Machine capability index: Reference for accuracy assessment

Accuracy is crucial, as it describes the ability of a measuring instrument or a machine to deliver values that are near to the true value. In manufacturing environments, accuracy assessment tools play a key role in qualifying production processes. To achieve this, the machine capability indices Cm and Cmk in manufacturing are often mentioned in connection with accuracy. These indices were established in the automotive industry as a method of assessing the quality and suitability of power tools – today, they are broadly referenced as part of the machine capability index approach.

What is the difference between machine capability index Cm and Cmk? 

• The first machine capability index (Cm value) is a crucial index used to measure the general ability of the tool to produce the required quality at the operating point. The Cm index illustrates the repeatability of the machine, without taking any systematic errors into account. It is therefore a cornerstone of Cm and Cmk in manufacturing when validating tool consistency.

• The second machine capability index (Cmk value) however, indicates the deviation between the operating point of the tool and the target value. In other words, it gives an indication of compliance with the required operating points. While Cm measures the machine's potential capability, Cmk also considers systematic errors, providing a more realistic assessment of tool stability and accuracy in real operating conditions.

A Cmk value of 1.67 is commonly targeted in industry to ensure the machine is both highly capable and well centered within tolerance limits. 
This threshold provides a robust safety margin, reducing the risk of non-conforming parts before moving to full process capability.

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What is the target machine's capability to ensure accuracy?

The lower the standard deviation in relation to the specified range, the higher the Cmk value and the more reliable the production process becomes. Accurate machine capability analysis is therefore a key part of precision engineering and quality assurance.

In other words, if you throw three darts at the board, they will always hit a specific field, but this will not always be the bull’s eyes. If the machine is set precisely to the center of the specified range, the Cmk value will be equal to the Cm value. As the setting moves away from this center, the Cmk value decreases. The objective is to achieve Cm and Cmk values that are as high as possible. 

These diagrams with the dartboards gives a graphic illustration of this point. 

High Cm value but low Cmk value

With reference to a nutrunner this would mean that the unit worked with high repeatability but always achieved the wrong results.

Low Cm value and low Cmk value

The nutrunner delivers a different result for almost every tightening. 

High Cm value and high Cmk value

In this particular case, the nutrunner always delivers the correct result with high repeatability or is always within the specified torque range.

A high Cmk value can only be reached with a high Cm value!