape then fill the cut spaces with resin to which white colorant has been added. Then pull off the masking after it cures.


That sounds like the winner idea to me!


Seeing how much of this there is, it’s quite possible that it would be easier and more accurate to to full-depth engraves. But I could be wrong, in which case you’d be wasting a bunch of time.

Hi Shop
Like your answer, and I did go to the physics website. Actually you are correct and so is palmercr and Scott Burns. The physics website is locked and can be updated, or I would answer the question on that site. Of course the physics website is discussing a hole in a plate.
In my earlier days I had a physics professor that loved to torture is students with this question. Half of the class said it would shrink the other half said it would expand. The answer was debated quite heatedly, but what it boiled down to was a few astute observations that the physics teach pointed out. Never assign physical properties to a hole. The “hole is a lack of everything including properties” and the correct way to think about this is “how does the remaining material react when thermally excited” most people commonly believe that when a solid is heated it expands and when it is frozen it shrinks. If this was strictly true then there would be no life on this planet. If a hole is small and the plate is large enough the hole will shrink (think infinite plate here) If the hole is large relative to the plate then the physics will be understood using boundary conditions. There is also the need to understand thermal gradients etc, but that goes into 3 dimensional thermal analysis and other things that I am not going to jump down that rabbit hole <== pun intended, 3D thermal analysis requires thermal circuits simultaneous equations etc…Hopefully the solutions above are correct and the modification of the cutting speeds etcfixes the issues. If you do get it post a picture if you can.


I think kerf will be more consistent when cutting slowly with low power than it is cutting fast with high power. The GF will need to slow down on tight corners but if it is already moving quite slowly it will be a smaller change in speed for a smaller duration.


Why would a small hole in a large sheet behave any different to a small hole in a small sheet? If it is uniformly heated all the atoms move apart. How far the hole is from the edge makes no difference to how much it expands.

If you constrain the outside edge then yes it can shrink inwards like a cookie but that requires it to deform.


I do love the aforementioned idea of using resin in the cavities. It would give a very interesting dimensional effect, since it’d be likely to bulge up at the edges due to surface tension. Also, you can add tints to transparent resin and get a very cool see-through effect.

Additional idea, cast something relevant inside the resin, like a Laser-Cut heart into the applicable anatomical cavity. Or an eye into the eye socket.

I’m putting this in my “hopper” for things to try when Glowforge arrives.


Hi palmercr,
What your describing falls into boundary conditions. My note regarding the infinite plate is just another boundary condition, but if you think about it the plate if it is infinite cannot expand at the edges and the only direction it can is in the direction of the hole. The tricky part falls when you don’t heat a plate uniformly which in the case of say a hole that is in the side of a building or ship. The material close to the hole wants to expand and the material farther away which is not heated does not creating a boundary condition that forces the material inward. The only way the metal will deform if it gets stressed beyond the yield strength of that material. In the case of say steel you are really on talking about a few thousands of an inch in expansion. The cookie is an interesting analogy but really doesn’t fit as the material during heating is actually changing density. Hope I helped, bottom line the results are dictated by the boundary conditions.

All materials change density if they expand, they get less dense.

If you keep increasing the size of the sheet and heat it uniformly the hole always gets bigger by a constant factor. If you extrapolate that to an infinite sheet I don’t see why it would suddenly change to shrinking. How would it be influenced by a boundary an infinite distance away? There is an issue with the boundary moving at infinite speed though.

Anyway the original question is about a finite sheet heated uniformly. There is no such thing as an infinite sheet and it would take an infinite amount of energy and time to heat it uniformly, so not really worth considering. For all real sheets the hole expands.


Or a new Glowforge name - Glueless Joe Jackson.

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Hi palmercr,
Yes expansion dictates the change, but the material in the case of the cookie is permanently modified. The boundary conditions you stated say the material is moving that is not the same as an infinite plate but one that is going to infinity which is a dynamic boundary condition. That actually changes the density etc. If the sheet is infinite and is not going to infinity then it forces the material to go in the only direction available. If you are heating the material it is expanding, the hole has no properties the distance measured across the hole reduces due to the expansion of the material. It is true that a true infinite plate does not exist but I have to point out that there are many theoretical situations that involve infinity that are used in real world applications… Calculus come to mind. The calculation of the area of a property has when it has a river as one of its boundaries. I agree with the conclusion that a finite plate when heated uniformly expands, but at the same time if the plate is large and the real world situation will not allow you to heat the plate uniformly then the boundary conditions change and the distance between the material edges shrink due to the expansion of the material. The material at the edge next to the hole for all intents and purposes “sees” and infinite plate. The boundary conditions determine the result

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So you’re saying that an infinite plate is essentially the same as a edge-constrained one, i.e., all directions meet at infinity.

HI Scott.Burns
Yes…see why I love this question, gets the juices flowing makes you think about “theory” and real world situations and where do they meet. It lots of fun kinda like determining who is the biggest baddest comic super hero. Everyone has there own thoughts on the subject and most everyone is right in some way or another.


Yes and if you use the same rules as calculus then as the sheet tends to infinity the hole still expands by a fixed amount. Therefore in the limit it still expands by that same amount. It doesn’t suddenly switch to contracting. If it did then by how much would it contract?


I got lost. Are we talking about chocolate chip cookies or snickerdoodles? Thanks!


HI palmercr
Again you are giving the hole properties, the plate is always expanding the hole does not have properties so nothing is switching to shrinking the material is expanding resulting in a smaller hole.