how to make the semi-circle section on a tube bender die using a lathe

======================= I don't mean to belabor the obvious, but as this is alt.machines.cnc, do you have a cnc lathe?

If so, an old technique may help. If you lathe has enough swing, rig a faceplate. It will be an interrupted cut so HSS may better than carbide, but G2/G3 and lots of patience may be a solution.

If a strictly manual lathe you can use the method described in one of Lautard's "Machinists Bedside Readers" to generate a radius using a series of plunge cuts. see

and several of the catalog tool supply houses also carry these books. This is not a actual radius but series of tiny steps that approximates a radius, most likely good enough as machined, or file/polish to suit.

What equipment do you have available and what are the sizes of the dies?

Unka George (George McDuffee)

...and at the end of the fight is a tombstone white with the name of the late deceased, and the epitaph drear: ?A Fool lies here, who tried to hustle the East.?

Rudyard Kipling The Naulahka, ch. 5, heading (1892).

The dies are about 6" in diameter.

CNC vertical mill, manual lathe.

I just figured out that I could put the die on a shellmill holder (in the vertical mill spindle), put the cutting tool in the vise and do a Z axis interpolation on the mill.

Thanks for your suggestions!