Originally Posted by EddyCurr
The ER collet blocks are interesting, but I think that 5C's are still preferable. Simply because 5C's ae available in other shapes not just rounds. They can be had in soft form to machine into custom shapes also. None of which is available in the ER collets.
To OP, unless you need to do certified work for a paying customer who requires a metrology paper trail, don't sweat it too much. Buy whatever is financially prudent for you.
If you think you understand what is going on, you haven't been paying attention.