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Make Super-Strong Through Mortises & Tenons

Mortise drawing

Joint-sizing pointers

Through-mortise-and-tenon joints typically go together one of the two ways shown at right. 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 above, 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 right. 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 right. 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.

Overlapping holes drilled with a brad-point bit should leave evenly spaced scallops and rounded corners.

Drawing of chisel guide

Chisel on top of board
Glue a guide cleat to your chisel guide so the guide edge aligns with the marked mortise edge. The guide holds the chisel 90° to the workpiece and prevents cutting outside the layout lines.

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 right. 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, bottom photo. Stop when the tenon slides through the mortise with only hand pressure.

Dados cutting tenon
Plan your tenon passes over the dado blade to avoid leaving a score line where you'll later bevel the end of the tenon.

Hand planer on tenon
To fine-tune a tenon with a rabbet block plane, shave away equal amounts on both faces. Check the fit after every two passes.

Finish tenon and assemble

Insert the tenon through the mortise and lightly scribe a line around the exposed end of the tenon, photo right. Separate the joint parts, and sand or plane a 45° bevel on each tenon face and edge until it reaches the scribed line, photo middle right.

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, bottom photo. Cut the dowels flush and sand the surface smooth up to 180 grit using a firm sanding block.

Knife cutting tenon
A marking knife with a single bevel helps you score around the tenon flush with the surface of the mortise.

Plane on side of tenon
Plane from the edge to the center to avoid tear-out when beveling tenons. Then sand the bevels and end to 180 grit.

Hammering dowel

Mortise drawing
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