Control limits for one sided control charts using WECO rules

J

jay1ph

#1
Hi,

How can WECO rules be applied for one-sided control charts, say for x bar-R chart? If I only have minimum specification limit, do I need to apply WECO rules on the upper control limit?
 
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Miner

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#3
Control charts are not one-sided. Your specification may be one-sided, such as a max or min requirement, but to control a process, you must control deviations from a mean value whether they increase or decrease regardless of the specification.

Therefore you can apply the WECO rules.
 
D

Darius

#4
There is no set of rules that apply to anykind of process. Just use the rules that apply to your process. Try to understand your process, wich patterns usually happen as part of the normal workings of your process (normal causes).

For one sided process is possible that runs happen offten without any special causes, specially if it is physically bounded (a natural limit in one site). If you put all rules, the operators will neglect the system if they don't know why this was something they have to take care.
 

Miner

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#5
For one sided process is possible that runs happen offten without any special causes, specially if it is physically bounded (a natural limit in one site). If you put all rules, the operators will neglect the system if they don't know why this was something they have to take care.
When you have a natural limit, such as zero flatness, you will typically have a non-normal process. If using an I-MR chart, you will see unusual patterns that will frequently violate the WECO rules.

If using an Xbar/R chart, this should tend to normalize the data and minimize WECO problems (unless you are running right up on the boundary and the data are extremely non normal).

However, if you are running at a distance from the natural boundary, normality is not typically an issue (unless precision machining, see bobdoering's blog for more info).
 

bobdoering

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#6
Hi,

How can WECO rules be applied for one-sided control charts, say for x bar-R chart? If I only have minimum specification limit, do I need to apply WECO rules on the upper control limit?
As usual, it would be handy to know what the characteristic is specifically and the process used to provide a more concise answer to the question.
 
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