Bowing in Pine Staves After Planing
Here's a good explanation of why lumber may move after being planed — and why that is not easy to correct. January 20, 2010
Question
I just started supplying a couple local lumberyards with wood screen doors. I had 5/4" pine planed down from 1-1/8" to 1" and my dad, who runs the shop where I had it planed down, told me he spaced it and took all the material off one side of the board, so now half my stiles are bowed. I am contemplating cutting up these stiles for rails and ordering in more stile material but I was wondering if anybody had any ideas on how to salvage these. I've got two ideas. One would be to wrap up stiles in bundles of two, long face to long face, and stick them out in the barn. It's been raining a lot and I thought maybe the humidity would straighten them out. The other idea is to do the opposite. Wrap up the stiles in bundles of two, short face to short face, and put them in my parent's sauna for an hour. Just so there is no confusion, when I say long face or short face, I am referring to the bow in the board. One face is effectively shorter, causing the bow.
Forum Responses
(Architectural Woodworking Forum)
From Professor Gene Wengert, forum technical advisor:
It is likely that what you are seeing is called longitudinal stress and it developed during drying. It is seen in softwoods, especially from smaller trees. It can be removed in drying. If not removed in drying, then when a piece of wood is machined unevenly (one side more than the other), this machining will unbalance the stress and cause immediate warp. (A second option is uneven moisture content, but this will result in warp over time and not be immediate.) Immediate warp is also called longitudinal casehardening. If not removed in drying, it cannot be removed later, except by machining the opposite face (long face) enough to have the stresses balanced again.
From the original questioner:
Thanks for the info, I actually didn't notice the bow until about three days after it was planed. Does that make a difference in your diagnosis?
From Professor Gene Wengert, forum technical advisor:
If it did indeed occur over a few days, then it is related to a moisture gradient. By planing or molding one side, that exposed wetter wood on that side, which then dried and shrank a small amount and caused bow. Unfortunately, this is also next to impossible to remove at this point. Rewetting the one side will cause swelling and straighten (partial or total or even reverse... hard to control), but then trying to hold it straight as it redries is hard to do, as when the force holding it straight is released, often the piece springs back enough to still be a problem. It is also possible that there is longitudinal casehardening or stress present at the same time. Such stress is seldom removed in drying for softwood species, as it is not expected that most softwoods will be machined heavily and unevenly.
From the original questioner:
I was pretty sure it was moisture content by the way it happened. It's funny - I used to work at my Dad's shop about ten years ago and I did a lot of planing. Both he and the owner were always very particular about having me plane even amounts off both sides of the material, so it surprised me when he spaced it. Anyway, I tried sticking them out in the humidity and that straightened them out, but like you say, I'm not sure how it will end up by the time they dry again. Fortunately, or unfortunately depending how you look at it, I don't have much work right now, so I can keep an eye on them as they dry and rewrap them if I notice one face drying faster than the other face.
From contributor D:
I've built thousands of doors over the years and I'm a believer in engineering the stiles. If that is too time consuming, a less elegant solution would be to glue your two short sides together (the ends touch before the middle). This sandwich can be de-stressed a bit by taking it down a little at a time, allowing it to acclimate at 1 1/2", 1 1/4", and so on. This cuts your yield in half, but two pieces of pine glued this way are about twice as stable as using one solid piece, which will forever be in the elements as a screen door.
From contributor O:
This is your product. You may not want to risk losing that customer. If that's the case, it may be wise to take an inventory of the situation, cut your potential losses, and start over. This time, take charge. Select the right material, joint and plane in a concerted manner - i.e. front-side, back-side, well balanced - and acclimate slowly. You're building exterior screen doors. The alternative is to ship butchered doors. I say "butchered" because you're not happy with them.
From the original questioner:
I had considered doing engineered stiles. I'm actually trying to fill a gap left by a door distributor that went out of business a couple years ago. They were the only source in the area for stock wood screen doors, which they offered out of solid pine with luan panels. A lot of people are looking for a comparable door as far as price goes. That's why I decided not to do engineered stiles, since what the lumberyards want is a screen door for around $175 (their cost). I hate the idea of seeing my doors in the area if they're going to warp, though, so I think I will reconsider. Thanks for the food for thought.
From contributor O:
What would you use for the core, face and edges, adhesive, and machining?
From the original questioner:
I'm not sure what I would use for the stock doors. It seems kind of absurd to use pine as face material. Before I ran into this problem this time, I was planning on offering cheaper doors as stock doors out of solid pine and doing stave-core or Timberstrand-core stiles for any custom doors. As far as adhesive, I was thinking of trying Weldwood. I also ran through the idea of just offering solid pine, quarter sawn white oak or mahogany since all those woods are fairly stable. As you mentioned, if I am careful in selection and machining, I can avoid a lot of warp issues. I also liked the suggestion of face-gluing two boards together. I could use regular 1X if I did that so as to avoid doubling my material cost. This has helped me really think through my process more thoroughly - thanks for the responses. Keep them coming if you have any more suggestions.
From contributor D:
We don't use Timberstrand anymore. We tried it and had problems. We don't finish our doors, so have no control after the contractor gets them. The differing expansion rates of the face veneer and the Timberstrand core cracked the veneer if the bores for the hardware weren't sealed throughout. Even after going back to staved cores, which we had no problems with, I still swab the inside of every hardware mortise with some sealer.
