I see in the video that hex bar is used as the center axis, then the ends are turned down on the lathe.
What if instead long coupling nuts were used on thread rod, like m12? the Hex coupler nut can be put on the thread rod, then tack welded on the ends so it does not spin off again.
unless you used grade8 bolts… They are already much stronger and treated. Maybe steel epoxy the threads, and tack weld the end of the nut?
Locally where I am at, Hex bar requires a 3/4 day trip to buy it, then another couple days at the machine shop where they will charge me 40 or 50 dollars to turn it down, and the work is not very good.
So you would have to be very careful not to weaken the shaft from the current design. As in a shredder design usually the shaft is the weak link. As the shaft will generally fail in Torsion way before anything else fails.
actually now that i think about it. That has a serious problem. In order for the hex nut to be the same size as the precious plastic design the threaded rood that it would spin on would be tiny. Thus the threaded rod would be very weak in Torsion as it is very small in diameter and just twist apart under small load. so you would need a threaded rod about the same thickness as the current rod then the hex nut would be even larger on top of that meaning you would have to change the whole design and cut bigger holes.
threaded rod in like 1″ diameter is more expensive then hex bar with same width.
Then you have to buy large threaded hex nuts.