mm wrote:
bending 1/2 inch wide stock to a 3/8 inch radius

I need to make a bracket for rear turn signals for my new-to-me '69
Honda cb450. I'm going to use a steel rod? [I don't know what you
call it. The Home Depot cash register calls it "flats"] 1/2"W by
1/8"T x about 18"L, and I need to bend it twice, 90 degrees at each
location, to make a rather square C shape, but the corners have to
have a 3/8" radius (to look good where it wraps around the luggage
rack. Can't figure out how to do this, and don't want to try too many
times. I look to you sages.

Carve the desired radius into the edge of a hardwood block. Clamp
to your workbench over a leg. Beat the metal strip against the block
with a mallet.

A. My vice has a rounded part at the end away from the vice, but it's
sort of conical.

Ain't gonna work, way too hard to control the bend.

B. I could clamp it in the vice at the start of the curve and clamp
two 6-inch pieces of 2x4 (or 1x2, or thinner if I could find some
steel to use) the right distance from the start of the curve ( (2 pi
R)/4 = one quarter of the circumference plus a tiny bit more for the
circumference of the outside side of the flat), but this assumes the
metal bends evenly. Is it likely to?

Mild steel, less than 1/2% carbon, dead soft temper. You should
have no problem.

C. I could clamp the flat to the bike luggage rack and clamp two
6-inch pieces of 2x4 the right distance from the start of the curve
same as in B), but this assumes 1) bending it around the luggage rack
tube would make it bend evenly, and 2) that I could bend it in that
situation. Usually I bend this kind of stuff by hitting it with a
hammer, and here the luggage rack might be too springy to let it bend,
or I might break the luggage rack.

D. I could find some other rod or tube that is 3/4 inch in diameter
(2x3/8), clamp that in the vice, but then how to I hold one end of the
flat while I hit the other? This would be a problem for A too.

How would you do it?

Bend the strap you already bought.

If you screw up, go to the fence department and buy a few pieces
of "stretcher strip." Same stuff, except galvanized so it won't
rust, same size, cheaper cost.