A dado is a rectangular groove cut across the grain of a board (Fig. 37) into which another member is fitted.

Laying Out the Joint

In making all joints the lay-out of the work is of equal importance with the cutting operations. If the lines are not where they belong, no amount of skill with the cutting tools will produce good results. Hence, there is need for skill in the correct use of the marking gage, the try-square, the knife, and the rule, not occasionally, but always. Until you have mastered these laying-out tools you are not ready to cut joints.

When, however, you are sure that you can run straight lines and measure accurately proceed as follows:

1. Locate one side of the dado and square a knife line across the piece, remembering to square from the joint edge.

2. Locate and square the other side of the dado by measurement, or better, by superposition, i. e., by placing the piece A upon the piece B and marking its exact thickness.

3. Square the lines across the edges of piece B equal to the depth of the dado.

4. Set the gage to the required depth and gage between the two lines, on both edges. Be sure to gage from the face side.

Notes. In case of duplicate parts:

(1) Measure all distances first on one piece and then transfer them to the others by means of the try-square and knife.

(2) Gage all depths with one setting of the gage and remember to gage and square all lines from the two "marked surfaces," i. e., from the working face and the joint edge.

Fig. 37.

Cutting the Joint

1. Saw to the center of the knife lines keeping the saw kerf in the waste stock.

2. Chisel out the waste stock, working from both edges toward the center.

3. Test the bottom of the dado with a straight-edge to see that it is a true plane.

4. If the sawing was not accurately done you must now pare to the center of the knife line with a chisel.

Notes:

1. If the dado is a wide one, mechanics often tack or clamp a piece of wood across the board B (See Fig. 37) so that when the saw is pressed against the piece, it will cut just to the center of the knife line. Ask some good mechanic to show you how he does this.

2. If the groove does not run entirely across the board, it is called a gain. In this case the "blind end" of the gain must be bored or chiseled out and the rest of the joint sawed and chiseled as in the case of the dado.

Reference Work:

1. Ask some good mechanic to show you how he "gains" the treads and risers into the wall stringers in a flight of stairs.

2. Look up the definition of a rabbet joint, a grooved joint, a forked joint, and a cogged joint and make a sketch of each.

3. Find out and write up the method of making each of the above joints, and if you ever wish to use any of these joints in the construction of an article, be sure to try out your method first on some waste material.

4. As a supplementary problem make a screen and a box filing case, so that all joints that are made as preliminary practice pieces can be mounted and the written record of the method filed. This collection will gradually develop into a very valuable exhibit.