Hi Bernd,
I believe the brass dies will be up to their task with flying colours, for copper as well as brass tubing. The smaller roller just "wraps" the free tubing end around the bigger stationary bending block. Any possible deformation would come from the tubing, trying to widen the groove - thereīs no way I see that happening. And if contrary to expectations that would happen, I can always make new rolls, from something stiffer

I made the needed parts for 2, 4 and 6 mm tubing, I did the grooves with a rotary table and mill, the way Bogstandard suggested in another tubing bender post. An excellent, fast method to get the size just right.
I also tweaked the plans a little, made an extra cut to all bending blocks, I think that way thereīs no chance of the bushing through which the tubing enters the bender from moving, not even a little.
Also I made some extra long screws from allthread, with large "knurled" heads, and another large "knurled" nut to the lever screw. These 3 screws are quite fiddly to adjust with a spanner or a key, they should be finger-adjusted, IMHO. I hope G. Britnell approves

Couple more pics of the change parts for different tubing sizes, and the screws in place

