Difference between spec limits and control limits

This topic has 4 replies, 3 voices, and was last updated 11 years, 4 months ago by Don Strayer.

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March 12, 2009 at 5:59 am #26855

Ajetha
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Based on what do we get the spec limits like the way we get control limits based on past performance?What happens if the spec limits fall between the control limits

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March 14, 2009 at 2:47 am #65253

Don Strayer
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The spec limits are set by the customer and they do not change unless the customer changes them. If you’re getting the specification limits any other way, you’re doing it wrong. Control limits are calculated from the data points (past performance). If either spec limit falls between the control limits, the process is out of spec even though it may be statistically under control. Look up how to create and use control charts and the difference between specification limits and control limits should become apparent.
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March 25, 2009 at 4:07 pm #65256

Perryman
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To add to Don’s response, spec limits are not normally used on control charts. They are used to determine your process capability and sigma value. For example, if as you say, your spec limits are tighter than your control limits and your process is in control and producing consistent results, you may be producing defect-free products (within the spec limits). On the other hand, you may be in control (within the control limits) but producing defects nonetheless if you are falling outside the spec limits. Hope this helps.
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April 9, 2009 at 10:29 am #65269

Ajetha
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So, if both the spec limits are in between the control limits, then the process is stable and capable.If the observations lie between the control limits but not between the spec limits then the process is stable but not capable.Is this correct!!
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April 12, 2009 at 2:39 am #65272

Don Strayer
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Your second statement is correct. You want the control limits to be within the spec limits.
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