Wood movement and medium size boxes

[Wend]

Hi folks,

I want to make something that is essentially a box, and am trying to understand what measures I need to take against wood movement. I wonder if anyone can help me please? In case it matters, the light wood will likely be poplar/tulip (as I want to do pyrography on it); I haven't looked at options for the dark wood yet.

The top of the box will be broken into sections, something like this:

which I plan to make by getting rectangles of wood like this (probably 3mm or 5mm thick) and gluing them down onto a 375x220mm base:

Grain direction will be left-right apart from the vertical dark pieces. So the first question is, do I need to do anything about wood movement for those vertical pieces? If so, what? Just glue them in the middle? I'm a bit concerned they might curl upwards.

Then I need to attach the sides of the box. I think that the easiest way will be to cut a rabbet out on all 4 sides so that the top sits inside the edges like this:

But what's really confusing me is, how do A and B get attached to the top?:

As the top expands or shrinks, it will be the 220mm edge that will change size, right? So it seems like it is only safe to glue the top to the middle of B (and its opposite piece); is that right? It doesn't seem like it would be held on particularly securely in that case. And in case it expands, I would also need to leave a 1mm gap between the top and A, and between the top and A's opposite piece?


Thanks
Wend

 

[Wend]

Ah, I think I finally get it! What I actually want is to put the lid into a groove in all 4 sides thus:

with a little space for expansion, such that the fit is tight enough that it can't move around (I guess I may as well glue it in one spot too), right?

 

[Glynne]

In terms of fitting the lid to the sides you’ve worked it out for yourself. The main expansion of a solid top would be along your 220 side so a little dab of glue in the middle of this side will stop the top rattling around (if your grooves are a bit wide) but will still allow for movement.
However this is for a solid top and if your top is fixed to a solid substrate (plywood / MDF) then there shouldn’t be any expansion at all. Most of the boxes I make will have a veneered MDF top (and base) which avoids having to consider expansion but also allows you to fully glue them in place which ads to the overall strength, useful if your box sides has unsupported mitre joints.
The problem you may have is your solid wood pieces trying to move on the substrate? You could try glueing so as the “free” end is the one that goes into the grooves (rabbets) so as to allow for movement but if you glue solidly down on the whole panel you could well end up with the wood splitting.
I did ask a question on the forum a few years ago as to how thick a veneer you could reasonably glue down to a substrate without having to worry about movement and if I recall, the answer from Custard was probably less than ~1.5mm. Anything over that would be down to chance.
So if you are going to try for 3mm - 5mm Wood, then acclimatise the wood in the environment you are going to keep the box in before attaching it to the substrate and then keep your fingers crossed! That having said, you will see pictures of boxes with different types of wood and different orientations so it can be done.

 

[AndyT]

I reckon this could work ok, but only if you use a manufactured board for the pale coloured top. This could be plywood or mdf, with or without a decorative veneer of your choice. With a board like that you can ignore movement entirely. You could glue the top to the sides all round and you could glue the decorative strips onto it.

Your main difficulty would be finding a small piece, so you might have to buy a full 1220 x 2440 mm sheet.

Using solid wood would make it all much harder, which is why this is not a conventional approach to box making.

 

[custard]

There are loads of different ways of putting boxes together, but here's the method I use the most and which is most relevant to your project. Here's one of my boxes, almost all the boxes I make are variations around this theme,

The sides are solid timber. The top is plywood that I've veneered on both faces. Veneering is a pretty important skill to acquire for box making, but it's really not that difficult when you're working on relatively small components like this. I can veneer a box and I can veneer a wardrobe, I promise you the box is about a thousand times easier! The other nice thing about veneering for boxes is that you won't need to spend much money on tools (it's not often I can say that about woodworking!) and also that these smaller veneers are conveniently available via mail order (it's even less often I can ever say that about sourcing timber for woodworking!)

Here's the key thing though. Unlike your plan, the top panel has a rebate worked into its upper face. This means it can fit into a groove in the sides, but still end up flush on the top surface. This is the key difference to your plan.

The other key thing is that fitting the top panel this way will leave a tiny gap between the top panel and the sides. So you need a strategy for how you'll cover that gap. You can see the solution in the box in the photo. You let into the top surface a contrasting inlay band all the way around the top and mitred at the corners, this both covers up the gap and also adds a decorative element.

That, in a few sentences, is the secret of how most boxes are made.

If this is the way you want to proceed let me know and I can post some more information.

 

[custard]

I've just found a sketch that illustrates the constructional principles, apologies that it's bit rough but I think I've captured the important details,

 

[profchris]

I'd say that *if* this box top is subject to big humidity swings, then there is a good chance that it will curl up or down (the longitudinal axis of the curl will be along the grain direction of the wide parts). If the substrate it's glued to is something like plywood then it will resist that curling up, but assuming the substrate is only 3-5mm thick then it won't be able to resist completely. Your top layer will swell (or shrink), the substrate won't move, so the stresses have to go somewhere.

*However*, if the box is going to live somewhere where the humidity swings are lower, like a living room, then you might get away with it. The trick would be to glue the pieces down to the substrate when they are all at the same humidity as the place where the box will live - i.e. bring them into that room, leave them there for a few days, and then glue up in that room and don't move the piece until the glue has dried.

Of course, if you then take it into an unheated shed to finish the box, it might move after a day or two. But you can deal with that by not leaving the work piece in your shed unless you're working on it.

The other option is to pick only wood with vertical grain (looking like this seen end on - ||||||||||||||||||||||||| ) for both your top layer and the substrate. Then the whole thing will expand or contract roughly together. If the grain is slanting a lot (\\\\\\\\\\/////////////), then each piece will try to curl up. Opposing the likely direction of curl in the two layers:

\\\\\\\\\\\\\\\////////////
///////////////\\\\\\\\\\\\

might prevent this - or it might not!

The dark wood with the grain running the other way looks too small, in comparison to the rest, to have much effect, so I'd glue it down all the way along to top it moving up and down!

But the best way to get the effect without having to worry about movement is to use thin veneers on plywood or MDF, as someone else already said.

 

[Wend]

Thanks, I think this is all making a lot more sense now!