The "Accuracy" Myth
Machine tool builders often quote a single number: "Accuracy: ±0.0002". This is misleading. In the world of ISO 230-2, accuracy is a statistical probability derived from multiple bidirectional passes.
The Big Three Metrics
1. Positioning Accuracy (A)
The maximum deviation between target and actual position across the entire travel.
A = Systematic Deviation + 2σ (Random Scatter)
2. Repeatability (R)
The ability to return to the same spot. Usually much better than 'A'.
R = 4σ (Standard Deviation range)
3. Reversal Error (B)
Lost motion when changing direction. Also known as Backlash.
B = Mean difference between approach directions
ISO 230-2 vs JIS B 6338
Be careful when comparing Japanese machines (JIS) to European/American ones (ISO).
- ISO 230-2 (The Strict One): Uses 2-sigma or 4-sigma statistics. Requires 5 bi-directional runs. Reports the "Performance Band".
- JIS B 6338 (The Lenient One): Typically reports max deviation without the full statistical penalty. Numbers look better on paper.
Reading a Laser Calibration Report
When you buy a high-end machine, you get a Renishaw (or similar) laser report.
Look for the "Slope": A consistent slope error (e.g. growing by 0.001" over 40") usually means thermal expansion or linear scale scalar mismatch. This is fixable in parameters.
Look for the "Hysteresis": A gap between the forward and reverse lines indicates mechanical backlash (ballscrew wear or thrust bearing play). This is the "B" value.
The 20°C Rule
All accuracy standards are defined at 20°C (68°F).
Steel expands at ~6.5µm per meter per °C. If your shop is 30°C (86°F), a 1-meter ballscrew has grown by 0.0025" (65µm). Without glass scales or thermal compensation, your "accurate" machine is cutting scrap.