Joints, Box Lids and Puzzle Pieces

I’ve been playing around with interlocking pieces cutting them out with my Shapeoko, but I’ve found using exact matches makes the pieces too tight.
I’ve made lids to boxes, but the tolerances are so tight it was difficult to pull the lid off. So I decided to try testing interlocking pieces in the sample that is attached.

I was wondering if anyone has figure for how much “slop” I should put between mating pieces. I figure since a good scroll saw blade has a 0.01 inch width that might be a good place to start.

Here’s a sample with a “perfect fit”

joint-test.c2d (20.8 KB)

I had the same issue and tried reducing the inner piece .002 and that seemed to work. Play with it to get the fit you want.

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If you make things out of wood that have to fit together, then the word “precision” is defined differently than if you make something out of metal.

I got tired of using sandpaper to correct for being too precise. So, your educated guess of 0.010 inch is fine. That’s not even 1/64 inch, but considering that for a box lid the offset is doubled you’ll be closer to tight than loose. Also, you’ll find irritation with a box lid that is so tight that getting it off involves breaking a vacuum!

It isn’t impossible to create a small lip or ramp that gets engaged right before the lid settles into the closed position. I’ve inspected some old hand-made boxes where the craftsman worked out the perfect angle for that joining.

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For a given box design/size/material combination, my solution for this is to machine one part, usually the base and finish it, then make the lid, sizing it a bit oversize, then I do a test fit, and run a pass around the perimeter based on measuring the parts — repeat w/ slightly finer passes until a friction fit is achieved.

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I just tried gradually trimming away the male side of the joint. Looks like 0.005 - 0.01 in is good for joints. 0.01 is good for interlocking pieces you don’t want to come apart too easily, which is what I was looking for. I bet 0.02 or 0.03 would be good for jigsaw puzzle pieces. I guess the thickness of the stock would also be a factor, you probably don’t need as much slop for a thinner piece.

I did develop a “cheat” for getting the slop in. When I contoured the male side of the joint, I copied my 1/4 endmill tool and changed the diameter to 0.23. That caused Carbide Create to push the tool 0.01 inches across the contour.

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That cheat is done quite a bit more than is advertised! :smiley: But its not so obvious from within your toolpath, that you did that. Makes it a bit hard to troubleshoot later on.

Vectric software has an “Allowance” field that makes this adjustment more obvious. You can put “+” or “-” allowance in there. This does get a bit brainy depending on whether you’re doing a profile/contour or a pocket operation. :thinking:

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