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Using air dried wood and warping2/10
Does anyone have experience mitigating the cupping that happens on a table top due to moisture content? I recently bought ash from my local lumber yard at 12% moisture and the table tops warped upward by 1/4" over 24". I'm wondering if the same type of coving that was a technique for keeping solid wood flooring flat might work for the underside of a table top. These tops are for a restaurant and will have a centrally mounted metal base, so no skirt can be used to help control the warp.
I assume it was flat cut? Quarter sawn is much better, almost never cups. How wide of boards did you glue up? Did you finish the bottom surface of the table top exactly like the top surface? Wood moves a ton this time of year with furnaces running full blast. How long did you let it acclimate to your shop? Did you stack the tops tightly as you worked them, or did you let air circulate all around the tops. Tight stacking lets one surface to dry out more than the other. Not a good time to bring wet wood into the shop and make a table. Buy kiln dried wood!
Yes, all of the precautions were taken, letting all side have full air exposure, proper finishing etc,. It is flat sawn and 6" with alternating orientations of convex and concave grain orientations- quarter sawn was not a possible aesthetic option here. The wood was kiln dried wood, it just so happened that this batch came in a little higher moisture than normal.
As a side note, so much of the kiln dried lumber available these day is dried way too fast and results in case hardened boards that corkscrew after being ripped, requiring re-joining and planing if forcing into place with clamps glue and fasteners, is not practical. The air dried lumber I use, for the most part, stays the shape it is, if it has time to acclimate post joining and planing.
I'm wondering, specifically, if coving, heavier finishing, paint or any thing else might be used to pull the tops in the opposite direction.
If all the tops are cupping concave upward despite alternating ring direction that indicates some factors in the shop are contributing to the problem. Normally a table would tend to cup down due to less finish applied to the underside and the quicker drying on the bottom. Forced air heating in the shop is a killer. You may have had the tops laid out when sanding or finishing and had the heater blowing predominantly on the top surface. That would do it.
Would it be possible to attach cleats with slotted screw holes to the undersides clear of the metal base? Or to add some slotted steel tabs to the bases to serve the same purpose?
These are all good comments. My Shop is in California on an old military base without any heat and we always do the same amount of finish top and bottom. But in my experience, Just finishing the tops and not the bottoms always results in a cup, regardless of moisture content. So, I'm not sure I understand why less finish on the bottom would be better? The only difference between our tops and bottoms in terms of treatment is the level of sanding- Tops get to 220 or 300 grit and bottoms get 120-150. Maybe this has something to do with it?
Hey Kevin Janess,
This is an interesting idea, to use a metal cleat. It would have to be fairly beefy to not deform, and the top of the restaurant base is only 8" across, so they would have to extend from the platform. It would also need to not be a knee banger/clothes catcher, but I like the direction. Thanks!
You are correct, I don't think it is good practice to do fewer coats on the underside. I thought in your case it would allow the bottom side to dry. Is it possible for the heat of the higher grit sanding to accelerate the drying of one side? My only question is do you feel that sanding ash to that fine a grit is necessary?
You quite simply are losing moisture from the face, and/or gaining moisture on the underside of the tops. This will continue until the tops reach the EMC for the area they are to live in. See the map.
When lumber is kiln dried to the correct MC, it is 'fixed' somewhat, making the material more stable.
I suggest the OP find a better lumber supplier - or dryer - if he is seeing as many problems as he describes. My experience with commercially dried lumber is very good, with only a few exceptions over the years. Most of those are from reaction wood, but some are from poor drying. They are easy to spot, right at the first rip.
Checking for case hardening is easy to do before the wood is received. If it is present, reject it all. Once they get a load or two rejected, they will either straighten up or go away.
Another method that can promote cupping is taking thick stock and removing more material from one face than the other when surfacing. If the interior is wetter than the surface, and you remove 1/16" from one side and 1/4" from the other side, you now have 1 wetter face than the other. It will quickly cup.
Rich is correct, though I believe the effect he describes is another indicator of case hardening.
Case hardening being the difference in the thickness of the wood where the 'shell' is dry, but the center is wet, relatively.
I have found on properly dry lumber that there is no difference in how much wood is planed - or sawn - off of one face or the other. Same with ripping - rip down the center of an 8" wide board, and the kerf should remain the same the full length, neither opening or closing.
This map is fantastic. I happen to live and ship most of my furniture to the "11%" region of the western United States. Do you know what the %R.H. stands for?
*another anecdote advocating for air dried wood- last week I band-sawed 17.5" high 1/8" veneers of plain sawn elm that didn't cup or spring at all, where trying to do anything larger than 8" in plain sawn, kiln dried, oak, walnut and ash, has always been not so great. Quarter sawn is always OK, but I haven't seen a piece of quarter sawn anything wide that 12" from my suppliers.
RH = Relative Humidity
Kevin - You will find a copy of Bruce Hoadley's "Understanding Wood" very helpful.
One big difference between the little vendor vs the larger commercial vendor is the width and grain in boards. I live in the middle of the best Walnut and White Oak in the world, but I can't get anything of width, length, color or nice grain. The very best lumber that is sawn is dried, put into a container and sent overseas. The money is wired to the mill before the truck picks up the container. They love it, obviously. We will never see it, unless a smaller operation gets the log somehow and saws it and makes it available.
You can strongly advocate for air dried lumber up until you get a call from a customer about fine sawdust coming from a hole in the wood. One call about powder post beetle in their home and a fumigation bill, then everything changes!
There is no doubt that properly air dried lumber is better behaved than poorly kiln dried material, but if it is not conditioned close to its average equilibrium moisture content in service you are going to have trouble. Find a good supplier of kd stuff or get your air dried stock in hand well before you need it so it can reach a usable moisture content when you need it. A kiln can help to speed things up.
As for adding cleats to your existing tops, I was thinking of 2-3" thick wooden cleats, but angle iron would work as well. The small size of the metal bases would preclude adding tabs to them. 24-25" above the floor is typical for the bottom of apron on a dining table, so knee clearance would not be an issue. Whether the tops can be flattened by this method is subject to experiment. The fundamental problem seems to be the initial moisture content. You might find that kerfing or coving the bottom surfaces would make it easier to pull the tops down to the cleats.
Somewhere in the trade rags of yesteryear was a knowledgeable wood person (name escapes me) who stressed, over and over, in glue ups, NOTHING wider than 3". I'd start there and see what could be made to work.
Ever look at a Boos butcher-block? Their laminations are about as wide as they are thick. I am guessing there is something to that.