Thanks for the comment. I didn't intend to taper the treads or risers for the mortise, just the stringers. If I could post the link to the TrenddotCOdotUK website, I could show you how their 'staircase routerbit' cuts the mortise. The tread and riser ends remain flat. Its basically the same bit as I bought.
Bosch offer a 7/8" diameter Staircase bit also, with 7-degree angle, but my risers would be flapping in the Prairie wind.
Yeah, I'm looking forward to some expert input, for sure. As it is, especially for the top end winder steps, I'm relying on the ONLY resource I've found on the methodology, from an 1905 book, titled, "A manual of carpentry and joinery" that I found on another site called "evenfallstudios.com/woodworks_library/".
Excellent reference on all things wood! Even describes how these masters cut their mortises with tenon saws, by hand, after 'boring' the nosing profile in the stringers. Routers were pedal-powered and strictly floor-based! We've come a long way, eh?
I look at the studs and techniques used in my old house, and am amazed that this wood is over 100 years off the tree, which probably took 100 years to grow big enough to harvest (all Douglas Fir and REAL 2 X lumber! Even the old 'square peg' iron nails are still good! Amazing when I think that we often get 60 MPH and higher winds!
Right now, I've got my drawings in Punch Software's Architect series format. I hope to start cutting and prepping the treads on Monday, and the stringers shortly after.
My plan is to build the winder section separately and mounted on its own four posts, connected to the two side walls for the main stringers and the end wall, shared with the kitchen. The new pantry is below these stairs and above the new basement stairs. The outer stringer will be mortised and tenoned into the winder's outer stringer, all 2 X 12 fir.
Most modern stairbuilders make the straight stair winders at the bottom of the staircase, using almost box-like frames below each kite tread. That book I mentioned, does add intermediate tread bracing, but only below the tread to allow 90 degree bracing back to the staircase headers at the main staircase and at the top of the flight. Much lighter, although a bit more technical.
But I'm open to ideas.