3 Plenty-strong plywood joints
Plywood's multiple layers (or plies) make it more economical and resistant to seasonal movement than solid wood, especially for large panels. But those plies also weaken butt joints in project construction. These three joinery methods maximize the strength of plywood joints.
1. Full-width dado or groove
Strong, reliable, and easy to make, a full-width dado (across the grain) or groove (along the grain) perfectly captures the mating workpiece with glue surface all around. As a general guideline, cut a dado to a depth about half the thickness of the plywood. A cabinet, bookcase, or dresser built with snug-fitting, glued-together dado joints will last for decades.
You can cut dadoes and grooves with a tablesaw or router. We like using a stacked dado set on a tablesaw because it's quick and easily repeatable.
Stack the right combination of chippers, shims, and outer blades, shown below, to match the plywood thickness, and install that setup on your tablesaw. Make test cuts in scrap until you get the right fit—the inserted workpiece should slide in and out of the dado with moderate hand pressure but not fall out when held upside down. Add or remove shims as needed.
Once you get your stack set up, you can cut all your dadoes for stock of that thickness. Registering against the rip fence, as shown second below, guarantees that all cuts made on matching workpieces will be perfectly aligned.
For corner joints, this channel becomes a rabbet. Because you lose one of the glue surfaces of a dado, it's best to use a rabbet in conjunction with an additional form of support, such as screws or a solid-wood face frame covering the exposed edges.
2. Shouldered dado
This joint matches a narrow dado with a tongue, created by cutting a rabbet on the insert workpiece. And when you use this joint at a corner—where it shines best—it becomes a lock-rabbet, providing more glue surface, greater strength, and superior rigidity to a standard rabbet joint.
Cutting a shouldered dado requires machining both mating pieces. First, cut the dado with your tablesaw's dado blade or a router with a straight or downcut spiral bit, as shown below. Next, cut the rabbet on the mating piece (center photo) so the tongue fits precisely in the dado. (Make cuts in test scrap to fine-tune the fit.)
3. Splined butt
To avoid the problem of inconsistent and nonstandard plywood thicknesses, go with this can't-miss joint. Because you custom-make it to suit your stock, plywood thickness proves irrelevant. For never-fail matching channels, cut both with a router and the same straight or spiral bit. Make a router jig from scrap based on the one shown below, sized to match your plywood's thickness. (The dimensions of the jig components can vary, but the fences must straddle the plywood snugly while allowing smooth movement.)
You could also cut the dado easily on a tablesaw with a dado set, but cutting the groove in the mating piece's edge can be tricky, especially for workpieces longer than 2', because you stand them on edge where they can be wobbly. So a router works best.
For maximum strength, size your dado and groove width one-third the thickness of the plywood and the depth half the thickness. With the channels cut, plane a length of hardwood to fit snugly in each, rip it to width, and crosscut to length. Glue all sides of each channel for a strong joint.
Watch a FREE video showing an easy method for matching a dado blade to plywood thickness at woodmagazine.com/perfectdadoes.