Twin-Blade JoineryComments (0)
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.
Blades, spacers, and shims. The key to twin-blade joinery is a pair of matched blades. Full-sized blades provide maximum depth of cut, but the outer blades from a dado head will also work. What’s important is that the blades include some raker teeth–which are squared across, rather than beveled–to create a flat-bottomed cut. Spacers of 1⁄8"-thick aluminum create the gross blade offset, with shims of various thickness added to fine-tune the cut width. You can use commercial dado set shims, or make your own from aluminum cans, manila folder paper, and flat board from notepads. Mark or code your shims for easier setup.
Tenon jig and ZCI. To cut tenons, you’ll need a tenon jig. A commercial model will work, but I prefer a shop-made version that rides the rip fence. You can outfit a jig with hold-downs if you like, although this can slow down production. Whatever jig you use, outfit it with a replaceable backer to eliminate exit tearout. It’s important to outfit your tablesaw with a ZCI (zero-clearance insert) to prevent work from slipping downward into the throat plate opening, especially if you’re not using hold-downs on your jig. (For more on ZCI’s, see issue #53.)
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.
Smooth the perimeter. To clean up the edges for safe handling, first mount each spacer on a 1⁄4"-diameter bolt, securing it with a nut. Then chuck the setup in a drill press, and sand the edges with coarse-grit sandpaper.
Enlarge the arbor hole. Clamp each spacer to a backer board at the edge of your drill press table, and widen the center hole to 21⁄32". Sand or file away any remaining burrs afterward.
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.
Keep a record. For future reference, keep a chart of the various spacer/shim combinations that work with your dedicated saw blade set. I keep my chart with my hollow chisel mortise set.
Set up, and test the cut. To set up for a test cut, mount your inner blade, followed up with spacers that total no more than your desired joint thickness. (Two 1⁄8" spacers for the 1⁄4" tenon desired here.) Then add enough shims to account for twice the tooth offset. Add the outer blade, a ZCI, and then make a test cut. Try the fit in a sample mortise; it should be very close. Remove or add shims, and recut until the test tenon fits perfectly, requiring just a bit of hand pressure to fully insert it in its mortise.
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.
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.
Single-pass work. If tenon shoulders are designed to be narrower than the width of the blade teeth, the joint can be cut in one pass. To prevent scoring the face of your jig, use double-faced tape to attach a sacrificial panel to the jig.
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.
A safe chute for shoulder cuts. To prevent the freed waste from shooting out when sawing tongue shoulders, clamp a raised auxiliary fence to the rip fence for a pinch-free cut.
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.
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