Hi Steve
A pozilock chuck is fine if all your going to do is use it for milling ,,,, Small shops need versatility IMOP .
Clarkson autolock chuck ,,,,,, all it holds is 4 metric collets and four imp collets ,,,,,,, thats all it dose .

this is not an ER collet system ,, but close ,,,,,,,, lots of collets

A few additions ,,,,, two holders for the lathe ,, two for the mill all using the same collets and closer

because there they are bored right through long work can be passed through them on the lathe ,, they also fit the dividing head .

On the mill table or angle plate or rotary table

in the mill vice , HZ or vertical say for milling a square on the end of a shaft ,,,,,,, or two parallel flats

I have a hex one that need to be finished
more versatility

Rob