Hardware Joinery

Screws and nails have been used for centuries in furniture of all kinds. Studies of antiques suggests some clear limitations on their use.

Wood expands and contracts seasonally in width and thickness. Nailed connections often allow movement - the nail bends and/or the wood crushes easily around the narrow width of nail shafts. Screws make a much less forgiving connection. A screw hole elongated in the direction of movement can accomodate seasonal expansion and expansion. A firm screw connection with no provision for wood movement often results in splits and breaks.

Screws have been used probably as long as they have existed for non-structural applications such as joining a tabletop to its supporting apron. This is perfectly acceptable as long as the connection allows for seasonal movement. Structural joinery in best traditional woodworking practices was a well glued wood-to-wood joint, most often a well designed mortise and tenon or dovetail joint.

Factory furniture today almost exclusively uses screws and hardware for structural joinery. The simplest example may be angled pocket screws that connect for example a table's apron rail to a leg. A slightly more involved example may be a metal bracket or plate that is attached to the apron rail and leg with screws. Factory bed frames often are assembled with interlocking metal pieces, one of which is screwed to the rail and one screwed to the post.

The first problem with structural hardware joinery in wood is that wood has limited screw holding capability. Understanding Wood: A Craftsman's Guide to Wood Technology by R. Bruce Hoadley (2000) suggests that maximum withdrawal load (p) in pounds can be estimated by

p = 15700 * G * G * D * L where

G = specific gravity of the wood being used,

D = screw shank diameter (inches), and

L = the depth of threads into the receiving piece of wood (inches).

As an example, consider a #10 x 1 1/2" wood screw attaching a 3/4" piece of red oak to a thicker piece of red oak. The specific gravity of red oak is .63, the diameter of a #10 wood screw was measured at .15", and the threads penetrate the 2nd piece 13/16". The withdrawal load is calculated to be 759 pounds.

Red oak is too hard to dent with a fingernail. Cherry, with a specific gravity of .50, can be marked with a fingernail. Substituting cherry in the above formula, under the same conditions, the withdrawal load is calculated to be 478 pounds, only 63% of that of red oak. Substitute an even softer wood and the withdrawal load is even less.

More important, Dr. Hoadley adds "Similarly, holding power diminishes over time, and withdrawal resistance for loading conditions of long duration might be as little as 20% of the values estimated by the formula given above." (Page 191) Coupled with the significant leverage built into an apron rail-leg joint of a dining table, it's understandable that dining tables assembled with screws can fail prematurely.

Factory beds typically are assembled with interlocking metal hangers screwed to the rail and the post. Considering the weights involved and the leverage exerted by long bed rails, it again is not surprising that these hardware joints don't last as long as one might otherwise expect. Traditional bed joinery uses a strong mortise and tenon joint between the rail and post, and a bed bolt simply to hold the joint together - the structural strength is provided by the mortise and tenon.

To prolong the life of hardware joinery, regularly tighten screws, being careful not to overtighten and tear the wood fibers. Minimize leverage on the joints by not pushing sideways on the tabletop or apron. When moving the table, pick it up by its apron and move it, rather than sliding the table across the floor. And look for heavier woods that are not easily dented with a fingernail.