Routing accurate dadoes for plywood
If you've ever cut a 3⁄4 " dado for 3⁄4 " hardwood plywood, you've ended up with a sloppy joint because the plywood is actually thinner than that. I get great results using a spacer like the one shown in the drawings at top.
First, I clamp a straightedge to a scrap piece of plywood and rout a dado with a 1⁄2 " straight bit. Next, I measure the actual thickness of the finish-sanded wood going into the dado and subtract 1⁄2 ". I plane or rip a strip of wood to this thickness to make the spacer.
Laying the spacer next to the straightedge, I make another pass with the router and test-fit the joint. It should fit perfectly.
To rout the dado in the actual workpiece, I mark the critical edge of the dado (the edge that must measure exactly) on my workpiece. Then, I clamp the straightedge to the workpiece so that the first pass routs the critical edge. Finally, I lay the spacer in place and make the second pass.
—Jan Svec, WOOD® magazine staff