[FrankJ]

I thought aluminum doesn't really have a fatigue limit. You load it in a reversing stress, it will eventually fail. (Granted maybe long past an FRC bots life time.) One reason air frames have a service life with inspections.

Maybe some one from VEX could comment, but didn't they switch a few years ago from 6061 to 7075 shafting because of fatigue issues?

One reason to use thunder hex even with a lathe available is you can replace a failed shaft in the field by cut to length. A general efficiency in production rule is don't custom make when an off the shelf product is available. OK two reasons.

Although the thunder hex bearings are sort of a odd size, I like round bearings better than hex bearings since you are not putting a stress riser in a brittle bearing race.