What the standard defines
The ER collet system: tapered, slotted collets that close around a tool shank when a nut draws them into a matching taper. The standard fixes collet sizes, their tapers and the clamping ranges each collet covers. ER11 is one of the smaller sizes, suited to the shanks common on desktop routers.
What it does not promise
No runout figure for a particular spindle, which depends on manufacturing quality and is published by some makers. No spindle power or speed. No cutter suitability: flute count, coating and material are properties of the end mill. No holding force under every cut; the nut torque and cleanliness matter, as makers describe.
What it works with
End mill shank diameters, which must fall inside the collet’s range. Spindle power and speed published by the router maker. Rotary tools, which use their own smaller collets or keyless chucks.
Questions makers ask before buying
Does one collet fit all shank sizes?
Each collet covers a small range of shank diameters. A spindle needs the collet that matches the end mill’s shank, and sets of collets are sold for that reason.
Can an ER11 spindle use larger end mills?
Only if their shanks fall within the ER11 range. A larger cutting diameter on a small shank is possible; a larger shank is not.
Last reviewed 15 September 2026