http://www.holdridgemfg.com/
Here is the link to the manufacturer of this ball turning tool. They have 4 sizes, and also a series of over-the-top sherical turning tools that mount in an Aloris style QCTP.
The 3S and 3D, (D being deluxe) that fits smaller lathes where the compound rest height to centre line is 1"
The 4S and 4D which will cut up to a 4" dia ball. Compund height to centre line is 1-5/8 to 1-3/4"
The 8D which will cut up to an 8" ball, compound height to to centre cine is also 1-5/8 - 1-3/4"
and the 12D for lathes larger than 15" and will turn a ball that is 12" dia. The compound height to centre line is also 1-5/8 - 1-3/4"
These units will cut internal or external ball shapes and will cut half round internal or external grooves around the OD of a cylinder to make bead rollers or tubing bending dies or rope sheaves etc. See the manual on the holdridge site for pictures and more details.
Now even though my lathe is a 14" the centre line to comppound height is 40 mm or 1.5748" so it was too low for the 4, 8 or 12" units without this modification which have a minimum distance of 1.625".
At the factory they have a jig that is made up to allow them the machine the frame quickly and easily. It locates the part and allows the operator to be cutting off that face in minutes. With my setup, it required trying to make a setup that would hold the part in alignement so that the bottom could be squared off, and maintain that hold even though a fly cutter set to about 3-2/4" diameter was attacking it and trying to knock it out of square.
The main alignment was with the Tee Nut for the lathe with a 1" diameter rod in it that is used to mount my Tool Post Grinder. this would have been better if the rod was longer and centre drilled to catch the pivot pin in the lower leg of the frame, but even as it was it allowed me to move the frame up and down and clamp it to the 1" dia rod. I backed this up with a machinist screw jaclk and a step clamp over top to hold it solid. The Tee Nut that I made to clamp down the Radii cutter was used as a jack screw set up with two 1/2" carriage bolts on the top of the swivel vise jaws, which allowed me to support and level out the flat area. A toolmakers vise was clamped in the main vise and and another clamped in it to lift my clamping point. Parallels were used to space them so that the upper support was clamped straight. I used a lay up of parallels between the tool maker vise and the frame to make sure that it was aligned so that as the fly cutter passed along the Y axis of the mill it stayed parallel with the initial cut that had been made on the frame.
I used a 6" Starrett #98 level to align the face of the frame in both X and Y axis, which works well as the mill is leveled. Should have dialled it in, but this seemed to work OK.
Overall, I was pleased with the way that the setup worked out and the frame was cut flat to within tolerance.
Walter