Thermal Expansion Calculator for Machined Parts
How much a part grows with temperature — and the reverse that actually matters in a machine shop: how much temperature swing your tolerance can survive. Uses typical expansion coefficients for the materials we machine, from aluminium at 23 µm/m·°C to Invar at barely anything.
From the 20 °C reference — a warm afternoon on the shop floor is easily +5.
Why temperature is a tolerance, and 20 °C is the law
Every dimension on every drawing implicitly says “at 20 °C” — that is the reference temperature all gauge blocks, CMMs and standards are calibrated to. Away from 20 °C, a part is genuinely a different size: ΔL = α × L × ΔT, where α is the expansion coefficient. A 500 mm aluminium part just 5 °C warm has grown 57.5 µm — nearly six times our ±0.010 mm standard precision — while the same part in steel grew 29 and in Invar barely 4. Nothing was machined wrong; the part was simply measured at the wrong temperature.
The mixed-material trap is the one that catches workshops: measure that warm aluminium part with a steel micrometer and you do not see 57 µm of error, you see the difference between the two expansions — about 29 µm — which is exactly the kind of discrepancy that starts arguments between a supplier and a customer whose inspection room is air-conditioned. Same part, same instruments, different temperatures, both “right”.
The reverse mode is the planning tool: given the tolerance, how much temperature swing is survivable? Holding ±10 µm over 500 mm of aluminium allows well under 1 °C of drift — which is why tight work on long light-alloy parts is machined, allowed to soak, and inspected in controlled conditions, and why our materials guide flags thermal stability notes per family. If your tolerance and your material land this tool in the red, the answer is process and measurement discipline, not a tighter machine.
Thermal expansion — FAQ
How do you calculate thermal expansion of a part?
Growth = coefficient x length x temperature change. With the coefficient in µm/m·°C: a 500 mm aluminium part (23 µm/m·°C) warming 5 °C grows 23 x 0.5 x 5 = 57.5 µm.
What is the thermal expansion coefficient of aluminium, steel and titanium?
Typical 20–100 °C values: aluminium alloys about 23 µm/m·°C, carbon steel 11–12, stainless 304/316 about 16, titanium about 8.6, Invar about 1.5. Plastics run far higher — PEEK 47, POM around 110, PTFE 125.
Why are parts measured at 20 degrees C?
Because 20 °C is the internationally agreed reference temperature for dimensional metrology. Gauge blocks, CMM scales and the tolerances on drawings all assume it. A dimension checked on a hot part with a cool instrument mixes two expansions and reads wrong.
Does thermal expansion matter for tight tolerance machining?
Enormously. At ±0.010 mm over a few hundred millimetres, a couple of degrees of temperature difference between machining, part and instrument can consume the entire tolerance. Long aluminium parts are the classic case.
Which materials are most dimensionally stable?
Invar 36 is the standout at roughly 1.5 µm/m·°C — used for metrology fixtures and optical structures. Among common engineering metals, titanium (8.6) and alloy steels (11–12) are the steadier choices; aluminium and most plastics are the liveliest.
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