Tooling and hardware
Collet chart: ER, 5C and R8 collet sizes
An ER collet grips from its marked size down 1 mm (ER16 to ER40): a 1/4 in (6.35 mm) shank goes in a 7 mm collet (7 to 6 mm) or a 1/4 in collet. 5C collets hold round stock to 1 1/16 in (26.99 mm).
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Source: DIN 6499 and ISO 15488 (ER collets), with sizes from REGO-FIX technical data and Leitz collet data; Hardinge spindle tooling catalog (5C and 16C); PG Collets R8 data; Machinery's Handbook (collets).
| Series | Collet outside diameter D1, mm (in) | Collet length L, mm (in) | Grip range per collet | Smallest collet listed, grips (mm) | Standard set sizes, smallest to largest (mm) | Larger sizes also listed (mm) |
|---|---|---|---|---|---|---|
| ER8 | 8.5 (0.3346) | 13.6 (0.5354) | 0.5 mm | 1.0 to 0.7 | 1.0 to 5.0 in 0.5 mm steps | None |
| ER11 | 11.5 (0.4528) | 18.0 (0.7087) | 0.5 mm | 1.0 to 0.5 | 1.0 to 7.0 in 0.5 mm steps | None |
| ER16 | 17.0 (0.6693) | 27.5 (1.0827) | 1 mm | 1.0 to 0.5 | 1 to 10 | None |
| ER20 | 21.0 (0.8268) | 31.5 (1.2402) | 1 mm | 1.0 to 0.5 | 2 to 13 | Half sizes such as 12.5 to 11.5 mm |
| ER25 | 26.0 (1.0236) | 34.0 (1.3386) | 1 mm | 1.0 to 0.5 | 2 to 16 | Half sizes such as 15.5 to 14.5 mm |
| ER32 | 33.0 (1.2992) | 40.0 (1.5748) | 1 mm | 3.0 to 2.0 | 3 to 20 | 21 and 22 |
| ER40 | 41.0 (1.6142) | 46.0 (1.8110) | 1 mm | 3.0 to 2.0 | 4 to 26 | 27, 28 and 30 |
| ER50 | 52.0 (2.0472) | 60.0 (2.3622) | 2 mm | 6.0 to 4.0 | 6 to 36, even sizes | None |
A collet marked N grips from N down to N minus the grip range: an ER32 7 mm collet grips 7.00 to 6.00 mm. Inch values are mm / 25.4, to 4 places. Set sizes are the sizes REGO-FIX packs in its sets; the smallest and larger collets are REGO-FIX collets outside some sets.
| Inch collet (in) | Grip range (in) | Grip range (mm) | Series listed |
|---|---|---|---|
| 1/4 | 0.2500 to 0.2100 | 6.35 to 5.33 | ER25, ER32 |
| 3/8 | 0.3750 to 0.3350 | 9.53 to 8.51 | ER25, ER32 |
| 1/2 | 0.5000 to 0.4600 | 12.70 to 11.68 | ER25 |
Ranges as printed by REGO-FIX in inches; mm = in x 25.4, to 2 places.
Which ER collet holds your shank
ER32 collet
mm
d_mm = d_in x 25.4 = 0.25 x 25.4 = 6.35 mm; low = N - s = 7 - 1 = 6.00 mm
Grip range 7.00 to 6.00 mm (0.2756 to 0.2362 in); the shank is 6.35 mm. Inch option: 1/4 in collet, 0.2500 to 0.2100 in (ER25, ER32). Every series is in the table below.
| Series | Collet (mm) | Grip range (mm) | Grip range (in) | Shank position |
|---|---|---|---|---|
| ER8 | None | - | - | Too large for this series (largest collet listed 5.0 mm). |
| ER11 | 6.5 | 6.50 to 6.00 | 0.2559 to 0.2362 | 0.35 mm above the low end |
| ER16 | 7 | 7.00 to 6.00 | 0.2756 to 0.2362 | 0.35 mm above the low end |
| ER20 | 7 | 7.00 to 6.00 | 0.2756 to 0.2362 | 0.35 mm above the low end |
| ER25 | 7 | 7.00 to 6.00 | 0.2756 to 0.2362 | 0.35 mm above the low end |
| ER32 | 7 | 7.00 to 6.00 | 0.2756 to 0.2362 | 0.35 mm above the low end |
| ER40 | 7 | 7.00 to 6.00 | 0.2756 to 0.2362 | 0.35 mm above the low end |
| ER50 | 8 | 8.00 to 6.00 | 0.3150 to 0.2362 | 0.35 mm above the low end |
The smallest listed collet whose range holds the shank. A shank at the top of one range and the bottom of the next takes the collet it tops out.
Capacities are the makers' published maximums; read the collet and its range before you clamp.
| Item | 5C | 16C |
|---|---|---|
| Max round capacity | 1 1/16 in (1.0625 in, 26.99 mm) | 1 5/8 in (1.6250 in, 41.28 mm) |
| Max hex capacity | 29/32 in (0.9063 in, 23.02 mm) | 1 13/32 in (1.4063 in, 35.72 mm) |
| Max square capacity | 3/4 in (0.7500 in, 19.05 mm) | 1 9/64 in (1.1406 in, 28.97 mm) |
| Back bearing diameter | 1.250 in (31.75 mm) | 1.889 in (47.98 mm) |
| Overall length | 3.281 in (83.34 mm) | 4.312 in (109.52 mm) |
| Draw thread | 1.238 in x 20 TPI RH, external (Hardinge); another maker lists 1.240 in x 20 TPI RH | 1.870 in x 1.75 mm pitch RH |
Fractions show their decimal to 4 places; mm to 2 places. Two makers print the 5C draw thread diameter differently, so both are shown.
| Item | Value | Source |
|---|---|---|
| Drawbar thread | 7/16-20 (0.437 in x 20 TPI RH) | PG Collets R8 data |
| Head diameter, max | 1.250 in (31.75 mm) | PG Collets R8 data |
| Inch collet sizes | 1/16 to 7/8 in (1.59 to 22.23 mm) in 1/32 in steps | PG Collets R8 data |
| Spindle | The Bridgeport-type mills in two university machine shops have R8 spindles; check an older head before ordering collets | University machine shop pages for their Bridgeport mills (MIT, UC Berkeley) |
R8 body diameter, taper angle, overall length and metric sizes are not listed yet.
Step and emergency collets
An emergency collet has a soft face with a pilot hole; you bore or step it out to the size you need. Step collets hold work larger than the standard collet capacity and take threaded positive stops. Step collet capacities are not listed here yet.
How the lookup works
d_mm = d_in x 25.4; low = N - s; low <= d_mm <= N
| Symbol | Meaning | Unit |
|---|---|---|
| d_in, d_mm | Shank diameter in inches and in mm | in, mm |
| N | Collet nominal size, the top of its grip range | mm |
| s | Grip range per collet: 0.5 mm (ER8, ER11), 1 mm (ER16 to ER40), 2 mm (ER50) | mm |
| low | Smallest diameter the collet grips | mm |
For each series the lookup takes the smallest listed collet whose range holds the shank. The first collet in a series can have its own range, as listed in the ER table: an ER8 1.0 mm collet grips 1.0 to 0.7 mm.
Worked example: 1/4 in shank
A 1/4 in end mill shank.
- d_mm = 0.2500 x 25.4 = 6.35 mm.
- ER32: N = 7 mm, range 7.00 to 6.00 mm (0.2756 to 0.2362 in); 6.35 mm sits 0.35 mm above the low end.
- Same 7 mm pick in ER16, ER20, ER25 and ER40. ER11 picks 6.5 mm (6.50 to 6.00 mm); ER50 picks 8 mm (8.00 to 6.00 mm); ER8 does not reach it (largest collet 5.0 mm).
- Inch option: a 1/4 in collet, 0.2500 to 0.2100 in (ER25, ER32).
Result: ER32 7 mm collet or a 1/4 in collet.
Worked example: 3/8 in shank
A 3/8 in end mill shank.
- d_mm = 0.3750 x 25.4 = 9.525 mm.
- ER32: N = 10 mm, range 10.00 to 9.00 mm (0.3937 to 0.3543 in).
- Same 10 mm pick in ER16 (its largest collet), ER20, ER25 and ER40. ER50 picks 10 mm (10.00 to 8.00 mm); ER8 and ER11 do not reach it (largest collets 5.0 mm and 7.0 mm).
- Inch option: a 3/8 in collet, 0.3750 to 0.3350 in (ER25, ER32).
Result: ER32 10 mm collet or a 3/8 in collet.
Spot checks against the sources
- ER32 7 mm collet: REGO-FIX prints 7.00 to 6.00 mm, 0.2756 to 0.2362 in, matching the lookup.
- ER32 body: 33 mm in diameter and 40 mm long; REGO-FIX and Leitz agree.
- 5C: 1 1/16 in round = 1.0625 in = 26.99 mm; Hardinge prints 26.99 mm.
Shop notes
- A shank at the bottom of a collet's range is held on less of the collet bore. When a shank sits at both a range top and a range bottom (6.00 mm is the top of the 6 mm collet and the bottom of the 7 mm), use the collet it tops out: 6 mm.
- Not every ER collet grips a full 1 mm: some makers make 0.5 mm range collets (Leitz lists a 6 mm ER collet that clamps 5.5 to 6 mm). Read the range etched on the collet.
- 5C collets have a short closing range: match the collet to the stock size. Hardinge lists 5C decimal collets in 0.001 in steps from 0.015 in to 1.0625 in.
FAQ
What ER32 collet holds a 1/4 in shank?
A 7 mm collet (7 to 6 mm) or a 1/4 in collet (0.2500 to 0.2100 in); 1/4 in is 6.35 mm.
What ER collet for a 3/8 in end mill?
A 10 mm collet (10 to 9 mm); 3/8 in is 9.525 mm.
What is the largest 5C collet?
1 1/16 in (26.99 mm) round, 29/32 in hex, 3/4 in square (Hardinge).
How far does an ER collet collapse?
1 mm on standard ER16 to ER40 collets, 0.5 mm on ER8 and ER11, 2 mm on ER50.
What drawbar thread does an R8 collet take?
7/16-20.
What is the ER32 collet size range?
3 to 20 mm in standard sets, 1 mm per collet; 21 and 22 mm collets are also made.
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