Decent dies should be steel, good dies shold be hardened. Material costs and heat treating a 20 pound die are substantial.
You can do them on a manual lathe and cut off the part you don't need. To get the perfectly round cross section you need requires the special swivel tool holder. You can to it on a CNC lathe and skip the special tool holder.
You can do them on a manual mill and a roatary table: use the table vertically with a long roughing mill, vertically with a ball mill, or horizontally with a suitable half round cutter. CNC mill can be done horzontally or vertically with a 4th axis.
If I were running low to medium volume production I'd run them horizontally in a CNC mill using a 1/2 round cutter with a cutting edge radius much smaller than the desired cut, then off to heat treat.
I've seen the the > I'm curious (Gunner, please don't make the cat pun here ;)
Hm. I have seen all kinds of sections rolled into steel at mills. I wonder if you could heat just the rim of a die to soften it, and *roll* this half-round section into the die? I have seen forged dies, and this would be rotary forging.
Then you'd just quench the already hot part, and it would be hard surface, mild core, just what is needed for a rolling die.
This would be a mass-production process requiring considerable tooling, not much of a method for a one-off.
For a nice set of dies, one might take a forged set and finish by grinding to precise radius. Many tubes are shiny finished, and this finish could be preserved by ground dies.
I think the HF tool you reference is intended to go on the 10T pull-back ram that is an accessory for the hydraulic autobody utility kits. The pull-back ram has threaded ends and comes with chain type hooks. Looks like this accessory lets you pull from bolts.