What is the relationship between Specification Limi

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    Titu John

    What is the relationship between Specification Limits & Control Limits in context to SPC ?


    Johnny Guilherme

    Specification limits are normally limits given by the customer for a product whereas control limits are established once a capability study has been done on a machine/process. So you might find that after doing your capability study that your control limits dont look or line up with the spec limits. Control limits are then in essence a funcion of what the machine or piece of equipment is capable of. If you need an example of how to do a capability sstudy let me know.


    M. Elhefnawi

    I believe Johnny is right. The specification limits normaly are defined by the designer, and these limits are mandatory to follow, while the control limits are calculated from the  actual performance of the process and used to tell you if the process become unstable and you will, but not yet, produce defectives. 



    Spec limits and Control Limits are about two separate and different issues that are often confused. The process owner should understand the purpose and differences of each and link these two issues correctly and logically.
    Spec limits are “voice of the customer” defining requirements that product delivered to the customer must meet.
    Control Limits are “voice of the process” telling you if the process is stable and predictable over time.
    Control Limits tell you nothing about meeting specs. Most control charts plot averages and, therefore, spec limits should not be included on the chart (unless your spec is for averages and not individual observations).
    To evaluate capability to meet spec limits, typically you use individual measurements taken over a significant period of time to calculate the proportion that expected to be outside the spec limits.
    When measuring capability, it may help to think of this as a histogram of individual data points collected over a period of several days or weeks with spec limits on the histogram. It should be obvious that the histogram of individual measurements will be much wider than the distribution of averages on the control chart. (Methods for calculation and description of capability depend on the form of the data, customer requirements, industry standards, etc.)

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