Understanding Wood – Hoadley
Chapter 7 – Coping With Dimensional Change in Wood
Chapter 7 of Understanding Wood is possibly one of the most important for both the furniture-maker and the prospective client. Many pieces of furniture have been ruined because no allowance was made for the inherent attributes of wood. A little education in this matter is crucial for the aspiring artisan.
The following three topics mentioned in this chapter will be the focus of this lesson:
Preshrinking is a simple concept that is universally applied in the wood industry by means of kiln drying. By drying wood to moisture levels encountered indoors it is possible to negate the impact of wood shrinking as it loses water content. The catch here is that “typical” indoor environments vary widely depending upon geography and other factors such as seasonal heating and cooling. Unless a building is strictly climate-controlled, the indoor humidity will go up and down. Pre-shrinking the wood does reduce the overall movement and is thus very important. Except in the driest locations, air-dried lumber does not get dry enough to be ready for immediate use in making furniture. Instead, it should first be cut into over-sized parts and allowed to dry indoors until it is at the appropriate moisture content.
Moisture absorption control
Moisture absorption control refers to protective coatings that slow the rate of moisture loss and gain in wood. No coating outside of a vacuum-tight capsule will completely stop this process. A coat of finish however, will slow it down enough to prevent furniture from suffering the worst effects of sudden humidity spikes. Certain varieties of wood (especially the very hard and dense) are prone to checking and cracking if they are exposed to rapid humidity changes. Slow and gradual changes seldom cause problems in well-designed furniture.
Good furniture design is the most important factor in accommodating for wood movement. Since expansion and contraction are a given it becomes necessary to direct that movement in a harmless direction. There are many techniques that have been in use for centuries for this purpose.
A frame and panel consists of a solid wood panel inside of a stable frame. The frame takes advantage of wood’s longitudinal stability to eliminate door shrinkage and expansion. The panel should fit snugly into the groove but is not glued in. The frame keeps the panel flat but allows it to expand and contract inside the grooves. Note that the panel should also be smaller than the opening to accommondate expansion of the panel.
One in the center at both top and bottom.
Spaceballs are a useful product. Inserting them into the grooves before gluing up the door ensures a rattle-free and snug fit. The panels will always stay centered and the balls are soft enough to permit expansion.
Breadboard ends on a table top help keep it flat yet permit inevitable wood movement to occur without stressing the wood. Apply glue to the first several inches only so as to permit the top to slide in the groove as humidity changes. The back end of the breadboard is sometimes fasted with a screw through an elongated hole but using a sliding dovetail in place of the tongue and groove is a cleaner approach.
As humidity changes the top will expand and contract. This becomes obvious when looking at the alignment of the breadboard end and the back edge of the top. Their relationship will continuously change over the seasons.
Installing drawer runners across the grain of a solid wood cabinet can spell trouble if provision is not made for seasonal movement. Simply gluing the runner to a wide board will probably cause splitting in the future.
This is an example of how not to install a solid wood table top. A 20 inch wide top will change dimension approximately a 1/4 inch over the course of a year. Screws will not prevent this but eventually they will cause a crack to appear.
This table will avoid the splitting problem. Pocket screws are still used in the centers at both ends for alignment. The rest of the screws are driven through blocks of wood that fit into grooves in the apron. The tongue and groove permits them to slide as the top expands and contracts.