Thermal expansion in CNC machining: why +/-0.01 mm is not only about the tool
How thermal expansion affects CNC machining tolerances, material coefficients and measurement strategy for precise aluminium, steel and brass parts.
Temperature can move your tolerance
During metal cutting, the tool and the workpiece heat up. A large part of cutting energy becomes heat, and metal expands with temperature. If you measure a part at 28 °C and it works in an assembly at 20 °C, the dimension may no longer be the same.
At +/-0.01 mm, a few degrees can matter. That is why precision machining is not only about a sharp tool and a rigid machine. Temperature control is part of the process.
How much materials expand
Linear thermal expansion is described by the coefficient alpha, usually expressed in micrometers per meter per Kelvin.
| Material | alpha (µm/m·K) |
|---|---|
| Aluminium | 23 |
| Brass | 19 |
| Stainless steel | 16 |
| Carbon steel | 12 |
| Titanium | 8.6 |
| Invar | 1.2 |
Aluminium expands almost twice as much as ordinary carbon steel. That is why large aluminium parts with tight tolerances need careful process planning.
A practical example
Length change is simple:
Delta L = alpha · L · Delta T
For 100 mm of aluminium with a 10 °C change, the movement is about 23 µm = 0.023 mm. That is already more than twice a +/-0.01 mm tolerance.
For 500 mm of aluminium with a 20 °C change, the movement is about 0.23 mm. If that part is measured hot and then used cold, the difference can be far larger than the allowed tolerance.
Where the heat comes from
Most heat comes from cutting and friction. Chips carry some heat away, but the cutter, holder, machine and workpiece can all warm up during longer operations. The shop environment matters too: morning versus afternoon temperature, sunlight, heating and coolant temperature can all affect measurement.
This is one reason why aluminium CNC machining needs a different mindset from steel or brass.
How we control it
Thermal expansion is controlled in layers. Coolant removes heat and reduces friction. Machines and measuring tools should reach a stable working temperature before fine work. Finish passes use appropriate feeds, speeds and sharp tools.
CAM strategy also matters. Where useful, the program can account for movement. For precise parts, the component should stabilize before final measurement. Measuring immediately after a heavy cut can tell you more about heat than about the finished part.
If you have a tight-tolerance part, send the drawing through the quote form and include the operating temperature or assembly function if it matters.