Make Strong Through Mortises & Tenons
By: Chelsea Rodgers
The mortise and tenon joint is one of the strongest and most appealing woodwork joints able to be made because of its flush fitting design. Like the dovetail joint this woodwork joint can be difficult to properly construct but it is incredibly strongand aesthetically pleasing if built well. This woodwork joint consists of a tongue that is secured into a slot and
it is used in areas such as table legs. For structural areas like these the joint must be tight fitting to ensure maximum strength but also to achieve a neat flush look.
Through-mortise-and-tenon joints typically go together one of the two ways, wither 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 ". Here’s a little 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, 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, like the MLCS Drill Press Table pictured left, (MLCS item #1273) mount a bit about 1⁄16 " smaller than the mortise width and if not already attached, attach the fence to the drill-press table. Clamp
the workpiece using the T-Track Hold-Downs, pictured right, (MLCS item #9441) and drill through the workpiece along the length of the mortise right. The same can be accomplished using a solid carbide spiral bit and plunge router.
To clean up the mortise, first with a narrow chisel like one from the Mortise Chisel Set (Eagle America item #400-7005) pictured left, to square the ends. Then assemble a chisel guide. 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. 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.
Glue a guide cleat to your chisel guide so the guide edge aligns with the marked mortise edge. The guide holds the chisel 90degrees to the workpiece and prevents cutting outside the layout lines.
Cut tenons to fit
Install a Bottom Cleaning Bit (MLCS item # 7942) or a similar bit in your router table. Set the rip fence a distance from the blade equal to the tenon length and mount an extension on the miter gauge (MLCS item #9443). Ensure the Fence is square and lock it in place. Check the cut with a scrap to make sure both the cut depth and distance from the fence is set correctly. It is very important not to move the fence after this. Like with the mortise make several small cuts to ensure a tight fit. Use a scrap piece of wood when pushing the tenon through to keep it from kicking off square. Cut both sides of the piece, starting with the cut against the fence first. Make several passes adjusting slightly higher on each pass. Dry test fit the corner of the tenon as you go removing a tiny bit at a time, this will ensure a tight fit. Raise the bit by half what you want to remove. The tenon needs to be cut equally on both sides. Therefore, when the bit gets raised it will remove twice as much material as it is raised.
To fine-tune the tenon, remove material from each side equally using a scrap block with 100-grit abrasive on one face (not the edges) or a rabbet block plane. Stop when the tenon slides through the mortise with only hand pressure. Plan your tenon passes over the dado blade to avoid leaving a score line where you'll later bevel the end of the 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. With the correct size chamfer bit you need, (MLCS item #5373-5376, 5378-5379, 7675-7681) route each tenon face and edge until it reaches the scribed line. A marking knife(MLCS item
#400-3327) pictured right, with a single bevel helps you score around the tenon flush with the surface of the mortise. Plane from the edge to the center to avoid tear-out when beveling tenons. Then sand the bevels and end to 180 grit.
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. Cut the dowels flush and sand the surface smooth up to 180 grit using a firm sanding block.