Coefficient Of Thermal Expansion

Coefficient of thermal expansion (CTE) quantifies how a part changes size per degree temperature change, expressed as ΔL = αL₀ΔT, where α is CTE, L₀ original length, and ΔT temperature change. In machining, CTE explains why a part measures correctly at the machine but goes out of size after thermal soak or temperature shifts; aluminum expands roughly twice as much as steel.

On a CNC cell, CTE is used to predict thermal growth in the workpiece, fixture, spindle, ballscrews, and probe stylus, ensuring tool offsets and inspection results remain valid across temperature swings. In millwork, the same principle applies to metal inserts and dissimilar-material joints; an aluminum extrusion bonded to wood moves much more with temperature than the wood substrate, so slotting, allowance, or floating fasteners are used to avoid stress buildup and seasonal fit issues. For precision jobs, the 20 °C / 68 °F reference temperature is the standard metrology baseline, requiring parts and gauges to reach thermal equilibrium before final measurement. A 1 m aluminum bar at roughly 23 µm/m·°C grows about 23 microns per 1 °C rise; over long machine travel or large work envelopes, this becomes a measurable source of taper, hole-position drift, and bore-size error.

• ⚠️Measuring a warm part as if it were cold: A freshly machined aluminum component can be several tenths oversized until it equalizes; if shipped or accepted before cooldown, it finishes undersize after contraction.
• ⚠️Ignoring temperature mismatch between part, gauge, and reference standard: If the CMM, height gage, and workpiece are not at the same temperature, measurement bias can exceed the tool’s nominal accuracy.
• ⚠️Long-part machining without thermal compensation: Large steel or aluminum parts grow along their length during the cycle, causing positional drift, hole misalignment, and out-of-flatness conditions as features cut early no longer match the programmed coordinate system.