How to prevent cupping (warping) of resawn wood

[Rob Cheetham]

I am trying to edge joint some american white oak boards together.

Basically I planed up one face and one edge of a two pices of american white oak. Then I resawed a 10mm strip of of each piece. I then left these boards (which had one perfectly flat face each) overnight. The boards were stickered to give even air circulation. Also im pretty sure the boards were dry as they have been standing for about a year.

When I came back the next day the boards had cupped. Not majorly but enough to be frustrated. How can I stop this from happening. I want these pieces for scoll saw projects and dont want to keep planing them down to flatten them out which will result in losing thickness on the boards.

Should I be sealing the endgrain straight after cutting? If so what would be the best thing to use to seal the endgrain.

Any other tips would be greatly appreciated. Thanks

 

[Glynne]

Moisture isn’t the only reason that boards cup. There can be internal tensions and resawing can release these. If you’re looking to get 2 x 10mm boards, you will have to start with something like 2 x 15+mm boards, let them settle and then plane both sides to get to your desired thickness. Converting timber is a very wasteful process and you would have to be extremely lucky to get to your ideal dimensions immediately after resawing unless you are say cutting thin veneers.

 

[Rob Cheetham]

Thanks for the reply. I did compensate for the thickness. Was going to get down to 6/7mm eventually. One question though. How long would you let a resawn board settle for after resawing. Thanks again

 

[Glynne]

There isn’t a standard time. For me, I resaw and leave at least overnight. When I come back (with fingers crossed) I then look for any cupping or warping. If there isn’t any, I will probably plane (thickness) both sides but not to the final dimensions. I leave again and, all being well I can then go to my actual dimensions. But this is the ideal scenario. If the wood has moved it needs to be left for longer.
Sorry that there isn’t a more precise answer. I historically have bought a lot of green, waney edged timber as it is a lot cheaper than PAR timber on the basis that I have a good bandsaw and planer / thicknesser so I could convert my own timber.
What I didn’t appreciate is that it takes forever to get your timber dimensioned before you can even start your project let alone the waste involved.
I still will buy “rough” timber but it will be dry and generally dimensioned to what I want to make.

 

[profchris]

I regularly resaw to around 4mm, to end up with 2mm boards for instruments. Because they are so thin I can cope with a certain amount of non-flatness, but ideally they'll end up flat.

24 hours or so will see most of the movement, but not all, so in your situation I'd plane the boards to remove the worst (but not all) of the cupping and then leave another 24 hours. If there's no further movement I'd remove all of the cupping, then wait another 24 hours. Then I'd plane to final thickness, working *both* sides of the board - if you get it flat and then work one side only it might move again, so try to take equal amounts of both sides.

Basically, creep up on the final dimensions so as not to alarm the wood!

 

[AndyT]

The other big factor here is the original position of the board when it was part of the tree.
Broadly, if you look at the end and see a series of arcs, the board was tangential to the trunk and is very likely to cup. If the end shows short lines across the thickness, its original position was more like radial and it can stay fairly flat when drying. But those 'quarter sawn' boards are less common and can cost more.

 

[Rob Cheetham]

Thank you eveyone for the replys. Ill welcome any help or advice after this aswell. Ill take on board about everything and see how I get on. One thing ive never done is thicknessed the board down gradually by working each side. Ive always just done one so I will definetly start working both sides now and see how that works out.

Few more questions though if anyone can help?

* Does it help if you wrap the piece in plastic as Ive read that can stop the wood from gaining or loosing moisture too quickly? If so anything special to use or would just a plastic bag for example be good enough?

* As I would cut to final dimensions at the end, would sealing the end grain with anything help to stop the wood from loosing or gaining moisture to quickly? If so whats the best thing to use to seal the end grain? (I know it can loose or gain from the face but I know end grain it can go from quicker)

* I only have a garage workshop (no heating or anything). Should I keep my wood in the house and then just bring it into the workshop when working on it. Then when done bring it back into the house. Or should I just keep it in the garage where the wood has been sitting for a year anyway?

Any help to these questions would be great. Thanks

 

[profchris]

Plastic wrap and sealing the end grain are to slow down the drying from freshly cut, and so reduce the risk of checks and splits. That wouldn't help you.

Initial drying from cutting down the tree is said to take an inch per year, so your wood should be at equilibrium with its environment. But that equilibrium changes - see Humidity below.

You have two separate things to think about here:

1. Cupping/twist etc caused by releasing stresses already in the wood. This has already been explained - plane from both sides, give time for movement between planing sessions.

2. Cupping caused by humidity changes. The humidity of the atmosphere changes hourly, and in the UK it might be somewhere between 50% and 95%. That's a wide range.

Wood absorbs moisture and releases it, swinging along with the atmospheric humidity but with a time lag. Thicker wood does this slower than thin.

If wood gets more humid it expands across the grain. If the end of your board show vertical grain then the plank (probably) expands equally on both sides and stays flat. If you have radial grain lines at the end then your board might cup towards the inside of the curve.

There is no way to prevent this expansion and contraction, you have just to allow for it in what you do. So floating panels in doors aren't glued in on all sides, table tops have radial grain alternating up and down, etc.

If your scroll saw projects are to live indoors, and must move as little as possible, then I'd acclimatise the wood indoors, work on it and then take it back indoors. Indoor humidity usually moves less than outdoors. But be aware of central heating, winter can be arid in a fully heated house!

If they have to be flat when you scroll them but can move a little after, then assuming your workshop isn't wildly different in humidity from where the final work will live, acclimatise the boards in the workshop.

I build musical instruments, and humidity changes can literally tear them apart if you don't take that into account when building. But I'm working with boards 2mm thick or less.

If it helps, I find that 10mm boards don't move a huge amount, once taken to final thickness, unless the air becomes wringing wet or Arizona dry. But they still move a little.