A radius turning tool lets you make ball shapes or radiused ends on your metal turnings. With the tool you can make convex curves up to 3/4″ in diameter in brass, aluminum steel, or plastic — you just mount the jig on your lathe’s tool post and swing the handle, which rotates the cutting tool in an arc.
With a quick search, I could find two versions of this tool: one from Micro Mark and one from the Little Machine shop. The Micro-Mark version, which runs $50, is made of blackened steel and includes the high-speed steel cutter.
The Little Machine shop version costs $90 and is completely machined from solid metal, not stamped, cast, or pressed, by OMW corp in the U.S. The frame on the Little Machine Shop version is machined from a high-strength aluminum alloy, the handle is tool steel, and the hardened pins pivot in bronze bearings. This care to detail is probably the reason it’s twice as expensive as the Micro-Mark version.