ER stands for the DIN 6499 collet standard — a spring collet that closes evenly around a shank when the collet nut is tightened, giving good concentricity across a range of diameters rather than needing one collet per exact size like a 5C. The size number (ER8 through ER40) refers to the collet's outside diameter, not the clamping range, and determines which collet chuck or toolholder it fits.
ER32 is the size most jobbing shops reach for first — it's the standard fit for BT30 and BT40 toolholders and covers roughly 2-20mm clamping range across the full collet set, which handles the majority of cutter and workpiece shanks on a typical mill. ER16 and ER20 suit smaller BT30 setups and lighter-duty spindles; ER40 and above are for larger shanks and heavier cuts.
Standard precision collets (around 0.010mm TIR) cover general milling and drilling. Ultra precision (0.005mm TIR or better) is worth the extra cost for finish milling, thread milling or anything where tool runout shows up in the surface finish. Coolant sealed versions add a seal ring so through-spindle coolant reaches the cutting edge instead of leaking out around the collet.
See the full ER Collets range for all sizes and grades in stock, or our collet buying guide if you're not yet sure ER is the right system for your machine.
