The Simple Secret to No-Fuss, Dead-On Dadoes
Stack the dado set slightly oversize
Forget tedious trial-and-error testing. The trick to tight tablesawn dadoes is to subtract-not add—shims.
To start, you'll need two scraps, about 6" square of the same thickness. In one of those squares, center a 5⁄8 " dowel about 4" long. Now follow these three steps anytime you need to cut a dado.
Finish-sand all workpieces that will be inserted into the dadoes, and lay one on the nondoweled scrap. Stack the blades and chippers on the doweled jig until they're just less than the thickness of the workpieces. Then add shims between the blades until the dado stack becomes slightly thicker than the workpiece, as shown in photo. Install the blades, chippers, and shims in your tablesaw, and make a test cut in scrap. (You don't need to worry about the dado height at this point.)
Discard the shims that fit
Remove enough of the dado set to retrieve all of the shims, then set them aside. Insert the workpiece end in the test dado, which should fit loosely. From the shims removed from the dado set, fit as many as possible into the gap between the workpiece and dado edge, as shown below. Set aside these shims and install the remaining shims as you reassemble the dado set.
Cut the perfect dado
Set your blade height for the finished dado depth, and make a second test cut to confirm the correct width and depth. Now you're ready to cut dadoes for gap-free joints like the one shown below.