Basic Woodworking Processes

Herman Hjorth

Milwaukee; The Bruce Publishing Co., 1935
Fifth Printing

Chapter XII: MORTISE-AND-TENON JOINTS

The mortise-and-tenon joint is, no doubt, the oldest, strongest, and most appropriate method of joining two pieces of wood together. It is used extensively in all good cabinetwork, in carpentry and interior woodwork, and also in patternmaking.

The mortise-and-tenon joint is made with many variations, some of the most important being the blind, the through, the haunched, the keyed, and the slip joint. All mortise-and-tenon joints consist of two parts, the "tenon," which is a rectangular projection on the end of one member, and the "mortise," which is a rectangular hole chiseled on the edge or side of the other member, and into which the tenon is fitted.

92. Making a Blind Mortise-and-Tenon Joint.

This joint is used mostly in cabinetwork, because it is neater in appearance than the other types. In a blind mortise-and-tenon joint, the tenon is concealed within the other member. The processes of making a mortise-and-tenon joint may be divided as follows: laying out, making the mortise, making the tenon.

Laying Out.

1. The pieces are squared to dimensions and their faces are marked. The tenon is called member A, and the mortise, member B (Fig. 238).

2. The length of the tenon is laid out on piece A, and a line is squared all around it at this point. In ordinary cabinetwork, tenons usually are from / to 2 in. long, depending upon the size of the mem­bers. The longer the tenon is the more holding power it has.

3. The total width of piece A is laid out on, piece B, and a line is squared all around it at this point. In this case the width of A is laid out from the end of B.

4. The thickness of the tenon is determined. This usually is from one third to one half the thickness of the stock. The points of a mor­tise gauge (Laying-Out Operation 14) are set to this distance, and the head is adjusted so that the double line will be marked in the middle of piece A. Piece A is gauged on the end and edges from the face, as far as the line squared around it.

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5. The width of the mortise is the same as the thickness of the tenon, and should be gauged at the same time and from the face. If the pieces are to be flush on the face side, the setting of the mortise gauge is not changed. If the tenon member is thinner than the mortise member and is to join it in the center, as for example the rail and leg of a table, the block of the gauge is moved correspondingly, but not the setting of the two points (Dowel Joints, Art. 83, step 6, and Fig. 220).

6. A tenon usually is both thinner and narrower than the piece on which it is cut. This is done so that the mortise will be entirely covered when the joint is assembled. When the mortise is made at the end of the piece as in Figure 238, at least / in. is taken from the width of the tenon to prevent the end wood of the mortise from tearing out. These measurements are laid out simultaneously on both the tenon and mortise members.

On ordinary work the width of the tenon usually is from / to 1 in. less than the width of the tenon member. On pieces more than 8 in. wide, a double tenon is made in order not to weaken the mortise member by cutting too large a hole (Fig. 256).

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7. In mortise-and-tenon construction there usually are several joints of the same dimensions to be made. In order to make a good job, all these joints are laid out at the same time before changing the setting of the gauge (Fig. 240).

8. When laying out the mortises and tenons on a table, stool, or other structure having four legs, the legs are placed together and are marked right front, left front, right back, and left back. To further guard against mistakes, the outside surfaces are marked "out." The mortises are gauged from these outside surfaces (Figs. 239 and 240).

Summary of laying-out processes:

a) The length of all the tenons is marked, and lines are squared all around.

b) The total width of all the tenon members is marked on the mortise members, and lines are squared all around.

c) The thickness of all the tenons is gauged.

d) The width of all the mortises is gauged with the same setting of the gauge.

e) The width of all the tenons is marked and laid out on all mortises.

f) The work must be checked carefully before starting to cut.

Making the Mortise. 9.

The mortise can be made in two ways, either by boring a series of adjoining holes and cleaning out the reining wood with a chisel, or by making the hole entirely with 1 chisel.

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10. Using the first method, the piece is clamped in the bench vise, and a line is gauged exactly in the center between the two lines already gauged for the mortise. A number of holes are bored as closely to­gether as possible along this center line using an auger bit or a dowel bit of the same diameter as the width of the mortise. A gauge is used on the bit (Boring Operation 59) so that the holes are all bored to the same depth, which should be slightly more than the length of the tenon. Care must be taken to bore the holes straight (Fig. 241).

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11. The small pieces of wood left between the holes are chiseled out. A chisel with a short, wide blade is used for the sides of the mortise, and a narrow chisel for the ends. The chiseling must not go beyond the gauge lines (Fig. 242). If the bit has gone a trifle outside the gauge lines here and there, this will not be seen when the joint is finished and will not materially affect its strength.

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12. Using the second method, a mor­tise chisel of the same width as the mortise is selected. A mortise chisel is very thick below the handle to prevent it from breaking when the shavings are forced out of the hole (Fig. 130). The piece is clamped on top of the bench, and cutting is begun by chiseling out triangular pieces in the center of the mortise until the full depth is reached. A strip of paper glued around the chisel serves as a depth gauge (Fig. 243).

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13. The chisel is held perpendicularly with the bevel toward the center of the mortise which is chiseled to both ends (Fig. 244). The shavings are broken loose by pushing the handle toward the center of the mortise. A try-square placed alongside the work aids materially in making perpendicular cuts.

Making the Tenon. 14.

The piece is clamped diagonally in the bench vise as shown in Figure 245. In this way the gauge lines on the end and on one side can be watched at the same time. A backsaw is used on small pieces. On large pieces a ripsaw may be used. The piece is sawed outside the lines until the line marking the length of the tenon has been reached. The piece is reversed in the vise and is clamped in a vertical position with the edge not sawed toward the worker. When finishing the saw cuts it is only necessary to watch one gauge line, because the saw cut already made serves as a guide for the saw. "These w cuts are called cheek cuts.

15. Next, the other cheek cuts are sawed, marking the width of the tenon (Fig. 246).

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16. Now the tenon member is sawed across the grain along the line squared around it. These cuts are called the shoulder cuts. As it is very important that the shoulders of the tenon are sawed straight and square, either a triangular groove is cut with a chisel, in which to start the saw (Figs. 247, 248, and 249), or a block is clamped right on ,lie line to serve as a guide for the saw (Fig. 73).

17. The piece is clamped in a vise or it is held against a bench hook, and the shoulder cuts are made with a hacksaw (Fig. 250).

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18. Any unevenness is cleaned up with a sharp chisel, and the mortises and tenons are fitted together. The ends of the tenons are beveled slightly so that they will enter the mortises easily. If the tenon fits too tightly, the joint should not be forced together, but the sides or the mortise or the tenon should be pared with a chisel. The join should fit so that it can be pressed together with the hands. Each corresponding mortise and tenon is numbered so that the joints can b, correctly and rapidly assembled when gluing.

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19. A blind mortise-and-tenon joint often is strengthened by driving a pin through it (Fig. 251). When the appearance is unimportant this usually is an ordinary dowel, otherwise a square or triangular pin is used.

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20. If the shoulder cuts on a tenon are uneven, they may be made straight as follows: The piece is clamped firmly, and a line is cut all around the shoulders of the tenon with a knife or a sharp chisel (Fig. 252). It is chiseled down vertically on this line on both sides with a /-in. chisel and a mallet (Fig. 253). The piece then is clamped in the vise with the tenon up, and the remaining parts are removed on both ends with a /-in. chisel (Fig. 254). Corresponding tenon members must be cut to the same length.

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