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3 Plenty-strong plywood joints

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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 right, 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 below right, 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.

Finger on shim ring
Stack the dado set's outer blades, chippers, and shims next to the plywood and feel for a combination equal in thickness.

Pushing board thru table saw
You can safely use both the rip fence and miter gauge for making dadoes because you're not cutting through the board.

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 right. 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.)

Router on board
Make a simple right-angle T-square from scrap and use it to guide your router for cutting dadoes. Use the channel in the arm to line up the next dado.

Up close of router
Use a rabbeting bit to cut a precise tongue length; change bearings for different lengths. Adjust the bit's cutting depth to control the fit of the joint.

Crack in joint
For maximum strength, place the tongue on the bottom of the insert piece. This joint split when weight was placed on it.

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 right, 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.

Jig

ROUTER ON LONG BOARD
Make and use the router-mounted jig to ensure perfectly centered grooves on 3/4" plywood ends and edges. Then fill with a made-to-fit spline.

3 joints together
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