"Rightsize" undersized plywood with a rabbet and dado.
Plywood is a convenient material for building casegoods shelving, and the strongest way to install it is to fit the ends into dadoes. Of course, that means you'll have to contend with today's undersized plywood thickness. But, sizing a dado to accommodate 23/32" sheet stock doesn't mean you have to invest in special undersized bits for your router. You also don't have to fuss with shimming a dado set to match the sheet thickness. Instead, turn to this simple shallow-shouldered rabbet joint—it's quick and easy to make on the table saw.
Step 1: Start by installing a dado blade in your table saw. Stack it to a cutting width that's narrower than the shelving plywood is thick—5/8" works well (see Photo 1). There's no need for shims with this joint. You'll adjust the thickness of the rabbet's tongue to fit the dado.
Step 2: Cut the dadoes to a depth you prefer for your project. Back up the workpiece against your miter gauge and a long auxiliary fence (see Photo 2). Or make these cuts with the workpiece on a crosscut sled equipped for dadoing.
Step 3: Once the dadoes are completed, clamp a sacrificial fence to the rip fence to prepare for rabbeting the ends of the shelving. Adjust the blade's projection out from the sacrificial fence to match the depth of the dadoes. Raise the blade to form the rabbet's shoulder and to establish the thickness of its tongue. Make the tongue match the dado width. Install a featherboard to press the shelving down firmly against the saw table when you make the rabbet cuts. Now, make a test cut on a piece of extra plywood to dial in the blade settings farther (see Photo 3).
Step 4: Check the fit of your test joint. The rabbet's tongue should seat fully into the dado, and the shallow shoulder should contact the plywood face (see Photo 4). If the tongue is too thick, raise the dado a tad. Adjust the fence left or right if the tongue is too long or short. Aim for a nice "push" fit of the joint parts.
Step 5: Proceed to rabbet the ends of the shelves and assemble the joints (see Photo 5). With a sharp dado blade, you'll produce shelving joints that fit together as well or even better than those made with undersized router bits. Reinforce these joints with glue and brads or countersunk screws.