Stock Loss when Planing Cupped Lumber
From contributor S:
Another trick is to rip the board up the middle. This should reduce the cup quite a bit and may be your only choice it you don't have enough thickness.
From Gene Wengert, forum technical advisor:
Run a straight line from edge to edge on the concave side. Measure the distance between the concave surface and the line. This measurement, which is how much you need to plane off, subtracted from the overall thickness will tell you the thickness that you can get after planing, but you actually need to deduct a bit more for surface roughness.
In practice, you need to deduct even more as you will likely plane a bit more off than you really need to. In any case, when you do this calculation on a piece that is 6" wide, you will find that even a slight amount of cup is very costly in terms of final thickness. For example, 1/4" of cup on a 6" wide piece (which is hardly any) will require 1/4" minimum planer removal and maybe 1/32" for roughness.
For this reason, ripping in half or even into several strips is commonly done. Just remember that when ripping, the edges will not be at 90 to the face (except rarely), so if the ripped pieces are glued into a panel, there will be some flat panels with poor joints or unflat panels with good joints that need a lot of planing. It would be a good idea to re-machine the edges to get them at 90 degrees in most cases.
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