Designing parts to fit inside other parts

Had a little trouble getting a project done this weekend, a heart shaped box with a cover that slides down over the box. Things didn’t fit as I thought they would.

Is there a rule of thumb when setting a dowel hose size that works best for gluing? 3/16" (0.1875) dowel fits super snug into a 3/16" hole, but what do you add for glue? I drilled a hole on my drill press with a 13/64" (0.203) bit and it was sloppy. I might have to make a dowel size jig to know for sure.

When designing a part to slide down over another part, like a box cover, what offset do you find works best? For example a box with a 1/2" side wall, how thick would the lip be from the inside of the box and outside of the cover to get a snug but not sloppy fit? 0.245" lip on each did not seem to work too well.

We have a few links on this at: and

This is one of the reasons I like to use a dowel plate to make dowels — it tends to compress them a bit, then they will expand with the moisture from the glue.

Best thing is to do a test fit in a prototype made of scrap.

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Unfortunately, I think the amount of clearance you compute depends on a too many factors for a simple rule of thumb to be any more than a starting point.

For example, the friction/smoothness of the surface finish, sanding loss, finish thickness, tool run-out/precision, material ability to hold that precision, humidity, temperature, etc.

All that said, I generally aim for something like 0.2mm gap when done for an easy fit that touches, but doesn’t resist, in plastic or a little bigger in wood if I’m going to add finish.


That’s for a lid that goes on and off. For a friction fit that isn’t supposed to come apart during normal operation: definitely tighter.

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