NEWS & GUIDES
ER, R8, C type or SK: which collet family fits?
The right collet is the one that matches both the holder and the tool shank. Buying only by a bore such as 10 mm leaves the connection unidentified: several unrelated collet families can hold a tool with that shank diameter. Begin with the existing holder code, then verify the collet’s body geometry and individual bore.
Four families, four identification checks
| Catalog family | First identification check | Common ordering mistake |
|---|---|---|
| ER | ER size on the holder, nut and collet | Ordering a bore without specifying the ER series |
| R8 | Machine spindle and drawbar arrangement | Confusing a direct R8 collet with an R8-to-ER holder |
| C type | C body family plus the catalog dimensions | Treating the body code as the tool bore |
| SK collet | SK collet designation, diameter and length | Confusing this collet label with a complete SK spindle holder |
Our C type collets are organized as C20, C25, C32 and C42. Our SK collets use SK10, SK16, SK20 and SK25. These are catalog family names, not a promise of interchangeability with every similarly named product.
For ER, separate the series from the bore
ER16, ER25 and ER32 identify different sizes of collet system. Once that system is confirmed, select an individual collet for the actual shank. The coverage of a whole series is not the clamping capacity of one collet. Do not clamp a shank above the selected collet’s stated range or assume the same collapse range applies to every design. REGO-FIX’s ER assembly and sizing instructions explain the importance of correct shank sizing, engagement and nut assembly.
For a new purchase, list each bore and quantity separately. Specify metric or inch dimensions explicitly, and use the tool’s shank size rather than its cutting diameter. Our ER collet page is a starting point for choosing the family; the quotation should confirm individual sizes and inspection requirements.
For R8, decide whether you need a collet or a holder
A direct R8 collet and an R8 ER holder are not the same assembly. The latter adds an ER clamping end and requires its own ER collet. Hardinge lists these as separate tooling formats in its milling tooling catalog.
What to send if the collet is unmarked
- Clear side and end photographs of the collet and the mating holder.
- Measured overall length, relevant body diameters and bore, with units.
- A photograph of the nut, retention groove or draw-thread arrangement.
- Tool shank size, machining operation and quantity for each size.
- Required inspection conditions, including where runout should be measured if it is a purchasing requirement.
Do not force a near match into a holder. Request a dimensional comparison first. Keep the holder, nut and collet identified as one system in your stock list so replacement orders do not mix families.
Prepare your size list
Browse the four collet families and send the sizes you use. References above were checked 5 September 2026. Supplier-specific accuracy values and tightening instructions must be confirmed for the products actually supplied.