Make Super-Strong Through Mortises & Tenons
Through-mortise-and-tenon joints typically go together one of the two ways shown above. Either both parts are the same thickness—a 3⁄4 " rail mating a 3⁄4 " stile, for example—or the tenon fits into a mortised part of greater thickness, such as a 3⁄4 "-thick table apron against a 1 1⁄2 " square leg.
To join parts of equal thickness, cut both the mortises and tenons one-third the part thickness. On a joint using 3⁄4 "-thick parts, for example, the mortise width and tenon thickness both measure 1⁄4 ".
Quick Tip! Allow some wiggle room. If your smallest chisel measures 1⁄4 " wide, lay out mortises a hair wider than that to simplify chiseling out waste later.
For parts of unequal thickness, make the mortise and tenon up to a third the thickness of the larger part. To join a 3⁄4 "-thick table apron a 1 1⁄2 "-thick leg, for example, cut tenons 3⁄8 "-1⁄2 " thick on the apron ends.
Make the tenon 1⁄32 " longer than the width of the mating part if you'll sand the tenon end flush with the mortised part after assembly. For beveled-end tenons, like the one shown below, make the tenons 1⁄4 " longer than the mating part width.
Begin with the mortises
Tenons can be fine-tuned easier than mortise widths, so cut your mortises first and match the tenons to them. First lay out a mortise on two opposite faces of the workpiece and check that they're equal distances from one end. On your drill press, mount a bit about 1⁄16 " smaller than the mortise width and attach a fence to the drill-press table. Clamp the workpiece to the fence and drill through the workpiece along the length of the mortise right.
To clean up the mortise, first use a narrow chisel to square the ends. Then assemble a chisel guide as shown, middle below. Slide the flat face of a chisel that's 1" or wider down the edge of the guide and just deeper than halfway through the mortise to smooth the scallops left from drilling, bottom below. Switch the guide to the opposite side and clean up that mortise wall, then flip the workpiece and do the same from the opposite side.
Cut tenons to fit
Install a dado blade as wide as your tablesaw accepts and set the height to leave tenons slightly thicker than your mortise widths. Test the settings on scrap the thickness of your tenon parts.
Set the rip fence a distance from the blade equal to the tenon length and mount an extension on the miter gauge. Cut tenon passes with minimal overlap to reduce scoring on the face cheeks, photo below. Then turn the workpiece on edge to cut the edge cheeks.
To fine-tune the tenon, remove material from each face cheek equally using a scrap block with 100-grit abrasive on one face (not the edges) or a rabbet block plane, second photo below. Stop when the tenon slides through the mortise with only hand pressure.
Finish tenon and assemble
Insert the tenon through the mortise and lightly scribe a line around the exposed end of the tenon, photo below. Separate the joint parts, and sand or plane a 45° bevel on each tenon face and edge until it reaches the scribed line, second photo below.
To assemble the joint without smearing glue on the exposed tenon, lightly glue the center of the face cheek, insert the tenon, and clamp until dry. To mechanically reinforce the joint, drill 1⁄4 " holes 1" in from the mortise ends and deep enough to pass through the tenon. You can also drill through both faces of the mortised part if you take care to avoid blow-out when driving dowels through.
Bevel the ends of two 1⁄4 " dowels and drive them into the holes, as shown, below. Cut the dowels flush and sand the surface smooth up to 180 grit using a firm sanding block.