Changing specification limitation limits - What effect on Process Capability




Ps tell me what are the considerations to be taken care of before changing specification limitation limits say, 0.5 sigma on either sides. What is the effect of this change in process capability ? What steps are necessary to ensure that all production lots are accepted with these modified limits.


Rick Goodson

Changing the specifications by increasing 0.5 on either side will give you a better process capability number as the process capability (Cp) is calculated by dividing the tolerance by 6 sigma. With Cpk the average is subtracted from the upper specification limit and the lower specification limit is subtracted from the average, and then each divided by 3 sigma. The smaller number is the process capability. Once again increasing the tolerance gives the a larger number and the capability is better. If you want all production lots to be accepted, keep increasing the specification until everthing passes. The product may no longer work or meet customer use requirements, by the lots all pass.

The purpose in calculating process capability is to determine if the process is in control and capable of manufacturing parts that meet specifications given that all processes are dynamic and will vary. If the process is not capable you need to address the process, not the specification (assuming the specification is fact realistic).
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