Table-Leaf Warping Problem

Why are extra leaves from a tabletop warping? Experts explain causes and cures. June 28, 2005

I am looking for some ideas on how to keep extra-finished table leaves from warping. We make extra leaves with each table batch that we do to help us with matching the colors of the leaves with the tables. What we are having problems with is stopping the extra leaves from warping between batches. Does anyone have any suggestions?

Forum Responses
(Furniture Making Forum)
From contributor R:
If you have warping issues after finishing, then you also have hidden moisture issues. I'd look into the moisture content of the stock prior to machining/finishing. It sounds like it's too high. I'd also look into the humidity of the storage room where the extra leaves are kept it could possibly be too wet in there.

Finally, look at the storage and stacking system. For some situations, stickering and weighting may be the key to keeping stuff flat. In other cases, good airflow or even forced airflow around all sides of the stored items is necessary. Which system you choose depends on your location, the weather cycle, and the elements that are present in your shop.

From Gene Wengert, forum technical advisor:
I agree totally with Contributor R. Wood does not shrink, swell, warp, or move unless the moisture content changes. You have a moisture issue.

I suggest that you go to Radio Shack and get a temperature/humidity sensor ($25), and then you will know what the RH is around the leafs when they are stored. Temperature itself is not an issue.

Most storage will be around 35% RH, which is 7% MC. Check your MCs; 6.5 to 7.0% MC is the best in the wintertime. Low readings are as bad as high readings. A few pieces of 9% MC lumber can end up in many leafs, giving you the problems in every one.

Do not shoot for MCs much higher than 40% RH. That will be more humid than most homes and offices, so you will only be delaying the problem until the purchaser gets the table. Solve your MC problem (and maybe a storage problem), and you will solve your warping problem.

From contributor E:
About a month ago I was experiencing a similar problem. I build small living room tables in Montana where humidity is generally not a concern with properly dried wood. I noticed that my tops were coming out of the clamps after glue up with a slight cup. I checked all the tools for square and the wood for moisture content, which was between 6-7%.

I ran a series of boards through each step and checked for square and flatness. It was after I ran the boards through the planer that I noticed, regardless of plain sawn or quartered, the boards (4 to 6 inches wide) would cup ever so slightly. I knew the blades were getting dull, so I changed them and that corrected the problem. I would not have believed it if I had not seen it. I'm not sure why it occurred, maybe someone else might know?

From Gene Wengert, forum technical advisor:
To contributor E: If wood warps immediately upon machining, it is because of drying stresses (commonly called casehardening). It can also rarely be due to processing wood with a moisture gradient. Such a gradient will disappear with time, so the problem will go away by itself.

The greatest problem with moisture is always in a dry environment. Wood will typically be too wet for the air. This is why we see moisture related problems more often in the wintertime than summertime. If you check the MC after the problem shows up, the MC will always be correct. This is because if the MC is too wet, the wood dries a little and warps, and when you check the MC, the high MC has already left.