SK collets (SK6 – SK25)
The high-grip alternative to ER, sold in the same size range. SK collets use a longer body and a steeper angle than ER, which is where the extra clamping force comes from — and they need a dedicated SK chuck, not an ER one.
Dimensions by size
| Size | Clamping range (mm) | Body Ø (mm) | Length (mm) | Typical application |
|---|---|---|---|---|
| SK6 / SK06 | 2 – 6 | 10.5 | 25 | Small engraving and high-speed spindles |
| SK10 | 2 – 10 | 15.6 | 30.5 | Small mills, drill-tap work |
| SK13 | 3 – 13 | 20.1 | 39 | General small-shank milling |
| SK16 | 3 – 16 | 24.6 | 45 | The most widely stocked SK size |
| SK20 | 4 – 20 | 30 | 50 | Medium roughing, 40-taper machines |
| SK25 | 6 – 25 | 35.6 | 57 | Largest standard SK size |
- Body diameter. One maker's table gives 10.5 / 15.6 / 20.1 / 24.6 / 30 / 35.6 mm; another publishes 8.9 / 13.0 / 17.0 / 20.5 / 25.0 / 29.7 mm for the same six sizes. The two sets most likely measure different points on the tapered body (large end versus small end), but no source states which, so we have not picked one for you.
- SK20 length. 50 mm in one table, 54 mm in another.
- SK25 clamping range. Chinese makers list 3–25 or 6–25 mm; one major Western maker lists SK25 as 16.0–25.4 mm only. That is a big practical difference. Confirm the range with the seller before ordering an SK25.
One naming note, since both appear in the same catalogues: SK is also used for the German 7:24 tool holder taper — SK40 / SK50 to DIN 69871 and ISO 7388. That is a shank, not a collet. In practice the two rarely collide, because the collet series stops at SK25 while holder tapers start at 30. If you landed here looking for an SK40, see tool holder tapers compared.
Why SK grips harder than ER
A maker of SK systems states that the longer body and steeper collet angle give two to three times the gripping force of an ER collet, and publishes a runout guarantee of 0.0002 in (5 µm) at 4×D on on-size collets, with speeds up to 30,000 rpm on their chucks. We could not find an independent standard that pins the exact included angle of an SK collet, and sources describe the geometry as “8 degree versus 16 degree” without agreeing on which figure applies to which system — so treat the angle as maker-specific.
What is well supported across sources is the design intent:
| SK collet | ER collet | |
|---|---|---|
| Body | Longer, steeper angle | Shorter, 8° per side |
| Gripping force | Higher (2–3× claimed by one maker) | Baseline |
| Nut | Collet sits flush with the nut face | Collet recessed below the nut |
| Chip nesting | Reduced by the flush fit | More exposed |
| Coolant | Through-tool available on the chuck | Depends on the nut type |
| Chuck required | SK collet chuck | ER chuck / ER nose |
| Standard | Maker-defined; not covered by DIN 6499 | DIN 6499 / ISO 15488 |
Which sizes are worth stocking
Most shops that adopt SK do not buy all six. SK16 and SK25 cover the great majority of milling shanks; SK6 and SK10 earn their place only on high-speed engraving or small drill-tap work where the slim nose is needed for clearance. SK13 and SK20 sit between the two and are the sizes most often skipped.
Collets are ordered by bore, in 0.5 mm or 1 mm steps, in the same way as ER. Unlike ER, SK collets are usually bought “on size” — the runout guarantee quoted by makers applies to on-size collets, and a collet forced well away from its nominal bore will not deliver the accuracy you are paying the premium for.
SK collets and SK chucks
Comparing the high-grip options? See OZ collets for the DIN 6388 heavy-duty series, and DC / SDC rear-pull collets for the no-nut design. Both solve a different problem than SK does.
Because the SK collet standard is maker-defined rather than covered by DIN 6499, buy the collets and the chuck from the same supplier whenever you can. Mixing an SK collet from one maker into another maker's chuck is the single most common cause of “it doesn't grip like it should” with this system.