They are not designed to be blade holders. They are 7/8" internal diameter. They are meant to slide OVER a 3/4" ID tube, then be locked in place.
Machining them out is not going to be easy because you need to take enough material out of the inner diameter. There are holes bored around that perimeter. Sure you could mill it out, but why go to all the trouble when you can just machine one yourself if you have access to a lathe anyway?