Flattening warped boards

[Kyle Kaldor]

I'm making some drawer fronts with some inset hickory panels. After I planed them to 1/2" thick, they warped and will no longer sit flat in the drawer front frame. The only idea I can come up with is to shim under the cupped portion of the board (to keep the planer from flattening them) and run them through the planer again. Does anyone else have any other ideas?
Thanks in advance,
Kyle

[Lee Schierer]

First off, your board is cupped. Your piece may flatten out on its own accord in a week or so. But, place it on stickers with some weight also on stickers to encourage it to flatten while it adjusts its moisture level.

A possible solution would be to wet the concave side and then clamp it flat while it dries. That may of may not work.

When planing boards you want to remove equal amounts of material off of each face whenever you can to reduce the chance of cupping. Removing material this way keeps the moisture levels of the exposed wood approximately the same. You also want to stack your planed wood with stickers between layers and any flat surface so that the newly exposed surfaces can adjust to the room evenly.

[Kyle Kaldor]

Lee, thanks for the advice, much appreciated. It has been a few weeks since I planed the boards, so I don't think they will flatten out on their own, but it is possible since they are in my very dry MN shop and they will be coming into the house where there is more humidity. I think I'll try to wet one side and then clamp them flat as you mentioned. And, you're right, the proper terminology is "cupped", my brain is a little slow today apparently.

[daryl moses]

Worse comes to worse rip it down the middle, turn one piece upside down and re glue it back together. Be sure to sticker after it comes out of the clamps.

[Phil Mueller]

Your thought on shimming and planing will work...I’d probably go after it with hand planes and finish on a thickness planer, but the issue is that you are going to end up with a thinner panel. I can’t tell exactly from the pictures, but looks to be about an 1/8”+ gap...meaning you may have to reduce the thickness up to 1/4” or more...leaving you maybe a 1/4” panel. Will that still work?

[Scott DelPorte]

I recently had some boards develop a slight cup after removing thickness primarily from one side. The boards had been sitting in my shop for 5 years so the moisture was uniform, but must have had some internal stress in it, perhaps from improper kiln drying. I removed some thickness from the other side and got the cup to go away. The boards have stayed flat.

[Alan Lightstone]

I've often wondering why this doesn't work. Why not thoroughly wet the board, and then put it in a steam box for a few hours. Then remove the board and quickly clamp it flat with some long clamps that clamp the top and bottom of the board (a number of companies make these. Woodpeckers are the ones I have). Keep the boards clamped for a few days or a week while they dry (the clamps act as stickers themselves). After this, will the boards re-cup? I mean some woods can clearly be steam bent to dramatic curves (though there is some level of spring-back).

I've never heard of this discussed, so I'm assuming it doesn't work. But why wouldn't this solve Kyle's cupping issue?

[Lee Schierer]

I've often wondering why this doesn't work. Why not thoroughly wet the board, and then put it in a steam box for a few hours. Then remove the board and quickly clamp it flat with some long clamps that clamp the top and bottom of the board (a number of companies make these. Woodpeckers are the ones I have). Keep the boards clamped for a few days or a week while they dry (the clamps act as stickers themselves). After this, will the boards re-cup? I mean some woods can clearly be steam bent to dramatic curves (though there is some level of spring-back).

I've never heard of this discussed, so I'm assuming it doesn't work. But why wouldn't this solve Kyle's cupping issue?

It might work, but you need a steaming chamber and one that is large enough to hold the piece you need to flatten.

[Joe Jensen]

I've often wondering why this doesn't work. Why not thoroughly wet the board, and then put it in a steam box for a few hours. Then remove the board and quickly clamp it flat with some long clamps that clamp the top and bottom of the board (a number of companies make these. Woodpeckers are the ones I have). Keep the boards clamped for a few days or a week while they dry (the clamps act as stickers themselves). After this, will the boards re-cup? I mean some woods can clearly be steam bent to dramatic curves (though there is some level of spring-back).

I've never heard of this discussed, so I'm assuming it doesn't work. But why wouldn't this solve Kyle's cupping issue?

Wood cells have a shape that is defined in part by the moisture level in the wood. More or less moisture and they move. Because the part is cupped, I'm 100% sure that panel moved due to a humidity change in the wood. Might be the shop is dryer than where the wood was store before. Might be that the wood was not fully dry and ready to use. Wetting and clamping might flatten while in the clamps, but once the clamps are removed and the moisture content of the board normalizes it will move again.

The problem here is trying to use a thin and wide board that is flat sawn. Flat sawn wood cups way more than quarter sawn. The book "Understanding Wood" is fantastic and will help you understand how wood moves and why. There are also tables that show the radial and tangential movement for every species.

[Brian Tymchak]

I recently had some boards develop a slight cup after removing thickness primarily from one side. The boards had been sitting in my shop for 5 years so the moisture was uniform, but must have had some internal stress in it, perhaps from improper kiln drying. I removed some thickness from the other side and got the cup to go away. The boards have stayed flat.

Even with the wood sitting in your shop for a long time, when you expose fresh faces by planing, those faces are always more moist than the unplaned faces. Moisture levels are not consistent through the board, with the surface being drier than the core. The issue most likely is that by planing mostly one side, you ended up with a moisture imbalance from side to side, causing the cup as the new face dried out. As someone else mentioned, you should always try to remove material equally from side to side.

[Andrew Seemann]

Hi Kyle,

With the deep freeze we have had in MN since Christmas, the humidity levels in your shop probably dropped from low to almost nothing. The board will go back to flat when they rise again. The main problem is that when summer (hopefully) comes, the humidity will be back to 70 - 90 percent indoors and the board may actually have enough moisture to cup in the opposite direction. Minnesota has the most climatically extreme weather in the country between summer and winter for both temperature and humidity (its actually one of the worst in the world that is actually inhabited to any extent), so any problems with wood movement will be worst case scenario here.

Are you attaching the front to a drawer box? If so, the cupping isn't that bad, and you may be able to draw the front flat. You will need to screw it top, middle, and bottom (you might want to make the top and bottom holes elongated if it is a plywood box). If it is an integral front, the sides will probably hold it flat enough, although you'll may have trouble forcing it into position. If possible, use a piece of wood that is less flat sawn, even diagonal grain will be better. For a tall drawer front, you may need to break down and make it out of multiple pieces.

Another thing to consider is the expansion across the grain. If it is going into a space fixed by a frame that can't expand (or minimal expansion), you will need to consider how much the front will expand in summer. As a rough rule of thumb per foot, figure 1/4 inch for quarter sawn, 1/2 inch for flat sawn, and somewhere in between for diagonal or rift sawn. There are charts that give more exact numbers by species. Though a pretty wood, hickory, unfortunately, moves a little more than most other common woods.

Actually the cupping you show in the pictures is pretty common here for any flat sawn board that was planed and let sit for a week or two. In this climate, it is something that you just need to take into account in your designs.

[Scott DelPorte]

Even with the wood sitting in your shop for a long time, when you expose fresh faces by planing, those faces are always more moist than the unplaned faces. Moisture levels are not consistent through the board, with the surface being drier than the core. The issue most likely is that by planing mostly one side, you ended up with a moisture imbalance from side to side, causing the cup as the new face dried out. As someone else mentioned, you should always try to remove material equally from side to side.
Interesting. I always thought that the boards eventually reach an equilibrium if left in the same RH long enough. In this case my shop never goes below 40pct or above 45pct. I am still thinking it had more to do with internal stress in the boards due to improper kiln drying. Two things led me to think this. The first was that the bow occurred toward the surface that was planed. This meant that that surface was in compression. If the surface were drier than the core, it would shrink and it would be in tension, and the board would cup away from the planed surface as you relieve that tension. If they dried it wrong and case hardened the wood (as I think they did), it acts this way because the center of the board is in tension and the outside is in compression. I also left the boards in their freshly planed and slightly cupped condition for several weeks didn’t result in the boards changing shape again, as you would expect if there was extra moisture on the new surface.

[Andrew Seemann]

Hi Scott,

Improper kiln drying can cause warpage, and wood will indeed reach equilibrium if left in the same relative humidity long enough. The problem in a climate like Minnesota, is that the relative humidity doesn't stay tend to stay the same long enough for a board to reach equilibrium. Interior humidity can range from 10 percent in the winter to over 80 in the summer, and can drastically swing over a few days in any season. Here, the inside and outside of a board are almost always at different moisture contents, unless the board is very thin. The cupping shown in the picture is very typical of humidity change. It would have likely moved nearly that much even if it had not been planed.

Planing will change the internal tensions of the wood in that the outside will be closer to the moisture content of the air, and the inside will be closer to what it was further back in time. Removing that outside layer tends to put the wood more out of sync with the current RH, and as it starts to equalize, it moves again.

In general, you try to use the parts as soon as you can after cutting them. If that is not possible, you have to take that expected warpage into account in your design.

[Brian Holcombe]

Ok the panel is cupped, but more I’m more curious to the assembly. Rather than inset them it seems to make sense that they should be made as a floating panel set into a groove.

I like to make panels 3/8” or 5/16”.

[Derek Cohen]

I'm making some drawer fronts with some inset hickory panels. After I planed them to 1/2" thick, they warped and will no longer sit flat in the drawer front frame. The only idea I can come up with is to shim under the cupped portion of the board (to keep the planer from flattening them) and run them through the planer again. Does anyone else have any other ideas?
Thanks in advance,
Kyle

Kyle, if you use the boards in a dovetailed set up, where they are captured by the dovetails, then the joinery will straighten the boards and keep them flat. As they dry and stabilise, the internal tensions will slow.

Regards from Perth

Derek

[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/Understanding-Wood-Craftsmans-Guide-Technology/dp/1561583588/ref=sr_1_1?ie=UTF8&qid=1514938953&sr=8-1&keywords=understanding+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".