Q. I am planning to build a large dining room table, about 10 x 3.5 foot. I have some 2 inches x 8 inches x 12 feet Honduras mahogany (beautiful pieces) I plan to use. I will glue them up, edge to edge. I have questions: 1. Is this project feasible or advisable? 2. What adhesive should I use? 3. What about the table frame? 4. How much will the table top expand or contract in width? 5. How do I finish the ends and top to prevent cracking and warping?
A. You have certainly asked some key questions. My answers would be best given over a cup or two of coffee, as there are many fine points I could discuss and even follow-up questions that you might have. Here is a start.
1 and 3. The project is certainly feasible, but if you have any moisture change, especially if top and bottom of the table change differently due to unequal finishes and moisture properties, you can expect warping. Further, it is very important to have a frame for the tabletop that allows the top to move as some moisture change is unavoidable. We can expect moisture from 6 percent MC in the wintertime to 9 percent MC in the summertime in many locations (that is 30 percent RH to 50 percent RH). Depending on the mahogany that you have, shrinkage and swelling with moisture changes is half as much as oak; true mahogany is indeed quite stable.
4. As an example of the amount of movement, if we assume that the wood changes by 3 percent MC and that it is flatsawn Honduras or true mahogany, then the wood will move about 0.4 percent, which is under ¼ of an inch for a 3.5 foot wide table. The length will not change at all.
2. You need to use a top-of-the-line adhesive. If you properly prepare the wood, the joint will be 1.5 times stronger than the wood. I would use a quality pva or pur adhesive. You do not need the more expensive versions that are designed for outdoor exposure. You need to follow proper procedures, which is true even for the most expensive adhesives, to achieve strength greater than wood.
5. The key to a finish, in addition to looking good and being non-polluting, is that it have good resistance to moisture vapor movement. Today, we might consider several coats of polyurethane. In any case, avoid a permeable finish--one that does not form a film or thick coating. I cannot give you more information on finishing, as that is not my field of expertise.
Have something to say? Share your thoughts with us in the comments below.