I built this small ring bender this weekend. It doesn’t have a crank so 3/16th rod is about the biggest material that can be fed through it by hand. It’s really made for bending rod that is part of a large rolling ball sculpture while the rod is already attached to the sculpture on one end.
The thumb screw was hard to turn so I ended up grinding the end of it down to a point. That results in much less friction than the off-the-shelf flag end rubbing as the screw is turned. A bigger thumb screw would be needed if thicker material was used in this. I also think that fine threads might be a better choice for the thumb screw to make it easier to turn.