Reading a Collet Catalog: Sizes, Bores and Specs
Short answer: A collet catalog is read in four passes — series (ER, 5C, R8, TG, auto-lathe), size code (ER11 to ER50, or the 15/25/36/46 auto-lathe types), bore (round, hex, square, triangle, pin-slot), and accuracy grade (typically 0.005–0.015 mm TIR). Match the series to your holder, the bore to your stock, and the grade to your tolerance. Then check clamping range, not nominal size: an ER32 collet marked 10 mm grips roughly 9.5–10.0 mm. BQUQ manufactures collets to ±0.005 mm and quotes custom and standard collets in 12 working hours with flexible MOQ.
Most buyers get into trouble with collet catalogs not because the data is missing, but because the data is dense and ordered in a way that assumes you already know the family tree. A catalog page for an ER32 collet might list a dozen numbers, and only three of them matter for your application. This guide walks through the reading order that experienced process engineers use, then covers the traps that cause scrap parts and spindle damage.
What are the main collet series, and which one do you need?
The first decision is always the interface. Your machine tool, chuck or holder dictates the series before any other specification matters. You cannot substitute an ER collet for a 5C collet, and an auto-lathe collet will not fit a milling chuck.
| Series | Typical use | Common size codes | Clamping style |
|---|---|---|---|
| ER | Milling, drilling, tool holding | ER8, ER11, ER16, ER20, ER25, ER32, ER40, ER50 | Double-angle, nut-driven |
| 5C | Lathe workholding, fixtures | 5C (single body size) | Drawbar or lever, 1 mm steps |
| R8 | Manual mills, Bridgeport-type | R8 (single body size) | Drawbar, keyed |
| TG | Heavy milling, high grip | TG100, TG150 | Nut-driven, single angle |
| DA | Light milling, drilling | DA100, DA180, DA200 | Double-angle, nut-driven |
| Auto-lathe | Swiss-type and cam lathes | 15-type, 25-type, 36/46-type | Drawbar or spring push |
The size code in an ER series is not a bore size — it is a body size. ER11, ER16, ER20, ER25, ER32, ER40 and ER50 describe progressively larger outer diameters and clamping capacities. An ER32 collet body is nominally 32 mm across, and that family covers bores from about 1 mm up to 20 mm. If you need to hold a 25 mm shank, you move up to ER40 or ER50, not to a "bigger ER32."
For lathe work, the 5C family is a single body size with a very wide bore range, typically 1/16 inch to 1-1/16 inch in imperial catalogs and 1 mm to 26 mm in metric versions. Because 5C collets are usually supplied in 0.5 mm or 1 mm increments, the catalog will list dozens of part numbers that differ only by bore.
Auto-lathe collets follow a different logic again. The 15-type, 25-type and 36/46-type designations refer to the machine's collet capacity class rather than a body diameter, and the catalog will normally separate them by machine brand and model. For Swiss-type work, the guide bushing is a separate catalog line from the collet itself, and the two must be ordered as a matched pair.
How do you read the size and dimension columns?
Once the series is fixed, read the dimension table in this order: bore (d), body diameter (D), overall length (L), and clamping range. Catalogs usually print these in a compact grid, and the clamping range is the column buyers skip.
| Parameter | What it tells you | Why it matters |
|---|---|---|
| Bore d | Nominal gripping diameter | Selects the part number |
| Body D | Outer diameter, e.g. 32 mm for ER32 | Must match holder bore |
| Length L | Overall collet length | Clearance and nut engagement |
| Clamping range | Actual grip window, e.g. 9.5–10.0 mm | Prevents under/over-clamping |
| TIR | Runout at specified gauge length | Sets achievable part accuracy |
| Compression | Axial travel under clamping force | Affects grip force and spring-back |
A collet is a spring. Its bore is ground slightly larger than nominal so the segments can compress onto the workpiece or tool shank. That is why an ER32 collet labeled 10 mm typically accepts 9.5 mm to 10.0 mm, and why forcing a 10.5 mm shank into it destroys the elastic range. Once a collet has been over-compressed, it does not return to its original bore, and runout degrades permanently.
The same logic applies to under-clamping. If you hold a 9.0 mm shank in a 10 mm collet, the segments only close partway, contact is concentrated on a narrow band, and grip force drops sharply. The correct approach is to step down to the next bore size rather than stretch the range.
For 5C and R8 collets, the clamping range is much narrower — often only 0.05 mm to 0.1 mm of usable compression — because these are drawn into a tapered seat rather than compressed by a nut. This is why 5C catalogs are organized in tight bore increments and why buying a "close enough" size rarely works.
What do the different bore types mean?
Bore geometry is the second-most-common source of ordering errors. A round bore is the default, but catalogs also list hex, square, triangle and pin-slot variants, each ground for a specific workpiece profile.
Round bore
The standard configuration for holding round stock, drill shanks, end mill shanks and pins. Ground to a nominal diameter with a defined clamping range. This is what most buyers mean by "collet."
Hex and square bore
Ground to accept hex bar or square bar directly, so the collet grips on the flats rather than on corners. Hex bore collets are common in Swiss-type and auto-lathe work where hex stock feeds through the guide bushing. Square bore collets are used for square bar, taps and special fixtures. Both are typically made to order in specific across-flats sizes.
Triangle bore and pin-slot
Triangle bore collets hold triangular stock or specific fastener profiles. Pin-slot collets carry an internal or external keyway that engages a pin on the holder, preventing rotation — used where the workpiece must be indexed or where torque transfer is required.
Extended, mirror-polished and quenched variants
Some catalogs list extended collets for reaching into recessed features, mirror-polished collets for reduced friction and better chip evacuation, and quenched (hardened) collets for extended wear life in high-volume production. These are process variants of the same base geometry, not separate series.
If your application involves a non-round profile and you are unsure whether the collet or the bushing should carry the profile, the compatibility rules in collet and bushing compatibility are worth reading before you place the order.
How do accuracy grades and TIR numbers work?
Accuracy grade is where catalogs differ most between suppliers, and where the price gap usually comes from. A grade is a promise about runout, measured at a defined distance from the collet face, under a defined clamping condition.
| Grade (typical) | TIR at face | TIR at 2×D | Typical application |
|---|---|---|---|
| Standard / economy | 0.015 mm | 0.030 mm | General drilling, rough milling |
| Precision | 0.010 mm | 0.020 mm | Production milling, lathe work |
| High precision | 0.005 mm | 0.010 mm | Finishing, tight-tolerance parts |
| Ultra precision | 0.003 mm or better | 0.006 mm | Grinding, gauging, optics |
Three cautions apply. First, the measurement distance matters: a 0.005 mm claim at the collet face can become 0.020 mm at 3×D, which is where your cutting edge actually sits. Second, TIR is only meaningful with a specified test bar and a specified nut — swapping to a lower-grade nut can add more runout than the collet upgrade removed. Third, grade claims are only verifiable with inspection equipment, so ask for the measurement method. The accuracy grade comparison covers how grades are tested and what to request on a certificate.
For most production work, precision grade at 0.010 mm is the practical floor, and high precision at 0.005 mm is what you specify when the part tolerance is under ±0.02 mm. Ultra precision is reserved for grinding and metrology.
What else should you check before ordering?
Nut and holder compatibility
The collet nut is not a commodity. Nuts differ in thread, bearing type, taper angle and clamping force, and a mismatched nut will change both grip and runout. If your catalog lists collets and nuts on separate pages, confirm the pairing. The collet nut types guide explains the differences between standard, bearing and sealed nuts.
Material and hardness
Catalogs may specify through-hardened spring steel, case-hardened alloy steel, or specific hardness ranges in HRC. Higher hardness extends wear life but reduces elastic range, which matters if you clamp near the edge of the bore range. For high-volume Swiss work, quenched collets are usually the better economic choice despite the higher unit price.
Concentricity versus parallelism
A collet can be concentric at the face and still hold the workpiece at an angle. Catalogs rarely print parallelism, but it matters for long parts and for thin-wall components where clamping distortion is a risk. If you machine thin-wall parts, the guidance in collet workholding for thin-wall parts is directly relevant.
Quantity, lead time and MOQ
Standard ER and 5C sizes are usually stocked. Auto-lathe collets, non-round bores and extended variants are typically made to order. Ask for the lead time separately for standard and custom lines, and confirm whether the MOQ applies per size or per order.
How does BQUQ handle collet sourcing?
BQUQ runs four production lines in one Dongguan factory covering CNC machining to ±0.005 mm, metal stamping, custom springs and heat sink production, under ISO9001. Collet and chuck work is produced on the same site as the tooling that holds it, which shortens the loop between a drawing revision and a delivered part.
For buyers, the practical points are:
- Standard and custom collets quoted in 12 working hours
- Flexible MOQ, including mixed-size orders
- Round, hex, square, triangle and pin-slot bores
- ER, 5C, R8, TG, DA and auto-lathe 15/25/36-46 type series
- Inspection reports with measured TIR on request
If you are specifying a collet for a new process, send the holder drawing, the workpiece diameter and the required TIR. That is enough to select the series, bore and grade without a back-and-forth.
Frequently Asked Questions
Q: What is the difference between an ER collet size code and a bore size?
A: The ER size code describes the collet body, not the hole. ER11, ER16, ER20, ER25, ER32, ER40 and ER50 refer to progressively larger outer diameters and clamping capacities. Within each body size, the catalog lists many bore options — an ER32 family covers roughly 1 mm to 20 mm. Always read the bore column separately from the series name.
Q: Can one collet hold a range of workpiece diameters?
A: Only within its stated clamping range, which is typically 0.5 mm for ER collets and much narrower for 5C and R8. Exceeding the range in either direction concentrates contact, reduces grip force and permanently distorts the collet. For accurate work, select the bore size that puts your workpiece near the middle of its range.
Q: How do I choose between standard and high-precision collet grades?
A: Match the grade to your part tolerance. Standard grade at roughly 0.015 mm TIR suits drilling and rough milling. Precision grade at 0.010 mm suits general production. High precision at 0.005 mm is appropriate when part tolerances are under ±0.02 mm. Remember that runout grows with distance from the collet face, so check the specification at your actual gauge length.
Q: Do hex and square bore collets cost more than round bore?
A: Yes, typically. Round bores are produced in volume and often stocked, while hex, square, triangle and pin-slot bores are usually made to order because the profile must be ground to your specific across-flats dimension. Expect longer lead times and a higher unit price, though MOQ can often stay flexible for mixed orders.
Q: What information should I send for a collet quote?
A: Send the holder or chuck drawing, the workpiece or tool shank diameter and profile, the required TIR and measurement distance, the machine model, and your annual quantity. For auto-lathe and Swiss applications, include the guide bushing specification as well. With those details, BQUQ can confirm series, bore, grade and lead time in 12 working hours.
Related Resources
- About BQUQ and our Dongguan factory: /about/
- Auto-lathe collets and spring collet series: /auto-lathe-collets/
- Tool holder collet chucks and ER assemblies: /tool-holder-collet-chucks/
- Power chucks and Swiss-type workholding: /power-chucks-swiss/
- Industry trends in precision manufacturing: /industry-dynamics/
- Technical articles and engineering guides: /bquq-blog/
- Frequently asked questions: /faq/
- Case studies and applications: /case/
- Contact the engineering team: /contact/
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
