This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
This joint is so important and so constantly employed in one modification or another in almost all branches of carpentry and joinery that it deserves special description at some length. The gauge used for marking out the mortice has been spoken of on p. 186; and the use of the chisel in cutting it out has been explained on p. 232. In cutting the tenon, a very sharp and accurately set saw should be used, so that the edges left will need no paring or trimming of any kind. The simple mortice and tenon have been shown on p. 233. In sawing the shoulders of a tenon, there should be just a tendency to undercutting them, as a safeguard against rounding them. A few words may be said about wedging and pinning. Suppose that a tenon nicely fitted is to be wedged and glued. Taking it out of the mortice, the latter has a wedge-like portion cut out on each side to be filled in by a pair of wooden wedges of similar form. If these are made short and blunt, they will not be able to be driven home, but will jump back, and have no effect in tightening up the joint by drawing the parts together. The wedges should be long in proportion to their thickness. The object is to convert a straight tenon into a dovetailed shape, which cannot be drawn back out of its mortice.
The whole tenon and the wedges are carefully glued with hot glue, about as thick as cream, the wood having also been well warmed. The joint is driven up, wedged, and left to dry. In pinning a tenon and mortice through (which is always the method used in heavy carpentry), having cut and fitted the parts accurately, bore through the mortice carefully at right angles, having just removed the tenon. Use for this a shell or nose bit in a brace. Now insert the tenon, put the nose bit in again, and just begin to bore the tenon sufficiently to mark it. Take it out and bore the hole about 1/20, in. nearer the shoulder of the. tenon than you would have done if it had been left in its mortice and bored while therein. Then make a nice oak pin, and not too tapering, but a tight fit; as it enters the hole in the tenon, it will draw it in close in the endeavour to bring its hole true with those of the mortice. It must not be so bored that it cannot draw in, and so will be in danger of tearing and splitting: but must almost tally at the outset with the other holes.
This forms a perfect joint that can (if need be) be at any time separated by knocking out the pin, which is sometimes left long that it may be more readily driven back
As an example of more difficult fitting, it sometimes happens that the mortice is cut in a piece of hexagonal form, or rather section of that nature, and that a rail has to be fitted in which the shoulders of the tenon must be so made as to embrace the parts about the mortice. Fig. 511, a and 6, represents such. The shoulders c, d, are specially difficult to pare, owing to the angular direction of the grain, as the natural way of cutting such a surface smoothly would be to work from x to y of e, and this cannot be done in this case. It may be pared with a chisel more readily when laid down on its side, as at f, the chisel cutting perpendicularly; but the angles frequently prohibit the chisel from cutting into them closely. Still, there is no help for it, and there is no job which requires a sharper tool deftly managed. When the work is small, the finest saw, used carefully, may suffice without any subsequent paring, and is the safer tool to use. When, however, the parts are to be constructed of wood of more than usually curled grain, it may suffice to cut a recess into the standard, to receive the hexagonal rail itself beyond its tenon, Fig. 512, a, b, and c, where the mortice is shown quite black, and the recess is shaded.
Neatly done, the effect is the same as when the shoulders are cut, as in the previous case; but allowance must be made in the length of the rail, or it will, of course, be too short when fitted into its place. The first plan, even if well done, is not so strong as the second, and, in an outdoor job, where exposed, the latter would be far less liable to admit rain to injure the tenon; but there are many cases in which the same kind of fitting is needed where a plan similar to that first described is essential. It should be borne in mind that a mortice and tenon ought to just slide stiffly into place, without requiring a lot of knocking with the mallet.
A curious form of mortice and tenon is shown in Fig. 513, and is made in the following manner: - Get 2 pieces of clean, straight-grained yellow pine, recently cut from the log that is not seasoned, 9 in. long, 1 1/4 in. broad, and 7/8 in. thick. In the middle of one of these make a 1/4-in. mortice 1 1/4 in. long, as at a; and on the other piece, after it has been dressed to § in. thick at 3 in. from one end, make a tenon 1/4 in. thick and 1 1/4 in. long, as at b, and taper the other end as shown, so as to make it easy to introduce into the mortice. Then get both pieces steamed, and while they are heating prepare something to support the sides of a, so as to prevent it from splitting when b is being driven through, and a strong cramp or vice to compress b. When the wood is thoroughly steamed, place b in the vice or cramp, with a piece of hard wood on each side, so as to press its whole surface from the tenon to the tapered end equally, and screw up as hard as possible. Withdraw a from the steam, and place it in its prepared position; try the screw again on b; then take it out, enter its tapered end into the mortice, and drive through until the shoulders that have not been pressed rest on a; put them into warm water for several hours, then take them out and dry; afterwards cut all the arms to an equal length, and clean off.
It will allow of examination better if the tenon on b is made 2 in. long, so as to enable o to be moved along, us when all is firmly together it will be at once asserted that the cross is made of 3 pieces. Obviously no practical carpenter would use such a joint, as the wood must suffer much in the unequal compression and expansion of its fibres, besides giving no particular strength. It is a sort of puzzle in joinery.