When wood moves quickly.

[Andy Kev.]

About three months ago I bought a board of American poplar (tulipwood) and had it cut into 1 m lengths at the timber yard. Then it sat in my workshop. At the weekend I flattened one face and one edge of each piece so I could run them through my new bandsaw to get to a bit more than final thickness. Then it was a matter of planing down the newly sawn side and squaring up the other edge and the ends. The original board was fairly straight grained, had very little wind and a normal degree of cupping. Therefore the work was done quickly.

However, within half an hour of coming off the bandsaw, the cupping returned to the pieces. Am I right in thinking that this can only be due to stresses being relieved by the bandsaw cut? Given the amount of time it takes wood to dry out and given that there have been no sudden humidity changes over the last few days, I'm inclined to think that changing moisture content can't be at the root of this.

I don't anticipate this leading to problems and I intend to give the pieces about a fortnight to settle and then re-establish square and quickly get the dovetailing done before any further movement can occur. Is this a sensible course of action?

 

[custard]

You don't give the actual dimensions of the various cuts, but I'll take an educated guess and say you bandsawed off far more from one face than you planed off from the opposing face.

The net result is that the board was wetter on the bandsawn face than the planed face, so it cupped.

The moral of the story is remove equal amounts of material from both faces of any board, in other words bring it down to final thickness just using the thicknesser, flipping it over on each pass to keep it equalised.

 

[Andy Kev.]

That explains it. I hand planed probably about max. 1/8" from one side and took off the better part of 3/8" with the bandsaw from the other side. The subsequent cupping is not massive (all the boards are from a fairly well behaved plank) but enough to scupper any thought of dovetailing without further treatment. That's a lesson learned for the future. As it is, given that it is very near final thickness, I reckon that if I leave it for a fortnight to settle, it will be just a minor planing job to get it sorted out prior to dovetailing.

I was aiming for a finished thickness of 3/4" but I don't mind if I end up a bit under that as it is destined to be a shoe storage chest which will never be sat on.

 

[custard]

Sorry Andy, I didn't realise you'd planed the piece by hand, I thought you had used a machine.

When you look at antique furniture, that pre-dates the machine age, there are often quite large variations in the thickness of different components. I'd follow suit, flat off both faces equally with a hand plane and then see if the design can be adjusted to accommodate whatever thickness you just happen to end up at!

 

[MattRoberts]

Custard, out of interest, is this likely to happen regardless of how dry you believe the wood to be?

 

[custard]

Custard, out of interest, is this likely to happen regardless of how dry you believe the wood to be?

Every piece of wood is different, but it's certainly pretty common, even with kilned timber, to find a noticeable moisture gradient that means the centre of the board is wetter than the outside.

I try and take a pragmatic approach, thinking about what components in a piece of furniture really need to be stable and what components can handle a bit of movement. So, for example, when starting a project I'll often get out the drawer sides first, picking quarter sawn and straight grained timber, then working the components down to finished thickness in stages over a week or more, and storing them in stick right up until they're needed. A top on the other hand will get chosen for grain and appearance rather than stability, and because it's held down with buttons all round it's generally less of an issue if it tries to cup slightly.

Another thing I'll sometimes do with a high value or highly figured board, is bandsaw off a couple of veneers from each face, rather than just whack it through the thicknesser. If I was thicknessing with a hand plane I'd be tempted to do this more often.

 

[MattRoberts]

Thanks for the insight

 

[Andy Kev.]

The following might be of interest as a postscript to this. On Sunday I had to cut six rails each of 16" x 1 1/2" x 3/8". I had a piece of tulipwood/American poplar which was 17 1/2" x 6" x 1 1/8" all planed, squared up and ready to go. So from the reference face I marked a cut line 7/16" in and a second line the same distance in from the other face (but also marked from the reference face).

I ran it through the bandsaw and got a piece of the desired thickness. Then I moved on to cutting the second piece. It had developed a noticeable curve within about 10 seconds of cutting. I only got away with it because there was more wood in the piece than I needed and I'd deliberately marked a bit on the thick side. The piece left in the middle stayed absolutely flat in accordance with theory.

The reason I mention this is the speed with which the cupping happened. I'd reckoned with minutes and so thought that I'd be beavering away with my plane long before any curve appeared. It really was an eye-opener. The lesson which I draw from this is simply to deep/recut a piece right down the middle and then be prepared to plane away the excess.

 

[MusicMan]

There's two sorts of cupping/warping that can occur, due to different gradients being in the wood. Custard describes accurately what happens when there is a moisture gradient. When the wood is cut so as to disturb the balance of the gradient, then you get slow warping over minutes or even hours. A good reason why turning should be completed in one go not left on the lathe overnight. As a moisture gradient evens out, it will cause a stress gradient or warpage. Custard shows above how to minimise this effect in practice.

When there is a stress gradient, the cupping will occur almost instantly on cutting. Unfortunately you can't find out if there is one until you cut it. But if you look at the sawn section right across a log, and you see that the growth rings are not concentric, you can be sure that there is reaction wood present giving a stress gradient. This is the wood in tension (softwoods) or compression (most hardwoods) that hold the tree up if it is leaning, or if it is a branch.

The stress gradients are usually bigger near the edges of the log because of this reaction wood. The test in the timber industry is in fact to cut a plank into fingers (like a feather board) and see if the fingers distort.