Temperature changes cause accuracy drift, especially for precision metrology equipment like granite plates and CMMs
Simple conclusion: Temperature is one of the core factors causing accuracy drift in precision measurement, understood in two parts: the thermal expansion and contraction of the material itself the temperature gradient of the entire equipment.
1. Temperature effect of granite (Jinan Blue)
The linear expansion coefficient of granite is very low (about 4.5~6 ×10⁻⁶ /℃), far superior to cast iron, but not zero.
- Example: For a 1000mm Jinan Blue plate, a 1℃ temperature change results in a length change of ≈4.5
6μm; a 5℃ temperature fluctuation can cause a drift of up to 2030μm. - Key point: It is not the average temperature, but the temperature gradient (one side of the plate hot, the other cold) that is more harmful. It causes bending deformation and a sharp deterioration in flatness, which is the most common form of accuracy drift.
Advantages of granite: Slow heat conduction, delayed deformation after temperature changes, and short-term temperature fluctuations do not cause immediate deformation; Disadvantages: Once the overall temperature rises or falls, deformation will persist for a long time before stabilizing.
2. Cast iron bases / machine tool castings
The expansion coefficient of cast iron is about 11~13 ×10⁻⁶ /℃, roughly twice that of granite.
- For the same 1-meter component, a 1℃ change results in an elongation of about 11μm; the warping caused by temperature gradients will be more pronounced than in granite.
3. CMM (Coordinate Measuring Machine) overall drift
A CMM is not just a base, but also includes scale rulers, aluminum alloy beams, bearings, and air bearings:
- The expansion coefficient of aluminum alloy is very high (≈23×10⁻⁶/℃), making the beam extremely sensitive to temperature;
- Scale rulers themselves also have temperature drift;
- Non-uniform ambient temperature: For example, direct air conditioning, sunlight, or motor heat generation creates local hotspots → causing axis drift and deterioration of perpendicularity / straightness.
Two forms of drift
- Steady-state temperature drift: Uniform heating and cooling of the entire system, where dimensions change linearly according to the expansion coefficient, which can be corrected through temperature compensation (software compensation);
- Gradient drift: Inconsistent temperatures across the workpiece / plate / equipment parts cause bending and twisting, which is difficult to completely eliminate with software compensation and belongs to hard deformation.
4. Industry standard requirements (Metrology lab)
For 00/000 grade granite plates and high-precision CMMs, constant temperature workshops generally require:
- Nominal temperature: 20℃ (reference temperature for geometric measurement)
- High-precision scenarios: Temperature fluctuation ≤0.5℃/h, spatial temperature difference ≤1℃
Deviating from these conditions will cause continuous accuracy drift; even for a 000 grade plate, placing it in an ordinary workshop with a day-night temperature difference of over ten degrees will cause significant drift in flatness and dimensional accuracy.
