This is the part of the spindle that will hold and grip the end mill at the spinning part of the spindle. This is a collet for smaller spindles with the ER-11 specification. This collet will accept tools with a 4mm shank.
This collet has a high precision TIR (Total Indicated Runout) of 0.0002". The bore measures 4mm to hold end mills that have a 4mm shank. The shank of an end mill is the smooth part of the end mill that is inserted into the collet of the end mill.