The ER20 precision collet holder has a 2MT shank with an M10 thread for a draw bar. ER collets have the unique ability to grip over a diameter range of 1mm without any loss of accuracy. This flexibility means that you need fewer collets compared to other collet-based systems. An additional benefit is that there is no requirement for a separate imperial listing; a 1/4" end mill, for example, will fit into the 7mm to 6mm collet.