When beams or pieces of wood stand square with each other, and the strains are also square with the pieces and in the plane of the frame, the most common junction is the mortise and tenon.
A mortise is an opening, which may be square or oblong, intended to receive the tenon, and which may go into the work only a short distance, or may go all the way through. Where it goes only part way through it is called a blind mortise, and where it passes all the way through, a through mortise. A tenon is a projection on the end of a piece and fits into the mortise. The tenon usually has two shoulders formed by cutting away the sides, and should be about one third the thickness of the piece.
Fig. 64.
There are a number of different styles of this joint and methods of fastening, which we will consider later in our work.
The working drawing shown in Fig. 64 gives the dimensions of the pieces, the material of which is to be worked out in one piece, as directed in the previous exercise, and then cut up into lengths suitable for the exercise. The student should commence work on a piece with a full understanding of what is required to be done in order to finish the work as called for, and not try to make any kind of work do in order to proceed to the next task.
Notice what is required in this exercise:
1st. That the pieces be perfectly straight and square.
2nd. That the tenon piece be exactly in the center of the mortise piece, and that the angles be right angles or “square.”
3rd. That the work be laid out systematically, with the lines in their proper places.
4th. That the tenon be made altogether with the saw.
5th. That the mortise be cut out with the hand mortising chisel.
6th. That the tenon fit into the mortise, and not be squeezed.
7th. That the joint fit closely, and that the work be finished off smoothly on the sides, with all the corners sharp and the end of the pieces sawed square.
The following methods if carried out will help the student to finish the work as required.
It will be unnecessary to repeat hereafter the method of planing, as the student by this time should have learned to plane the pieces properly to dimensions.
After the material has been planed, mark the piece to the desired lengths as shown in Fig. 65, and saw off the pieces square on the ends. Use the knife to mark the lines. In sawing, care must be taken to saw on the right side of the line, for the saw will cut out its own thickness and reduce the length of the piece that much if the piece is sawed on the wrong side of the line.
Fig. 65.
Leave the tenon piece about ⅛ inch longer than the drawing calls for so that the tenon will protrude through and be finished off even with the mortise piece.
Fig. 66.
Proceed to lay out the work. Take the mortise piece, which is 6 inches long, and mark the distance from one end (6″-1½″ = 4½″.) 4½″ / 2 = ⁹/₂ × ½ = ⁹/₄ or 2¼″; locate the first point on the face edge; then measure from this point the width of the tenon piece, which is 1½″. Through the points just found draw the lines square to the face side. Place the square against the face edge and mark (on the opposite edge on the corner), a small cut for both lines (see Fig. 66) and square from the face side across the edge; lay the piece aside; take the tenon piece and point off the distance from the end of the piece to the shoulder, and mark across the face and back, using the square and the knife in marking. Prepare the shoulder lines for the back-saw, as shown in Fig. 53, taking care that the notch is cut on the right side of the line.
Take the gauge and set it to the distance from the face side to the first side of the mortise, and gauge the lines for the mortise on both edges; gauge the lines for the tenon. (This is for a single gauge.) Take the mortise chisel, Fig. 67, and make a mark from this line (see Fig. 68), which will give the thickness of the tenon and the width of the mortise; set the gauge out to the width and gauge the rest of the lines.
Fig. 67.
Fig. 68.
Take the rip-saw and saw down the outside of the lines on the tenon piece the length required; cut off the sides with the back-saw. In sawing split the line so that the tenon will be as thick as the mortise is wide. This means that half the line is to be left on the work. Fasten the mortise piece in the vise, putting a piece below to keep it from going down when cutting.
Place the mortise chisel about the center of the mortise; hold it vertically, and with the mallet drive the chisel down into the work; release the chisel and make a new cut, keeping the flat side of the chisel towards the end to which the mortise is being cut. Fig. 69 shows how the cutting should be done.
Having reached the end, turn the chisel around, and cut towards the other end in the same manner. (Where the mortise goes through it will be unnecessary to take the chips from the first side.)
Fig. 69.
Fig. 70.
Turn the piece over, and repeat the operation on the other side, when the chips can be easily removed. Proceed to test the work; see that the mortise is straight on the ends. Generally the student will leave the ends rounding as shown in Fig. 70; this, if the tenon is driven into the mortise, will squeeze the edges out of true (Fig. 71) and leave an opening on the ends of the mortise, as shown in Fig. 72.
Fig. 71.
Fig. 72.
Care should be taken to avoid this fault in this exercise. (A mortise gauge such as a joiner uses is shown in Fig. 73; it has two spurs, one being adjusted by the thumb screw at the end of the shank. We will use a mortise gauge in our work later on.)
Fig. 73.
After the pieces have been cut, put them together, having the face sides together, and finish smoothly.