The Real Thickness of Plywood
How do you pick a router bit if you can't trust the size?
I'm planning to build a cabinet and have been looking for a set of router bits to make grooves for plywood shelves. But I've found that bit manufacturers don't always agree on the size of bit required for a particular thickness of plywood. They don't even seem to agree on the actual thickness of plywood. For example, one bit manufacturer says ½-inch plywood is actually 31/64 inch thick, while another says it's only 15/32 inch thick. What's going on here?
— Dink, Grapton, W. VA
Norm Abram replies: I come up against this problem all the time, and there's no simple answer. For starters, plywood is not like lumber; it's made to the thickness actually stamped on its face. The problem is, the manufacturing tolerances vary depending on the kind of plywood being made. For instance, sanded grades of softwood plywood that are ¾ inch thick or less — the type you'd probably use for utility shelves — are manufactured to tolerances of plus/minus 0.4 mm (1/64 inch). Most other softwood plywood, including the unsanded grades, is manufactured to a tolerance of plus/minus 0.8 mm (1/32 inch).
Hardwood plywood, which is used to build higher-quality cabinets and shelves, is a slightly different story. Panels ¼ inch thick or more are made to a tolerance of plus 0.0 mm/minus 1.2 mm (3/64 inch). A plus-zero tolerance is dictated by the cabinet and furniture manufacturers, the biggest users of this material, who can live with a panel that's slightly too thin but not one that's too thick, because of the assembly problems it causes.
On top of all this, add the fact that plywood swells and shrinks as it absorbs and loses moisture, like any wood product. That, too, can show up as a slight difference in thickness from what's stamped on the panel. Since the thickness you have to plan for is hard to predict, here's what you should do: Measure the actual thickness of your panels carefully, with calipers or a micrometer if possible, in various locations around the edge. Then choose a router bit that's going to make a slightly bigger dado (flat-bottomed groove) than the fattest edge you find. (If you don't have a bit that's just the right size, use a smaller one and make multiple passes.) I prefer to make dadoes on a table saw using a stacked dado head; I can shim it in or out to cut exactly the thickness I need. Whichever method you use, don't rely on measurements; make test cuts in the actual stock and try the fit to make sure it slips in snugly. But if you need a mallet to drive plywood into a dado, the fit is too tight.