ER collet size chart: ER8 to ER50, and how to choose the size
ER collets are standardised in DIN 6499 and ISO 15488. The ER number is roughly the collet’s outside diameter in millimetres. Each collet closes over a small range — 1 mm on most sizes, 0.5 mm on the smallest — so one chuck with a set of collets holds every shank inside its capacity.
ER collet sizes at a glance
Size
Collet OD (approx.)
Maximum clamping Ø
Range per collet
Mongtec chuck range
Typical use
ER8
8.5 mm
5 mm
0.5 mm
—
Micro drills, engraving, high-speed spindles
ER11
11.5 mm
7 mm
0.5 mm
1 – 7 mm
Small cutters, angle head and live tool outputs, Swiss-type
ER16
17 mm
10 mm
1 mm*
1 – 10 mm
Small end mills and drills, compact live tools
ER20
21 mm
13 mm
1 mm*
1 – 13 mm
General small-diameter work, live tools
ER25
26 mm
16 mm
1 mm*
2 – 16 mm
The usual live tool output; general milling
ER32
33 mm
20 mm
1 mm*
2 – 20 mm
The workhorse size on machining centres
ER40
41 mm
26 mm
1 mm*
3 – 26 mm
Larger drills and end mills, heavier milling
ER50
52 mm
34 mm
1 mm*
6 – 34 mm
Large drills and tools on BT50 / HSK100 spindles
* The smallest collets in each size close over 0.5 mm. Standard ER collets have a 16° included taper (8° each side). The “Mongtec chuck range” column is the published clamping range of Mongtec ER collet chucks.
How to read a collet size
An ER32-12 collet is an ER32 collet with a nominal bore of 12 mm. With a 1 mm range it closes on shanks from 11 mm to 12 mm. Clamp as close to the nominal size as you can — a shank at the bottom of the range is held on a smaller contact area and runs out more. Inch shanks need an inch collet or a metric collet whose range covers them; check before assuming.
How to choose the ER size
The largest shank you will hold sets the minimum size. Do not size up the whole shop for one large drill; a second chuck is cheaper than a lost rigidity on every small tool.
The nose diameter. If the chuck nut, not the cutter, is what hits the part or the fixture, a smaller ER size or a mini nut solves it.
Overhang and rigidity. For the same tool, a bigger collet in a shorter chuck is stiffer than a small one on a long extension.
Coolant. Through-tool coolant needs a sealed collet or a sealed nut (GER style) to keep the pressure at the cutter.
Mistakes that cost runout
Putting the collet into the nut after it is in the chuck. Snap the collet into the nut first so the eccentric ring engages; otherwise the collet cannot be extracted properly and does not seat evenly.
Too little shank inside the collet. The shank should reach well into the collet bore — a shank gripped only at the front is held on a fraction of the collet and runs out.
Tightening by feel. Use a torque wrench and the nut maker’s value. Under-tightening slips; over-tightening distorts the collet.
Dirt on the tapers. Chips in the chuck taper or on the collet show up directly as runout at the tool tip.
Mixing systems. ER, OZ (DIN 6388), TG and DA collets are different systems and are not interchangeable.
Accuracy classes
Standard ER collets are typically specified at 0.015 mm runout, high-precision versions at 0.010 mm, and ultra-precision versions at 0.005 mm or better, measured at a set distance from the collet face. The chuck and the nut count as much as the collet: a precision collet in a worn chuck is not a precision assembly.
Mongtec AA grade
0.005 mm runout — the standard Mongtec collet grade
Mongtec A grade
0.008 mm runout
0.010 mm grade
0.010 mm runout
Mongtec ER collet chucks and collets
Mongtec builds ER11 to ER50 collet chucks to DIN 6499 on BT30, BT40, BT50, BBT40 and BBT50 tapers and on HSK-A32 to HSK-A100, in gauge lengths from 70 to 300 mm, with UM, hex, mini, sealed GER and short-body nut styles. ER outputs are also used across Mongtec angle heads and driven tool holders, and Mongtec supplies the matching collets in AA grade (0.005 mm runout), with A grade (0.008 mm) and 0.010 mm grades also available. ER collet chucks →
It is approximately the outside diameter of the collet in millimetres. An ER32 collet is about 33 mm across and holds shanks up to 20 mm.
What is the clamping range of one ER collet?
On most sizes each collet closes over 1 mm, for example 11 to 12 mm. The smallest collets, and all ER8 and ER11 collets, close over 0.5 mm.
Are ER collets from different brands interchangeable?
ER collets are made to DIN 6499 / ISO 15488, so the size and taper interchange. Accuracy, sealing and nut design differ between makers, so mixing brands can change the runout you get.
How accurate are Mongtec ER collets?
Mongtec ER collets are AA grade, 0.005 mm runout. A grade (0.008 mm) and 0.010 mm grades are also available.
Which ER size is most common?
ER32 on machining centres and ER25 or ER20 on lathe live tools. ER11 and ER16 are common on angle heads, Swiss-type lathes and small high-speed spindles.
