Essential tenon tips

One of the best things about being a home workshopper is you can afford to indulge in woodworking luxuries that aren't feasible in industry. The tenon is a good example.

Though they're almost extinct on the commercial furniture scene, not so in home workshops across Canada. Since you're free from concerns about profitability, you can use features like tenons to enhance your work beyond anything that's measured financially.

In fact, home workshops are one of the last places where traditional woodworking practices survive. As useful as it is to have stores filled with mass-produced furniture at prices everyone can afford, it's also good to know that dedicated enthusiasts are still free to preserve old-style woodworking virtues.

Besides being strong and trustworthy, tenons can be fun to make. And just knowing they exist as hidden features of a project builds my admiration for the work and the woodworker. There are easier ways to join wood, but none better. Of the several options for cutting furniture-grade tenons, my favourite uses the tablesaw and a surgically-sharp 3/4" or 1"-wide bench chisel. By cutting regular grooves, called saw kerfs, part way through the ends of a workpiece, waste wood becomes easily removed with a chisel to create the tenon. A variation on this theme employs a hand-held circular saw to create similar kerfs on timbers too large to be moved over a stationary saw. Either way, the approach is safe, accurate and an efficient combination of hand and machine operations.

Tenons are strong because they mimic the way tree limbs intersect tree trunks, taking full advantage of the strength of wood found parallel to the grain. And the first step involved in making tenons is designing them. Tenon thickness should be one-third the thickness of the wood involved, and as wide and long as possible. One problem when discussing tenons is confusion around words like thickness, width and length. Tenon thickness is measured the same way as board thickness; tenon width is measured across the grain and tenon length along the grain.

Besides tenon size, you need to think about a joint feature called the shoulder. These are the step-shaped transition zones where tenons end, and since shoulders bear most of the stress and visual scrutiny in the joint, they demand special attention, for both aesthetic and structural reasons. First, it's crucial that tenon shoulders be in full contact with the surrounding wood for maximum joint strength. This means tenons must be 1/16" shorter than the depth of mortise they fit into. That's not a big deal early in construction because tenon length can be easily trimmed later, during trial assembly with a mortise. Just keep it in mind. Shoulders must also exist on at least three sides of every tenon, aligned flawlessly with each other where they meet at corners, and cut perfectly square to the surrounding stock. Intersecting parts will only fit together true if tenon shoulders are also true.

One plus about kerf-cutting tenons on the tablesaw is it creates absolute consistency from one part to the next. And this translates directly into excellent tenon shoulders. If you're using a mitre gauge to guide workpieces, for example, then your rip fence can act as a length-stop to regulate shoulder location. Clamp a scrap block to the end of the fence nearest you, then adjust the fence/block combination from side to side so the shoulder of the proposed tenon aligns with the saw blade. Slide your workpiece over until it hits the block, then push the mitre gauge forward into the spinning blade to make the shoulder cut. Roll the workpiece 90°, then cut the adjoining shoulder and repeat. Since the stop block exists only at the nearest end of the fence, your workpiece is not in contact with the block before it hits the blade. This is an important safety consideration. At no time should wood be in contact with a mitre gauge and rip fence at the same time. If that happens, the slightest twisting of the workpiece can cause binding and kickback.

Regular Canadian Home Workshop contributor Konrad Sauer offers a useful variation on the standard shoulder-cutting procedure that I like. By adjusting your tablesaw blade 1/32" higher for shoulder cuts than the height used for the kerfing cuts that follow, it becomes easier to chisel the tenon down to final thickness later, in the area where tenon meets shoulder. Ordinarily this zone is prone to developing frazzled wood fibres as you trim the tenon for final fit, as the wood hangs on to the shoulder face. Deeper-than-normal shoulder kerfs, however, make it easy to clean these corners, and that translates to a better fit between tenon and mortise.

With the precision-demanding shoulder cuts out of the way, it's time to kerf the waste before chiseling it off. Kerfs every 1/4" are fine in softwood; somewhat closer are ideal in hardwood. And since knots never chisel well, space cuts through them between 1/16" and 1/8" apart, regardless of wood type. Closer kerfs mean easier chiseling. With your workpiece held to the bench, chisel the kerfed waste away from all sides of the tenon with paring cuts across the grain. Take the tenons down to the point where the bottom of the kerfed grooves just barely remains, then stop. Before you chisel any further you'll need to cut mortises to test-fit the tenons into as you bring them down to final size. Tenons should slide into mortises with moderate pressure, and hold together without glue. Accidentally made a tenon too thin and loose? No problem. Cut a thin slice of wood and glue it to the thin side of the tenon (with matching grain orientation), then kerf again on the tablesaw and chisel.

Modern carpenter's glues are strong enough to hold any tenon without mechanical help from pegs or wedges. But adding a peg or wedge does add a nice detail, especially as a visible part of a traditional project. Tenons that fit into mortises that go right through a workpiece should start out 1/4" longer than necessary, then trimmed flush to the surrounding wood after assembly. External wedges of contrasting wood set in the ends of tenons like these offer a striking way of telling everyone that your creation definitely didn't arrive in a cardboard box. And isn't that the best thing about being a home workshopper?


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