The strategy here is to use a “ring tool” to “hog out” the spheroidal curve roughly. I’ve decided to grind to f/6.25 for a focal length of 78 inches.
The ring tool: a 6″ steel disk about 1/2″ thick. Folks on campus helped me with this. At the workshop we attached some sort of flange … tapping holes (8-32) into the steel disk, and then lathing (clumsily the wooden handle).
I began grinding with 60 grit in a circular motion. For the first hour or so, I ground exclusively around the center. Afterwords I began gradually working my way concentrically outward – center, middle, periphery.
At a certain point I noticed that I was not longer decreasing my radius of curvature even as I was achieving better spheroidicity (?). I changed my stroke to center-over-center, and the ROC began shortening again. In addition to the advice of the NCA-Chevy Chase workshop experts, I also consult Mel Bartels’s helpful webpage on rough grinding.