Question about warping of walnut/wood in general - Woodworking Talk - Woodworkers Forum
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post #1 of 29 Old 01-17-2012, 01:21 AM Thread Starter
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Question about warping of walnut/wood in general

About a month ago, I made the small box in the pics below out of some scraps or mahogany, walnut and padauk. The box is about 8" x 5 7/8", and the top is 7/16" thick. The top is made of 3 jointed pieces of walnut. About 24 hours after jointing and gluing up the top, which was about 1/2" thick at that point, I planed it to the final 7/16 & while is was still a rectangle, I made a ~1/16" deep dado for the paduak insert in the middle. I then cut it into the final triangle shape, and glued in the paduak. This all happened in about 3 days. So far so good. After finishing with water based poly, the top slowly began to warp at the outer points of the triangle near the "handle", as shown in the final photo. The top sat nice and flat on the box right after finishing. The gap at the outer edge as shown in the pic is about 1/16". The walnut scrap I used had been in my dry shop for about 3 years, though I did have to resaw it to the 1/2" thickness used in the glue up. I have used the same wood for several tables over the last 3 years with no problems so far. What would have caused the top to warp this way, and what might I have done to prevent this? Should I have left the jointed glue up sit for some days prior to planing to the final thickness? Or should I have left the resawed pieces sit even before jointing/gluing up? Should a waiting period always be allowed prior to final jointing/gluing/planing to allow and panel to "settle"? I'm planning to remove the cleats on the underside of the top, and try jointing it flat, than fitting a smaller triangle to the flattened underside to replace the cleats. I hope all the above is clear! Any suggestions appreciated.

It may not be clear in the pics, but the top glue up is 3 pieces, 1 about 1 1/16" wide in the center where the dado is, and one on each side about 2 3/8" wide at their widest points.

Dan

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Last edited by Dan9876; 01-17-2012 at 02:09 AM.
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post #2 of 29 Old 01-29-2012, 05:59 PM
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My guess would be that the material was under stress because it came from a place on the tree where there was a crotch or other grain change. When you resawed it the apossing stress was relieved along with cut off piece. I rarly use water based materials with figured walnut. For stability oil rubbed on all sides is best on figured material.
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post #3 of 29 Old 01-30-2012, 12:09 AM
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The resawing was the culprit. Anytime you resaw you need to sticker the wood for a week or so and then flatten and join. The moisture content on the inside of the board was different than the outside. A small kiln could speed this process and is what high end professional shops use.
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post #4 of 29 Old 02-03-2012, 01:25 PM
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Here is the old technical lesson on panel glueing we were taught in woodshop back in the old days. Look at the end grain of each board to be glued into the panel. Most of the boards wil show a curve to the end grain. Every other board has the curve going the same way...so curve up, curve down, curve up, curve down...across the panel. This process evens out the stresses as the boards expand and contract with humidity changes. If you have a series of boards glued with the curve going the same direction, they will cup or "warp". Not much help for you now, but look at the end grain and see if it is going all one direction. Your wood can be bone dry and have dried for 100 years, but it can still cup if you don't glue it up properly. It is due to humidity differences to each surface. The amount of finish you apply to each side affects this a lot. I see it all the time on trim for houses like baseboard and crown molding. The painters varnish the front and leave the back unfinished. It lays on the jobsite until the finish carpenter comes and by then the back has swelled and none of the pieces are flat anymore...talk about a nightmare!
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post #5 of 29 Old 02-04-2012, 05:51 PM
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Over the years the old rule of alternating grain has been found to be largely untrue. Once your lumber has dried to 6% then allowed to rest in a space, as long as your joints are perfectly square your panel shouldn't cup or twist. If it does the alternating method probably wouldn't have stopped it.
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post #6 of 29 Old 02-04-2012, 06:20 PM
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Quote:
Originally Posted by TylerJones
Over the years the old rule of alternating grain has been found to be largely untrue. Once your lumber has dried to 6% then allowed to rest in a space, as long as your joints are perfectly square your panel shouldn't cup or twist. If it does the alternating method probably wouldn't have stopped it.
+1 agree. That's what I've read. Built my dining table walnut 3/4 x11"x 7' I didn't alternate the end grain.chose the best side, Built it about 3 years ago & no cupping.
Maybe I got lucky.

When it's rustic......it's rustic
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post #7 of 29 Old 02-04-2012, 07:11 PM
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I think the culprit was probably the water based poly. Warpage is usually caused by the moisture content of the wood being greater on one side than the other. If you got a little more on one side it would cause that side of the wood to swell making it warp. You can probably straighten it out by steaming the cup side but do it a little at a time or you might reverse the warp.
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post #8 of 29 Old 02-05-2012, 10:53 AM
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Quote:
Once your lumber has dried to 6% then allowed to rest in a space
Here in the Pacific NW, our wood never even gets close to 6%
BTW, nice looking box.

That bowl was perfect right up until that last cut...
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post #9 of 29 Old 02-05-2012, 01:04 PM
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You guys don't have kilns there? Its not about keeping the wood at 6-8% its about getting it there once to get the free water out from between the cells. Once you do that the wood permenantly gains hardness and stability. After that just let it rest in your shop for a while and sticker it after rough machining.
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post #10 of 29 Old 02-06-2012, 10:56 AM
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Just because wood has been kiln dried to 6 - 8% doesn't mean it will stay there. If that were true we wouldn't have all these warping problems. Unfinished wood is like a sponge and takes on humidity after a while and can end up back to 11 - 18% which is about where air dried wood is at. Once a good finish is put on, making sure the end grain is really saturated, this process of humidity transfer is greatly reduced, but not totally eliminated. Proper flip flopping of grain, as well as proper width of boards in a glued up panel does make a difference in my world, at least. If the other guys get by without doing it, lucky them.
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post #11 of 29 Old 02-06-2012, 08:47 PM
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I'm not talking about the wood staying at that MC. Wood changes once it reaches that MC, it gains hardness and stability. These properties remain after the wood has taken in moisture. This is because the distribution of water within the wood fibers changes permenantly after reaching a certain MC.
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post #12 of 29 Old 02-06-2012, 08:56 PM
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And as far as panel stability goes I don't see luck having anything to do with it. Alternating end grain doesn't even enter the equation when composing a panel for me. Its appearance, joint quality and hugely important is stock prep. I also don't pay attention to keeping boards a proper size for the most part. I very rarely make a tabletop with more than four boards. I worked on one a few months ago with Andy Rae who literally wrote the book on wood andthe two center pieces of the table were right around 18" each. That piece was being shipped from Asheville to Lake Tahoe and I can tell you, we weren't overly worried about warpage. Many of these so called rules came about because people weren't looking at what was happening to the wood on a molecular level. When you do that it expands your options endlessly.
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post #13 of 29 Old 02-07-2012, 12:55 PM
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I would like to know what species of wood that table was made from and whether those wide boards were plain sawed or quarter sawed. All these things do matter. We are trying to solve a warping (cupping) problem here, not brag about how our projects haven't warped. I agree that warping doesn't happen often and that it can happen on a flip-flopped grained panel if not treated and handled properly, and many factors go into why a panel warps. I also agree that appearance takes priority over grain pattern. Here, we are trying to investigate which factors caused the warping problem so we need to go into the physics of water absorbtion in the wood. Most likely, not as many coats of finish were put on the back side which allows more moisture to be absorbed on that side, which swelled the cells and expanded that side or the opposite; the top side lost moisture and shrank, and it had nothing to do with grain orientation. That being said, if the panel has several boards which were plain sawed, and the bows of the end grain are going the same way, the chance of cupping increases considerably. You can't deny that boards cup and wider boards cup more. Plain sawed boards cup more than quarter sawed boards. Every species of wood differs in the tendancy to cup. Saying you were "lucky" was a loose phrase. When we are staring at a panel that has cupped and we are cussing because all our work has been wrecked I won't say we are unlucky, either. I just don't like to see anybody in that position. I've been there.
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post #14 of 29 Old 02-07-2012, 03:22 PM Thread Starter
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Thanks for the great replies. I finally did get around to reworking the top, as shown in the pic below. I removed the cleats (which were glued & nailed), and jointed the top flat, then fitted a triangular piece of walnut to replace the cleats. I think that should work much better. The triangle was more difficult to size as exactly for the fit I wanted then attaching the cleats was, but the end result fits the box quite well, and I think looks much nicer, too.

When attaching the triangle to the bottom of the top, I ran the grain in the same direction as that of the top itself. As I write this, I'm wondering if stability would have been enhanced if I had run the grain of the insert 90 degrees to that of the top, plywood fashion?

What is meant by "sticker" the wood?

Thanks again,

Dan

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post #15 of 29 Old 02-07-2012, 09:15 PM
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To sticker the wood is placing sticks of wood between stacks of lumber to let air flow between the wood allowing it to dry evenly. Like 1x1 pieces of wood between them.
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post #16 of 29 Old 02-12-2012, 09:28 PM
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To review, the board was resawed then finished with a water based finish. It warped within a few days.
Yes the resawing may have contributed. It is wise to let resawed wood rest and equilibrate for a period of time depending on its thickness. 1/2 inch wood would probably only take a few days. There can be tension in the wood from its growth or the interior may not have been as dry as the exterior.
The water based finish may have contributed.
It appeared from the picture that the wood was tangentially cut with a fairly tight radius to the growth rings. This is the tell that the wood will change in shape when it's moisture content changes. The curves will flatten as they dry. Even coming from a normal basement in a centrally heated home to the upper floor will change the humidity enough to change the moisture content a bit. This property NEVER goes away. We manage it with structure that hold the slight cupping tension. Your original design has no such support. Breadboard or frame and panel construction are examples of such stabilising. Plywood is another way wood panels are kept flat. Veneering saves valuable wood but also prevents warping if the substrate is well chosen.
If there is no external stabiliser then using radial cut wood is vital.
It was written ( how do you get the quotes in the pretty blue boxes?)
"the wood staying at that MC. Wood changes once it reaches that MC, it gains hardness and stability. These properties remain after the wood has taken in moisture. This is because the distribution of water within the wood fibers changes permenantly after reaching a certain MC."
This in not accurate. There is no substantial permanent change.
Wood retains its movement forever. You can use centuries old tangentially cut wood as a humidity monitor as its movement is directly related to the surrounding humidity if it has time to equilibrate. It will cycle with the seasons. Drawers and doors that bind or stick in summer when humidity is high will be OK again next winter. Finishes can slow but none stop the movement. Check the graph in Bruce Hoadley's book Understanding wood for interest and confirmation.
Another rant but the topic keeps coming up on one way or another. Especially with the current use of wood from small trees that have the small radius growth rings.
I hope your new box behaves.
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post #17 of 29 Old 02-13-2012, 04:33 PM
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MidlandBob, I disagree with your statement that no permanent change occurs. Here is why: Within wood lies two types of water. There is free water and bound water. The free water is the first to go when drying and comes from the capillaries, the bound water is bound to the wood by a hydrogen bond. This is caused by free negatively charged Hydroxyl in the cellulose and lignin walls. Water is attracted and thus bound by a hydrogen bond. Once the mc of any given wood begins to dip below the FSP (fiber saturation point) of that wood the bound water begins to be released via evaporation and the wood gains strength and hardness. This is also when the bulk of shrinkage occurs as free water doesn't cause very much shrinkage or swelling. This doesn't mean that the wood never changes MC again. Far from it, but the wood is going to retain the hardness and stability because it will never again reach it's FSP.

Can wood still move? Yes. But handled correctly you are able to negate most of these old rules of needing support or max board widths. Why? Because wood that is correctly dried and acclimated will take adjust to the changes in moisture slowly and fairly evenly. Shrinkage is about 5-10%max in the tangential plane and 2-6% in the radial. This is absolutely enough to bind a drawer or door but doing the math on those numbers shows that they aren't enough alone to warp a top or a board that badly if that board has an even MC throughout when it's introduced to its new area. This happens when the shrink or swell comes on suddenly because the wood wasn't acclimated well before it was opened or surfaced. Or if the wood is case hardened, which comes from basically the same thing.

Also the observation that the growth rings want to straighten is problematic as a rule. They sometimes DO straighten some but this is caused by the variance in radial and tangential swelling and shrinking as well as the natural variance of bound water from heart to sapwood. Not from the direction of the growth rings, the rings are simply an indicator. This can be almost entirely negated by drying the wood more slowly after it is first sawn. If you have a three foot wide slab, yeah this is gonna happen some, but on an 8" wide board the effects are negligible if you handle the board and it's acclimatization correctly.

I'f I'm wrong on this last point I would love to be corrected, I just prefer science to age old rules or hearsay.
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post #18 of 29 Old 02-13-2012, 04:34 PM
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Also the wide boards on the table I mentioned earlier were walnut. They were flitch cut slabs so they contained both planes.
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post #19 of 29 Old 02-14-2012, 04:47 PM
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Tyler
You are wrong. Your understanding of the position of the bound vs the unbound water in wood seems flawed as is the role of tangential vs radial components of the wood. Reaction wood can also contribute to unwanted and asymmetrical wood movement
I don't want to get in a pissing match but if you do want to better understand the chemistry, I will be happy to try to be more specific.
Sorry if you are upset. I find it interesting to try to try inform other workers but the urge to be succinct is a challenge. The misunderstandings about wood movement come up often. Wood is an amazing product and the reasons are not often intuitive .
Knowledge can set us free. Now I'm sounding pompous and I'm not, I'm really shy.
Bob
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post #20 of 29 Old 02-14-2012, 11:32 PM
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Hey man if I'm wrong I will accept a correction. I truly hope that I didn't sound like someone who doesn't care about the truth. I would truly truly love to be corrected on the relationship and chemical bonds of water in wood. Elaborate as much as possible or PM me if you don't feel like posting in the thread. Dude I'm 26 years old, I have studied and trained under masters ONLY because I wish to be corrected. I certainly don't think I know it all. I just want to help others like I was helped when starting out. If I have misled others I apologize. To be great we must understand our medium more than everyone else.

I won't say that what I posted ease incorrect ...until you prove it to me. When you do that I will humbly withdraw. The moment we stop seeking a mentor or a master in the craft is the same moment we stagnate.
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