Cupping in boards

[Blackswanwood]

I am making a cupboard for my daughter out of pine. It is 350mm deep and I have flattened two boards both of which were cupped and edge jointed them to make the carcass. Coming back to the glued up boards today one has cupped again by 7mm but the other three are perfectly flat.

Am I correct that this isn’t anything to fret over as I am going to use dovetails and it will pull it back into shape provided I hold the board flat with a clamping caul while marking out the dovetails?

Cheers

 

[MikeG.]

You won't pull them back to shape, I'm afraid. Besides, trying to set out dovetails with cupped boards will give you nightmares.

I'm guessing that you just bought the wood and started working on it straight away. Is that right? It really needs to stand indoors for a week or two, to move as it is going to move, and then to stabilise. At that point you re-assess, and it may well be that you have to rip it down to narrower widths, flatten those, and then re-join into wider boards again.

 

[Blackswanwood]

I have actually had the boards for over a month Mike so they should have stabilised. It’s a bit of a mystery as the other three are fine and all made up from the same original boards.

I have enough to glue another one up but wondered if I could get away with it as it’s just a pain as I thought all the prep was done last night!

Thanks

 

[MikeG.]

Have you had the boards in the house, or in the workshop? If in the workshop, then that's not really going to have given them a chance to stabilise.

 

[AndyT]

You might get away with it, if it's fairly thin wood, the piece is quite large, the joints are good and you can tame it into flatness to work on it.
Can you bend the problem board flat with just hand pressure? Have you made a cabinet like this before?

I've got away with a bit of cupping on shelves which I forced into housings, but that's no help to you.

The only way to find out about your boards on your project is to try it.
How would you feel if you put the extra effort in and it still went wrong? Would it have been easier overall to buy another bit of wood instead?

Nobody else can answer those questions. Sorry!

 

[MusicMan]

The mystery will be because they came from different points in the tree, most likely the bad one was nearer the surface. There is usually some tension reaction wood there, if the tree was not grown perfectly vertical. This shrinks more than the inner wood. and will shrink differently at the edge and the surface hence cupping.

Then you flattened them, likely before they were in full equilibrium with the external humidity (that takes months not weeks). Two things can happen. The board may have had uneven stresses internally, balanced by opposite stresses at the surface. When you removed a surface, the balance was removed and the board immediately cupped again. If it did not do so immediately, then the cause is likely an uneven distribution of moisture in the section, but still a stress to balance. Over a few days, moisture will migrate to make the wood swell or contract to balance the stresses again, resulting in distortion of some sort. (Boxwood is very prone to this - lathe work must be done in one go if dimensions are critical). The three good boards were obviously more evenly stressed and with a more uniform water content.

Whether you can flatten them enough to work with depends, as has been said, on the thickness. In general though, the forces involved in wood expanding with moisture are enormous: remember, the Egyptians used this to split stone.

Plug for Richard Jones' book, 'Cut and Dried'.

 

[MikeG.]

.....that takes months not weeks.......

That depends on the starting point. Also, of course, on one's judgement of the strength of the joints, speed of work, and so on. Let's just say that wood should be acclimatising in their final environment at least for weeks.

 

[NickM]

That depends on the starting point. Also, of course, on one's judgement of the strength of the joints, speed of work, and so on. Let's just say that wood should be acclimatising in their final environment at least for weeks.

Mike, after the boards have acclimatised, what's the best process from that point? Obviously the best thing would be to get out to the workshop and crack on with the job as quickly as possible, but is there a "best practice" if you can't do that - e.g. let's say you can only work for short periods at weekends and the whole project will take several weeks/months to complete? Should you be bringing the wood back indoors between workshop sessions? Thanks.

 

[transatlantic]

Lets say you buy the timber, and then bring it into the house to acclimatise for a month. Then you take it out to the garage (not insulated) to work on the project at hand, and this takes 2 - 3 weeks. Isn't it then going to move again as it'snow in a completely different environment?

How should you work in these cases? bring the project back into the house each evening?

 

[That would work]

Clamp the boards flat, mark the dovetails and assemble with them clamped flat. 7mm over 300 is just about workable with.
OR better still... damp the concave side of the board and expose the convex side to gentle warmth and they will flatten in no time. Now go ahead joint cutting wasting no time and all should be fine.

 

[Blackswanwood]

Thanks guys. I have played it safe and flattened and glued up a new one. The wood had been indoors (house) for three weeks and four days and was 22mm but brought down to 18mm thickness when flattening.

I have clamped the cupped one between a couple of pieces of three by two at each end as an experiment to see if it ends up nearer to flat again.

 

[MikeG.]

.......OR better still... damp the concave side of the board and expose the convex side to gentle warmth and they will flatten in no time. Now go ahead joint cutting wasting no time and all should be fine.

That's fine for something utilitarian, like shelves in a workshop, but is absolutely not the thing to do for something you have any pride in in your house. You would simply be storing up trouble. The board will dry out and go back to the way it was before, tearing joints apart in the process. Even if the joints held, the middle of the board away from the joints will cup.

 

[MikeG.]

Lets say you buy the timber, and then bring it into the house to acclimatise for a month. Then you take it out to the garage (not insulated) to work on the project at hand, and this takes 2 - 3 weeks. Isn't it then going to move again as it'snow in a completely different environment?

How should you work in these cases? bring the project back into the house each evening?

Not each evening, but if you weren't going to be working on it for a week or more I'd be looking to bring it into the house. Again, it depends on the nature of the job. Some painted shelves in a spare bedroom may not be as precious as a dining table, for instance.

 

[That would work]

They will only be flat for as long as they are clamped flat. So as I said, either adjust using movement inducing conditions or hold it flat during marking and assembly.

 

[That would work]

.......OR better still... damp the concave side of the board and expose the convex side to gentle warmth and they will flatten in no time. Now go ahead joint cutting wasting no time and all should be fine.

That's fine for something utilitarian, like shelves in a workshop, but is absolutely not the thing to do for something you have any pride in in your house. You would simply be storing up trouble. The board will dry out and go back to the way it was before, tearing joints apart in the process. Even if the joints held, the middle of the board away from the joints will cup.

The process I am describing replicates what actually happens to timber according to atmospheric conditions.
Storing up trouble??? That's simply not the case. Somewhat over dramatising Mike!
I can assure you that the advice I have offered does work... over many years with all types of work (but not workshop shelves :lol:

 

[That would work]

Am I correct that this isn’t anything to fret over as I am going to use dovetails and it will pull it back into shape provided I hold the board flat with a clamping caul while marking out the dovetails?

Cheers[/quote]
You are totally correct!

 

[MikeG.]

......The process I am describing replicates what actually happens to timber according to atmospheric conditions......

Funnily enough, not many bits of furniture get rained on on one side and frazzled in the sun on the other side. Those that do don't usually look too special.

.....I can assure you.......

That's where you lost me.

 

[That would work]

You are right.

 

[woodbloke66]

Am I correct that this isn’t anything to fret over as I am going to use dovetails and it will pull it back into shape provided I hold the board flat with a clamping caul while marking out the dovetails?

Cheers

You are totally correct![/quote]
I've done this before now where it was absolutely necessary, but it's very awkward and extremely time consuming. As such, I can't recommend the process - Rob

 

[thetyreman]

it's actually not too bad if it happens on the tails on a wide board as long as it was flat when you marked it out and cut the joinery, this happened with my tool chest and whilst not ideal it can be worked around, pine does compress a bit and I managed to knock into flatness with help from a few clamps.