Joint types for very thin plywood

Question

I have some sheets of very thin plywood (beech). By very thin I mean 1/8 inch (3.00 mm). I need to make some small boxes out of it (movable compartments in drawers etc.).

What would be a good way to joint such thin plywood?

Answer 1

What would be a good way to joint such thin plywood?

The simplest joint of all, butt joints, could be sufficient here given the possible sizes of boxes you're making and the use they have to withstand. Particularly if you prep the joining surfaces well* and use an adhesive that can fill gaps (so, not any type of PVA).

PVA might yield strong enough joints, I've made some small storage boxes for the workshop in 3mm hardboard glued with PVA that have held up to light/medium use for a number of years, but with plywood you can expect to have some voids and tiny gaps on the edges no matter how carefully you cut and prep the material and they could undermine the strength enough to be an issue, if you don't reinforce the joints in some way.

The base adds a lot of strength
All boxes can gain strength from their bottom/base, and in thin material you specifically want to add strength by any means available. If glued in (or on) well the base goes a long way towards preventing the box from being able to splay or rack out of square and the shallower the box the greater the effect.

Reinforce further if you need the strength
You can further reinforce the corner joints if you want to err on the side of caution, using one of a number of possible methods. These include epoxy fillets, and triangular or square glue blocks. Do note that these options take up some of the internal space, it can be minimal in the case of epoxy fillets and fairly modest with triangular glue blocks, but the payoff in corner strength would be huge.

In order to not lose any internal space you may also be able to successfully use pinning or dowelling, even in material this thin, but you'd have to be very careful in how you drill the holes, e.g. by drilling with a hand drill and not be tempted to drill too deeply — only 3-4mm would be perfectly adequate.

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*The surfaces should be freshly worked, within the hour at least, and the ply edges should be as smooth as you can easily make them, if necessary by shooting with a hand plane (my preference) or sanded well to approximately 320 grit/P400.

Answer 2

one-eighth inch beech plywood is a common material for laser cutting, specifically for creating custom boxes as you describe. There are extensions to Inkscape for making boxes with tabbed edges, also described as finger joints. You would not need the software to create your boxes, unless you had a laser cutter. You could use the software to make paper templates to assist in the cutting and/or marking of the wood.

The image below is of a box made with 6 mm wood, but the concept is the same. Image courtesy of James Stokebrand.

One could use a coping saw, a hobby-grade desktop jigsaw or even a table saw to notch the edges of the panels to 1/8" depth spaced apart the proper distance. It would be extremely detailed work and easy to make an error if doing this manually, however.

A dovetail jig with a small cutter would reduce the possibility of error.