I use them all the time; a 5C as a matter of fact, which can only hold stock up to 1-1/8. Tony and others pretty much hit the nail on the head. I work on a lot of shafts and most are finished when I get them but damaged on the end or the bearing race is worn-out. Almost always, they're between 1 to 1-1/8 inch (about half of them are a metric diameter). The holding ends are precision machined and polished and thus cannot be damaged by a regular chuck. They usually come to me straight as arrows so when you chuck e'm in a collet they line-up basically dead-on with the center.
A few more positive aspects: The chuck is usually pretty small and you get to use more of the bed if needed. Likewise, most lathes can accommodate a 5C collet right in the spindle -giving you even more usable bed length. When you do that though, the shaft cannot protrude into the spindle because of the collet closing mechanism resides there. Also, the chucks tend to balance very well. My 5C balances so well, the machine simply doesn't vibrate at any speed.
So, instead of the upsides that Tony pointed-out, the downsides are that you need a full set in 1/64" increments to hold all possible size shafts. That's something like 50+ collets (at 6-10 bucks each). You cannot use them to turn tapers with the tailstock method (taper attachements work fine with them) -but this is also true of any jawed chuck. Limited range (For 5C, 5/64" I think at the small end and 1-1/8 at the large end). They can be damaged easily. If a piece spins-out, that could be the end of that collet. I believe someone else mentioned this but, you can't put rusty/scaley stock in them if you care about them, lest you transfer pit marks in the surface. If you're not careful, it's possible to get a collet stuck in the chuck (knock on wood, it hasen't happened to me) and getting them un-stuck can be an ordeal.
I like e'm... they save me a lot of time.
Ray
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