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Routing accurate dadoes for plywood

If you've ever cut a 34 " dado for 34 " 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 12 " straight bit. Next, I measure the actual thickness of the finish-sanded wood going into the dado and subtract 12 ". 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

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