Shrink-fit tool holders

A shrink-fit holder has no collet, no nut and no set screw. The holder nose is heated until the bore grows a few hundredths of a millimetre, the tool is dropped in, and the bore clamps onto the shank as it cools. That is the entire mechanism — and it is why this system has the slimmest nose and the shortest overhang of anything on this site.

The one hard requirement: h6

Every maker says the same thing: the tool shank must be h6. Independent catalogue entries for HSK, BT and HSK-E shrink chucks all specify “for clamping cutters and drills with straight shanks in h6 tolerance”, and list the holder's own bore tolerance as H6. A shank ground to a looser commercial tolerance will not clamp to rated force, and a shank that is oversize will not go in at all — the interference window is only a few hundredths of a millimetre wide.

This is the single biggest practical objection to shrink fit. It means the system quietly forces you to buy better tooling. Cheap end mills with inconsistent shank diameters are the reason shops try shrink fit once and go back to ER.

What the numbers actually are

These are maker claims repeated across several catalogues, not values from a standard. We have flagged where sources disagree.
Property	Published value	Agreement
Runout at the nose	< 0.003 mm (3 µm)	Four independent makers
Balance grade	G2.5 at 25,000 rpm	Three makers agree; one lists 30,000 min^-1 max speed
Shank tolerance	h6	Unanimous
Body hardness	HRC 48 – 57	Conflict: published as 52±2, 54−2, 48–52 and 55±2
Max coolant pressure	80 bar (one maker)	Single source

Because the figure quoted by all four makers is the same 0.003 mm, it is fair to use it when comparing systems. What it does not tell you is at what projection length it was measured — see runout and accuracy classes for why a runout number without a measuring distance is unusable.

Reading a shrink-fit part number

Makers encode nose diameter and projection length directly in the designation. One published HSK-A series reads like this:

Worked example from one maker's HSK-A shrink chuck table. Other makers use their own prefixes — the logic is what transfers, not the letters.
Designation	D1 (bore)	D2	D3	L1 (projection)
`HSK63A-SRS06-80`	6	12	17	80
`HSK63A-SRS06-120`	6	12	21	120
`HSK63A-SRS06-160`	6	12	26	160
`HSK63A-SRS12-90`	12	18	24	90
`HSK63A-SRS12-160`	12	18	32	160

Note that D3 grows with length: the nose gets fatter as the holder gets longer, because a longer holder needs more section to stay stiff. Long versions are sold precisely for deep cavities, but the rigidity penalty is real — see overhang and rigidity.

Where shrink fit wins, and where it loses

	Shrink fit	Hydraulic	ER
Nose profile	Slimmest	Slim	Bulkiest (nut)
Parts needed	None	None (or a reduction sleeve)	Collet + nut
Tool change	Slow — needs a shrink machine	Fast — one hex key	Fast
Shank tolerance	h6 mandatory	h6 required	Nominal ± collapse range
Vibration damping	Low — solid metal path	High — oil chamber absorbs it	Moderate
Capital cost	Holder + heating unit	Holder only	Lowest

The practical summary: shrink fit is the answer when you need to reach deep at high speed with a small tool and you already own h6 tooling. Hydraulic is usually the better buy when you want the same accuracy with faster tool changes and better damping.

Shrink-fit holders

We deliberately do not publish heating times or temperatures here: they depend on the heater, the holder size and the alloy, and every maker's procedure differs. Follow the heater manufacturer's cycle. For the spindle side of the interface see HSK holders and 7:24 tapers compared.