ER Collet Sizes Explained: ER11 to ER40 Capacity and Three Common Mistakes

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ER collets (DIN 6499 / ISO 15488) are the most common workholding element on the shop floor, and picking the wrong size or the wrong type is one of the most common causes of runout and tool pull-out.

Three ideas up front: the number is a maximum capacity, not a "bigger is better" rating; one collet only covers a 1 mm clamping range; and end mill, tapping and sealed types are not interchangeable.

What can ER11 to ER40 actually hold?

Maximum clamping diameter by size, per DIN 6499:

ER8 — up to 5 mm
ER11 — up to 7 mm
ER16 — up to 10 mm
ER20 — up to 13 mm
ER25 — up to 16 mm
ER32 — up to 20 mm
ER40 — up to 26 mm
ER50 — up to 34 mm

Small sizes (ER11, ER16) have slim bodies and suit Swiss-type lathes and compact driven tool holders where space is tight. Large sizes (ER32, ER40) give better grip and rigidity for heavy milling but take more room.

Why does one collet only cover 1 mm?

DIN 6499 defines the standard clamping range of an ER collet as 1 mm. An ER32-16 collet is designed to hold 15 to 16 mm shanks.

This follows from how the collet works: it grips by elastic compression of a slotted body. Compress it beyond its design range and contact is no longer even around the full circumference — only a few segments carry load. The practical rule is to work near the top of a collet's range; grip quality falls off noticeably in the last 0.3 mm at the bottom.

So squeezing a 16 mm tool into an ER32-20 collet is not saving a collet — it is trading away runout and holding force.

The three most common ER collet mistakes

Using one collet for every diameter — the most common by far. More than about 2 mm difference in shank means a different collet, not more torque on the nut.
Not seating the shank deep enough — if the tool does not fill the collet's effective clamping length, only the front section carries load, and the tool pulls out or runs at an angle under cut.
Not snapping the collet into the nut — the ER nut contains an eccentric ring; the collet must click into the nut first, then the assembly goes onto the holder. Dropping the collet into the holder and then threading the nut on is the classic assembly error: runout goes out immediately and the collet jams on removal.

Do end milling, tapping and coolant need different ER collets?

Yes. The common specialised types are:

Standard ER collets — drills, reamers, general shank tooling
End mill collets — gripping surface designed against the axial pull of an end mill, better pull-out resistance
Sealed (coolant-tight) collets — for high-pressure coolant that must not escape through the collet slots
ER-GB tapping collets — square drive inside to stop the tap slipping, usually with a small amount of axial float

Tapping with a standard ER collet will slip eventually; running high-pressure through-coolant with a standard collet loses the coolant out of the slots before it reaches the cutting edge.

How much runout is acceptable?

DIN 6499 defines tolerance classes: standard grade is typically within about 15 μm, high-precision grade within about 10 μm. Mongtec's precision collets are rated to 3 μm.

To check in practice, clamp a known-good ground bar and indicate it. If runout is well outside the expected range, check cleanliness of the collet taper and the holder bore first — swarf and oil residue cause more runout complaints than worn collets do.

Which ER size goes on a driven tool holder?

That is set by the holder, not chosen freely. A driven tool holder is built with an ER16, ER20, ER25 or ER32 interface; changing the size means changing the holder.

So the order of selection is: confirm which holders the machine/turret accepts → read that holder's ER size → then pick collets by tool shank diameter. Starting from the collet is how people end up with combinations that will not mount. For confirming the turret interface, see VDI vs BMT.

What does Mongtec supply?

Mongtec's precision collets cover BT/NT series ER collets, ER end mill collets, ER sealed collets and ER-GB tapping collets, with even clamping force and accuracy to 3 μm. Used with driven tool holders or static tool holders — if you are unsure which size you need, send us the holder model and the tool shank diameter.