A board that bends across the width of its face is cupped. Cupping, which occurs mainly in plainsawn lumber, affects a board’s finished thickness.
Boards that are slightly cupped are easy to flatten. Joint them with the concave side down. This keeps both outside edges in contact with the jointer’s bed, for stability.
To flatten a severely cupped board without sacrificing its thickness, rip it in half and joint both pieces separately. Don’t rip a cupped board on your tablesaw, however as it’s too likely to cause a kickback.
Use a bandsaw, circular saw or jigsaw.
Glue the pieces back together, after jointing their mating edges.Then make a final smoothing pass on the glued-up face.
A board that bends across the length of its face is bowed.
You’ll lose length when you flatten bowed boards,because you have to remove more from the ends than the middle.
Joint with the bowed side down, and don’t press the board flat against the jointer bed.
The only way to deal with a board with a pronounced bow is to cut it into shorter lengths.The short pieces will still bow, but not as much, so they’ll be easier to flatten. If the bow is confined to one end of the board, cut it off or make repeated jointing passes on that end only.
A board that bends along its length is crooked. Straightening a crooked board reduces its width. Boards with a minor crook are common. They can be straightened by jointing and/or ripping.
(Again, don’t rip a crooked board on the tablesaw without a sled to hold it; kickback is too likely.)
Jointing or ripping won’t work on a board with a major crook—you’d end up with nothing. Instead, cut the board into short pieces.
A board with one high corner has twist. It’s best to let twisted boards be someone else’s nightmare.
They’re difficult to flatten, and even if you’re successful, the twist has a tendency to return. If you must use a twisted board, cut it as short as possible, to minimize the deflection.