I needed to cut stopped grooves on a round surface – and while I could have cut them on the stock while it was square, then proceed to turn it on the lathe, I didn’t want to worry about catching my turning gouge on a groove and causing tearing out (or worse).
So, after considering (then rejecting) some kind of router jig, I figured out a way to use my router plane. I often use “joiners saddles” (V-shaped blocks) to hold round work at the bench. So, I cut one of my saddles in half at the apex of the “V,” then screwed each piece to my router plane on either side of the blade.
A little trial and error got me to the correct angles for the fences for my particular workpiece.
La Crosse, Wisconsin