How to make a wide flat bottom dado?

[Peter Christensen]

Don't see how you can use a riving knife since cut is a through cut.

The blade is buried during the cut except when it enters and exits the cut, therefore not a through cut. A riving knife is like a shark fin sitting just behind and a touch below the top of the blade, so it works when the blade is inside the wood. A splitter sits higher than the blade, usually holding a blade guard and anti kickback pawls, so it can only make through cuts.

[George Bokros]

The blade is buried during the cut except when it enters and exits the cut, therefore not a through cut. A riving knife is like a shark fin sitting just behind and a touch below the top of the blade, so it works when the blade is inside the wood. A splitter sits higher than the blade, usually holding a blade guard and anti kickback pawls, so it can only make through cuts.

Since I do not have a riving knife I assumed it was similar to a splitter. I stand corrected.

[Frank Pratt]

A rabbet plane will flatten the bottoms all to exactly the same depth quicker than you can set up a router & with a lot less mess & noise. They are a wonderful tool.

[Greg Hines, MD]

I would do this in two stages. First, use your table saw to cut the depth of the rabbet at the inside edge. to give you a crisp edge there. Then I would use a router, either in a table or handheld, to hog out the rest with a flat bottomed bit, starting at the end of the board and working my way to the saw cut.

Doc