Cp and Cpk for straightness and parallelism

#1
Hi

I am going through dimensional reports for 32 plastic samples. My task is to calculate cp and cpk for all dimensions but i am not sure how to calculate it for straightness (0.15) and parallelism (0.30).
I understand that 0 is nominal and 0.15 or 0.30 is upper limit but tere is no lower limit - 0 is perfectly straight or parallel. Is there a cp and cpk formula for unilateral tolerances ? How to work out ptocess capability. Minimum cp and cpk >1.33 .

Can anyone help please ?
 
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Jim Wynne

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#2
First, have a look at these Cove search results. The subject has been discussed extensively here. Cpk at its best is of dubious value, but it has no value at all unless the samples have been taken in rational subgroups from a running process and chronological order is maintained and the process is normally distributed and statistically stable. It's not something that can be derived from measurement results of a bunch of samples of unknown provenance with regard to the requirements stated above.
 
#3
Sampes are colected in even intervals during Operational Qualification. Samles are numberd and than send to met lab for measuring. Process is statistically stable and normaly distributed. I only don’t know how to calculate cp and cpk or is it not posible for unilateral tolerances
 
#5
If straightness is tolerance zone is 0.15 than 0 will be nominal, 0.15 is upper limit and there is no lower limit ? And when all results are 0.001 - 0.008 it is perfectness but cpk values are low . Can cp and cpk be calculated atcall for unilateral tolerances ?
 

Jim Wynne

Staff member
Admin
#6
Sampes are colected in even intervals during Operational Qualification. Samles are numberd and than send to met lab for measuring. Process is statistically stable and normaly distributed. I only don’t know how to calculate cp and cpk or is it not posible for unilateral tolerances
Did you collect samples in subgroups, or just single samples? Was there any charting (x-bar/R, e.g.) done to establish that the process is stable and normally distributed? If not, how did you make that determination? Did you look at any of the threads in the search results I provided?
 
#7
Did you collect samples in subgroups, or just single samples? Was there any charting (x-bar/R, e.g.) done to establish that the process is stable and normally distributed? If not, how did you make that determination? Did you look at any of the threads in the search results I provided?
Samples are collected in subgroups top half x 4 cavities x 32 samples and bottom half x 4 cavities x 32 . So we have 32 samples for each cavity x 2 diferrent pieces. Yes charters were done before. All measured results are within tolerance too.
 

Jim Wynne

Staff member
Admin
#9
Samples are collected in subgroups top half x 4 cavities x 32 samples and bottom half x 4 cavities x 32 . So we have 32 samples for each cavity x 2 diferrent pieces. Yes charters were done before. All measured results are within tolerance too.
To say the least, you're sending mixed messages here, as what you've posted isn't consistent with the dimensional report form you posted. Assuming that you're just concerned about capability calculation for unilateral tolerances (and not whether what you're doing makes sense), have you looked through the search results I posted?
 
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