Depending on atmospheric conditions of temperature and moisture, solid wood will naturally expand and contract. The wise woodworker will anticipate this phenominon and head it off at the pass before it destroys his or her fine woodworking project.
Solid boards are sensitive to changes in moisture content and it will expand and contract accordingly, more in terms of width than length. If you take your table from the rain forest (damp air) to the desert (dry air), it will get smaller. If you move it from the desert to the rain forest, it will become larger in size.
I have built many tables in my lifetime and I discovered early-on that it is a very bad idea to fasten a solid wood table top rigidly to the table’s apron. The wood will expand and contract over time and if the movement is considerable, it will tear the apron and the table apart. That is why I design all of my tables with a dado groove near the top, inside of the apron all around. I then create wooden clips that will connect the top to the apron. The clips are screwed to the underside surface of the top. A “finger” is made by cutting away part of the clip at one end. This “finger” is inserted part way into the dado in the apron before being screwed to the table top.
Because the screw holes in the clips vary slightly in location from one clip to the next, I always mark the clips in numerical sequence with a felt tip pen. I mark the same number under the table top where it will be hidden by the clip. This way, the top can be taken off for refinishing and be put back with all the screw holes lining up with the clips.
Movement of solid wood table tops can also be observed in the form of warping or cupping. 1 1/2" x 2” cross bars can be mounted across the grain, under the top. They should be installed with #14 round head brass screws (with brass washers) screwed into the bottom of the table top. The holes through these cross bars must be slots to allow for wood movement. Never glue these cross bars to the table top, as that will curb the top's natural movement. Three cross bars should be enough for the average dining table, two for a coffee table. The cross bars can be easily taken off and put back for table top refinishing.
Bob GillespieWoodworker
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© 2010 Robert M. Gillespie, Jr.
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