So I got some ER11 collet nuts. Had a piece that needed to run quite true. When I used one of the new collet nuts, it was out by more than 0.05mm. Used the genuine ER nut and it was fine.
So I set up made a spacer and checked the original with my set up and the taper on the nut ran within 0.01mm, the new ones were out by as much as 0.1mm. So I set up and recut the 30 deg front taper using the thread and the back shoulder as a reference. Picture shows the inverted bore tool and the compound set at 30 deg. Using a Regofix ER11 collet holder with the sleeve spacer for the nut to seat against.
I also set up and trimmed the outside with it just on the thread of the mandrel 1st. Cutting the inner 30 deg taper was a second op.
Now they are just as good as the original collet nut.