Why ER40 chuck and not 3 of 4jaw?

Quick question. Why would you rather use an ER40 chuck on the lathe than a 3 or 4jaw scroll chuck. I know it has exact collets for certain sizes,but that is not something the other chucks can't do. I am asking because of ignorance about it and have always used 3 and 4jaws on the lathe except on the mill of course ER32 collet chucks are great. Please shine some light on this for me. Thanks

Michael

My very first project after getting a lathe was turning the backplate for an ER-40 chuck. It has less than two tenths runout, and is my most-used accessory. It's quick, obviates the need to indicate, and works great for holding a wide range of work, but especially for small parts and tubing. I only mount a chuck if I have to hold something over an inch. I ground the jaws on my 3-jaw, and the ER-40 chuck still has a fraction of the runout for anything it will grip.
 
It was one of my first projects too.I made a er 50 collet chuck but bought the nut.I had a bash at making a collet as well
This on my to do list now. It's a very long list I must add.
 
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