10.1 · Accuracy & Performance
Accuracy vs Repeatability
These two numbers are on every spec sheet, but people confuse them constantly. Accuracy is how close you are to the truth. Repeatability is how consistent you are.
The Analogy
Key Terms
| Term | Meaning |
|---|---|
| Positioning Accuracy | How close the commanded position is to actual position across the travel. Usually ±0.005–0.02 mm. |
| Repeatability | How consistently the machine returns to the same position. Usually ±0.002–0.005 mm. |
| Backlash | Lost motion when an axis reverses direction. Worn ballscrews increase it. |
| Thermal growth | Spindle and axes expand as they warm up, shifting positions. |
Repeatability is more important for production. You can calibrate out systematic errors (laser compensation). What you can't fix is random scatter. A machine that consistently makes parts 0.02 mm over — but always by the same amount — is fine. A machine that varies ±0.03 mm randomly is scrap.