My “standard” bronze frame reel has two surfaces that are cut to a spherical shape, one is the spool retaining screw head (convex) and the other is the waist on the knob (concave; at least, concave motion of the cutter). When I had a Sherline mill I could use it as a lathe by turning the headstock 90 degrees and mounting the cutter to a rotary table. But now I have the more robust Mini Mill, and axis of the headstock can only be vertical. It is still possible to turn, but it seems quite awkward.
So I sought a way to do the turning on my Mini Lathe. First I bought the standard ball turning tool:
But it failed on both counts; it was not big enough to make an 0.8 inch convex radius or a 1.8 inch concave radius.
On Pinterest, I have been seeing many home made ball turning tools. So I have made my own, of a generally similar design.
The cutter is a 3/8 inch round carbide insert.
Cutting a convex surface – screw head.