For holding all ER20 collets. Collet Range: 1/16 - 1/2" (2-13mm). The ER Collet Chuck System Simplifies tool changing on conventional milling machines and eliminates awkward use of draw bar. In addition, you can use the same chuck for milling, drilling (holding on flutes), reaming, tapping, boring, etc. Wide .040" Holding Range of each ER collet guarantees that undersize and oversize cutting tools can be held without sacrificing accuracy or gripping power.