R8 is the industry standard. I would guess that a mill that comes thru with an ER16 taper in the spindle is too small for an R8 taper. If you have an R8 taper ouc can use ER 16, 25 , 32 or 40 collets in it with the proper collet chuck. My HF 33686 has an R8 taper but I bought an ER32 collet chuck set and use the ER32 collets 90% of the time. They go from 1/16 to 7/8 by 32's. They also have a greater range for each collet as they are slotted for both ends and the sides stay parallel. Every person I know that doesn't have an R8 spindle regrets not having one.