OZ collets — OZ16 to OZ50 (DIN 6388)

A heavy-duty milling collet series built for rigidity rather than for range. OZ covers larger shanks than ER does in the same body size, and the slot layout means it clamps along the tool differently — which is the part nobody mentions until a cutter starts chattering.

Dimensions by size

Body Ø and length are consistent across the makers we compared. The clamping-range column is not — see the note below before you rely on it.
Size	Body Ø D1 (mm)	Small Ø D (mm)	Length L (mm)	Published series range (mm)
OZ16	25.5	22.65	40	2 – 16
OZ20	29.8	27.4	45	2 – 20
OZ25	35.05	32.9	52	2 – 25
OZ32	43.7	41.3	60	4 – 32
OZ40	52.2	49.7	68	6 – 40
OZ50	63.8	61.1	80	8 – 50

Two things to check before you order.

The lower end of the range is not agreed. For OZ32 we found 3 mm in one catalogue, 4 mm in another, and 3 mm in a third; for OZ40 the published lower bound is 3 mm or 6 mm depending on the maker. Only the upper bound (the size number) is reliable.
Some sellers label OZ as DIN 6499B. DIN 6499 is the ER standard. The OZ collet series is specified to DIN 6388 — that is the reference given by European chuck makers. If a listing claims OZ is DIN 6499B, treat the rest of its specification with suspicion.

Body diameters and lengths above matched exactly across independent maker tables, which is why we treat them as the reliable columns.

How OZ clamping differs from ER

Both are split sleeves compressed by a nut, but the slots are cut differently:

	OZ collet	ER collet
Slot direction	From one end only	From both ends, alternating
Compression pattern	Folds inward at the bottom — tangential clamping	Closes evenly along the length — uniform clamping
Grip distribution	Weakest at the rear of the bore	Even along the bore
Stated strength	Higher gripping force and rigidity	Baseline
Standard	DIN 6388	DIN 6499 / ISO 15488

Why that matters in practice. The uneven grip of an OZ collet is a real trade-off: you gain rigidity and clamp force at the nose, and give up some holding consistency deeper in the bore. For short, stiff cutters in a rigid setup that is a good deal. For a long-reach tool where the shank is doing the work, an ER collet clamped evenly along its length is usually the safer choice.

OZ chucks and the accuracy figures quoted

European chuck makers list OZ chucks to DIN 6388 with a maximum concentricity of 0.005 mm, a taper angle tolerance better than AT3 (per DIN 7187 and DIN 2080), case-hardened bodies at roughly 60 HRC, and dynamic balancing to G 2.5 at 25,000 rpm. OZ collet chucks are offered in HSK and in the usual 7:24 tapers.

Those are chuck figures, not system figures. The runout you actually measure at the tool tip also depends on the collet, the bore match and the nut torque — see our page on runout and accuracy classes.

Choosing between OZ and ER

Choose OZ when you are running larger shanks in a rigid machine and want more clamp force than ER gives at the same body diameter — drilling, heavy milling and tapping on SPM or dedicated machines are the usual reasons.
Stay with ER when you change sizes constantly, when you need the widest possible coverage from few collets, or when your tooling is long and slender.
OZ25 is the size most people start with — it covers the common 6–25 mm milling range and is the most frequently stocked across suppliers.

Order OZ collets by bore, in the same way as ER. One maker describes OZ as covering a wider diameter range per collet than ER's roughly 1 mm of collapse, but no published figure was given, so order on size and do not rely on a collapse range.

OZ collets and OZ chucks

If the goal is clamp force rather than bore size, look at SK collets; if it is nose clearance, look at DC / SDC rear-pull collets. All three sit above ER on grip and below it on convenience.