Joints and Joining

One basic skill of woodworking is the art of joining pieces of wood to form tight, strong, well made joints. The two pieces that are to be joined together are called members. The two major steps in making joints are (1) laying out the joint on the ends, edges, or faces and (2) cutting the members to the required shapes for joining.

The instruments normally used for laying out joints are as follows:

• Try square
• Miter square
• Combination square
• Sliding T bevel
• Marking or mortising gauge
• Scratch awl
• Sharp pencil or knife for scoring lines

For cutting the more complex joints by hand, the hacksaw, dovetail saw, and various chisels are essential. The rabbet and fillister plane for rabbet joints and the router plane for smoothing the bottoms of dadoes and gains are also helpful.

Simple joints are used mostly in rough or finish carpentry. They may be used occasionally in millwork and furniture making. They include:

• Butt joints
• Lap joints
• Miter joints

More complex joints are used mostly in making furniture and cabinets and in millwork. They include:

• Rabbet joints
• Dado and gain joints
• Blind mortise and tenon joints
• Slip tenon joints
• Box corner joints
• Dovetail joints

Edge joints are used mainly in furniture and cabinet work. They include:

• Dowel joints
• Spline joints

Plain butt joints and tongue and groove joints are used in practically all types of woodworking.

The joints used in rough and finished carpentry are, for the most part, simply nailed together. Nails in a 90° plain butt joint can be driven through the member abutted against and into the end of the abutting member. The joints can also be toenailed at an angle through the faces of the abutting member into the face of the member abutted against, as shown in Figure 18. Studs and joists are usually toenailed to soleplates and sills.

The more complex furniture and cabinet making joints are usually fastened with glue. Dowels, splines, corrugated fasteners, keys, and other types of joint fasteners can provide additional strength. In the dado joint, the gain joint, the mortise and tenon joint, the box corner joint, and the dovetail joint, the interlocking character of the joint is an additional factor in fastening.

All the joints we have mentioned can be cut either by hand or by machine, and are shown in Figure 19.

Whatever the method used and whatever the type of joint, remember, to ensure a tight joint, always cut on the waste side of the line, never on the line itself. Preliminary grooving on the waste side of the line with a knife or chisel will help a backsaw start smoothly.

Butt Joints

A 90° plain butt joint is two boards glued edge to edge or face to edge without overlapping as shown in Figure 20. It is mostly used for thin wood, under 1 inch thick.

A butt joint can have a wood or plywood piece called a fishplate used to fasten the ends of two members together with nails or bolts, shown in Figure 21. These are sometimes used at the junction of opposite rafters near the ridge line.

Half Lap Joints

For half lap joints, the members to be jointed are usually of the same thickness, as shown in Figure 22. The method of laying out and cutting an end butt half lap as shown in Figure 22 is as follows:

1. Measure off the desired amount of lap from each end of each member and square a line all the way around at this point. For a corner half lap, as in Figure 22, measure off the width of the member from the end of each member and square a line all the way around. These are called shoulder lines.
2. Select the best surface for the face and set a marking gauge to one-half the thickness. Score a line, called the cheek line, on the edges and end of each member from the shoulder line on one edge to the shoulder line on the other edge. Gauge the cheek line from the face of each member to ensure the faces of each member will be flush after the joints are cut.
3. Make the shoulder cuts by cutting along the waste side of the shoulder lines down to the waste side of the cheek line.
4. Make the cheek cuts along the waste side of the cheek lines. When all cuts have been made, the members should fit together with faces, ends, and edges flush or near enough to be made flush with the slight paring of a wood chisel.

Other half lap joints are laid out in a similar manner; the main difference is in the cutting method. A cross half lap joint is best cut with a dado head or wood chisel rather than a handsaw. Others may easily be cut on a bandsaw. When you cut a half lap joint this way, be certain to cut on the waste side of the lines and make all lines from the face of the material.

Miter Joints

A miter joint is made by mitering the ends or edges of the members to be joined together as shown in Figure 23. The angle of the miter cut is one half of the angle formed by the joined members. In rectangular mirror frames, windows, and door casing boxes, adjacent members form a 90° angle, so the correct angle for mitering is one half of 90°, or 45°. For members forming an equal sided figure with other than four sides, such as an octagon or a pentagon, find the correct mitering angle by dividing the number of sides the figure will have into 180° and subtracting the result from 90°.

For an octagon, a figure with eight sides, determine the mitering angle by subtracting 180° divided by 8 from 90°. This is shown by the following formula:

90° – (180° ÷8)
or
90° – 22.5° = 67.5°

For a pentagon, a figure with five sides, the angle is:

90° – (180° ÷ 5)
or
90° – 36° = 54°

End miter members to 45° in the wooden miter box and to any angle in the steel miter box by setting the saw to the desired angle, or on the circular saw by setting the miter gauge to the desired angle. Edge miter members to any angle on the circular saw by tilting the saw to the required angle.

Sawed edges are sometimes unsuitable for gluing. If the joint is to be glued, the edges can be mitered on a jointer, as shown in Figure 24. Please note that the saw guard does not appear in this figure to better show the relationship of the blade to the stock.

CAUTION

This is a dangerous operation and caution should be taken.

Since abutting surfaces of end mitered members do not hold well when they are merely glued, they should be reinforced. One type of reinforcement is the corrugated fastener. This is a corrugated strip of metal with one edge sharpened for driving into the joint. Place the fastener at a right angle to the line between the members, half on one member and half on the other, and drive it down flush with the member. The corrugated fastener mars the appearance of the surface into which it is driven, so use it only on the backs of picture frames and the like.

A more satisfactory type of fastener for a joint between end mitered members is the biscuit. This is a thin piece of wood or veneer that is glued into a kerf cut in the thickest dimension of the joint. Use the biscuit in the following manner:

1. Saw about halfway through the wood from the outer to the inner corner.
2. Apply glue to both sides of the biscuit, pushing the biscuit into the kerf.
3. Clamp it tightly and allow the glue to dry.
4. After it has dried, remove the clamp and chisel off the protruding portion of the biscuit.

A joint between edge mitered members can also be reinforced with a spline. This is a thick piece of wood that extends across the joint into grooves cut in the abutting surfaces. A spline for a plain miter joint is shown in Figure 25. Cut the groove for a spline either by hand or with a circular saw.

Grooved Joints

A three-sided recess running with the grain is called a groove, and a recess running across the grain is called a dado. A groove or dado that does not extend all the way across the wood is called a stopped groove or stopped dado. A stopped dado, also known as a gain, is shown in Figure 26.

A two-sided recess running along an edge, as shown in Figure 27, is called a rabbet T. Dadoes, gains, and rabbets are not, strictly speaking, grooves; but joints that include them are generally called grooved joints.

Cut a groove or dado with a circular saw as follows:

1. Lay out the groove or dado on the end wood (for a groove) or edge wood (for a dado) that will first come in contact with the saw.
2. Set the saw to the desired depth of the groove above the table, and set the fence at a distance from the saw that will cause the first cut to run on the waste side of the line that indicates the left side of the groove.
3. Start the saw and bring the wood into light contact with it; then stop the saw and examine the layout to ensure the cut will be on the waste side of the line.
4. Readjust the fence, if necessary. When the position of the fence is right, make the cut.
5. Reverse the wood and proceed to set and test as before for the cut on the opposite side of the groove. Make as many recuts as necessary to remove the waste stock between the side kerfs.

The procedure for grooving or dadoing with the dado head is about the same, except that in many cases you can build up the dado head to take out all the waste in a single cut. The two outside cutters alone will cut a groove 1/4 inch wide. Inside cutters vary in thickness from 1/16 to 1/4 inch.

Cut a stopped groove or stopped dado on the circular saw, using either a saw blade or a dado head, as follows:

1. If the groove or dado is stopped at only one end, clamp a stop block to the rear of the table in a position to stop the wood from being fed any farther when the saw has reached the place where the groove or dado is supposed to stop.
2. If the groove or dado is stopped at both ends, clamp a stop block to the rear of the table and a starting block to the front. Place the starting block so the saw will contact the place where the groove is supposed to start when the infeed end of the piece is against the block.
3. Start the cut by holding the wood above the saw with the infeed end against the starting block and the edge against the fence.
4. Lower the wood gradually onto the saw and feed it through to the stop block.

A rabbet joint requires two cuts; the cut into the face of the wood is called the shoulder cut, and the cut into the edge or end is called the cheek cut. A rabbet can be cut on the circular saw. Make the shoulder cut first, as follows:

1. Set the saw to extend above the table to the desired depth of the cheek.
2. Be sure to measure this distance from a sawtooth set to the left, or away from the ripping fence. If you measure it from a tooth set to the right or toward the  fence, the cheek will be too deep by an amount equal to the width of the saw kerf.

By using the dado head, you can cut most ordinary rabbets in a single cut.

1. Build up a dado head equal in thickness to the desired width of the cheek.
2. Set the head to protrude above the table to the desired depth of the shoulder.
3. Clamp a 1-inch board to the fence to serve as a guide for the piece, and set the fence so the edge of the board barely contacts the right side of the dado head.
4. Set the miter gauge at 90°. Set the piece against the miter gauge, hold the edge or end to be rabbeted against the 1-inch board and make the cut.

On some jointers, a rabbeting ledgeattached to the outer edge of the infeed table can be depressed for rabbeting, as shown in Figure 28. Please note that the saw guard does not appear in this figure to better show the relationship of the blade to the stock.

The ledge is located on the outer end of the cutterhead. To rabbet on a jointer of this type, depress the infeed table and the rabbeting ledge the depth of the rabbet below the outfeed table, and set the fence the width of the rabbet away from the outer end of the cutterhead. When the piece is fed through, the unrabbeted part feeds onto the rabbeting ledge. The rabbeted portion feeds onto the outfeed table.

Various combinations of the grooved joints are used in woodworking. The tongue and groove joint is a combination of the groove and the rabbet, with the tongued member rabbeted on both faces. In some types of paneling, the tongue is made by rabbeting only one face. A tongue of this kind is called a barefaced tongue. A joint often used in making boxes, drawers, and cabinets is the dado and rabbet joint, shown in Figure 29. As you can see, one of the members is rabbeted on one face to form a barefaced tongue.

 

Mortise and Tenon Joints

The mortise and tenon joint is most frequently used in furniture and cabinet work. In the blind mortise and tenon joint, the tenon does not penetrate all the way through the mortised member. This type of joint is shown in Figure 30.

A joint in which the tenon does penetrate all the way through is a through mortise and tenon joint, shown in Figure 31.

Besides the ordinary stub joint seen in Figure 31 view A, there are haunched joints, as seen in view B, and table haunched joints, as seen in view C. Haunching and table haunching increase the strength and rigidity of the joint.

The layout procedure for an ordinary stub mortise and tenon joint is shown in Figure 32.

The shoulder and cheek cuts of the tenon are shown in Figures 33 and 34. Please note that the saw guard does not appear in these figures to better show the relationship of the blade to the stock.

To maintain the stock upright while making the cheek cuts, use a push board similar to the one shown in Figure 34. Tenons can also be cut with a dado head by the same method previously described for cutting end half lap joints.

Mortises are cut mechanically on a hollow chisel mortising machine like the one shown in Figure 35. The cutting mechanism on this machine consists of a boring bit encased in a square, hollow, steel chisel. As the mechanism is pressed into the wood, the bit takes out most of the waste while the chisel pares the sides of the mortise square. Chisels come in various sizes, with corresponding sizes of bits to match.

If a mortising machine is not available, the same results can be attained by using a simple drill press to take out most of the waste and a hand chisel, for paring the sides square.

In some mortise and tenon joints, such as those between rails and legs in tables, the tenon member is much thinner than the mortise member. Sometimes a member of this kind is too thin to shape with shoulder cuts on both faces in the customary reamer. When this is the case, use a barefaced mortise and tenon joint. For a barefaced joint, cut the tenon member on one side only. The cheek on the opposite side is simply a continuation of the face of the member.

Mortise and tenon joints are fastened with glue and with additional fasteners, as required.

Dovetail Joints

The dovetail joint, shown in Figure 36, is the strongest of all the woodworking joints. It is used principally for joining the sides and ends of drawers in fine grades of furniture and cabinets. In a Seabee unit, you will seldom use dovetail joints since they are laborious and time consuming to make.

A through dovetail joint is a joint in which the pins pass all the way through the tail member. Where the pins pass only part way through, the member is known as a blind dovetail joint.

The simplest of the dovetail joints is the dovetail half lap joint, shown in Figure 37.

Figure 38 shows how this type of joint is laid out, and Figure 39 shows the
completed joint.

A multiple dovetail joint is shown in Figure 40; Figure 41 shows how the waste is chiseled from the multiple joint.

Box Corner Joint

With the exception of the obvious difference in the layout, the box corner joint shown in Figure 42 is made in a manner similar to that of the through multiple dovetail joint.

Coping Joints

Inside corner joints between molding trim members are usually made by butting the end of one member against the face of the other. Figure 43 shows the method of shaping the end of the abutting member to fit the face of the other member:

1. Saw off the end of the abutting member square, as you would for an ordinary butt joint between ordinary flat faced members.
2. Miter the end to 45°, as shown in the first and second views of Figure 43.
3. Set the coping saw at the top of the line of the miter cut, hold the saw at 90° to the lengthwise axis of the piece, and saw off the segment shown in the third view, following closely the face line left by the 45° miter cut. The end of the abutting member will then match the face of the other member, as shown in the third view. A joint made in this reamer is called a coping joint. You will have to cut coping joints on a large variety of moldings.

Figure 44 shows the simplest and most common moldings and trims used in
woodworking.