Paragraph 66. The closed mortise and tenon joint is one of the oldest and most commonly employed joints of woodwork. It is formed by cutting an opening, or mortise, in one piece of material B and shaping the end of another piece of material A to enter this opening. This joint is usually assembled by the use of a wood pin, which gives it great strength in every direction. It was formerly the most important joint in house, barn and bridge construction, because houses were originally built with large timber in the frame. This joint is not common in modern house construction because the large timbers have gone out of use. It is, however, used considerably yet in mill and bridge construction where timbers are employed.
Be sure that the material is perfectly square. In laying out mortises a special gauge is generally used. This gauge is similar to an ordinary marking gauge, except that it has two points, both of which are adjustable. Set the mortising gauge so the two points will be as far apart as the size desired for the mortise and so the head of the marking gauge will be as far from the first point as the distance you desire the mortise from the working face. From the working face gauge the width of the mortise on the working edge of piece B. Determine the width which the mortise is to be and with a try-square, square these lines. In a blind mortise or tenon joint the depth of the mortise is not laid out, but you should determine how deep the mortise is to be and use a bit-gauge or some other device to determine how deep to bore. As the wood is to be cut away in forming the mortise, select a bit which will bore a hole about the size between the gauge lines. Bore as many holes as convenient without over-reaching the layout. With a sharp chisel cut out the wood to the gauge line. Be sure that the sides of the mortise are cut down exactly perpendicularly in every direction. Determine the length of the tenon and square a line this distance from the working end of material A on the working face.
In most projects the length of the tenon is determined by the nature of the work, the strength required, etc. With a try-square and lead pencil carefully square this line on both faces and edges of material A. With the mortising gauge set exactly as it was in laying out the mortise, gauge the width of the tenon across the end and down the edges to the square line. Hold material on the bench hook and with the back saw, saw down to the gauge line. Place the material in the vise and saw the tenon as illustrated in Chapter 2, Paragraph 14. If there is to be a relish on one or both sides of the tenon, lay out this relish and saw out with the back saw. Test the tenon to make sure that it exactly fills the mortise; pare with a keen chisel until it will enter the mortise without any danger of splitting. Remove the tenon from the mortise and bore a hole which is to receive the fastening pin. Bore this hole first through piece B, which contains the mortise, then in the tenon bore a hole a tiny bit closer to the shoulder than in the mortise. This will have a tendency to pull the tenon tightly into the mortise when the pin is driven. This process is called draw boring. It was always used in the framing of buildings in order to make sure that the joint would be as tight as possible when assembled.