Replies

1. 
   RalphBarker | Sep 16, 2012 11:02am | #1

   Irresistable forces

   Remember the old adage about the irresistable force that met an immovable object?  ;-)

   The approach to resolving the problem may depend on how the slab became cupped. The cupping could be a result of internal stresses in the wood, or it might be the result of improper drying - e.g. it laying flat on some other surface, causing unequal drying.

   You might try moistening (not "wetting")  the concave side of the cup and then stand the slab on edge to see if the cup is reduced. I've also heard of people laying a board on the lawn, concave side down, in the sun for a few hours. If equalizing the moisture content between the two surfaces doesn't remove the cupping, it may be internal stresses causing the problem. In that case, flattening with a plane may, or may not solve the problem. Over time, wood will do whatever it wants to do.

2. 
   howtoworkwood | Sep 24, 2012 11:34pm | #2

   It is very unlikely that you will be able to pull out a 1/2" cup over a 25" span without something giving way.

   The board is cupped because at some point the moisture left the board unevenly and uneven stresses were built up. It is likely that the board has cupped towards the pith due to the different shrinkage rates between tangential and radial wood.

   Is it one piece? If it is then you won't be able to do much to get that cup out as even if you wet it again, once it dries the different shrinkage rates will remain the same and it will re-cup!

   Is it at a stable EMC and can you re-plane it?