Usual preface, I’m with PreciseBits. So while I try to only post general information take everything I say with the understanding that I have a bias.
Don’t know if this will even help honestly. But in the hopes that it does…
So… ER collets were originally designed by Regofix. It’s actually what the “R” in “ER” stands for. The original design was for a hex headed nut that had the part numbers end in “UM” and the “standard thread”. After that mostly for things like toolholders a new type of nut was made with a finer thread. They are called “Mini” nuts. The mini’s used what is called a slotted or castle style nut.
From what I’ve been able to piece together, other manufacturer’s later came out with the “A” designation that meant the original coarser thread and a hex. They then use the “M” for the mini. That turned into what was probably 90% of the nut designations (e.g. ER16A, ER16M).
I should also note that for some there’s also an “E/slotted” type that is the standard thread with slots. Along with this there are specialty types for VERY high balancing or bearing nuts. In fact as far as I’m aware Regofix doesn’t even make anything with a hex style anymore.
Much more recently there have been these hybrid versions using a mini thread with a hex head. It greatly confuses the market in my opinion.
Regardless, one important thing to take away from this is that ANY nut that is using the mini spec’ed thread has a MUCH lower max torque rating before you start to damage things (bore size dependent). e.g. for a ER16, 1/4" bore, MAX torque:
- Mini thread: 24Nm/18ft-lb
- standard/UM/A thread: 56Nm/41ft-lb.
Hope this is marginally useful to someone.