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I've been making laser-cut objects out of 1/8" birch plywood using a variety of different joints (mostly finger and lap joints). When a joint turns out a little too tight, I can sand it lightly to get it to fit together properly.

How can I treat a joint to make it fit just a little tighter, without fixing it permanently together?

Wildly speculating, I'm guessing paint, wax, some kind of glue that doesn't bond too well, mud...

Joe
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  • Is there some innate inaccuracy in the machine or how it's set up that results in joints ending up tighter or looser than intended, or is it an issue with the original toolpath(?) that causes it? If it's the latter I suggest that sorting that out is the ideal solution here, because fixing the result is far from easy (particularly on thin material). Follow-on question in next Comment. – Graphus Jan 09 '19 at 07:51
  • Does the need to avoid fixing the pieces together permanently mean the items you're making need to be disassembled sometimes, often or frequently? If it's often or frequently I'm not sure there is any suitable solution other than ensuring the cuts are done better to begin with. – Graphus Jan 09 '19 at 07:55
  • @Graphus, there's some variability in the thickness of the wood, so one batch might have joints that are a bit tighter while another batch is a bit looser. Some of the long pieces can be slightly warped as well, causing the joint fit to vary. – Joe Jan 09 '19 at 07:56
  • @Graphus, the pieces might need to be disassembled at some point in the future, but neither often nor frequently. – Joe Jan 09 '19 at 07:57
  • wedges are used for all sorts of "knockdown" joints - joints intended to be disassembled - as well as part of permanent joinery. Note that generally that shallower the wedge angle, the stronger the joint will be. – aaron Jan 09 '19 at 14:55
  • Do laser cutters or the software that controls them have the notion of an "interference fit" for a given material? This would be the ideal solution. –  Jan 09 '19 at 17:01
  • @jdv, I'm afraid I don't know what you mean. – Joe Jan 09 '19 at 17:19
  • @Joe, what you are after is called an "interference fit". It might be useful to study your hardware and software and see if there are recommendations for doing this, as it is not an uncommon requirement. You may have to come up with your own way of doing this for a given material, but once you do that it should be easier to laser cut more accurately to allow for this sort of thing in the future. Laser cutters tend to be a "measure X times, cut Y times", where Y>X in many cases. –  Jan 09 '19 at 17:21
  • @jdv, ok, looking it up, I believe that's what I'm doing already. The problem is variation in the material from one batch to another. I can't recalibrate the cut pattern for each batch. – Joe Jan 09 '19 at 17:24
  • @Joe, that's the problem. So-called press-fit joins have very tricky tolerances that might be outside what most wood can offer (but that is a guess) because wood changes shape. I suspect some (finished?) hardwoods would do better, but natural variation probably means assembly variation with custom shimming. –  Jan 09 '19 at 17:28
  • I think dovetailing the joint (both kinds) would help quite a bit since it wouldn't require them to fit tightly to hold things together. – martineau Jan 10 '19 at 15:56
  • how about applying some pre-stain wood conditioner to swell the grain a little bit. – justin cress Jan 14 '19 at 19:17

3 Answers3

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without fixing it permanently together?

This is the real challenge here as the ideal conventional fix for many loose joints forms part of the final assembly process where the pieces are being glued together. One fix that is a separate operation, glueing on thin slips of wood to fatten fingers/tenons/dovetails may be too fiddly to be practical in material this thin — I'm visualising the fingers in 1/8" ply as being numerous and small. Plus you need a hand plane and source wood to make the packing material yourself since commercial veneers are almost certain to be too thick.

So, since you've clarified in the Comments that the items don't need to be disassembled often and thinking outside the box, I think a possible solution would be to use hot-melt glue. Hot-melt glue sticks wood together surprisingly strongly but yet is easily reversible using heat of course, and even if not fully meltable at low temps a hairdryer will usually prove sufficient to get it to weaken its hold.

In addition hot-melt glue can easily be dispensed in various ways to tack adjacent surfaces together, in a manner akin to welding. This isn't normally how conventional woodwork would be fixed* and in this glue the joint formed wouldn't be particularly strong, but it might be sufficient for the type of things you're building. So you could for example assemble anything with finger-jointed corners and then apply dots of glue, or a continuous bead, along the inside corner. Lap joints could be glued conventionally, then heated through with an iron when they need to come apart.

Another option might be to actually glue the joints the normal way, but using a reversible adhesive. Hide glue (the OG hot glue although room-temperature versions can be made or bought) is probably perfect for this since with heat and moisture joints can be separated at any point in the future. But the moist heat needed does pose a risk of warping the thin ply.

*Although something like this is done in epoxy.

Graphus
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Wildly speculating, I'm guessing paint, wax, some kind of glue that doesn't bond too well, mud..

That is pretty wild. Wax would probably make it functionally looser. Paint has a nasty habit of welding to other contact surfaces. Wood filler might give you a functionally tighter joint (and allow you to tune it) but it will stop you gluing that surface. Many fillers will deform under pressure of a stressed joint too, though probably at an acceptable level in a finger joint where the load and direction is limited.

Tacky glues and hot glue are okay but again, they have functional stress issues when used as a filler.
Again, that might be acceptable in your specific joints. My main problem with them is they're visible.

One option you may not have considered is —and I expect this to attract the full wrath of the WW.SE Gods— something like physical fasteners. An internal steel brace. A careful screw through the underside of a lap. They don't have to be metal. Even (lower grade) antique furniture has its share of wedges and pin dowel to hold things in place.

There are some great options you can borrow from modern flat-pack designs to really pull and hold two pieces together. They're usually designed for glueless butt joints so adding in the strength from your existing joint leaves you with something that should survive well and be reversible.

Oli
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  • Metal brackets are a great idea in principle, except that the material is only 1/8" / 3mm in thickness. How do you visualise attaching them securely to plywood this thin? I think screws are ruled out. Through bolts fastened with nuts could certainly work but they might be a hard sell to the OP. – Graphus Jan 09 '19 at 16:30
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You might try wood veneer in the joint or even paper.

Monte Glover
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  • Monte, FYI the system has flagged this (and previous Answers of yours) because of their length or content. As it stands this is more suitable to a Comment. SE values self-contained Answers which explain as clearly as possible what the solution is and very short Answers will rarely allow this, hence the auto-flags for single sentences. I know where you're going with this suggestion but the OP might not and many future readers won't either, so some explanation of where (and how) to attach the veneer or paper would flesh this out into a proper Answer. – Graphus Jan 09 '19 at 16:42
  • That being said, it actually is the start of a good answer. –  Jan 09 '19 at 17:22