Question
Does anyone know of a formula to use to determine the amount of cup that will still plane out to standard given a certain thickness?
Forum Responses
(Sawing and Drying Forum)
From contributor D:
Don't know about a formula but I do the following: measure from a flat surface to the inside of the deepest part of the cup, say a 1/4". So I will need to joint face a 1/4" off of the bottom side of the board and since you have a uniform inverted cup on top of the board, you will need to remove 1/4" off of the top of the board. So therefore, 1/2" of material, if this example board is 1.25", 5/4, in theory it should plane out to 3/4".
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.
