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wchuey
Hi fellow quality practitioners,
I am not sure if any one of you has encountered this problem before.
Based on the AIAG manual, control limits for SPC charts are set at +/- 3 sigma. However, as the process improves, the sigma decreases and the control limits start to get closer and closer to the centre line with each review.
I am currently in a situation where points fall out of the control limits with no assignable cause (the only logical explanation is that the control limits are too tight). The Cpk of the process is greater than 4.
Is there a way in which I can justify for an widened control limit?
My colleague suggested increasing the control limits to +/-6 sigma, since the Cpk obtained at 6 sigma is greater than 2. I have always know the formula for Cpk as (USL-mean)/3sigma and was surprised to hear that when the control limits are set at +/- 6sigma, the Cpk for the process changes to USL-mean)/6sigma. Is this a statistically sound method?
I am not sure if any one of you has encountered this problem before.
Based on the AIAG manual, control limits for SPC charts are set at +/- 3 sigma. However, as the process improves, the sigma decreases and the control limits start to get closer and closer to the centre line with each review.
I am currently in a situation where points fall out of the control limits with no assignable cause (the only logical explanation is that the control limits are too tight). The Cpk of the process is greater than 4.
Is there a way in which I can justify for an widened control limit?
My colleague suggested increasing the control limits to +/-6 sigma, since the Cpk obtained at 6 sigma is greater than 2. I have always know the formula for Cpk as (USL-mean)/3sigma and was surprised to hear that when the control limits are set at +/- 6sigma, the Cpk for the process changes to USL-mean)/6sigma. Is this a statistically sound method?