Problem with cupped boards of table top

[Erik Stol]

In the hall of my parents is this nice little octoganal table of oak. The top consists of two peaces of oak glueed together. Unfortunately, exactly at the glueline the boards splitted. This is maybe caused by the boards that started cupping over the years. Does anyone of you have a solution to reduce the cupping or to get rid of the cupping, so i can restore this table.

[Jerome Hanby]

If you can remove the top I'd be tempted to use it as a template and build a new top. I suppose you could try to use some angle iron screwed into the bottom side and force the cup out, but my gut feeling is that it is just as likely to split the top as remove the cupping.

[Erik Stol]

Jerome, the problem is that I can't rebuild or copy the table top. It has some nice cutting/molding on the surface edges. I will post some pictures later on. Do you think that putting the boards in a kind of steambath and then clamping it stiff, would reduce the cupping?

[Jerome Hanby]

I'll let someone else jump in, I'm out of my depth. If there were no other way to "fix" it, could you cut a section out of the middle of the top and replace it with some contrasting (and flat) wood? That could save the edges and possibly do away with the cupping sections. That would probably destroy any intrinsic value the table had because of age, but if the value was sentimental or ornamental, then it might be a good last resort thing to try...

[Erik Stol]

Thanx for the possible solution but the table has a sentimental valeu, so i I'm not opting for "killing" the tabletop. let's wat an see where fellow SMC's will come up. Thanx so far for your input.

[david brum]

I'm pretty sure that steaming the tabletop would make all the glued joints come loose, among other things. If the top isn't veneered, can you sand or plane it flat? If it is veneered, you could always flatten the top, then veneer it again.

[Jerome Hanby]

Let us know what works. This has got to be a pretty common problem and heck I may actually live long enough for some of my stuff to suffer similar problems...

[scott vroom]

Remove the top, rip cut along the split line, joint and plane the 2 boards, glue up and finish. If that's not acceptable then leave it alone and continue to appreciate it as-is for it's sentimental value.

[Terry Beadle]

I think Mr. Vroom is probably correct. The table probably was not jointed properly when built or has unusual water content or exposure.

If you can make a pipe clamp close the crack, and do not or can not do the procedure recommended by Mr. Vroom, then I would wedge the crack open with a scrap wedge, use epoxy to reglue, reclamp and let it cure 24 hours. Then I'd router a butterfly half lap joint on the bottom of the table joint, make a butterfly out of strong hardwood, red oak, hickory, rock maple come to mind. Glue and seat the butterfly with some epoxy. Let cure, plane flat and then cross your fingers for 50 years hoot! Gorrilla glue would also be a possible choice if the mating surfaces spring back under clamping easily. If not, the epoxy is the strongest bonding medium I know for wood.

[Carl Beckett]

I dont think steaming is a good idea (agree that pretty much everything will fall apart if steamed). Unfortunately, there isnt much that can be done with relatively large wide board to prevent cupping.

You might put a stiff runner underneath as suggested, but if you use this to 'pull down' the warped pieces to a flat plane it will very likely crack in a different spot (speaking from experience). I have some success where a glue-up was relatively flat to begin with and adding runners or a rail has help maintain the flatness (the best thing is always to machine the piece in the same climate conditions it will be used/stored). But seasonal humidity swings on wide boards are going to cause cupping (there are a lot of posts on here about trying to slow the rate of absorption by finishes, but this only slows, doesnt prevent)

I like the half lap on the underside - this might be your best bet. If your 'bridge' piece was relatively thick, the 'top surface would be relatively thinner and here you might add a little steam and be able to get it to conform down to this lap piece underneath. Essentially making a section of the top into a veneer like material - then clamping it down against some good glue (I have been warned off epoxy... but still use it sometimes... I bet basic wood glue would work here) maybe even contact cement is ok since its a veneer type construct)

Trying to pull the full thickness flat might be a losing battle....

A tough one....

[Steve Friedman]

Jerome, I have no idea if this would work, but wanted to throw out a thought.

What if you glued up some quarter sawn oak to make an octagon that is slightly smaller than the current table top. Then, take off the existing top and plane it to make it as thin as you can without losing the edge carvings that you want to keep. Finally, laminate the thinned top to the new "undercarriage" as if it was a piece of veneer. Hopefully, you could rout and stain the edges of the undercarriage to match the original top.

My thought was that using the same species of wood and thinning the old top would solve the problem of it slitting apart in a year. Just a thought.

Steve

[Erik Stol]

Hello everybody, thanks for all the input. The idea of Terry is one that I am going to study over the weekend together with all the other input. I will sit down in my workshop and review all of your ideas. I will let you know what I cam up with.

[phil harold]

Remove the top, rip cut along the split line, joint and plane the 2 boards, glue up and finish.
+1 on this method

if you can you may want to try and rib each board in half again and plane/joint to reduce the cupping sometimes you luck out and cannot see the cut afterwards

but each cut and planing will reduce your width

good luck

[Erik Stol]

Hello guys,

Here is the update on my cupped problem.

I listened to all of your advises and decided to do the following.
1. I gently swiped the curved cracked sides of both boards with my planer. I had to take off about a mm of each side to get them smooth.
2. I made a backer board out of a piece of 18 mm plywood. (sorry i didn't have such a big piece of oak around the shop. The size of the backer board has an offset of about 4 cm's to the original outer size.
3. I glued one of the panels to the backer board, with enough clamping pressure to eliminate the cupping. The next day i glued up the second panel.
4. Cleaned the table top with some woodoil, and now it looks ok again.

And I discovered that I need to go back to primaryschool. I told you guys that the table top was 8 sided. Well to my shame I miscounted: 9 sides!!! :D

See also the picture. I had some more of the process, but something went wrong during transferring from cell to pc.
207803207804207802

[Chris Fournier]

If you are glueing a solid wood top to plywood, whatever your intent you have or are going to destroy the top. Stop now.

If this piece has sentimental value then leave it alone and enjoy it as it is.

[Erik Stol]

Chris,

I disagree with you. It is a piece of, what we in The Netherlands cal WBP plywood (water boiled proof) it is used for exterior purposes on facades and so on. This particular piece was in my workshop already for years so it is OK, for its behavior. By the way, when gluing the top on the plywood I also used screws from the back to secure it.

[Kevin Stricker]

Erik, the challenge is that the plywood is dimentionally stable and will not move to any measurable extent with changes in humidity. The solid top will move significantly over the seasons. Now that you have the top rejoined I would go back and remove the plywood scab to keep from having any problems in the future. Considering it is glued and screwed I would remove the screws then score it with a skillsaw a few dozen times then chisel the remains off. The plywood has done it's job and the only thing more it will do right now is cause more problems for the table.

[Chris Fournier]

Chris,

I disagree with you. It is a piece of, what we in The Netherlands cal WBP plywood (water boiled proof) it is used for exterior purposes on facades and so on. This particular piece was in my workshop already for years so it is OK, for its behavior. By the way, when gluing the top on the plywood I also used screws from the back to secure it.

This is very fundamental stuff and if you disagree with me, you also disagree with years of generally accepted woodworking knowledge. Plywood by nature is dimensionally stable, solid wood by nature is not. Fasten the two together and the plywood will restrict the movement of the solid wood, ultimately causing it to tear itself apart. I've seen this style of "repair" myself and I can confirm that it is flawed in concept and practice - the pieces were ruined.

[Mike Henderson]

My experience is in California which does not have big swings of either temperature or humidity. Given the size of the top, and if glue that allows some creep was used (like PVA), it would probably be okay here.

Some one who has access to wood data can look up the expansion of that wood (is it oak?) and tell you how much it might move per inch of width.

I probably would have done it a bit different, however. I would have glued it in the center and put screws in elongated holes on the outside where the wood would expand and contract.

Mike

[Chris Fournier]

Bruce Hoadley provides the formula that you need to calculate wood movement from X mc to Y mc in his book "Understanding Wood". Guess work is not required. Mike's point is valid, as you get closer to the equator where the seasons all blend into one, you will see less movement. The Netherlands will see significant movement - lack of palm trees and all.

[Carl Beckett]

For sure, solid wood contracts/expands differently than plywood. It can be calculated to a certain degree.

Having said that - I have done a number of projects where I have glued/screwed wood together knowing there was a mismatch. Such as ; 12" to 18" of the center line of a breadboard end, chest top cleats (to keep the tope flat), and even solid wood to plywood. Short distances, where perhaps the absolute differences in expansion dont cause failure to the degree that can be noticed (by me). So far so good in climates of Ohio, Washington State, and Massachusetts.

Would I have used the plywood? No. But my experience has been that if you fix only short distances, the relative movement isnt so catastrophic that it warrants taking it all apart and risk causing more damage.

$.02 YMMV

[ian maybury]

It'd certainly seem very risky to glue the oak top down on plywood, especially since the cupping is a sign that the oak is moving anyway. One rule of thumb that's about suggests that much more than a 2mm thick veneer glued down over a substrate like MDF or ply that doesn't move very much is at risk of splitting - especially if the adhesive is a very rigid type like say an epoxy.

It's on the other hand fairly common practice to glue up boards face to face to make thicker pieces - although it's preferred to use matching pieces of similar thickness and grain placed cup to cup, and with the grain in the same direction, and in a similar state of dryness.

It's hard to see from the photos how bad the cupping is, and whether it's likely to be possible (or not) to get the boards to press flat without splitting. It looks like maybe there has been some lengthwise movement too which may or may not be capable of being rectified/need disassembly of the top.

The question arises too as to whether the cupping is the result of the conditions the table is exposed to having changed - become damper for example. I guess it'd be a lot better if the wood has equilibriated to whatever conditions it's going to be residing in ongoing. If that involves a change in wood condition then you never know - maybe it'd reduce the cupping too.

A light pass through a wide belt sander (both sides to equalise any moisture effects caused by the finish) might be an option to flatten everything out without the need for disassembly - if it could be done without damaging the carving, and if you feel able to re-finish to match the rest of the table.

An off the wall possibility. If it was felt essential to preserve the existing finish it might be worth trying cutting slits in the underneath of the existing top - similar to those used to prevent cupping in solid wood flooring. They might relieve enough of the stresses causing the cupping to significantly improve the situation, should anyway make pressing it flat a lot easier, and could maybe be stopped so they don't show at the edges.

Failing this, and if the wood is in a stable state (is equilibriated), and is not much cupped then face gluing boards of a similar thickness to the back under a lot of pressure (lots of clamps, good thick pads) might possibly be enough to straighten them - it'd need to be carefully chosen so the grain and type was a good match, and placed so that it'll tend to cup in the opposite direction. It'd need slitting on the mating face to match the strength of the two boards if that was done to the top too.

If the extra thickness of backing boards is an issue, or splitting is felt to be a high risk, or slitting can't be hidden behind a skirt or something it might be an option to machine away maybe half of the existing top thickness before face gluing the backing boards on - this should make it easier to straighten the cupping without splitting too.

It's all risky and unpredictable territory. Plan B might be to leave well alone....

ian