Twin-Blade Joinery

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The fast way to make perfect tenons and tongues on the tablesaw

Joinery is probably the most exacting aspect of woodworking. It’s where thousandths of an inch matter. No kidding. If a tenon, for example, is even .004" (the thickness of a dollar bill) thinner than its mating mortise, the joint’s strength has been compromised, no matter how well glue appears to hold it together. Because cutting joints can be fussy and time-consuming, it makes sense to take advantage of any efficiencies where you can. 

This is where twin-blade joinery comes in. By stacking two identical blades together, you can reduce the time it takes to cut tenons, tongues, and other joints with parallel faces. At the same time, you’re ensuring accuracy. For example, a common way to cut tenons is to saw one cheek, then flip the stock around on your tenon jig to saw the opposite cheek. The problem with this is that any inconsistency in your stock thickness translates into inconsistency in your tenon thickness. As you’ll see, twin-blade joinery solves this problem, eliminating the need to fine-tune the tenon thickness for final fitting. The technique is just as effective at cutting accurate tongues on panel edges for the same reason. Although a different principle applies, twin-blade joinery is also the perfect approach to cutting double spline slots.

All the technique requires is two identical saw blades and some shop-made spacers. The small expense pays off big time, as you’ll find once you try the method.

Make the spacers

You can make spacers from aluminum, brass, hardboard, or any other non-compressible material of a consistent thickness. For minimum material investment and greatest set-up flexibility for common tenon sizes, make six 1⁄8"-thick spacers. They don’t have to be perfectly concentric or of a precise diameter. (I make 3"-diameter spacers using 1⁄8"-thick aluminum plate.) Begin by using dividers to scribe out the perimeters, and then drill a 17⁄64"-diameter hole at the center of each.

Determine the winning combination

The thickness of a tenon or tongue is usually a specific increment (1⁄4", 5⁄16", 3⁄8", 1⁄2", etc.), based on the width of the chisel or diameter of the router bit that cut the mortise, dado, or groove. It takes some initial fussing to arrive at the correct combination of spacers and shims for any given thickness, but you only have to do it once, as long as you record it for future reference. You can do this totally by trial-and-error, or you can take a measured approach, as I’ll show you here, regarding the setup required for making a tenon.

Cutting typical tenons

Typical tenons (with their relatively deep shoulders) involve a 2-step process: sawing the cheeks, then the shoulders. To ensure proper joint alignment during assembly, make sure to first accurately dress your workpieces to their final thickness, and mark the “show” face of every piece so you can orient it properly on the tenon jig. Once you’re set up, you can cut every tenon to precisely match its mortise width every time. Quickly follow up with the shoulder cuts, and you’re done.

Cutting shallow-shoulder tenons

When the tenoned member of a frame is thinner than the mortised member, you can get away with minimal tenon shoulders, in which case, you can cut the shoulders at the same time as the cheeks, making for really fast work. To ensure square shoulders, make sure to use blades that include raker teeth.

Tools & Supplies:

See Buyer’s Guide on Page 66.

Cutting tongues

Frame-and-panel joinery often involves sawing the panel edges to create a tongue that fits into a groove in the edges of the frame. The blade/spacer/shim setup is exactly the same as for sawing a tenon. The only difference is how the workpiece is fed.

Better spline joinery

Gluing splines in a miter joint is a great way to reinforce what would otherwise be a weak connection. Although a single centered spline will work, two splines are much better. The joint strength is doubled, and the splines can be located closer to the faces of the stock, where they’ll better resist separation due to wood movement.