Rob, was there any particular reason you went with the hex stock instead of round? Or was it "available material?"Here are a couple of pics of a ER32 collet chuck that I made. I used 2" hex stock to make it. Yes you can purchase the collet nut, it will be a metric thread. I purchased the collets and nut from Amazon.
What is the deciding factor between ER32, ER40 , and others?
The idea of making my own ER32 collet chuck to fit the spindle of my lathe is intriguing. I thought the tapered bore that accepts the collet was super critical and had to be ground at a precise angle. Can I get away with setting up the angle on my compound and using a boring bar?
Size and cost. I choose ER32 because it went to 3/4" which is the size of the headstock on my Atlas lathe and also the nut was smaller than the 2" hex stock that I used.