Some stairs are made by cutting a groove in the stringers for the threads to fit into. Is this done with a router? If so, what kind of jig does it require?
Yes, some treads are housed in the stringer. A router, circular saw, radial arm, etc. can be used to do this. For a router, a custom jig, such as a piece of plywood with a hole in it, is used. This jig would guide the router, fitted with a guide bushing, around the hole that would be the same dimension as the groove or dado plus the thickness of the bushing edge to the router bit.
I prefer to cut out the stringers completely for a couple of reasons: the stringer is not weakened and especially on outside work, the water can run away better, rather than get trapped in the dado and cause problems with rotting. I live on the West Coast with lots of liquid sunshine (rain).
With inside work, on a fancy staircase, I cut out the stringers for good support, then nail the exposed stringer on the outside of the support stringer; then assemble the two together on the wall. The extra cost of material usually offsets the cost of labour.
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