This is an excerpt from “The Woodworker: The Charles H. Hayward Years: Volume III” published by Lost Art Press.
“We’ll glue those wedges and tenons!” How often is this explanation heard when gluing up framed work. A usual response being to dip the wedges into the glue and drive them hard home, unless the wedges break off or bottom badly.
If we analyse the reason for wedging a joint we find that the wedges are provided to ensure a compression in the fibres of the tenons to equalise the inevitable movement due to age and conditions. At the same time it is necessary to provide a mortise with parallel sides for the tenon, so allowing for movement.
Take as an example a through mortised and tenoned wedged joint, the shoulders being tightly fitting to ensure rigidity in the work. In framing up we glue the shoulders and a small adjacent area only of the tenon, to allow the movement along the tenon (see Fig. 3 B). It will be obvious that to solidly glue the whole joint is defeating the essential object of that particular joint.
The logical method would be to glue the shoulders as usual, place the long grain edge of the wedge to the tenon, but do not glue (it may in fact be slightly greased), but gluing the remaining parts of the wedge into the mortise of the stile, making a parallel path for the tenon, but under compression. A joint made in this manner will not open at the shoulders.
In the case of double tenons, drive the outside wedges first to set the compression, the inner ones then being driven to equalise the compression on the tenons.
Good quality work of the old days had the tenons and wedges cut back in the stiles to allow for shrinkage clearance, and a pocket piece let in and flushed off in the stiles, making a workmanlike job (Fig. 1 A).
A Bad Fault. An odious method becoming prevalent to-day is tenon splitting and wedging the tenon out into a fantail in the mortise; it is apparent that the least shrinkage will pull the shoulders right open, when all rigidity in the work vanishes (see Figs. 1B and 3A).
Such a method is only permissible when diagonal wedging in thin material such as carcase construction, shelves to ends (Fig. 2), or in fox-wedging in the appropriate joints.
Selecting suitable joints and framing them up is a complicated matter at times, but consideration on the foregoing lines will amply repay the craftsman in the quality of the work he produces.