Shrink-fit toolholders
Thermal interference clamping, shank and heating compatibility, engagement limits, and the distinction between friction grip and pullout protection.
Tags: tooling, toolholders, thermal-clamping
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How thermal clamping works
MAPAL's ThermoChuck instructions (ninth issue, July 2025) list 145 N·m transmitted torque and 37 mm minimum clamping length for a 12 mm shank at the h6 minimum size and 20 °C. The torque describes resistance to rotation in the cited condition. Axial pullout needs separate evidence. Follow the model's approved heating, engagement and operating limits. MAPAL, technical data, pp. 45–46.
WTE describes an induction coil heating the holder locally to enlarge the bore. A cold shank enters the bore, and cooling establishes the interference grip. WTE/MAPAL, shrinking technology, printed p. 164.
The bore, shank and heating unit form a system
HAIMER's Tool Holders, first edition 2020, describes its standard HSK-A40 shrink-fit chucks for HSS and solid-carbide tools with h6 shanks. Its 3, 4 and 5 mm short versions have different supplied features from larger sizes: no back-up screw or balancing-screw threads, and slits along the bore for external cooling. These are catalog-specific distinctions, not permission to use any small tool with any heating unit. HAIMER, printed p. 80.
BILZ's standard ThermoGrip page describes a counterbore that permits initial cold insertion before shrinking and lists cylindrical shank forms DIN 1835 A/B/E and DIN 6535 HA/HB/HE. Its statements concern that design; initial insertion is not the final interference grip. Shank-form compatibility and dimensional tolerance remain separate requirements. BILZ, standard ThermoGrip shrink chucks.
As engineering synthesis, a holder's wall thickness, shank material and heating arrangement affect how quickly the bore and shank change temperature. A heating cycle suitable for one geometry cannot be inferred merely from a matching nominal bore. Cooling also affects when preset length and assembly dimensions become stable.
Engagement, torque and temperature: one bounded example
MAPAL's ThermoChuck instructions, ninth issue July 2025, specify h6 cylindrical shanks and suitable shrinking equipment for carbide or HSS. Its table gives these values at 20 °C with a shank at the h6 minimum size. They are transmitted torque values, not screw-tightening torques. MAPAL, technical data, pp. 45–46.
| Nominal clamping diameter | Minimum clamping depth | Permissible transmitted torque under the stated conditions |
|---|---|---|
| 6 mm | 26 mm | 24 N·m |
| 12 mm | 37 mm | 145 N·m |
| 20 mm | 42 mm | 450 N·m |
The same manual distinguishes a maximum 500 °C shrinking temperature from 20–30 °C operating temperature for solid-carbide tools. It also makes operating speed subject to the machine-side connection and selected tool. These figures belong to this manual's products, not every shrink-fit holder. MAPAL, temperature and speed conditions, pp. 44–45.
A heating maximum is not a target temperature for every tool change, and it is not a machining-temperature rating. Likewise, the table does not establish axial pullout resistance, cutting-tool strength or a machine's power limit. Those are separate physical constraints.
Friction grip and positive pullout protection
HAIMER explains that high axial cutting loads can cause gradual tool movement even in a strongly clamped assembly. Its Safe-Lock design combines frictional clamping with a positive connection between grooves in the tool shank and corresponding holder features. This is an added tool/holder design, rather than a property implied by ordinary shrink fit. HAIMER, Safe-Lock explanation, printed p. 216.
Thus concentric grip, torque transfer and resistance to axial movement are related but distinct. A runout figure does not quantify any of the latter load capacities. Optional pullout protection also requires the matching shank geometry; its name alone does not qualify an arbitrary tool.
Worked example: clearance from thermal expansion
Original simplified model. Assume a free, uniform holder bore of diameter D = 12 mm, a constant linear expansion coefficient α = 12 × 10⁻⁶/K, and a uniform temperature rise ΔT = 200 K. The assumed coefficient and temperature rise are illustrative inputs, not material data or heating instructions for a cited product.
Diameter increase ≈ α × D × ΔT
= 12 × 10⁻⁶/K × 12 mm × 200 K
= 0.0288 mm = 28.8 µm
This estimates the free bore's change in diameter. Actual insertion clearance also depends on the initial interference, shank temperature, transient heat flow and dimensional tolerances. The calculation gives neither clamping pressure after cooling nor a sufficient heating cycle. A real holder is not uniformly heated, and α need not remain constant across its temperature range.
Coverage and source dates
The MAPAL manual explicitly identifies ninth issue July 2025, code KAL-SC-D/E-09-0725. HAIMER's cited catalog states first edition 2020. No issue date was established for the WTE catalog or BILZ page; download-directory and filename years are not treated as editions. Sources were inspected on November 2, 2026.
This reference explains thermal clamping and selected product conditions. It does not establish a universal heating power, dwell time, cooling interval, insertion depth, allowable speed, runout, gripping torque or fatigue life. Holder geometry, tool shank, machine interface, balance and the heating/cooling equipment remain part of the actual assembly.