Problem with cupped boards of table top

[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