What’s the difference between a groove and a dado? Aha! You didn’t know there was a difference?
A groove is — well — a groove… in wood… along the grain. A dado, on the other hand, is well, a groove. Except it runs across the grain. Okay, okay. Unless you want to be a real stickler, use dado. It’s understood to mean both (and sounds more “carpenter-ish” than groove.)
A dado joint is good for, among other things, joining book shelves to uprights.
You can cut a dado with a table saw or a circular saw. A dado blade will do the job in one pass, but unless you need to make a lot of dadoes, you may want to use a regular blade and make a number of passes. To use a table saw, set your blade to the proper depth, and set your fence so the first pass will cut one edge of the dado. Then move your fence in small increments before making successive passes. A circular saw will do the same thing, and will work best if you use the “shoot board” we’ve talked about in the past.
There’s a little thing you should know about wood. Board thickness may vary, depending on where it was milled, and plywood varies among manufacturers. So for a tight fit, make sure you double check the thickness before cutting your dado.
If you’re using a router, you may want to try this technique. Instead of making one pass with a bit of the right thickness, make two passes with a smaller bit. Clamp a guide to one side of the cut, and make your first pass. Now, clamp a guide to the opposite side of the cut, and make your second pass along this.