Minimising cupping

[stuartpaul]

Solid oak table top 970 x 865 (actually two, - it's a flip top table). Mixture of planks from 130 to 225. Worried about the longer term prospect of cupping.

Obviously arranged with 'alternate' grain to minimise cupping. I'm wondering if ripping the wider planks down and adding those to the alternating process would help? Don't want to if I don't have to as I'll loose some nice grain but the cupping issue is probably more important.

Thoughts?

 

[marcros]

how are you fixing them- breadboard ends would help keep it flat. Also, is the timber seasoned to the location that it is going into correctly? Personally, I wouldnt rip down the wide boards, unless you wanted the asthetic of single sized boards, or if they wouldnt fit through the planer or thicknesser.

 

[Phil Pascoe]

I made up some wide boards (2') of mahogany some years ago for a friend and the stuff he supplied me came cheap, but it had a mind of its own. I actually ran router grooves down the underside to half the thickness. It's not twisted or cupped yet.

 

[Sgian Dubh]

You're right to be wary. There's a reason why unsupported solid wood table leaves tend to be made out of the more stable wood species such as walnut and mahogany. The oaks in general are less stable because the tangential to radial shrinkage ratios are higher than in either walnut or mahogany, and the greater the ratio (disparity) between the two shrinkage factors the greater the tendency for instability. Mahogany for instance has T/R factor of 1.4, whereas American white oak has a T/R factor of 1.9. In European oak it's a factor of 1.7 T/R.

Before committing yourself to doing this I suggest you machine up the boards flat, leaving them fat, and sticker them up in an atmosphere similar to, or the same as the circumstances in which the finished table is going to live for a while, the longer the better, perhaps a few weeks. It would probably be beneficial to clamp the stack flat too. This will allow the boards to acclimatise to those conditions and you'll be able to assess any distortion that develops over that time.

If they show a limited or non-existent tendency to cup or otherwise warp you can assess if it's a reasonable risk to go ahead with your plan, even if this does entail ripping boards to narrower dimensions and rejoining. If there's been significant movement it might be better to move on to using a veneered panel of some sort. Slainte.

 

[andersonec]

Is there no way you could add a couple of bracing pieces underneath which are screwed to each board with two screws but the screw holes have to be an oval shape to allow for any sideways movement of the boards.

Andy

 

[stuartpaul]

Thanks Gents,

Breadboard ends isn't something I want to do as it won't match the style of another piece we're trying match. May have to go that way if all else fails.

Bracing won't work as it's a flip top so I'm sort of stuck with the design I have. For the same reason I cut put any relief cuts in either.

Richard, - its now indoors having spent a couple of months in a dry garage and been thicknessed to a few mil oversize. Time will tell but I am expecting movement.

We'll see ........

 

[Peter Sefton]

If I was buying the timber I would try to go for boards of about 150mm wide and by preference 1/4 sawn for stability, in this case I would most likely cut the wide boards down to minimise the distortion.

With any expanding tables you are reliant on the leaves staying flat for the tables mechanism to work well, you may indeed suffer in the future if the leaves move at different rates or don't stay flat.

I would also treat the timber as advised by the others, dry down and pre machine before final sizing.

Cheers Peter