Fortify wood edging on sheet good shelves with wedged tongue-and-groove joinery.
Plywood and melamine board are practical options for building shelves, but they tend to sag under the weight of books or other heavy display items. Then there's need for a solution to hide those unsightly edge plys, wood pulp or voids in the material thickness. The usual remedy is to apply veneer edge tape or a thin strip of solid wood to cover the edges, but that treatment adds little to no strength. The next time you're building shelving, here's a better solution: install a wider strip of hardwood that bolsters shelf stiffness, plus gives your shelf edges the look of solid wood. This wedged tongue-and-groove bit set from Freud (stock number 99-032 and 99-033) makes installation easy, and the interlocking joint provides a strong connection as well as a self-aligning glue joint.
Notice in the top photo that the bit on the left cuts a wedge-shaped tongue and the right-hand bit cuts the mirror-image groove. You can choose whether to mill the tongue into the shelf edging — which we'll do here — or into the shelf edge. Either orientation will work fine. Whichever option you pick, these bits are substantial and present a full cutting edge to workpieces with no pilot bearing. They should be run only on a sturdy fence-equipped router table and not in a handheld router. Here's how to use them safely and effectively.
Step 1: These router bits are designed to completely remove the contact surface of a workpiece during routing, so you'll need to account for that situation when setting up the router table fence. The solution is to apply a shim to the outfeed (left) side of the fence, then register the bit against the amount of offset this shim provides. A piece of plastic laminate, fixed in place with double-sided tape, works well (see Photo 1).
Step 2: We'll use the groove-cutter first to mill the plywood shelf edges. Install the router bit, and close up the fence around the bit as much as possible to minimize tearout. Raise the bit so it's easy to access, and adjust the fence in or out until the deepest portions of the cutter (the shoulders on either side of the wedge-shaped groove) are flush with the fence's outfeed shim. Check this setting with a straightedge, and lock the fence in place when the shoulder portions of the cutter just "kiss" the straightedge (see Photo 2). This way, the bit will remove the laminate's thickness from the workpiece edge on the infeed side, and the outfeed shim will support the shelf's new edge as it passes through the cut.
Step 3: Carefully lower the bit until the end of the wedged groove cutter is centered on a piece of scrap that matches the thickness of your shelf material (see Photo 3).
Step 4: Install a pair of featherboards on your router table fence — one near the infeed side of the cut and a second on the outfeed side — to ensure that workpieces will be pressed down firmly during routing. The cut should not waver up or down along the plywood's length, or the edging won't fit properly. You can now mill the groove along your plywood shelves, but make a test cut on scrap first, pressing it flush against the fence as you proceed through the cut. If the test cut is consistent and centered, go ahead and mill the shelf edges now (see Photo 4). Feed the work smoothly past the bit and at a moderate speed to prevent overloading the machine.
Step 5: Once the grooves are milled, switch to the tongue cutter. Using one of your shelves as an index, raise or lower the bit until the widest portion of the tongue profile aligns with the groove opening in the plywood (see Photo 5). For this cutter, you'll also need to adjust the fence in or out until the deepest point of the cutting surface (the end of the wedged tongue) is flush with the fence's outfeed shim.
Step 6: Before milling your shelf edging, be sure to make a few test cuts on scrap material. You may need to raise or lower the tongue cutter slightly, or move the fence in or out a tad, to create a perfect mirror-image tongue profile for the shelf grooves. You can see two test cuts made on a piece of scrap in Photo 6. Once the tongue cut is dialed in, run your shelf edging strips. If you are applying edging narrower than about 2 in., mill it from wider stock to keep your fingers, push sticks and/or push pads well clear of the router bit. After the tongue-routing step is completed, you can rip your edging strips to final width.
Step 7: Spread some glue into the grooves and over the tongues, and you're ready to clamp this interlocking edging in place. Once the glue dries, you'll have an attractive and sturdy "strut" to help keep your shelves flatter under load. You can leave the edging square, knock off the corners with a roundover bit or a tiny chamfer, or even mill a decorative profile to dress the edging up. The choice of ornamentation is yours!