I made a small radius attachment for my 9" lathe some years ago, but wanted one for my big Storebro lathe. One of the very few advantages of breaking your shoulder is that you have lots of time to catch up on the project list, subject to your reduced capabilities. I think it took longer to decide exactly what I wanted to make than it took to make it. I was originally going to cut the frames out of aluminum plate, but it was only 5/8" thick. I finally decided to weld up steel frames.

Here's the outer frame ready for welding, complete with bevels for the welds. Not long before the accident, I upgraded my MIG to a Miller 211. I'd used it a bit, but never really rung it out. Since the outer frame is made of 3/4" thick steel, I cranked the welder up to max on the automatic settings and had at it. Man, that's a nice welder! It filled in the bevels very quickly.
   

I made the attachment to mount in the four-way tool post, so I wouldn't be changing the height setting on my biggest QCTP block. That way, it's on lathe centre every time.
   

Since I like using square HSS bits instead of round ones, I needed a 5/16" square hole through the inner frame. I've done this before, for a boring bar. Drill a hole, then force a HSS bit blank through the hole and tweak it with a file. This time, it was much easier with the 20-ton shop press. I filed the entry corners before, so the bit would start straight.
   

I drilled the inner frame with two pivot lines to make it easier to do smaller rounds. I made it to handle up to a 3" ball, just in case.
   
   

You can't make a knurler without at least one knurled part and you can't make a radius turner without at least one ball handle. This one is almost 1 3/8" in diameter, with a 1/2" shaft.
   

Here's the finished attachment. The handle also serves as the setscrew to hold the handle and hub onto the top of the vertical shaft. You can see the spot beside the bit where I had to grind out some material to allow the ball to be turned close to the shaft. I'll probably make a second inner piece - a straight shaft with a 1/4" bit for turning small concave radii, such as you would need for a new head for the pipe bender to handle odd pipe sizes.