James Waldby wrote:

MetalHead wrote:

Bob May wrote:

[snip]

Thanks, I think it's time to try polishing the die. Since it was made on
a lathe, the tooling marks are concentric to the curve. I will try to
get a little bit of draft as well.

Artemia is correct that the inside radius of the tube is bulging out
against the walls of the die. There are prominent scuff marks on the
tube. Hopefully with better finish, this will work better.

...

If you have a larger die, say 9/16" instead of 1/2", you could
make an insert out of 1/16" sheet. Put tabs or a handle on it
so it can be pulled out to release the tubing after bending.
-jiw

I polished the walls of the die with a hand grinder last night. There
were pretty substantial tooling grooves. It is still far from perfect,
but it is a lot better. Since the die is not a full circle anymore, I am
reluctant to try to machine it in the lathe. It is now possible to get
tubing out without deforming it much.

I had not though about a liner, thanks.

Bob