CNC: Positioning accuracy and Repeatability – what exactly are they ?
A simplified explanation of these terms would be: Accuracy is achieving the correct position. Repeatability is achieving it every time.
A common way of defining these terms graphically is by comparison with a target practice chart, where you attempt to shoot at the centre.
Accuracy is how close your shot is to the centre.
Repeatability is how close to each other your shots are when you shoot multiple times.
On a CNC machine, the destination position in a motion command (e.g., X100.0) is the equivalent of the centre of the chart.
Accuracy is improved by reducing errors caused due to distortions from loads, thermal variations, calibration, calculation and kinematic errors (motion errors – velocity, acceleration, deceleration). Repeatability is improved by reducing static friction, backlash, and thermal variation.
Achieving accuracy is far more costly than achieving repeatability.
If Accuracy is not good, you won’t get any good parts, even if the Repeatability is OK.
If Accuracy is good but Repeatability is not, you may get one good part but not a whole batch.
If Accuracy and Repeatability are both OK, you’ll get a whole batch of OK parts.
Text and pics. source: CADEM NCyclopedia multimedia CNC training software.
