Joinery Basics

The best way to hold together a high-end woodworking projects, whether you're building a timber-frame home, putting together a trestle table or making a step stool, is with a bit of glue and hand cut joints.

It is easier to use mechanical fasteners, like nails and screws, but to add that look of first-class work, with smooth, metal free visible areas, nothing will do the job like a direct joint between parts, bonded with glue. Of course, the type of joint you need depends on a variety of factors, like the nature of the materials, the function of the joint, strength and appearance, available equipment, and your own level of skill. Joinery may be intimidating to the beginner, but, like any other building skill, all it takes is a bit of practice.

Here we dissect the workings of two primary joints, the edge joint and the mortise and tenon. WIth these joints, you can build a wide array of furniture and tackle a number of woodworking projects without having to rely on unsightly (although time-saving) nails and screws.

The first requirement of a good edge joint is that the two mating surfaces must fit together perfectly with no discernible gaps. Second, the mating surfaces must be either on the edge or the surface of a board. End grain is not a candidate for edge joining because of its open cellular structure. When glue is applied to these cells, they act like straws, pulling the glue deep into the wood instead of leaving it near the surface where the bond takes place. When end grain must be joined to edge or face grain, the joint of choice is the mortise and tenon.

For edge joining, the mating surfaces must be flat and square to both faces of the board. To achieve this, first scribe a straight reference line on one surface, using a long straightedge. Then clamp this board to the side of your worktable and use a bench plane to flatten the edge. Check your progress relative to your reference line frequently. And check for square frequently with a combination square.

Once you'e satisfied with the edge on the first board, repeat the same process on the mating board. When you've flattened this edge, lay the two boards together on a flat surface and check for fit. Usually some additional work will be required to get a perfect joint. When you've achieved it, just spread glue on both mating edges.

Clamp the boards together until the glue sets.

One common problem with edge joining is that the glue often acts as a lubricant, causing the boards to slip, and ruining a flat joint. There are three common solutions to this problem: dowels, joining plates and splines. Dowels are the best choice for the beginner. All you need for the job is a drill and a doweling jig.

For standard 13/16-in.-thick stock, 1/4-in.-dia. x 1-in.-long dowels are a good choice. Start by laying out the dowel locations every 6 in. to 8 in. along the joint. Next, install the 1/4-in.-dia. bushing in your doweling jig and center the hole in the jig bushing over your first mark. Tighten the jig in place and bore a hole in the edge. Make sure that the hole is deep enough to allow a 1/16-in. space at each end of the dowel for excess glue. Repeat the same procedure for all the holes along the joint.

Birch dowel stock, in 36-in. lengths and in diameters from 1/8 in. to 1 in., is commonly available at hardware stores and lumberyards. When using this material, it's a good idea to cut a narrow groove down the length of each piece to create an escape route for excess glue. You can use the corner of a sharp chisel to scratch the side of the dowel. You also should slightly bevel both ends of the dowel with a piece of sand-paper. This bevel makes aligning the dowels in their mating holes easier.

Once your dowels are cut to size, spread a thin layer of glue in all the dowel holes and along the edges of the mating boards. Then gently tap the dowels into the holes.

Align the mating board so the exposed dowels meet their corresponding holes and use clamps to pull the joint tight. Tighten the clamps slowly to allow any excess glue to escape and leave the joint clamped until the glue sets.

The mortise-and-tenon joint is the best way to join end grain to long grain. The tenon is the male portion of the joint that is cut on the end of one board. It's designed to fit into an identically sized slot, the mortise, in the mating board.

Beginners often avoid this joint because the skills required seem out of reach. But if you take care in layout and cutting, you can easily achieve good results. Of course, it's always a good idea to practice on some scrap wood first. The tools you'll need are a combination square, marking gauge, drill, doweling jig, backsaw and sharp chisel.

To lay out the joint, begin by marking the tenon shoulder line. This represents the length of the finished tenon, which is usually 1 in. to 1 1/2 in. long. Use a square and pencil to extend this shoulder mark to both sides and edges of the board.

Set up your marking gauge to scribe the tenon width on the center of the board end. For 13/16-in.-thick stock the tenon is usually 3/8 in. thick with 7/32-in. shoulders on both sides. But a 5/16-in.-thick tenon with 1/4-in.-thick shoulders is also perfectly acceptable. Scribe these guide lines across the end grain and down the two edges till they meet the shoulder line.

Clamp the board in place with the joint end pointing up and use a backsaw to cut along the guide lines. Be sure your saw kerf always stays on the waste side of the line, and stop cutting when you reach the shoulder mark.

Clamp this board flat on your worktable and use a backsaw to cut along the waste side of the shoulder line. When this cut is complete, the waste should fall from the side of the tenon. Repeat the same process for the other side of the joint.

Then use a sharp chisel to pare the sides of the tenon (often called the cheeks) down to the guide lines.

For a guide to using a chisel, check out our previous instructable on chiseling.

Most tenons also have shoulder cuts on the top and bottom edges. To cut these, first lay out the guide lines using a marking gauge.

Cut along the length of the tenon using a backsaw, and finish by cutting along the shoulder line on both edges. Set the tenons aside for the moment and begin working on the mortises.

Use the marking gauge and square to mark guide lines for the mortise in the mating board. Because cutting a mortise requires accurately removing a great deal of stock, a drill and doweling jig are your tools of choice.

Just insert a bushing in the doweling jig that matches the width of your mortise. Then clamp the jig onto the board with the hole centered between your layout lines. Slide the drill bit into the bushing and bore a series of overlapping holes until all the waste is removed.

For the best results, set the hole depth on your drill bit by attaching the collar that comes with the jig to the bit. The hole should be 1/16 in. deeper than the length of the tenon. This provides some space for excess glue that would otherwise keep the joint from closing completely. Once all the holes are bored, square the ends and the sides of the mortise with a chisel.

Finally, test fit the joint. The tenon should be snug in the mortise, but you shouldn't have to force the parts together. If the joint is too tight, carefully pare the tenon cheeks with a sharp chisel until the fit is correct. If you need to remove just a bit of stock, use sandpaper. Once you're satisfied with the fit, apply glue to all the mating surfaces, and slide the pieces together. Clamp the assembly securely until the glue has dried.