Loose-Tenon Options

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This article is from Issue 28 of Woodcraft Magazine.

There’s little doubt that the mortise-and-tenon joint is one of the strongest joints in woodworking. But it’s not always the quickest or easiest to cut. There’s an alternative that’s a lot easier to make—the loose-tenon joint. This joint can be made quickly with a router, a cordless drill, or a high-speed mortiser, and when it’s assembled with modern adhesives, it’s plenty strong for most projects.

With a traditional mortise-and-tenon joint, you cut a mortise (a slot) in one of the mating pieces (for example, a table leg). Then you cut away material from the end of the other mating piece (the apron) to create a tenon. The challenge lies in getting the tenon to fit perfectly in the mortise. Since you’re cutting the mortise and the tenon with two different tools and two different setups, the possibility for a joint that doesn’t quite fit together perfectly is always present.

On a loose-tenon joint, you cut mortises in both pieces with the same machine or jig and the same setup.Then you cut a loose tenon (sort of a short spline) to fit halfway into each piece. It’s glued securely in place for a very strong face-grain-to-face-grain joint.

One benefit of the loose tenon is the instant fit you get with little or no additional handwork with a plane or sanding block. Another benefit is that by cutting the loose tenon narrower than the mortises, you have a “fudge factor” that helps you align pieces flush, even if you slightly miscut the mortise length or location. A third and very big benefit is that you cut all of the workpieces to finished length before cutting the joints. You don’t have to take into account the length of the tenons (i.e., no math). And, finally, you can easily use loose tenons with angled joints such as miters.

Here, we’ll cover three distinct approaches to cutting loose-tenon joints. They all have their advantages. While one saves time, another saves money. Hopefully, you’ll find one that best suits your needs and budget. (See the Convenience-Plus Buying Guide on page 31 for products mentioned.)

Guidelines For Sizing Mortises

WIDTH = Bit size = 1/3 the thickness of the thinnest mating piece

LENGTH = 2/3 the width of the narrowest mating piece

DEPTH = As deep as possible based on bit length, but at least 1/16" deeper than half the length of the tenon

The jig and router combo

Our low-tech, low-dough approach features a plunge-router mortising jig that you can make out of plywood, acrylic plastic, scrapwood, and some common hardware (Figure 1). Add jig-building to your set-up time, and you have an approach that takes the longest of the three to get started. And yet, the result will yield a long-lasting joint that’s quicker to make than a traditional mortise and tenon.

Building the jig

The heart of this mortising jig is a platform that holds and “corrals” your plunge router. The platform sits on top of a vertical clamping face and base that can be clamped securely to the top of your bench. The vertical support also features a toggle clamp and a work rest to position and secure the workpiece during machining. The 3/8"-thick clear acrylic plastic platform lets you to see through the “window” to set up exactly where you’re routing. Two rows of T-nuts in the clamping face help hold the workpiece vertically, horizontally, or even at an angle (Figure 2).

Build the jig to fit your plunge router by screwing a couple of hardwood fences along the edges of the platform. The distance between the fences should equal the diameter of your router base.

Mark the centerline of the mortise on the front faces of both pieces and transfer the line to the appropriate end or edge. Then lay out the length of the mortise.

Using the jig

To use the jig, hold the mating pieces together and then lay out a centerline for the mortise across the front (“show”) face and edges or ends of each piece as shown in Photo A. Then lay out lines showing the mortise length and the position of the mortise on the thickness of the piece as shown in the Inset.

Now, use the toggle clamp and work rest to clamp the workpiece in position under the short vertical hairline on the jig. (Note: Always check that the marked “show” face is against the clamping face.) Then position the platform so the long horizontal hairline aligns with your layout line and tighten the two knobs under the platform (Photo B).

Mount a spiral upcut bit in your router. (Note: A spiral upcut bit plunges, cuts sideways with a clean shearing cut, and lifts the chips up and out of the mortise.) Set the depth of cut taking into consideration the thickness of the acrylic platform and length of the tenon.

Next, set the router on the platform and position the stopblocks to determine the length of the mortise as shown in Photo C.

Finally, cut a mortise by plunging down 1/4" into one end. Then slowly rout side to side making increasingly deeper cuts. After the mortises are cut in the edges of all the appropriate pieces as shown in Photo D, reset the jig and cut the matching mortises in the mating ends as shown in Photo E. It’s always a good idea to make test cuts with scrap the same thickness as your finish cuts. This allows for any fine-tuning of the jig before cutting your project pieces.

Position both the vertical and horizontal hairlines, then toggle-clamp the workpiece in place and tighten the knobs under the platform.

Watch the bit to set the stopblocks and align with each end of the mortise, then tighten the knobs.

Plunge down then rout from side to side. Gradually rout the mortise in 1/4" deep increments.
Switch the position of the work rest until it’s vertical and cut mortises in the ends of the mating pieces.

The commercial jig and portable drill combo

Our second approach looks at using a store-bought jig and a portable drill. It costs a little more, but is a good alternative if you don’t have a plunge router or don’t want to take the time to build the shop-made jig.

Using a drill to rough out mortises is a common operation but it usually involves drilling a series of overlapping holes with a drill press and then cleaning out the scalloped edges by hand with a chisel. The JessEm Pocket Zip Slot Mortise Mill takes this to a new level. The Mortise Mill comes with a special 1/4" “mortising drill bit” (see the Inset in Photo F). It’s similar to a spiral upcut router bit, but with a longer shank. That means you drill the end holes with it and also work the bit sideways to clean out the mortise, eliminating the need for chisel work.

Expect some time loss in trial and error when setting up the Mortise Mill jig. The length of the mortise is slightly adjustable but you have to keep removing the workpiece and checking the length to match a specific width loose tenon. The maximum loose tenon width is 11/8" and the only width mortise (i.e., thickness of the loose tenon) this jig will cut is 1/4".

After marking the location of the mortise, secure the workpiece and jig in a vise. Then, with the setup established, work the jig’s lever back and forth with one hand while drilling down using a portable drill with your other hand as shown in Photo F. (Note: I found it best to use a corded drill at high speed unless you’ve got a powerful cordless drill and a recently-charged battery.)

The JessEm Pocket Zip Slot Mortise Mill sells for $99.99, but if you step up to its big brother, the standard JessEm Zip Slot Mortise Mill ($249.99), you’ll get easier adjustability, more clamping surface for workpieces, longer mortises, and the option of going with 1/4"-, 3/8"-, or 1/2"-wide mortises.

You can cut clean and consistent mortises simply with a JessEm Pocket Zip Slot Mortise Mill jig, an electric drill, and a special mortising drill bit.
Pushing the Festool Domino Joiner into the edge, end, face, or a miter creates a very accurate mortise that will accept a domino-shaped loose tenon.

The ultimate loose-tenon joiner at a stiff price

If you have the bucks for the high-tech end of the spectrum, you’ll find the ultimate in speed and precision in Festool’s Domino Joiner. Though from outward appearances it looks like a biscuit jointer, it works differently, with noteworthy advantages.

It features a router-like bit that plunges into the work and then oscillates from side to side. Within a few seconds, you’ve cut a perfectly clean mortise that accepts a custom-sized loose tenon. And here’s the real advantage—the loose tenon is much larger and thicker than a biscuit, providing greater face-grain-to-face-grain gluing surface.

Layout is as quick and easy as a biscuit jointer. You can work anywhere on the board, even on a mitered end as shown in Photo G. Just hold your two mating workpieces against one another and draw a pencil line across both pieces where you want the matching mortises. Then set the Domino Joiner for the desired depth and width. Secure the workpiece with clamps or in a vise, line up a cursor on the tool with your pencil line, and plunge.

Okay, what’s the downside? Well, other than the price ($775), you’re limited to five different depth settings and three different width settings. Also, you’ll need to work in metric sizes. All of the bits, depth and width settings, and domino thicknesses are in millimeters. If you’ve worked in millimeters, then it’s simple. If not, I’d recommend picking up a ruler or tape measure with both metric and inch settings across from each other.

Four Assembly Tips

• Sand slight chamfers on the ends of loose tenons so they insert more easily into the mortise.

• Dry-assemble your project to check that everything fits.

• Use a small brush to apply glue down into each mortise and on the first 1/4" or so of the tenon.

• Make your tenons 1/8" narrower than needed. This creates slight adjustability for a precision fit. That said, always check that mating pieces are flush on the ends and edges during clamp-up.

Round over all four edges of your tenon stock on the router table to match the mortise.
Clamp a stopblock to your rip fence to prevent kickback when cutting short tenons.

Making or buying loose tenons

After choosing a mortising approach, you’ll need to decide on one of the options for obtaining loose tenons.  These include making your own to fit any size mortise, or buying pre-made tenons. Festool tenons are available in metric sizes for mortises cut with the Domino Joiner. JessEm sells tenons in two different lengths and three thicknesses (all in inch measurements) to match their bits.

To make your own loose tenons, start by planing some scrap hardwood stock; aim for a slight friction fit in your test mortise. Rip off strips 1/8" narrower than the length of the mortise to allow some wiggle room during assembly. Next, round over all four edges of the tenon stock to match the rounded ends of the mortises. Do this with a block plane, sandpaper, or a router table and round-over bit as shown in Photo H. The final step is to cut the loose tenons to length with a miter gauge and auxiliary wood fence on the table saw as shown in Photo I. To allow room for glue, cut them 1/8" shorter than the combined depth of the two mortises. For more on assembly, see the tips above.

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