Shrink-fit, hydraulic, milling and collet chucks

A cylindrical shank can be gripped in several ways, and the four common ones work on different principles: a bore that shrinks onto the shank as it cools, an oil chamber that expands onto it, a tapered body that a nut squeezes onto it, and a slotted collet pushed into a taper. Most of what is published about them comes from the companies that make them, so this page marks maker claims as such. How runout at the holder turns into uneven chip loads is on the tool runout page.

Shrink fitHydraulicMilling chuckCollet

1. 1 · the nose is heated, the bore grows
2. 2 · the shank slides in
3. 3 · the nose cools and the bore grips the shank

Shrink fit

The bore of the holder is made smaller than the shank. Heating expands it until the shank slides in, and as the holder cools it grips the shank (CTE; Kennametal). Kennametal: the holder “grips the cutting tool shank 360° along the entire length of the bore”. How much smaller the bore is, no maker states; Scott Smith of UNC Charlotte writes in Cutting Tool Engineering that “Typical interferences are about 0.01mm”, and that more interference makes a stronger connection until the holder or the tool fails from the stress. Release is easier when the holder is heated more, and faster, than the tool and expands more than it; steel and carbide differ by a factor of about two, carbide expanding less (CTE).

• Heating. Kennametal calls induction “the easiest and fastest” of the induction, hot-air and open-flame systems; HAIMER and Haas build induction units, and CTE lists hot air and open flame as slower, cheaper alternatives. It takes seconds: 3–5 s on one HAIMER machine, under 10 s at Kennametal, and 5 to 9 s in Haas’s table for its HSF-325, depending on diameter and on whether a tool is going in or coming out. Kennametal gives controlled shrinking temperatures of 300–340 °C, “far below the temperature that could cause a microstructure change in the tool material”, and adds that temperatures typically reach only about 200 °C, without saying of which part; Haas gives “The maximum recommended heating temperature is 400°C.”
• Cooling. In air or liquid-filled heat exchangers (CTE), or by contact cooling (HAIMER; Kennametal’s ribbed cooling adapters with fans). The times differ: about 30 s at HAIMER, under 60 s at Kennametal, with 30–40 s to a safe handling temperature, and 2.5 to 6 s of air blast in Haas’s table, which does not say what temperature that reaches.
• The shank. HAIMER: “For all solid carbide tools with shank tolerance h6”; Kennametal lists h6 from 6 to 50 mm and h4 and h5 for its smallest bores. On high-speed steel the makers disagree: Haas writes “DO NOT use high-speed steel (HSS) cutting tools in induction heating operations.”, because they can be hard or impossible to remove, while Kennametal and HAIMER say their units shrink and release both carbide and HSS tools. A plain cylindrical shank gives the strongest connection (CTE); Kennametal accepts Weldon and Whistle Notch flats but recommends full cylindrical shanks.
• Runout and balance. HAIMER: “We guarantee < 3μm run-out at 3xD”, and G2.5 at 25,000 rpm for the chuck body. Kennametal: less than 0.003 mm can be achieved, with no measuring length. Both are maker claims.
• Limits. The tool length is set with an axial stop screw before shrinking (Kennametal). The grip weakens as the spindle speed rises, because of centrifugal force (CTE). How many heat cycles a holder survives, the sources put differently: CTE writes that typical holders “effectively function through 20,000 to 50,000 shrink/unshrink cycles”; Kennametal’s catalogue says its holders keep their concentricity and elasticity “after as many as 5,000 heat/shrink cycles” and elsewhere that one “can withstand over 5,000 cutting tool changes”.

Haas’s guide for its machine warns: “DO NOT approach or operate this machine if you are wearing a pacemaker or other device that can be affected by electromagnetism”, “DO NOT turn on the induction heating head without a toolholder present!”, and the toolholder temperature “will rise rapidly!”

Hydraulic chucks

Kennametal describes “a cylindrical body with an internal chamber filled with hydraulic fluid”: “When the clamping screw is tightened, it compresses the hydraulic fluid, which in turn expands the chamber”, applying uniform pressure around the shank. The screw is tightened by hand with a hex key until it reaches its stop; Camcut: “Full clamping force is only achieved at the stop.”

• The shank. Camcut: “The shank tolerance should be h6; in some specific interfaces, g7 is also acceptable.”; Kennametal gets the best accuracy with “a round shank with h6 tolerance and no bushing”. The tool has to go in to the minimum depth or the stop; “Typical minimum depths, depending on diameter, are 36–51 mm.” (Camcut). Reduction sleeves must be approved for the chuck and fitted up to the stop, and they may lower the torque that can be transmitted (Camcut).
• What the makers claim. Runout of about 3 µm (Kennametal, HAIMER, Camcut, SCHUNK) and under 4 µm at 2.5×D for Sandvik Coromant’s CoroChuck 930; vibration damping from all five, none with a number; torque of up to 520 Nm at 20 mm for SCHUNK’s TENDO E compact and up to 2000 Nm at 32 mm in Camcut’s instructions. SCHUNK claims tool cost savings of up to 40 % in roughing when switching from Weldon or shrink-fit holders.
• Care. Camcut: “Do not heat the chuck in a shrink-fit device.”; Kennametal: “Never attempt to adjust the stop pin while the hydraulic chuck is in the clamping position”. Camcut has the clamping force checked regularly with a test bar (for example every three months or 100 clampings), and a chuck taken out of use if the bar can be pulled out with two fingers or oil leaks.

Milling chucks

Here a nut runs on needle rollers (BIG DAISHOWA’s “needle bearing cage”; Nikken) over the tapered outside of the body (Nikken). Nikken: “Outside bearing nut mechanically induces tight taper of the milling chuck”, and the body’s “Thick wall and slots provide strong chucking torque and rigidity”. BIG DAISHOWA’s clamping notes: correct clamping on all its MEGA DS and most HMC chucks “is achieved when the nut contacts the holder body”; the nut must not be tightened on an empty bore (“You will deform the inner bore, which will likely render the tool unrepairable”); after a tool is removed the nut should be given “1-2 more turns”; and “Using a side-lock type of cutting tool with flats on the shank will also cause irreversible deformation”, so only straight shanks should be used. Nikken positions its milling chucks for heavy milling and gives a TIR of 0.0002 in (about 5 µm) at 4×D, elsewhere at 4 in from the nose.

Collet chucks

Camcut describes “a flexible, tapered collet that compresses into the chuck housing by means of the clamping nut”, with “±0.5 mm of flexibility per collet”, so a set of collets covers a range of shank diameters; REGO-FIX’s ER collets clamp from 0.2 to 34 mm (0.0079 to 1.334 in). Its standard collets are given a T.I.R. of ≤ 10 µm (0.0004 in) “over the entire clamping range”, its ER-UP collets ≤ 5 µm, without a measuring length; HAIMER claims < 3 µm at 3×D for its Power Collet Chuck.

Comparing them

The runout figures cannot be set side by side: HAIMER measures its shrink and collet chucks at 3×D (its hydraulic and Mini Shrink Chuck pages give no length), SCHUNK and Sandvik Coromant at 2.5×D, Nikken at 4×D or 4 in from the nose, and Kennametal, Camcut and REGO-FIX give no length at all. All of them are maker claims.

Independent tests with published numbers were not found for this page. Two studies from Gebze compared holders in milling and both ranked shrink fit first: Simsek, Kuram and Özçelik in dry milling of AISI 304 with shrink-fit, hydraulic and milling chucks, judging force, wear and roughness (“shrink-fit tool holder had the better performance than the others”), and a 2008 thesis by Sert with shrink-fit, Weldon and collet holders on a D2 tool steel, where the collet came last. Their abstracts give no figures, and both come from one university. The makers’ own claims pull in different directions: Kennametal writes that shrink-fit “Clamping forces have been found to be greater than those created by collet or hydraulic chucks.”, BIG DAISHOWA that where setup time matters “a quick-locking hydraulic chuck may be better”, and, for rough milling of tough alloys, that “a side lock end mill holder is a sound choice in order to prevent pullout”.