Here's another noobie question. I often see people with mills and lathes that have like r8 and mt3 spindle but then you see in their collection of tools that they have 5c collets. I also noticed that some indexing tools use 5c. Why?
R8 and MT3 collets have blind holes in which a tool such as a milling cutter or drill fits and they have smallish internal threaded holes at the opposite end for a drawbolt which pulls them into their taper. 5C collets have a through hole and an external thread so they screw into a draw tube. R8 and MT3 collets are intended for holding tools and 5C for holding a workpiece; very often bar stock which can pass all the way through the drawtube. 5C collets are often closed by means of a handle and a system of cams and toggles that work on a collar at the end of the drawtube and the thread that fits in the draw tube is used to adjust the amount of closing when the lever is operated. They can also be closed by air cylinders or closed by a spring assembly and opened by an air cylinder. Sometimes in things like indexers they are just closed by tightening a big hollow nut on the end.
5C has internal as well as external threads on the back end of the collet, which allows use of things like adjustable stops so that you can make sure every workpiece has the operations performed the same distance from the "back end". Being mounted in the internal thread, the stop doesn't interfere with using the external thread for collet tightening.
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