Issues with one-sided specification limits in process capability analysis
29 October 2012: Stephan Conrad
One-sided specification limits in the calculation of capabilities raise questions time and time again, especially when it comes to the difference between simple “one-sided limits” and “one-sided specifications with a natural limit”. This article illustrates the most important issues and provides you with possible solutions. However, first we are going to explain the meaning of “one-sided specification limits” and how to distinguish them from a “natural limit on one side”. The process capability results we are going to obtain can also be transferred to machine capabilities (Cm /Cmk) and preliminary process capabilities (Pp /Ppk), of course.
Two – one – zero – natural limits?
Two-sided specification limits
Two-sided specification limits consist of two limits between which a certain proportion of characteristic values falls. A characteristic may exceed both specification limits. In this case it proves nonconformity with specifications which leads to a product that is not okay.
Examples:
- Linear dimensions, e.g. the width of a license plate
- Line voltage required to operate a microwave
- Diameter of a threaded hole
One-sided specification limits
One-sided specification limits consist of a single upper or lower limit restricting the characteristic. The characteristic may exceed the limit (in case of an upper limit) or fall below the limit (in case of a lower limit) which proves nonconformity with specifications and thus leads to a product that is not okay...
Technological/physical/natural tolerances
There is a special case of one-sided specification limits. They just have a single specification limit; however, they also consist of a second limit – a natural limit the characteristic is not able to exceed for technological or physical reasons...
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