If you want to put the dado across the face of the board and relatively close to the end, you use the router table’s fence as a way to measure where you want to go and the same way you use a table saw’s fence to make a dado or rabbitt. Use the fence and a block of scrap wood clamped to the fence, with your sliding miter gage, to line up where the board is to go to put the dado in the right place. DO NOT run the end of the board to the fence as you likely will get very dangerous kickback. Just use the fence as a way to locate the position to hold the board against the miter gage. I would run this in 2 passes with either an upcut spiral cutter or a straight bit of a width to cut the dado width. For the depth of the first pass, I would take a little more than half the dado depth and finish the depth with the second pass.
If the dado is to go toward the middle of the length of the board, I would not run it on the table. If you have to make several boards with the same dado, clamp them together so you have a wide surface to set the router on as you make the cut. If you don’t need but one board with the dado, you still want to clamp enough other 5/4 boards together across their faces to get a wide surface to set the router on as you run to a straight board clamped to the top as a fence.
That is how you can make your cuts using a router but I would use either a table saw or a circular saw to cut the dado in several passes. Don’t run tight to a fence if you do use a table saw because of the danger of kickback.
Henry in MI
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