Of the various joints used by woodworkers in the several branches of the craft, none is more important than the mortise and tenon. Indeed, it may be classed as fundamental, for it enters into every sort of wooden construction from a Howe
truss or a trestle bridge to a Chippendale chair or a fancy cabinet.
A Universal Joint
Properly proportioned, it forms one of the strongest methods, and certainly the most all round effective one, for joining framing of almost every description. Some
Suggestions for the design and application of a universal joint familiar to all workers in wood
consideration of the various forms of this universal joint and of the rules for its design under varying conditions are therefore here presented, chiefly for the benefit of the beginner readers of this magazine.
The simplest form of mortise and tenon joint is to be found in the case of rough constructive framing where two pieces of timber are to be joined at right angles, uncomplicated by panels or other considerations which affect the form of the joint in almost all the finer work of joinery or cabinetmaking. In such a case, the rule is to make the tenon one-third of the thickness of the material as shown in Fig. 1
In passing it should be noted that this proportion ought never to be exceeded, for while the tenon is often made less than one-third the thickness of the material without detriment, a tenon greater than one third the thickness of the material leaves the “cheeks” of the mortise weak in proportion to the strength of the tenon.
Fig. 2 is one of the first modifications of the simple mortise and tenon joint found necessary in framing and is used where the mortise is at the end of a piece. Such a tenon is said to be “haunched” or “relished,” the idea being to leave a solid portion at the end of the mortised piece. It will be noted that a small piece of that part of the tenon which is cut away to form the “haunch” or “relish” is left on and fitted into a corresponding groove in the mortised piece. This small part is not always left parallel but is often cut back to nothing at the outer end, as shown in the case of the table leg and rail in Fig. 3.
Width of Tenons The width of tenons in joinery and cabinetmaking is another factor which must receive consideration, for it is bad construction to make a tenon too wide in proportion to its thickness. The effect on such a tenon when its wedges are driven in is shown in Fig. 4,
where the tenon is buckled by the pressure of the wedges and the cheeks of the mortise forced out. The rule for the width of a tenon is that it should not exceed five or six times its thickness. A familiar application of this rule is in the case of wide rails of framing where the tenons are formed at each edge of the rail, leaving a relished portion in the centre, as in Fig. 5.
For the bottom rail of an ordinary panel door a combination of Figs. 2 and 5 is necessary and is shown at Fig.6
Warning as to Wedges
To save complications in the drawings, no wedges, or provision for them, have been shown, but in joinery work at all events most mortise and tenon joints are well wedged. In this connection a word of warning as to the form of wedges may not be out of place, for it is a common fault with beginners to make their wedges of too great an angle. To be most effective a wedge should have an angle of not more than 5 or 6 degrees, as is shown in Fig. 7.
Fig. 2 was spoken of as the joint for the angle or end of a piece of framing but where the framing is oblique a special “open,” “slip” or “slot” mortise and tenon joint is used. This is shown in Fig. 8 and is used by joiners in framing the triangular panelling in the spandrel or “drag” often placed underneath a flight of stairs to form a closet or enclose a lower flight.
In cabinetmaking the through tenon is seldom used because of the unsightly appearance of its end grain on the edge of the framing. Given that the tenon fits properly so as to fill the mortise completely, there is no doubt that the short tenon is perfectly satisfactory for indoor work. If, however, the door or framing is to be exposed to the weather, the old-fashioned method of concealed wedging, known as “blind” or “fox” wedging, is to be recommended.
Fig. 9 is a sectional view of a fox- wedged tenon and shows several slim wedges inserted in saw kerfs in the end of the tenon, ready to be driven home as the tenon enters the mortise. The mortise is of course made slightly larger on the inside to allow of the consequent spreading of the end of the tenon, the whole arrangement forming a very effective joint.
Double Tenons of Doors A common requirement in architects’ specifications for first-class doors is that the lock or middle rail shall be double tenoned on the outer stile. Such an arrangement is
shown in Fig. 10, the idea being to allow of the insertion of a mortise lock without destroying the tenon, which would occur if the usual single tenons in the centre were provided. A very effective form of mortise and tenon formerly common in constructive work is
shown in section in Fig. 11 where one side of the mortise is formed to fit the dovetail shape of the edge of the tenon. Its chief use was in attaching to the backs of solid door frames the blocks which are built into the walls by the masons to hold the frame in its position.
Another joint familiar to everyone before the days of wire nails and “balloon” framing was the “stub” mortise and tenon used at the junction of the corner post with the sills of a building and shown in Fig. 12.
No account of the various mortise and tenon joints would be complete without a description of that much beloved one of the old carpenters, the tusk tenon joint. In fact, wherever floor timbers are properly framed today the joint is still used and is a most effective one.
Fig. 13 shows the joint and it will be seen that it is quite scientific, inasmuch as the wood in the mortised or bearing piece is cut away chiefly on the neutral axis, that is, in the centre of its depth where its fibres are theoretically neither in compression nor tension when the beam or joist is loaded.
Two or three methods of laying out the joint are used, probably the best being that shown in Fig. 13a.
The depth of the timber is divided into six equal parts and the tenon made equal to one of them and laid out in centre. The notch or step below is made a half of the remaining depth and the upper shoulder is sloped off from a point immediately above the line of the notch. The complications of the simple mortise and tenon joint arising out of the use of mouldings, panels, rebates, etc., are very numerous, but speaking broadly, the main things to be observed in designing this joint are the proper proportions of the thickness and width referred to in the early part of this article.