I am drawing for a manufacturer to bend steel plate into a U shape.

The steel is galvanised and 1.5mm thick.

How do I show the Radius of the bend?
What is the maximum radius I can bend with 1.5mm steel?

I am drawing for a manufacturer to bend steel plate into a U shape.

The steel is galvanised and 1.5mm thick.

How do I show the Radius of the bend?
What is the maximum radius I can bend with 1.5mm steel?
I would show the radius at the inside of the bend. Often times the material will be formed over a piece of a like radius.

I believe that you meant to ask what the smallest radius in the bend could be given the thinnest of your material? I can only say that tubing is subject to the rule of 1.5 times the diameter of the tubing such that 1/4" tubing should not be bent to a radius less than 3/8". Using that rule, 2.25mm would be your smallest radius.