Heat from cutting expands the workpiece and tool. In a long production run, this thermal drift can push parts out of tolerance. Calculate the expansion and decide when to let the machine warm up.
Calculate ↓ΔL = α × L × ΔT. Enter the material, part length, and expected temperature rise.
Every machinist has seen it: the first part of the morning measures perfectly at 20°C shop temperature. By part 30, the same program is producing parts 20-50 microns oversize. The machine hasn't changed — the temperature has. Cutting fluid warms up, the spindle bearings reach thermal equilibrium, and the workpiece absorbs heat from the cutting process. On a 200 mm shaft in 304 stainless, a 25°C temperature rise adds 85 microns of length — enough to push a close-tolerance feature out of spec.
Thermal expansion is fully reversible and predictable. The formula is linear: each degree of temperature change expands steel by 12 microns per meter, aluminum by 23, and titanium by 8.6. Once you know the temperature rise, you can calculate the dimensional drift and compensate in the program or in the tool offset table.
A CNC machine tool needs approximately 15-30 minutes to reach thermal equilibrium after cold start. During this period, the spindle grows vertically as the bearings warm up, and the ballscrews expand axially. On a Haas VF-2, the Z-axis growth during warm-up can reach 25-40 microns. On a Mazak VC-500, it's typically 15-25 microns due to the thicker cast iron construction which absorbs heat more slowly.
Running a warm-up program — alternating spindle speeds and rapid traverse movements — brings the machine to temperature evenly and reduces thermal drift during production. For parts with tolerances under ±25 microns, running a warm-up cycle before the first part is not optional.
How much does steel expand with heat? 12 microns per meter per degree Celsius. A 500 mm steel part that heats up by 20°C grows 120 microns — over 0.1 mm.
Does cutting fluid temperature matter? Yes. Flood coolant systems can heat up by 10-20°C during production, transferring heat to the workpiece and machine structure. A chiller unit on the coolant system is the standard fix for high-volume production with tight tolerances.
How do I compensate for thermal expansion in my CNC program? Two methods: (1) Let the machine warm up for 20-30 minutes before cutting the first part. (2) After warm-up, measure the first part and adjust your tool offsets to account for the remaining thermal drift.
Does tool expansion affect hole accuracy? A 12 mm carbide end mill heating up by 100°C at the cutting edge expands by approximately 5 microns — enough to affect H7 hole tolerances. Carbide's low expansion coefficient (5.5 μm/m·°C) compared to steel (12 μm/m·°C) means the tool expands less than the workpiece for the same temperature rise.