"Why do I need Dead-Length Collets or Step Chucks?"
"All draw-in collets have a diameter-to-length ratio when doing secondary operations. When using collets with a 10° head angle, the ratio is 1:3. For every .001" change on the chucking diameter, the part will either pull back .003" or stick out .003" from the setup part. This can mean a total length variation of .009" when the chucking diameter varies .003". If the print calls for a length tolerance less than .002", a standard collet cannot be used. This is where the Dead-Length collet or step chuck fits in. If the stock diameter varies .003", the finished part lengths will not vary more than a few ten thousands (.0002" to .0005")."