Mahogany Table-Top Glue-Up

      A discussion of how to avoid problems in a large round table-top slab. March 13, 2007

We have a client that wants us to make a 60" round mahogany table top that will sit on a center pedestal base for an accent table. We glue up lots of panels for doors, etc. What should we look out for or know about gluing up a table top it will be 11/4" thick when finished and finished on all sides?

Forum Responses
(Furniture Making Forum)
From contributor W:
If it is out of the solid at 1 1/4 just glue it up the same as your panels but obviously pay close attention to the grain and make sure your boards are going together to create a flat top and not a big dish. Make sure that when you attach it to the base that there is plenty of room for seasonal expansion. If there is going to be a decorative edge around it of some sort you may want to consider making it out of ply and maybe wrapping it to create the 1 1/4. A glue up is a glue up, you just have a bigger chance of compounded error on a big top.

From contributor D:
I would to need to know more design features to really give the in-depth guidance you need and the level of quality you’re going after to make it work economical. But here are some basics:

1. Try to use quarter-sawn wood or bastard sawn (it will give a nice look, but be little more stable than tangential grain and it will look much better than radial grain - which by the way is the most stable.

2. Look at the end grain and alternate the ring pattern as the earlier contributor mentioned.

3. Know the final environment's RH level and try to build it in the same conditions. Check the MC in the wood to predict the movement.

4. All for wood movement - fix the center so you can float the movement from the center out.

5. Balanced finish on both sided

6. To make easier in clamping and give you more glue surface and make alignment easier, tongue and groove the lumber edges like hardwood flooring. You want have to "wrestle" the top as hard.

From contributor W:
In my opinion, a tongue and groove or biscuit is a waste of time if the boards are milled properly. If your boards are flat and have a square edge, there is no wrestling involved. I agree with the other stuff Contributor D said.

From contributor D:
In my opinion it isn’t a waste of time because you give much more glue surface. I don't like the use of biscuit, wafer or dowel either. I work in production in the furniture industry and we make high-end. I have done product development on hundreds of these type tables. I do not have the luxury of grading and sorting through "perfectly milled" lumber.

From Gene Wengert, forum technical advisor:
A round table has more potential shrinkage than a rectangular one. This obviously leads to problems. The best you can do is to use very dry wood and check the MC of every piece (6.5% MC is ideal for most cases. Do you know the RH where the piece is going to be used? If the MC does not change, the top will not shrink or swell. Then, coat both top and bottom and edges with a water-vapor resistant coating (not just liquid water resistant). Avoid any cleats on the bottom of the table.

Regarding the pedestal, you will be happier if you can make sure that there is a large hole in the center to allow for shrinkage and swelling.

From contributor W:
Gene, I agree with you on coating both faces of the top but have a few questions on two other pieces of advice you gave. I personally would not avoid cleats. If done properly, they may prevent the top from cupping. They will however cause problems if all the screws are down tight and there is no room for expansion. I have seen many period tables that are in excellent shape with cleats.

I really don’t follow you on the hole in the pedestal? Is that to benefit the pedestal or the top and how?

From contributor G:
Gene - can you elaborate on what a "water-vapor resistant coating" would be? Is this something other than CV and catalyzed lacquers?

From Gene Wengert, forum technical advisor:
Vapor resistances increases with the number of coats and their thickness. Wax is the best of all; you need to look at the manufacturer's specs to know if the coating is a good barrier for vapor (or ask them directly).

The US Forest Products Lab evaluated a bunch of coatings 20 years ago, but coatings have changed, so this comparison data is not too helpful anymore. Maybe they can update their study.

In general, oils do not provide a vapor barrier effect, although multiple coats of tung oil do begin to have some barrier. Varnish (alkyd, phenolic, or polyurethane) has a much greater barrier effect, and are fantastic especially with thick coatings (multiple thin or several thick). Just make sure to coat top and bottom equally.

From Gene Wengert, forum technical advisor:
A cleat that can tolerate movement (such as screws that are in slots and can move) is indeed ok, but too many people using that design will either fasten he screws too tightly (imbedding the heads) or will glue the cleat as well. Dumb things indeed, but I see it often, so I tend to get away from cleats. Done properly, they are indeed ok.

The pedestal will perform better, and this also includes other round items such as lamps, if there is a sizeable hole in the center. This hole allows the outside to shrink (or swell) a bit without cracking. Further, the inside of the hole should be finished the same as the outside to allow the moisture inside and outside to change at about the same rate and at the same level. Uniform moisture in the wood is also helpful to good performance.

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