When mortise-and-tenon joints are not held together by external attachments, they are secured either by pins or wedges, and in joinery and small work are usually glued. In very light work glue alone is often relied on; but when this is not sufficient wedges are generally employed as well. Pins are better adapted for rough work, either with or without glue. Wedges hold the parts together by being driven either into, or alongside of the end grain of the tenon, making it club-or dovetail shaped, the mortise being tapered to suit, so that the parts cannot be withdrawn while the wedges are in place. To make a secure point, therefore, glue should be used with wedges. In Fig. A, an ordinary joint is shown with wedges partly driven in. The mortise is cut slightly larger, and tapered to receive them, and the glue holds them to the tenon after they are driven. The ends, of course, are trimmed off flush in finishing the work. In cases where extra security is required the wedges are driven into saw-cuts in the tenon itself. Sometimes four wedges are inserted, but two is the usual number.

Fig. 7. B is a barefaced tenon. This differs from an ordinary tenon in being shouldered from one face only. It is employed chiefly for thin rails which have to be made flush on one side with the posts they are tenoned into.

Fig. 8 shows how a stub tenon is fox-wedged. The wedges in such a case cannot be inserted after the parts are together, and so they have to be forced home by the closing of the joint. Generally, however-, stub tenons are not wedged. Fig. 8 also shows how the saw-cuts, both in stub and ordinary tenons, should be slightly out of parallel with the tenon, to lessen the risk of withdrawal.

When pins are employed for holding the joint, they are inserted from one face of the work, at right angles to the direction in which wedges are used. Screws or nails are often employed in this way, but more commonly wood pins or pegs, roughly pared to octogonal section, with a slight taper lengthwise, which are glued and driven into holes bored for them. Usually they do not go completely through the parts, but remain visible only from the face they are driven in at. When pins are used, they are made to assit in pulling the parts together by draw-boring; that is, by making small holes in the tenon slightly out of centre with the larger ones in the side of the mortise where the pins are entered. The work is usually cramped together before the pins are inserted, so that their function is not so much to pull the parts together as to maintain them so when the cramps are removed.

Mortise And Tenon Joints Continued 172

Fig. 10.

Mortise And Tenon Joints Continued 173

Fig. 11.

In some joints the end of the tenon is made to project considerably beyond the mortise, and a tapering pin is driven through the tenon only, as in Fig. 9. In such a case no glue is used, but the pin itself pulls and holds the joint together as tightly as may be required, and also permits it to be taken apart again by knocking the pin back. Fig. 9 is called a tusk tenon, and is employed for uniting heavy timbers which meet in a horizontal plane, the joint being designed specially to afford the maximum support to the tenoned end, with the minimum weakening of the mortised timber which supports it. As a beam is strained least in the central plane, it is only in that part that the tenon is allowed to pass through. Below the root of the tenon a short tusk is formed to prevent its geting sheared off, and it is similiarly strengthened above by a projection which tapers to nothing at the top, in order to cut as little as possible from the mortied beam. The weight of the tenoned timber is carried partly by the portion the tusk rests on, and partly by the longer support in which the slender tenon bears. In some cases, when the supporting beam is wide, or when timbers . enter on both sides, the tenon is not carried through, but held by a pin inserted through a hole in the mortised timber. All the parts of a tusk tenon are of the same width as the timber they are formed on.

Fig. 10, A and B show dovetailed tenons held in place by folding wedges. In A, one timber only is tenoned into the post, which is usually thicker than the timber that enters it. In B two ends meet, entering the mortise from opposite sides. These joints are employed more for work which has to be taken apart than for fixtures. They are suitable when the timber in which the mortise is cut is thick, and the tenoned pieces thin. The latter, in fact, are generally not, strictly speaking, tenoned at all, but enter at their full thickness, and are dovetailed on one edge. Stub tenons also can be secured in this way.

Fig. 11 is an open mortise and tenon with shoulders mitred instead of square, like Fig. 6 A. It is, of course, not so strong as the latter, and is mitred for appearance. In cases where only one face shows, its strength is often increased by mitring the front only, and leaving the back shoulder square.

In large well-equipped shops, mortises and tenons are cut by machines. By hand, mortises are partly bored out with a bit, and finished to the lines with a mortise chisel, or they are cut by chisel and mallet alone. Tenons are sometimes sawn to the line, but more frequently their shoulders are finished with a chisel, and their faces with a rebate-plane. Scribed and guaged lines are used everywhere in preference to pencil. The thicknesses of tenon and mortise are gauged with a mortise-gauge, which markes both lines simultaneously. The shoulders of the tenon and ends of the mortise are marked with square and scribe. If the tenon is a through one, it should be long enough to allow for dressing off after it is in place. A stub tenon should be a trifle short in its mortise to insure a close fit at the shoulder. The latter should be slightly undercut to insure close contact at the exterior.