wood cupping problem

[Lumberman]

Putting my new Planer/thicknesser through it's trials I was at first pleased with the results I surfaced planed one side then the adjacent edge, then thicknessed the remaining side, perfect! until I returned to the piece a few days later only to find the wood had a cup in it, , can anyone tell me what is wrong? The piece of wood is Ash, could it be that there was still moisture in the timber. I can re dimension it but of course the piece is going to be smaller than I intended, can any seasoned woodworker advise how can this be avoided.
Would be grateful for any advice.
Harry.

The cup is only slight but enough to ruin a project

 

[Steve Maskery]

Yes, it's moisture and the stresses in timber as a result of moisture transfer.
If the timber is now in a drier (or wetter) place than it was before you machined it, then it will move. Timber growth rings shink as they lose water and dry out and they expand as they take on water. This trnasfer happens all the time.
If the timber was stored at the same Relative Humidity before it was planed as it has been since, then the movement is a result of stresses being released as the wood is cut.
The remedy is to make sure that your timber is stored in the same sort of environment as that in which it will be used, and to take off an equal amount of timber from each side of the board as you plane it up. I realise that's not much help right now, but next time, eh?
S