Conference Table Disaster
One of life's hard lessons: Moisture-related wood movement has caused severe bowing in an unbalanced conference table. August 21, 2006
Question
I delivered a conference table to a client a couple of weeks ago. It is a 4-1/2' by 10' table. It is 3/4 pecan with an underlayment of 3/4 MDF, and a 3" wide 8/4 border of pecan. They left the table in a hot room stored horizontally (the grain also runs horizontally). It sits on 2 large pedestal bases.
The problem is that the table has bowed across the 4-1/2' section from end to end. That is to say, the bow goes from short edge to short edge, the whole length of the table. It bowed so badly that it actually popped the end borders off, and they were epoxied on. We picked up the table and brought it back to the shop put it up on saw horses. I took a piece of 2 x 2 pecan and placed it across the top with a C-clamp on each end and was only able to pull about 1/2 of the bow out. So I tried a piece of 1" wide 8/4 going across the 4-1/2' section and there was still about 1/4 - 3/8 gap under each end of the straight edge.
Does anyone have any suggestions on how to solve the bow in this table? Right now I am putting the table in the back storage area with clamps and cauls on it till I find a better idea.
Forum Responses
(Furniture Making Forum)
From contributor A:
You have a classic example of an unbalanced panel. The 4/4 solid expanded or shrunk, the MDF did not, thus that panel severely bowed. And yes, the ends should pop off. In my opinion, no amount of wrestling and strengthening the back side will help. As long as the solids undergo moisture exchange, that panel will bow, relax, bow and well, and come apart. I would scrap it and start over. Others may feel differently. You could, in theory, put an entire glue up of 4/4 pecan seconds on the back side to help balance the panel. The problem is, it is heavy and your panel is bowed already.
I would do the top from pecan veneer, both sides and put the border around it. Finish both sides too. If you choose solid stock, do not put the underlayment on the back. That is what caused your problem. You will not be able to put a border around the table unless the end borders are allowed to float. One way is called breadboard ends. The wood will expand and contract across the width, not length. So, a border glued to the endgrain will eventually crack off or the top will split. Epoxy will not help. The wood always wins.
From the original questioner:
I’ve made a lot of items this way - tables, desks etc., and never had a problem until now. The wood was dry. There was also a 6-1/2 x 6-1/2 walnut conference table I did for them at the same time stored in the same room but it was standing with the grain running vertically and no problem.
From contributor B:
Be grateful that the other tables didn't warp. Now that you know better it won't work for you anymore. That is how woodworking has gone for me too.
From contributor C:
Scrap it and start over again. Any time spent trying to fix this will be wasted. Either make the entire top of MDF and veneer it, both sides, or make it of solid stock again but do not put it on a substrate. Do not glue it. Had you not glued it, it would have just blown the ends off and not bowed the entire top. You are lucky that walnut does not move as much as that Hickory/pecan.
From contributor A:
I took a look at your website and I am having trouble reconciling the very nice designs I see there with the complete lack of understanding of wood movement demonstrated by the construction and your story above. I recommend that you get a copy of Bruce Hoadley's book "Understanding Wood" and pay careful attention to the recommended construction techniques. Maybe mesquite is an extraordinarily stable wood, or more likely your clients stay in the dry part of Texas, but this kind of trouble is bound to happen unless you change your methods.
From contributor D:
In fact, mesquite is considered one of the most, if not THE most stable domestic hardwood. It simply doesn't move any appreciable amount. I do things with mesquite I wouldn't dream of doing with regular wood.
From the original questioner:
I’ll admit the first time I saw this done and tried it I had some of the same concerns as have been expressed here, however again the old man that taught it to me has not had any problems with it nor have I. So I am hesitant to contribute the problem to construction, nevertheless I am giving it some serious consideration. I know all it takes is one time to make you rethink something. I will probably just go with using 8/4 only on woods other than mesquite.
I know the rules and laws of wood movement and I agree that this is a process that would seem to violate some of them and is causing me to rethink some things.
Let’s see if I can answer all the questions. First let me qualify that I have made quite a number of pieces over the past years this way, tables and desk tops etc, (always large tops) and to date have had absolutely no problems with it. It’s a technique that I learned from an old furniture maker some years ago. And just for what it’s worth there is a 6-1/2 x 6-1/2' walnut table built the same way in the room but it is standing with the grain vertical, no problems with it.
The table stayed in the shop for over a month after completion without any problems. I laminated (glued completely) 3/4 pecan down to 3/4 MDF. Each board is screwed from the bottom of the MDF while the glue is wet. Moisture of the wood was at 10% or less and was in the shop 3 weeks before I started the project. The border of 8/4 had mitered corners, so yes - the ends would have been in a cross grain fashion, however they did not bow. The table bowed so much so that one end popped completely off and the other end was loose at each corner but still attached in the middle. It took a 2 lb sledge and a block of wood to take it off the rest of the way. This end had not bowed as much as the other at least not at this point.
The bottom was sealed via the glue and the top was sealed with precat lacquer and the wood was below 10% MC. The glue used to attach the border is Sys 3 epoxy and the boards were edge glued and the bottoms and underlayment coated with Titebond 2. This is done one board at a time with screws and clamps. What I am looking for is fixing the current problem, and scrapping is not a possibility in my mind at this point - not at 5,000.00 for the table. Thanks for all of the input - I hope this answers some of the questions.
From contributor E:
Rebuild the table and do it right. I think you know you're looking for trouble doing things that way.
From contributor F:
I feel for you about this deal. $5000.00 is a lot of money. From my experience, you stand a high probability of losing even more money and time trying to get the warped table straight only to fail. It seems to me that once something that thick takes a set, it just won’t come straight again.
I, too, am taken aback by the methodology you are employing. Two things are in direct opposition to what I have learned about wood movement - first, restricting the crossgrain movement of the entire width of the table by glueing it to a stable laminate like MDF;
second, gluing edging across the endgrain of a wide body of solid wood.
From the original questioner:
To contributor F: I agree completely. I would have never tried a process like this if I hadn’t seen it with my own eyes and also been contracted to do an 8' round mahogany table with this process by the old guy to boot. Just for the record I never use this on any smaller project except mesquite desk tops. I follow the rules of expansion etc.
For the moment I am going to try setting it on saw horses supported on the outer edge of the MDF along the sides the full length of the table and put about 600 lb of weight (sheet goods) down the middle for a couple of days to see what if anything changes.
From contributor A:
As I mentioned in the initial post, you have, as you know, an unbalanced panel. No amount of coaxing will change that. Even if you could bring the MC of the wood back to exactly what is was when you finished it, it still would not be the same. Wood cells collapse as they dry and never spring back to there original configuration even when moisture is re-introduced. I assume there is a finish on the top too. This, being on one side, screws thing up too. Even hand rubbing one side more than the other introduces surface tension, causing cupping. No one wants you to lose your reputation as a craftsman with the client. Rebuilding it is the only right option. You'll be more frustrated trying to save the patient. We have all been there. I think that is why so many have taken interest in your scenario.
From contributor G:
What you don't seem to realize is that properly fixing the problem is even less of an option than scrapping the table, because the basic design is flawed. Also, as other posters mentioned, the fact that other tables built this way haven't failed does not mean they did not have the same design flaw. We had a suspension bridge up here, the Tacoma Narrows Bridge, that carried cars on it just fine - until the wind started blowing, setting up a resonance that made it twist and jump, until during one storm, it twisted itself apart. So they designed and built a new bridge that did not have that design flaw.
I did once correct a major bow in a solid wood table that was imported from Asia. It was also a table built to fail, like your conference table, because it did not allow for wood movement. I milled 2 wooden strongbacks, about 1-1/2" thick, as wide as I could without them showing below the apron, and a little shorter than the length of the table. I milled them so they were radically bowed in the direction opposite that of the table. Then I lag bolted them to the underside of the table top. Basically I was introducing a force opposite the force of the bowed table top. I made sure to tell my client that this was not a good fix, but that it was the best I could do considering the problems with the design of the table. You could try something like this with your conference table, although 3/4" isn't much for lag screws to bite into.
From contributor H:
Try taking the bow the opposite way, twice the amount of the bow. Leave it clamped in this strain for a couple of days and then release. If the gods are on your side it will come back somewhere near normal (flat). This method works for some things, but I've never tried it on a conference table. Basically it's going to be a lucky shot. I don't understand the need for gluing the wood to the MDF – it seems like the screws would be enough.
From contributor A:
You've been lucky in the past with mesquite, but this isn't mesquite. Your basic approach has caused this trouble. Just because an old guy tells you something doesn't make it true. He was lucky, too. I mean no disrespect, but you've just plain made a mistake and you won't be doing yourself or your client any favors if you keep messing with the bowed top.
From contributor I:
They are using this idea for wide plank flooring now where you glue and nail down solid wood flooring. They say it will only let the board shrink very little compared to having no glue. But their flooring is glued and nailed to a subfloor that is attached to joist. The reason this may work is if you have the right framing under the table but as I say, it may work. So with that I would try veneer for your center of your table and then wrap it with real wood or try building one heck of a frame for it. But remember if that table pops off of that frame you will be right back to where you started from.
From the original questioner:
I do agree with most of the posters as to the expansion and contraction issues. Except for large panel glue ups, as you have seen, I always follow these rules. This reply is not meant in anyway to be disrespectful to anyone, only as information and reasoning. I spoke to the old guy yesterday and questioned him about these things and this was his reply.
He’s really only about 64 and I'm 47 so who am I calling old right? I asked him to address the issue or expansion and contraction. He really knows how to make a short answer long but after getting the family history of woodworking and fame of his teachers, and family, it still boiled down to they never did anything this way, but that is the key. He put it sort of like this - most of the rules we follow in tradition about expansion and contraction have been written over several centuries and we all (me included) take them to be the absolute law. What he pointed out is that over this time until now (past 20 yrs or less) there has not been the climate control that we currently enjoy everywhere. So the circumstances to which the conditions apply have changed. It’s kind of like the F-117 stealth jet. When they built it they broke most of the rules for building a plane. As a matter of fact it shouldn't fly, but because of the new composite materials, technology, etc., they did it successfully. This is his basic reasoning. So, in short, you said it earlier -the customer’s environment was not a controlled environment.
Lesson learned. Don't leave pieces in an uncontrolled environment. I guess I was so frustrated at showing up and not having the help on hand to unload and set up, or rather not being able to setup, that when putting it in that room, I did think very briefly about the damage to the pieces and I just let it go. Never again. He thinks the same as most of the posts here - scrap it and start over.
From contributor J:
I would suggest removing the screws from the bottom of the table then rout end to end slots 1/4" to 3/8" wide right through or almost right through the MDF. This would release the tension in the top. Saw kerfs would work as well. Then insure you allow for the movement when attaching the base - maybe a strong back if you need it.
From contributor K:
I was going to say the same thing as contributor J. If I wanted to fix it (not that I would, but understanding your reasoning), then I'd cut a few or more saw kerfs down the entire length of the MDF. Make a piece of wood with a bow in it and clamp it up opposite the table. If it sits flat, then glue that piece to the table bottom, reattach your ends, but make them bread board ends and attach the top to the bottom of the table. Let it sit for a week and see what it does. If it stays flat, then send it.
From the original questioner:
When you build a round table and put 4" border of a different wood type on it. Realizing the table must be dead flat as must the finish, how do you attach the border?
From contributor F:
Typically when a contrasting wood type is used as a border to a round or a race track shaped top, the "field" portion of the top would be veneered. The attachment would be glued and any desired alignment devices such as splines, biscuits, etc. Alternately, if the center were solid wood then the center would have to float within the border with a tongue or a rebate or even a loose spline.
From contributor L:
I have a good friend in S Africa who makes solid wood front doors which are hand carved. They use various hardwoods and planks of 6 inch by 2 inch thick. The planks are all glued together without any mortice, splines, dowells, etc. They are then hand carved and sold. They do around 20 a month. They have never had a single return or problem with cracking, etc.
I also have a large 1.5 meter square, which is also made in S Africa from USA white Oak and it was in a container on a ship for 6 weeks. It was made in 1997, where I live in Australia ( Brisbane). The top temperature is 40C with humidity of around 95% on a bad day and moves to 8C and 45% in winter.
This table is one large glued up top and on the ends are short pieces of end grain glued together so that the whole top looks thicker than it is. There is no movement out of flat -the top is a good as it was the day it was made. I also have a coffee table about 1.8 meters by 1 meter wide that is totally one glue-up, from the same company, and also no movement or splits.
So the question is how do these guys get it right including the end grain glue-ons and we worry about movement? I also think that the glues today are vastly superior than say 100 years ago (I am 53 and when I was at school we used hot melt glue for all of our joints and there are still pieces I made in the family that have not separated) and I have visited a number of castles in Germany which are well over 100 years old and the huge solid wood doors are still in perfect condition. So the big question is, are we really using a "belts and braces" approach for something that is not that necessary?
From Dr. Gene Wengert, technical advisor Sawing and Drying forum:
The big reason why we see problems today is because of central heating and the warm interior temperatures.
From contributor J:
I would not suggest gluing the strong back to the underside of the table! That would be replicating the same problem that the MDF caused (restricting the natural movement of the wood). Screw it on with elongated holes or table top fasteners etc.
From contributor N:
Here's the fix. First, kerf all the wood every 4-6 inches along its length. Next, laminate the same thickness MDF to the topside of the lumber. This should pretty much balance things out. And if you don’t like the look of raw MDF you can always veneer it.
From contributor O:
There are two possibilities: 1) Build a new table. 2) Attempt to rectify the problems with the bowed table. To me the only rational thing to do is to try to fix the table that is completed. You know the table is bowed and the customer knows the table is bowed - that's a given. I like very much the recommendations of cutting kerfs in the bottom with either a saw or router. By doing this, there is no doubt in my mind that at some point the table will go flat (the MDF may be all gone, but it will be flat). Then some strongbacks can be added as needed. Hope you have success with this repair.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor T:
I experienced the same problem by putting green mesquite on MDF and even though mesquite only changes 1-2% from green to dry, 1% over 38 inches is .38 in. I fixed it by cutting groves with my Festool plunge skill saw every 4 inches (corresponding to the centers of the mesquite boards) and the complete thickness of the MDF. I was then able to flatten and screw to a backup. It still wasn’t perfect but it did save the project (a retail counter top). I don't know if it would be good enough for a dining room table.
