HSK tool holders (ISO 12164 / DIN 69893)

HSK stands for Hohlschaftkegel — hollow taper shank. It is not a bigger or smaller version of BT: the taper ratio is different, the shank is hollow, and it locates on two surfaces at once. Everything that follows comes from those three facts.

The three differences that matter

BT, CAT and SK are the same 7:24 family. HSK is a separate system, so almost nothing crosses over.
	HSK	BT / CAT / SK (7:24)
Taper ratio	1:10 (≈5.71° included, 2.86° per side)	7:24 (≈16.26° included)
Locating surfaces	Taper + flange face, simultaneously	Taper only
Shank	Hollow, clamped from inside	Solid, pulled by a retention knob
Retention	Expanding collet inside the bore	Pull stud gripped at the tail
Drive	Drive slots at the taper end engage spindle keys (form A)	Drive keys on the flange
Standard	ISO 12164 (DIN 69893 family, ASME B5.62)	MAS 403 / ASME B5.50 / DIN 69871

Why hollow works in your favour at speed

A solid 7:24 shank and the spindle bore expand at different rates as speed rises, and above roughly 8,000 rpm the spindle mouth grows faster than the shank — the connection loosens. An HSK shank is hollow, so the same centrifugal effect pushes the shank wall outward against the receiver, keeping contact. This is the reason HSK is the default on high-speed machining centres.

The performance figures below are maker claims, not standards. Published numbers we found include: repeatability of about 2 µm; axial repeatability around 0.2 µm versus roughly 15 µm for a 7:24 shank; weight about 50% lower than an equivalent steep-taper holder; and radial stiffness said to be up to five times that of CAT / SK / BT. These come from holder makers and secondary references, not from ISO 12164 itself. Use them to compare, not to specify.

HSK-A dimensions

Form A per DIN 69893-1 / ISO 12164-1. The size number is the flange diameter. Speed limits are the maker's recommended maximum, not a machine limit.
Size	Flange Ø (mm)	Taper gauge Ø (mm)	Recommended max speed (rpm)
HSK-A32	32	24.007	50,000
HSK-A40	40	30.007	42,000
HSK-A50	50	38.009	30,000
HSK-A63	63	48.010	25,000
HSK-A80	80	60.012	—
HSK-A100	100	75.013	16,000
HSK-A125	125	95.016	—

Makers label the intermediate columns differently, so we have only listed the two diameters that every table agrees on. One published table gives an additional diameter of 26 / 34 / 42 / 53 / 67 / 85 / 111 mm for sizes 32 to 125, but sources disagree on what that dimension measures and on its value for HSK-A32. Flange Ø and taper gauge Ø are the two numbers you can safely order against.

What the form letters mean

Form	Standard	Intended use	Confidence
A	ISO 12164-1 / DIN 69893-1	General milling with automatic tool change. Two drive slots, two flange grooves, a position notch and a data-chip bore.	Well corroborated
E	ISO 12164-2 / DIN 69893-5	High-speed. Symmetric with no drive slots and no chip bore, so it balances cleanly. Offered in 25, 32, 40, 50 and 63.	Well corroborated
F	DIN 69893-6	Face milling and turning. One maker lists an HSK-F63 and a Makino-pattern 80 with a 78 mm flange.	Moderate
T	ISO 12164-3	Stationary tools on turning / mill-turn machines. Torque through an end-face keyway plus taper friction.	Well corroborated
B / C / D	—	Sources disagree. One line says A and B are for automatic tool change and C and D for manual; another assigns B to high-torque boring, C to cartridge tools and D to extended reach.	Conflicting

HSK-A construction details

Torque is carried by two drive slots at the taper end engaging keys in the spindle — not by the flange.
Two flange grooves and a position notch are what the tool magazine and the changer orient on.
Coolant is central and axial, delivered through a coolant tube seated in the bore.
Identification is via a data-chip bore to DIN 69873 in the flange.
Material is commonly quoted as 58 + 2 HRC surface hardness with a core tensile strength of at least 1000 N/mm².

Choosing between HSK-A and HSK-E

Take HSK-A unless you have a specific reason not to: it has the drive slots that carry torque and the flange features an automatic changer needs. Move to HSK-E when spindle speed is the binding constraint and the tool load is light — removing the drive slots and the chip bore makes the holder symmetric and much easier to balance, which is why E-form is the one quoted at the top of the speed range.

Neither form is interchangeable with the other, or with any 7:24 holder. The taper ratio is different, so an HSK holder will not seat in a BT spindle and vice versa — this is not a pull-stud problem you can solve with an adapter.

HSK holders and HSK collet chucks

For the 7:24 side of the family see tool holder tapers compared and NT30 chucks. If you are adding reach rather than changing interface, read extension rods and long-reach chucks first — length costs more accuracy than the interface does.