Quote Originally Posted by njhussey View Post
Unless you have some sort of reverse collet where the bottom of the shaft is slotted and a tapered hole bored and tapped at the end so when you tighten it in it splayes the bottom to grip the tool? Hard to explain properly without a sketch...
You could do that by just having a thread which expands the parallel portion when the screw is tightened, by specially machining the screw or shaft so that one is oversize.