While demonstrating how to build a workbench and sawhorses with just a hand held circular saw, I cut a few dadoes with it. This led me to refine my technique, so I figured I'd make short article and video on it.
In this example, I want to cut a dado to fit a piece of 2x8 into the side of another 2x8. I made two guide fences to clamp to the 2x8 to help guide my circular saw. Each fence has the width of the edge of the shoe to the blade built into it, so I just need to put both fences around the piece I want to cut a dado for...
... and then clamp them to the workpiece.
A quick check to make sure that if I move the saw between the fences, the cut will stay between the lines.
Now making a series of cuts to hog out the material.
It can be tricky to get all the ridges with the saw, so I break these with a few a sideways taps with a hammer, then use the back of the hammer to scrape out the rest.
Next, moving the saw side-to-side to flatten out the bottom of the dado.
If the dado is wider than the shoe of the saw, this can be tricky. With the 1.5" (38 mm) wide dado I'm cutting here, when the saw is all the way to the left, the shoe no longer reaches over to the right side. Without support, it's all too easy to tip instead of slide the saw to the right, gouging the dado. The wider shoe on my Makita circular saw would have worked better.
A fairly clean dado for this one, no chisel required.
For dadoes 1.5" to 2.5" wide (40-80 mm), it may be a good idea to start from the left, clear that out completely, then work further right so there is always support on the right side. But for dadoes wider than 2.5" (8 0mm), there is no way the shoe will span the whole width, so it's better to completely clear an area maybe 1" wide at a time, clearing out successive sections of the slot from right to left.