Most
of the plywood that I use for my projects is slightly undersized. In fact the
last time I bought a sheet of 3/4"-thick plywood, it actually measured 23/32"
thick. And this was good. Sometimes it can be even thinner.
Dados that are cut to 3/4" would
leave a sloppy fit. So I custom fit the dado to match the plywood. But, if
you're like me, fiddling with shims and a dado set can get frustrating. So,
instead I do it with a simple jig and a router equipped with a pattern
bit
 To make the jig, I start by cutting some 1x4
cleats that are about 12" long. Then I cut two 1x4 guide boards that are
roughly 8" longer than the width of the piece getting the dado.Once the jig
pieces are cut, I assemble it using the workpiece for my projects.
To do this, I stand the
workpiece that will be the shelf (or mating piece) on edge on top of the
cleats, see the drawing at right.
Then without moving the vertical
piece, place a guide board on each side of it. Make sure the guides are resting
snugly against the vertical piece and screw them to the cleats. When the guide
boards are set at exactly the correct distance apart, you can remove the
vertical piece.
Now you can use the jig to rout the dado. First, align the gap in
the jig over the dado location. Equip your router with a 1/2" or 3/8" pattern
bit. Then set the router on top of the guide boards at the edge of the
workpiece and adjust the bit to the proper depth.
 All
that's left is to rout the dado. Just guide the pilot bearing down the guide board
and then cross over and come back down the other side, see the drawing and details
below. You should end up with dadoes that are perfectly sized for the plywood
you're using.
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