Method for making press-fit parts?

If you cut a shape, obviously the fit in the original material is loose because of the kerf. If you wanted to cut another copy that was slightly bigger so it would fit snuggly, how would you determine how much bigger it would need to be?

I assume the kerf for a given material and cut setting would be a constant, so you’d measure the kerf, and increase the size of the part you want to plug in by a tiny bit less than that.

Is this practical? Does the hourglass shape of the beam interfere with press-fitting?

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You are on the right track. The Tips & Trick category has “the Matrix” of tutorials, including these for kerf compensation in Illustrator and Inkscape:

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Here is the thing, kerf is a somewhat subjective thing in that you have to ask just how tight do you want it? Just tight enough to hold together as the glue drys? So tight that everything stays on its own?
What I do is make a small file with a joint like what I plan to use and play around with it until I have what I want. You only have to do this once for a material and joint type combination.
I call these kerfinators. :roll_eyes:
I need to collect them and post them.
But yeah adjusting for kerf is a thing.

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I’m hoping you can make flat press-fit decorative items out of 1/8 - 1/4" acrylic and have them hold together without adhesive. For example, imagine a green Xmas tree with some colored circles in it representing ornaments… Or a sign with embedded letters of another color.

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Acrylic is brittle, so getting the fit you want without breakage may take some work. It may also depend on the size of the parts you make; e.g. small fingers will be prone to break.

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What @jbpa said. I’d look for snug and then some adhesive.

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This will not work for every use case, but I have an acrylic case that uses a variant of the Stanford universal snap-it connector – will not stand up to opening and closing many times, but for closing a case “permanently” it’s not too bad…

https://tltl.stanford.edu/project/universal-snap-fit

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Kerfinators would be VERY helpful. Just an encouraging word.

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1/8" acrylic and press fit can work, but depends on stress and strain of part. Dog bones at corners help. I’ve done some tight kerf compensation for 1/4" acrylic that holds up very well as a stationary object that isn’t going to be getting a lot of strain. But you can make them tight enough so that an iPhone stand will hold together, especially if you differentially heat and cool the parts. Not as much as metal, but it does work.

Cast acrylic is fairly consistent, especially the Proofgrade, but you do have to measure with an exact calipers. I have a tab that I made in OnShape that I print and I adjust them accordingly in the parameters to ensure a very tight fit. One thing is to choose the tab or the slot to make adjustments on. It’s easier, or you can do 1/2 kerf for each, but it means more adjusting.

So cut your slot exactly 1" x .220" in the design software. Cut your tab 1.07 x .227" sized in the design software. There are a few things to think of when doing a slot to get 1/2 the kerf on a shoulder, but one way is just print and see how it fits and then you can see what to adjust.

I do a test print and then adjust accordingly. It does make a difference in 1/4". You can only insert the tab into the top of you are going for extreme tight fits since there is a slight draft to the cut. I just print two copies of this. I believe this one is sized for a .212" thick Chemcast acrylic I have.

tabslottest

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Thanks everyone!

That is super helpful–thank you! I still struggle with these issues. Keep thinking I have it right, then try to put parts together and something is either too loose or too tight. I think part of the problem is I have a version that works but forget to go back and update previous files to accomodate the change. Then do a reprint of the old file and have the same fit problem and forget where the solution is. Arrrgggh!

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I’m only posting this because I spent yesterday trying to come up with a press-fit design for a cell phone stand that visually wasn’t so obtrusive. I also wanted it to come apart fairly easily. It’s not like it’s a new earth shattering design or anything, I’m just not an engineering type so to design it from scratch as a bit of an effort for me and I’m proud of myself lol. I think I’ll be able to incorporate this into other projects such a box making too.

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Very good info marmak3261! (For whatever reason I can’t see you picture though?)

I will add that kerf will change depending on how narrow a piece is. I discovered this when trying to do this inlay and it seemed like each piece needed it’s own kerf adjustment. This is a pretty extreme example though and the pieces were pretty tiny so this won’t be an issues in a lot of projects.

https://community.glowforge.com/t/acrylic-inlay-question-making-tight-piece-fit/10189?u=kittski&source_topic_id=15753

It’s a very light, thin red line, hard to see in the white area… Should be able to right click and Save as. I use a .25 pixel width so that the stroke width doesn’t interfere with my measurements. Also in Inkscape choose in Preferences > Tools > Geometrice bounding box to get the measurements of the center of the path.