How to: wood movement

Methods for avoiding whoopsies

There are several things you can do to avoid problems, and at the same time have a bit of control. You need to allow for movement, and this can be done in several ways.

The first thing you should do is to acclimate your wood to the environment it will live in. First and foremost, the environment in your shop since your project will live there to begin with, ad it will undergo the greatest amount of changes. Ideally (but not absolutely vital), your shop should have a lower relative humidity level than your average living room. That way, your tight joints will be even tighter when you take the project from your shop to your living room. By accounting for movement, you won’t have any issues either. Leave your wood in the shop for a month, and you should be safe. Just remember to sticker and stack like we do for air drying lumber. If you pile your wood up 10”, the wood in the middle won’t change in a month’s time..

Here’s a few examples on how to allow for movement:

Loose joints: Normally, we want dead tight joints. But there are some cases where a slightly flexible joint is desirable – of course, this means joints that aren’t glued.

This cross lap joint is a good example. It is made ever so slightly on the loose side so that the legs may pivot a half degree or so. This small side table top will not change much, but it WILL change! A dead tight joint could’ve caused problems. Maybe not at once, but over the years – sort of like a cowboy wire-cutter – the cycles might break or loosen something.

Fasteners that allows movement: Another trick is to make the screw holes bigger than necessary or oblong. For that small side table, I drilled the holes 1mm oversized. This gives me at least 2mm play which, combined with the loose cross lap, will let the top move freely.

You can drill oversized holes and use big washers with smaller screws.

Turnbuttons: This is an old, tested and true method. These little brackets are easy to make in batches. They are inserted into oblong slots or a dado, and allows the wood to move. In this case, I’ve placed the turnbuttons along the front edge of my support table. The other side is screwed to the rail, which goes against the wall. The top is therefore allowed to move towards the front. The turnbuttons are angled a degree or two because they are slightly too low for the slot. This means that they squeezes the top tight to the rail. If you mount these on the end grain side, you can mount them flush to the rail since there will be no movement longitudinally. The ones mounted along the long grain sides, as in the image below, should have allowance for movement as shown. Observe the grain direction when making these!

You can also use figure 8 fasteners or metal turnbuttons, but your offcut bin is full of turnbutton blanks. Use’em!

Breadboard: One option you can use to keep a table top flat – and to dress up the end – is breadboard ends. The key is to lock the breadboard end in the middle, while the outer ends looks like they are firmly attached, but the holes in the tenon near the edges are oblong. The outer pegs can move in those slots and are only glued to the breadboard end. The tenon should be narrower than the mortise to allow for the movement.

The drawback with breadboard ends is that there will be a difference in width between the top and the end piece. This can be solved in a number of ways, for instance by making the ends deliberately too wide, rounding over the corners and let it be a design element. Form und funktion! The Shakers did this in their own way.

Note: this article shows ONE way to do breadboard ends. There’s more, so google it.

Sliding dovetail: This is a VERY useful technique. A dado is cut in the shape of a dovetail and a corresponding tail shaped piece of wood is inserted into the dado. This has a few advantages. It will hold the top flat (great alternative to C-channel steel inserts), and you can use it to fasten your table legs to.

Pro options: -make the sliding dovetail, but stop short of the outside ends of the table. The last plank for the table top needs to be glued to the top after you insert the tail shaped piece, and you need to come up with a game plan to avoid any glue squeezing out into the dado. Perhaps a channel cut around the outline of the dado on the edge of the two joining boards, finish and wax on the tail shaped piece… Or you can decide to let that end be the anchor point. Just remember to allow for the movement of your top; the sliding rail must be shorter than the dado. You can of course hide this in numerous ways, but why bother? Nobody is going to look under the table anyway – and this would look SO much better than a row of hex bolts screwed through some steel bars. YMMV.

-Drill a hole in the center of the tail shaped piece, and cut grooves longitudinally from the hole so that you can force glue in there. This secures the piece dead center, while the table top can move freely towards or from the sides.

-I have seen someone cutting the dado and tail pieces, then removed the tail form from the cross brace in sections: 5cm tail, 5cm square, 5cm tail and so on. By doing the same thing to the dado, you can insert the brace and knock it sideways into place. Seems a bit cumbersome to me, really. Gluing on the last board should be a breeze since you can use the sliding dovetails to guide the last board in place. You know – no need for biscuits, dominos, floating tenons, dowels or what have you. No need for machinery that cost a kidney either.

Joints you can knock tight again: The tusk tenon is such a joint. If it ever comes loose, just welly the wedge back in and it snugs up. Another property of this joint is that it is gravity assisted. Which means – providing you have installed it correctly, i.e. with the narrow end of the wedge down – it will self-tighten. The wedge needs to be a smidgen narrower than the width of the mortise, and it should stay solid without needing to jam the thing in there with a sledge hammer.

The humble tusk tenon

The tusk tenon is great for knock-down furniture. It is widely used in traditional Norwegian furniture – and I suspect all over the world. It is simple to make, and it is forgiving – even a sloppy one will work. To some degree, at least…

Alternative materials: For certain things, you can use materials that don’t move at all. Plywood “doesn’t move” due to the fact that it is all longitudinal movement because the layers are criss-crossed. This locks the cross grain to the long grain. Since the layers are so thin, movement is eliminated. Another option is MDF, which is a “dead” material that does not move at all. The drawback with these options is that the edges will look bad no matter what you do. But for things such as cabinets, bookcase backs, drawer bottoms and door panels, this is a really good option. There are veneered options so that you can have an oak back on your oak bookcase.

Placement of fasteners: If you secure a table top firmly in the middle and let it “float” (meaning the fasteners towards the edges allow movement), it will expand and contract equally to both sides.

If you have an alcove where you want a live edge slab style computer desk, secure the back end of the slab to the wall and let the front of it float. Remember my support table turnbuttons?

A single bolt in the center secures the leg assembly to my oak slab bench, allowing for movement.

On the next page, I’ll discuss what else we might want to consider.

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