Flattening warped boards

[Joe Jensen]

Here is some more info. I made a red oak kitchen table years ago that's about 36" wide. I live in Arizona and the humidity swings are not that wide here. My table changes 1/2" in width from the driest time of the year to the wettest. You can't prevent it, you just design to accommodate it. The top is mounted to the base in a way that lets it move freely. Red oak moves quite a bit. I also use Sapele and its much more stable. It moves less than half that of red oak. All woods move more tangentially than radially. Most woods move twice as much tangentially. I really recommend the book "Understanding Wood"...joe

[Lee Schierer]

Interesting. I always thought that the boards eventually reach an equilibrium if left in the same RH long enough.

The boards acclimate to the average humidity level in the space where they are stored. However, it takes a while for the center of a piece to change. If you were drying the wood from green, you would allow at least a year per inch to reach equilibrium. Hickory is particularly slow to change moisture levels. Hence the justification for removing equal amounts from each face of a board when planing.

[Scott DelPorte]

The boards acclimate to the average humidity level in the space where they are stored. However, it takes a while for the center of a piece to change. If you were drying the wood from green, you would allow at least a year per inch to reach equilibrium. Hickory is particularly slow to change moisture levels. Hence the justification for removing equal amounts from each face of a board when planing.

That has been my experience too. I mostly wanted to point out to the OP that cupping also occurs from non humidity related causes, and the way to treat it is different than when it’s due to moisture movement. It seems like there is sufficient info in the thread to help him diagnose the cause of his cupping.
I would also add that removing wood equally from both sides is a good strategy to avoid cupping, but there are times when it’s beneficial to plane unequally (when trying to book match, or to remove a defect on one side), so it’s worthwhile to learn different strategies to flatten boards.

[Joe Jensen]

It's not about equilibrium. If you do everything right and remove equal amounts of wood from both sides, and end up with a perfectly flat panel 1/2" thick and 6-8" wide, when the humidity changes the panel will cup. Since most wood finish does not hermetically seal the door, changes in humidity will cause the wood to move. Period. If I had to make a panel that wide and thin and I absolutely had to have flat sawn grain, I would make my own 1/8" thick veneer and put it on both sides of 1/4" plywood. If you tried to contain that part in dado grooves in the rails and styles the amount of cupping could easily break wood in the rail or style. I know because 40 years ago I used to try and had several failures. You really need to learn about the science behind how and why wood moves and then design around that.

https://smile.amazon.com/Understandi...rstanding+wood

[Lee Schierer]

It's not about equilibrium. If you do everything right and remove equal amounts of wood from both sides, and end up with a perfectly flat panel 1/2" thick and 6-8" wide, when the humidity changes the panel will cup. Since most wood finish does not hermetically seal the door, changes in humidity will cause the wood to move. Period. If I had to make a panel that wide and thin and I absolutely had to have flat sawn grain, I would make my own 1/8" thick veneer and put it on both sides of 1/4" plywood. If you tried to contain that part in dado grooves in the rails and styles the amount of cupping could easily break wood in the rail or style. I know because 40 years ago I used to try and had several failures. You really need to learn about the science behind how and why wood moves and then design around that.

I routinely seal both sides of every piece of wood with equal amounts of finish to mitigate moisture change differences. I respect your experience, and agree that a 1/2" panel in a 3/4" thick frame is likely to have problems, but most beginning wood workers don't have the equipment to make veneer 6-8" wide. I would opt for a thinner panel and narrower grooves. In my experience in wood working I've not had problems with solid wood panels, that were flat when milled, changing unless they were improperly stored between milling and installation in a frame. I have an entire kitchen full of 3/4" raised panels in cabinet doors that are still flat and intact that were made from solid plain sawn wood as wide as 18".