Advice on flattening a cupped slab

[tony_A]

A mate of mine is making a bench top from a natural edge blackwood slab and he is having problems with it cupping. The slab is about 30mm thick and 600 wide, full width of the tree but cut a bit off center so no heart wood, air dried for a couple of years he thought. Timber has been dressed and put on the bench and immediately started cupping. He clamped it down to try to flatten it and has been gradually tightening the clamps which is working to an extent. The bench is in a shed, so not a particularly hot or dry environment. Kiln drying isn't an option.
He was wondering whether running a series of cuts the length of the under side of the slab say 50mm spacing and half thickness would relieve the stress. Any thoughts or suggestions.

[AlexS]

Firstly, if you leave a board of any size flat on the bench, it will probably cup, as one side is exposed to changing humidity, while the other isn't. It should be left on stickers to allow air circulation around it when not working on it.
I'd cut the slab into 5 boards and flip the 2nd and 4th end for end and glue them up again. That way, any cupping will be minimised over the width.

[Midnight Man]

Is there any chance that this slab has been finished (i.e. any oil/varnish/poly etc) put on the top but NOT the bottom? If so, even if the design allows for it, moisture absorption from the air will be different on each surface and will potentially contribute to your cupping.

[auscab]

If it’s bone dry and no chance of more movement if kept in suitable environment I do one cut down the middle and re joint it .
Cut , buzz and shoot the joint till perfect with my no 7 razor sharp .
The joint becomes invisible . I sometimes have used biscuits on re glue as well . Your still going to have a small cup each side to flatten though .
All bark to bark slabs do this and the only ones I’ve seen stay straight after drying are the ones straight through the middle . 1/4 sawn each side and heart in middle . The heart usually shows splits even though it’s usually 50 mm thick boards . So I may remove that and re joint so there’s no problems later .
Id guess some types of wood are better than others but I’d think they all cup to some extent. I’ve mostly had Oak and Elm bark to bark to deal with . Some Blackwood as well .

Rob

[tony_A]

Thanks for the advice. Ill pass the comments onto my mate.

MM: No finish applied at this stage, will be applied to both top and bottom surfaces as you suggest.

Alex: he is hoping to avoid cutting the slab, well see how he goes.

Rob, Could pretty much guarantee its not bone dry, likely part of his problem. The tree came off a farm and with high rainfall and good soils so it would have been fast grown, that generally doesn't help with timber stability either.