Cutting dados on long pieces of wood can be done using either a table saw or a router. Both tools can produce clean, flat-bottomed dados in the material. A dado is a groove cut into a piece of wood and away from the edge of the material. It is meant to accept another board and make a strong, tight joint.
When using a table saw to make the dado in a long board, use either a stacked dado cutter, which employs several thin blades set between two thicker blades, or a wobble cutter, which uses a wedge of metal to cause the blade to become offset so it will move side to side.
For the router, choose a straight cutting bit without a pilot bearing.
On a table saw, set the rip fence the correct distance from the dado cutter. Measure the distance from the edge of the wood to where you want the dado to begin and set the fence so that it is equal to that distance from the inside edge of the cutter blade. Then, clamp feather boards to the rip fence and to the table so the board will be held in place as the cut is made. You will also need a roller at the infeed and outfeed sides of the saw to help prevent long boards from tipping.
For the router, use two boards that will act as straightedges. These boards need to be slightly longer than the workpiece and they need to have at least one straight, squared edge. Measure the distance from the inner edge of the straight bit to the edge of the router base. That will represent the distance the straight edge board needs to be set from the inner line of the dado. Transfer that mark to the workpiece.
Next, measure the distance from the outside edge of the bit to the the edge of the router base and transfer that measurement to the workpiece as well. There should now be two parallel lines where the two straightedge boards will need to be clamped in place to make the dado.
Making the Cut
With the board held tightly against the fence, feed the material into the table saw blade slowly making certain the board rests against the fence the entire time. The feather boards will help ensure this. Be sure to use push blocks to avoid getting fingers near the blades and to help prevent kickback.
The router cut needs to be made in several passes to avoid kickback and burning the wood. With the router bit set at about half the depth of the dado and the base held firmly against one of the straightedge boards, make a pass the full length of the workpiece. Then rest the router base firmly against the second straightedge and make a pass the full length of the workpiece. Remove any material between the two grooves by sweeping the router back and forth between the straight edge boards.
Adjust the bit to the full depth of the dado and repeat the process.