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casehardened chair seat6/2
Not sure if this is the correct forum, but I'll try to be brief.
I just carved a Windsor chair seat out of a 20" wide 8/4 walnut board. The board turned out to be casehardened and cupped towards the excavated side of the seat. Tried spraying the concave side of the seat with water, waiting till it flattened out and putting it in my little light-bulb powered kiln at 120 degrees, with minimal success.
I know that kiln operators can relieve casehardening in their kilns some way, but not sure how that works or if I can reproduce that in my little low-tech kiln.
Any help would be appreciated,
It's always been my understanding that case hardening is caused by drying wood too fast, and kiln drying will do that if it's not well air dried first. Maybe steaming or soaking the seat would reverse it, but I've never tried it. I'd stick with air dried material for slab seats.
Case hardening is a stress issue, so you can try to relieve the stress. The side that you machined has had much of the stress relieved, as you removed the casehardened wood. So, it is the bottom side that has the stress that is causing the warp. What we want to do is have this side try to expand rapidly with the addition of water. It cannot do so, however, and so it develops compression set (technical term) that the offsets the tension set that developed early when the wood was drying fast and could not shrink enough. After wetting, dry the water slowly. Ok?
Gene, When you said, "It cannot do so, however, and so it develops compression set (technical term) that the offsets the tension set" I was wondering if he should have the seat under pressure from clamps from the sides, as well as pressing the cup out? I've never tried this, so don't know.
However, I would like to ask how Elia came to the conclusion that the wood was case hardened, and what the MC is, and how it had been stored?
My first attempt to flatten, would be to place it out in the Sun, with the convex side up for an hour or two while preventing any airflow to the other, if the wood possibly had gained moisture after it had been dried.
Casehardening is a stress that results in immediate warp when machining, while moisture content issues cause warp over time.
To get rid of the stress or to offset it, we need to create the opposite type of stress and a lot of it. Note that Casehardening stress is only in the outer portions of the wood, so trying to bend it flat will not work for this reason as well. Further, once dry, the wood is so strong that takes twice as much bending to correct the stress. It is much easier to just bend or try to bend the outer shell with water instead of the entire piece. Ok?
Great questions and answers!
First, about the board. It was bought a month ago, prior to which it had been stored in a shed, then upstairs in my shop.
I excavated the seat and it immediately cupped, the excavated side of the board going concave. I tried the sunlight trick, without any permanent effect. I then made a fork test (correct term?) on a scrap and the forks bent in and have stayed that way for a week now.
After I posted my first query here, I found this article: http://www.woodweb.com/knowledge_base/Conditioning_Relieving_those_stresses.html
Left it out of the kiln two days and just took the battans off a half hour ago. Seems to have worked. Need to do it again to one end where the screws popped loose, but assuming it doesn't move, I think it is a success.
Question: What effect does the heat have on the process? I know heat plasticizes lignin, but does that help with these cross-grain issues?
A somewhat unrelated question: Are there any sources for info on what happens to wood in a kiln that could make it seem brash and hard to work with hand tools?
Thanks for your help!
The wood cells that are casehardened are only the outer layers, maybe outer 20% of the lumber. So, if you machine one side, you remove the Casehardening on that side and now the stresses are unbalanced...that is, the bottom in machined side still has all the casehardening stress. That is why working on the machined side does not help as there are no big stresses on that side. We can relieve the stress on the back side by planing or routing that side, or like is done with flooring, cut several grooves in the backside, removing some stresses.
The forces are very large...large enough to bend the piece, as you noted. In fact, if you cut the prong test, but before cutting the prongs measured the length, you would see the prongs, after cutting will be 1/8 to 1/4" longer and the center prongs will be shorter. It takes a lot of force to stretch wood that much.
Now what happens when you wet the un machined side with liquid water, the surface cells want to expand quickly, but they cannot due to the dry, unmoving core. So the outer layer develops compression set or reverse casehardening. This offsets the tension set or regular casehardening.
Heat makes things go faster.
Brash wood (that is indeed its name, but some people say the wood is brittle) can be from growth factors...mainly tension wood in hardwoods, but also very slow growth for woods like oak...and from drying factors...over drying (under 6.0% MC)and from high temperature drying (over160 F).
Thanks! That makes sense and confirms what I gleaned from that article. This second wetting of the seat, I think I'll try clamping it to a bench before wetting it and see what happens without heat.
Do you have a recommendation for a book that explains wood behavior related to drying and bending wood? I have read Hoadley's 'Understanding Wood' a number of times, but that only goes so far.
Thanks again for your help,
The early chapters of DRYING HARDWOOD LUMBER has what you need. It is in the archives here.
I left out "before wetting" in my question above, "I was wondering if he should have the seat under pressure from clamps from the sides, as well as pressing the cup out? I've never tried this, so don't know. " making it sound as if I was suggesting just clamping it dry.
What I was wondering, was that since there is already compression on the back surface, which wetting would probably increase, wouldn't there be a chance of internal splitting, since tension perpendicular to the grain is normally the weakest property in most woods?
You have it perfect...the bottom is in compression. Water will add to that compression. But here is the key...we want to have enough compression in the bottom to exceed the proportional limit so that we get "compression set" which will offset the tension set (which we commonly call casehardening).
Your concern about getting some damage is also correct. If we wet the bottom fairly deeply, then there would be a whole lot of the wood that would be expanding or trying to expand. (Isaac Newton correctly stated that for every force there is an equal but opposite force. So, with lots of compression, there has to be lots of tension.) However, casehardening (or tension set) is only in the outer layer and primarily in the outermost layers...let's say the outer 1/8" to 3/16". That is why planing the top causes so much warp, as you remove a lot of the set from one side. SO...this means that when wetting, whether with steam or water, we only want to wet the outer region and do it quickly. Long wetting (like with a very wet cloth for hours or long, slow steaming for over 12 hours) will not remove stress very well, but will increase the MC above the desired level.
Does this all make sense?
Interesting! I read somewhere that roughly 20% of a case-hardened board was compression-set and somehow I assumed that was from both sides, for a total of 40%.
I bet I've been wetting it way too long!
Any guess for how long I should put a soaking wet sheet on it in a 140F kiln? An hour or two?
P.S. I about had the cup removed, but I put it back in the kiln to thoroughly dry it and all the cup returned. Obviously I shouldn't do that. Any idea what happened?
A wet rag technique is not well documented as a process, so it is just a guess but a soaking wet rag might be needed for only 15 to 30 minutes. It depends on how wet, temperature, wood density, amount of casehardening, etc.
Pieces cup (or flatten) for two reasons... casehardening stress and normal shrinkage or swelling. Normal shrinkage caused the cup in drying.
The cup I got when drying in the kiln (and the cup I got from casehardening) made the side of the seat closer to the pith go concave, which is counter to my understanding of drying cup.
Elia, in addition to being a brilliant chairmaker you are clearly becoming well versed in wood drying.
The cupping towards the pith is something I have never seen. Like you I use local wood, mainly air dried and occasionally I dry some in my attic or a hot shed.
Let us know what else you learn - do you know whether or not that particular tree leaned when it was growing? There has to be something to account for how it is behaving now.