Control and Specification Limits

Submitted by chullan88 on Tue, 07/01/2014 - 01:35

Hi,

I know control limits are based on the project quality metrics and specification limits are based on customer input?inherent in the process.

So the control limits are to be stricter than specification limits.So does that mean if spec. limits are 3 sigma customer should be 6 sigma.

I cant relate this on the control chart as spec limit line is outside the control limit.
So can 3 sigma line be outside 6 sigma line??

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admin

Tue, 07/01/2014 - 08:08

Specification limits have to do with the voice of the customer while control limits have to do with the voice of the process.

So then, what are specifications? They define the allowable deviation from target or nominal. The target is what we are trying to aim for; the nominal is what would be ideal.

Say if you are manufacturing and filling biscuits, the nominal level is NET Weight as you dont want to give free biscuits, but if you are below net wt. it may mean huge loss if someone sues you. So you want your target to be set slightly above nominal.

Control limits are based on past performance. Its the pattern or process which tells you how much of variation process has produced in the past, with the intent of recognizing when a sufficient change from the past has occurred to justify adjusting the process

So in a nutshell , specifications are our promise to the customer of what we will provide and should be based on total system losses. Control limits show the range of variability we expect from the process and are based on actual process output. While process variability affects the total process losses, the specification limits in no way influence the control limits.

Check this post http://www.winspc.com/what-is-spc/ask-the-expert/248-what-is-the-differe...

Sandy10001

Fri, 07/25/2014 - 23:36

Sometimes I infer incorrectly but in this instance I believe the reference is that the Specification Limits (VOC) are to be plus or minus 3 sigma. Example: Stakeholders want the Spec. limits to be between 40 and 60 units on a 100 unit scale, they actually want 50 units but will accept that 20 point range. To Control the process you would set manufacturing limits closer to 50 - so perhaps 45 and 55. Doing so should ensure your process satisfies the stakeholders.