Accuracy and capability are two important factors to consider when selecting a torque controlled tool for an application. Accuracy tells us how close the tool comes to the desired torque value, while capability tells us how well the tool performs in the application.

Accuracy

Accuracy is calculated by using the following formula:

s=(n−1)∑i=1n(Xi−X)2

where:

• Xi is the value of each individual occurrence in the sample
• X is the average of all occurrences in the sample
• n is the total number of occurrences in the sample

The accuracy of a tool is often calculated as 50% of the natural variation (36) divided by the target value. This makes it possible to compare different tools at a certain target value, without relating them to a certain application (tolerances).

Capability

Capability is calculated by using the following formulas:

Cp=6σTolerance interval=6σHI−LO

Cpk = min [(HI – AVE) / 36, (AVE – LO) / 36]

where:

• HI is the upper tolerance limit
• LO is the lower tolerance limit
• AVE is the mean value
• σ is the standard deviation

The Cp relates the tolerance interval (HI-LO), to the process natural variation. The Cpk also relates the mean of the distribution to the target value of the application.

The Cp and Cpk requirements vary, but the most common is that they have to be greater than 1.33. A process that has a Cpk lower than 1.00 is never capable.

Importance of Accuracy and Capability

Accuracy and capability are both important factors to consider when selecting a torque-controlled tool for an application. Accuracy tells us how close we are to the desired value, while capability tells us how well the tool is performing in the application.

By considering both accuracy and capability, we can select the best tool for the job.

Practical Examples

Here are some practical examples of how accuracy and capability are used in torque-controlled tools:

• Automotive assembly: In automotive assembly, programmable torque screwdrivers and other torque-controlled tools are used to tighten bolts to a specific torque. The accuracy of the tool is important to ensure that the bolts are tightened to the correct torque. The capability of the tool is important to ensure that the bolts are tightened consistently.
• Aerospace assembly: In aerospace assembly, cordless torque screwdrivers and other torque controlled tools are used to tighten fasteners to a specific torque. The accuracy of the tool is important to ensure that the fasteners are tightened to the correct torque. The capability of the tool is important to ensure that the fasteners are tightened consistently.
• Medical device manufacturing: In medical device manufacturing, Electric Torque Screwdrivers are used to tighten screws to a specific torque. The accuracy of the tool is important to ensure that the screws are tightened to the correct torque. The capability of the tool is important to ensure that the screws are tightened consistently.

Conclusion

Accuracy and capability are two important factors to consider when selecting a torque-controlled tool for an application. Accuracy tells us how close we are to the desired value, while capability tells us how well the tool is performing in the application.

By considering both accuracy and capability, we can select the best tool for the job.