Radius Turning Tool, cut ball-shaped end or turns a radius on lathe, cuts ball-shapes up to 3/4 inch diameter, made of corrosion-resistant blackened steel
Clark Sphere Turning Jig
Assorts' Radius Ball Turning Tool for Lathe 3/8" Shank with HSS Tool Bit - Amazon.com
9x20 Lathe Ball Turners & Knurlers
Ball Turning Attachment For Lathe-Turns Round Concave and Convex Metal-Sphere Radius ball turning - Amazon.com