Stair saws, as the name implies, were originally designed to cut dado joints for stairs, but they are useful for dado joints of all sizes. I got to use my new stair saw a lot recently when I build a bookshelf, which required 16 dado joints. I build the saw from a kit that a friend put together for me. Through trial and error, I learned a few things about cutting with a stair saw, though I have a ways to go before I will get perfectly consistent results.
Setting up the stair saw is very easy. To set the depth, first tighten the nuts just enough that the saw plate does not move freely. Then you can use a ruler to set the depth. Check the depth on the front, the back, and the middle to make sure the saw is set the same depth the whole length of the saw.
An easier way to do this is to find a piece of scrap and cut a groove or rabbet the same depth as the dado. Then extend the saw plate a little way from the saw body, and set the saw plate into the groove. Press down gently until the saw bottoms out. Tighten the nuts, and it will be set to cut that precise depth.
Cutting with a stair saw can be awkward at first, but a little practice goes a long way. It really helps to establish a deep knife-wall first, so the blade has a surface against which to register.
Scribe a heavy line with a marking knife, or even a utility knife. Then, using a wide chisel at a very low angle, push the chisel toward the knife line. Be sure you are working from the waste side. This will define the saw cut and allow you to start the cut in precisely the right place.
This saw is designed to cut on the push stroke, which is fine for through-dadoes. However, if you are cutting a stopped dado, I highly recommend you turn the saw plate around and cut on the pull stroke. That will pull the sawdust out of the kerf rather than jamming it forward. You should still cut or drill a clearance hole at the front of the dado.
When starting the cut, place the saw right up against the knife wall. You should feel it come to rest in the right place. I think it helps to draw the saw back several times before starting to saw in earnest.
When starting the cut, you may find it easiest to hold the tote in your dominant hand while grasping the body with the other hand, much like you would hold down a wooden plane. Use a three-finger grip on the rear tote, with your index finger extended along the side of the saw body.
The handle at the front appears to be for steering only. Pressure on it will make trouble. So hold the front handle with your fingertips only, just to keep the toe of the saw from popping up out of the kerf.
Once you have established the kerf, saw as you would with any other small saw, taking long, smooth strokes. A well-sharpened saw will cut cleanly and easily, but you may have to stop to blow the extra sawdust out of the kerf once or twice, depending on how deep you are going.
Since the body of the stair saw blocks your view of the kerf, it can be difficult to tell when the saw has bottomed out. So watch the sawdust pattern on the workpiece. When the saw bottoms out, it will push the sawdust away from the kerf, leaving a little path on both sides.
This sawdust trail says, “keep sawing.”
This sawdust trail says, “you’re done!”
Now you can pop most of the waste out with a chisel, and clean up the bottom with a router plane.