CNC machines: Positioning accuracy, repeatability

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.


Astor Hotel Kolkata – popular with ghosts

I once spent a couple of nights at the Astor Hotel, Kolkata. A lovely red coloured heritage building that’s more than a 100 years old. Nice restaurants, lovely rooms and good service, but haunted by ghosts.

When I told someone that I was staying at the Astor, he said “Oh, the hotel that’s supposed to be haunted by ghosts”?. Turns out that the ghosts were a rumour, spread by the workers union that had a running battle with the management a couple of decades ago. They spread the rumour so customers would stay away from the hotel, thus causing financial damage to the management. Must be the most bizarre tactic used in a union-management fight.