Single Bid Ball Turning Attachment for Lathes Radius Turn Ball up to 38 mm Diameter - Amazon.com
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
Quick Change Radius (Ball) Turner for Metal Lathe : 10 Steps (with Pictures) - Instructables