C straight-shank power collets (C16 – C42)
Also sold as "power collets", "heavy-duty straight collets" or the SC prefix. These trade the ER system's flexibility for raw gripping force, and they are the usual answer when a cutter keeps pulling out under a heavy roughing cut.
Dimensions by size
| Size | Body Ø (mm) | Clamping range (mm) | Nominal length (mm) | Typical use |
|---|---|---|---|---|
| C12 | 12 | 4 – 8 | 45 | Small holders, deep blind holes, interference clearance |
| C16 | 16 | 4 – 12 | 45 | Small-diameter work where a bigger nose would foul |
| C20 | 20 | 4 – 16 | 50 – 55 | General purpose, widest useful range for its size |
| C25 | 25 | 4 – 20 | 60 – 63 | Medium roughing |
| C32 | 32 | 4 – 25 | 65 – 70 | The most widely used size; large end mills and drills |
| C42 | 42 | 4 – 32 | 73 – 80 | Heavy roughing, large forged or cast workpieces |
How a C collet differs from an ER collet
| C power collet | ER collet | |
|---|---|---|
| Outside shape | Straight — no taper | Tapered cone |
| Elastic collapse | Effectively none | About 1 mm (0.5 mm on ER8/ER11) |
| Size matching | Bore must match the shank | One collet covers a range |
| Gripping force | Higher | Lower |
| Contact length | Long bore, full-length contact | Shorter contact band |
| Suited to | Heavy roughing, interrupted cuts | General purpose, frequent size changes |
| Chuck required | Power collet chuck | ER collet chuck / ER nose |
Three rules that prevent most C collet problems
- Match the bore to the shank. Because there's essentially no collapse range, a 12 mm shank needs a 12 mm collet. Forcing an undersize shank doesn't just run out — it will not hold at all.
- Respect the engagement length. The tool must be inserted past the effective clamping length inside the bore. A shallow insertion feels tight on the bench and slips in the cut.
- Keep the bore clean and lightly oiled. Power collets work at higher clamping pressures; grit in the bore scores both the collet and the tool shank.
Most C collets are split into six slots to distribute clamping and resist pull-out, and are made from 65Mn spring steel. If you see a C collet with a visibly uneven slot pattern, that's worth questioning — the slot geometry is doing most of the work.
When ER is still the better buy
If your work is finishing passes, frequent tool changes, or you're covering a lot of different shank diameters on one machine, ER wins on cost and convenience. C collets earn their place when a specific large cutter keeps losing its grip. Most shops end up with a full ER collection plus a handful of C sizes in the diameters they rough with.