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I am conducting SPC studies (using Xbar/R charts) on a grinding process, using variable data from three characteristics:
(1) assembly thickness after grind
(2) parallelism of surface to backing plate after grind
(3) flatness of surface after grind
To make charting easier, I am coding the data...in other words 0.001" will be 1, 0.010" will be 10, 0.007" will be 7, etc...
I am also setting the nominal as zero (0), and then will chart the difference from actual target. For example, if a part thickness target is 0.650", and the measurement taken is 0.652", then I record "2"...if a measurement is 0.647", then I record "3". What this does is it allows me to use one standard set of charts, regardless of the actual part thickness.
That's just a little background on what I am doing...now on to the question.
Ok, for THICKNESS, the specification is bi-lateral, however, for PARALLELISM and FLATNESS, the specifications are uni-lateral. Thickness is typically 0.650" +/- 0.005" (ie: in my case, 0 +/- 5). Parallelism and flatness are typically 0 + 0.010" max (ie: in my case, 0 + 10 max).
Ok, for calculating Cpk and Ppk, the formulae are as follows:
(I know that there is no PPU or PPL, I just used those terms for the sake of this question...)
That being said, I realize that these formulae work fine for my thickness studies, since they are bi-lateral, but what about for the uni-lateral tolerances (parallelism and flatness)??
For parallelism and flatness couldn't I just say that Cpk = CPU and Ppk = PPU (since there is no lower specification)?
Does anyone have any ideas on a better way to monitor these two uni-lateral characteristics (are Xbar/R charts even appropriate for these)???
(I need a vacation!) :truce:
(1) assembly thickness after grind
(2) parallelism of surface to backing plate after grind
(3) flatness of surface after grind
To make charting easier, I am coding the data...in other words 0.001" will be 1, 0.010" will be 10, 0.007" will be 7, etc...
I am also setting the nominal as zero (0), and then will chart the difference from actual target. For example, if a part thickness target is 0.650", and the measurement taken is 0.652", then I record "2"...if a measurement is 0.647", then I record "3". What this does is it allows me to use one standard set of charts, regardless of the actual part thickness.
That's just a little background on what I am doing...now on to the question.
Ok, for THICKNESS, the specification is bi-lateral, however, for PARALLELISM and FLATNESS, the specifications are uni-lateral. Thickness is typically 0.650" +/- 0.005" (ie: in my case, 0 +/- 5). Parallelism and flatness are typically 0 + 0.010" max (ie: in my case, 0 + 10 max).
Ok, for calculating Cpk and Ppk, the formulae are as follows:
(I know that there is no PPU or PPL, I just used those terms for the sake of this question...)
That being said, I realize that these formulae work fine for my thickness studies, since they are bi-lateral, but what about for the uni-lateral tolerances (parallelism and flatness)??
For parallelism and flatness couldn't I just say that Cpk = CPU and Ppk = PPU (since there is no lower specification)?
Does anyone have any ideas on a better way to monitor these two uni-lateral characteristics (are Xbar/R charts even appropriate for these)???
