Dadoes and grooves are flat-bottomed recesses that strengthen supporting shelves and connecting panels. A dado runs perpendicular to the grain of the wood while a groove runs parallel to the grain. A rabbet is like a dado that’s missing a side. It’s essentially a notch cut into the edge of a board or piece of plywood.
You can cut dadoes, grooves and rabbets in many different ways. In this article, however, we’re going to show you a simple, foolproof cutting method that requires only a router, a pattern bit and two straight guides.
How a pattern bit works
The bit we’re using is called a top-bearing pattern bit or top-bearing flush-trim bit. The bearing follows along the straight router guide as the cutters carve out the recess. Since the bearing and the cutters have the same diameter, you just line up the edge of the guide or pattern with the marks for the dado, groove or rabbet and run the router bit along the guide. Using two guides as we show here allows you to cut an exact size dado for any wood thickness that’s greater than the bit diameter. The router bit can’t go off track either, since it’s trapped between the guides.
Most home centers and hardware stores don’t stock top-bearing pattern bits, but they’re readily available online and at woodworking stores. The bits come in different diameters and lengths. Make sure to buy a bit that’s narrower than the thickness of the spacers. We’re using a 1/2-in.-diameter bit with 1-in.-long cutters (Freud No. 50-102).
How to cut perfect dadoes
1. Make two straightedge guides
The dado cutting method we show here requires two perfectly straight guides. You can make your guides any length you want, and whatever thickness is required for your router bit. Since the bit we’re using has relatively long cutters, we needed a 1-in.-thick guide to give the bearing something to ride against and the clearance necessary for the bit to reach the bottom of the dadoes.
To make guides like those shown here, cut four 6-in.-wide by 4-ft.-long strips of 1/2-in.-thick plywood. Glue pairs together to make two 1-in.-thick strips. When the glue is dry, use a table saw or circular saw and straightedge to trim the guides and create one perfectly straight edge. Draw an arrow toward the straight edge of each strip to remind you which edge you should use for the guide.
When you’re done trimming the edges, check the straightness of the guides by placing them next to each other on your workbench, with the arrows facing, and press them together. The guides should fit tight with no gaps. If there is a gap, recut one or both pieces until they fit tightly together.
2. Mark the location of the dado
You only need to mark one edge of the dado. Then draw an “X” to indicate which side of the mark the dado goes on. A good tip is to make three, not just two, marks across the workpiece. Then when you line up your straightedge with the marks, if one of the marks doesn’t line up, you’ll know you’ve made a layout mistake.
3. Clamp the first guide
The method we’re using requires two guides, one for each side of the dado. This allows you to cut a perfect-width dado regardless of the thickness of the material. Line up the guide with the marks and clamp it securely. Be careful to position the clamps where they won’t interfere with the router base as you’re cutting the dado.
4. Add spacers
When you’re cutting out the parts for your cabinet or bookcase, save a few scraps of the material. Then use those scraps as spacers. This ensures that the thickness of the material you’re using will match the dado widths perfectly.
5. Clamp the second guide
Press the second guide against the spacer blocks and clamp it. Again, make sure the clamps won’t interfere with the router base. Remove the spacers and you’re ready to cut the dado.
6. Rout the first side of the dado
Since the dado we’re cutting is a little less than 3/4 in. wide, and we’re using a 1/2-in. bit, it will take two passes, one along each router guide, to complete the dado. First adjust the router depth. This will be the thickness of the guides plus the desired depth of your dado. We set the router to cut a 1/4-in.-deep dado. If you’re positioned as shown in this photo, start at the left side and move the router left to right, keeping the guide bearing pressed against the guide farthest from you. The rule of thumb is to rout inside cuts like this in a clockwise direction.
7. Finish up with a second pass
Complete the dado by moving the router right to left, keeping the guide bearing in contact with the guide closest to you. Before you remove the clamps and guides, inspect the dado to make sure both edges are straight and smooth. If you find any imperfections, run the router over that area again. That’s all it takes to cut a perfect dado.
See also, Router Basics.
How to cut perfect rabbets
1. Position the guide
There’s more than one way to cut a rabbet, including buying a special rabbeting bit. But since you already have the guide and a pattern bit, why not use them to cut rabbets, too? Here we’re cutting a 1/4-in. x 1/4-in. rabbet in the back of a cabinet side to accept the 1/4-in. plywood back. Use a scrap of the same plywood to set the position of the guide, then clamp the guide into place.
2. Rout the rabbet
Starting at the left end, run the router left to right along the guide to cut the rabbet. You can use this same technique to cut wider rabbets like the one along the top edge of the cabinet (see first photo), but you’ll have to make one pass along the guide, and then clean up the remaining wood using the router freehand. That just means holding the bearing a tad away from the straightedge to remove the rest of the wood. Be sure to keep the router base tight to the guide at all times.
This video shows an alternative way to cut a rabbet.