S
Scott McDonald
Good or Bad Sampling Plan
I have a question on whether or not a customer of ours is using appropriate sampling method when it comes to accepting or rejecting one of our lots.
Many of the parts we make come from muli-cavity tooling. These are the ones that generate more non-conformity reports with a needed corrective action response. When a multi-cavity tool is approved into production we always furnish capability studies of each individual cavity and shoot for a 1.67 CPK or > on the initial sampling. Everything looks good up until we send in our first production lot where the customer pulls 20 random samples pulling from several to all the cavities.
They crunch their results into a program that matches ANSI/SQCZ1.9-1993 Double Specification Limits, Variability Unknown - Standard Deviation Method, NORMAL (AQL-0.15%, S3), Sample Size = 20. Now by the randomness of the samples and dispersion of the different cavity averages, this range when applied to the program continually kicks out a small % either over or under spec limit generating a non-conforming material report with a request for corrective action. We are working with very limited tolerance of +/-.0008", so of course we're splitting hairs here.
Is their method correct for this situation?
This customer generates 70% of our sales, and I have a hard time with telling them that this may not be an appropriate sampling method for the situation. This is starting to cause a slight animosity between us.
Can anyone help me with this situation or at least guide me to a publication or articles that discuss this in more detail?
I have a question on whether or not a customer of ours is using appropriate sampling method when it comes to accepting or rejecting one of our lots.
Many of the parts we make come from muli-cavity tooling. These are the ones that generate more non-conformity reports with a needed corrective action response. When a multi-cavity tool is approved into production we always furnish capability studies of each individual cavity and shoot for a 1.67 CPK or > on the initial sampling. Everything looks good up until we send in our first production lot where the customer pulls 20 random samples pulling from several to all the cavities.
They crunch their results into a program that matches ANSI/SQCZ1.9-1993 Double Specification Limits, Variability Unknown - Standard Deviation Method, NORMAL (AQL-0.15%, S3), Sample Size = 20. Now by the randomness of the samples and dispersion of the different cavity averages, this range when applied to the program continually kicks out a small % either over or under spec limit generating a non-conforming material report with a request for corrective action. We are working with very limited tolerance of +/-.0008", so of course we're splitting hairs here.
Is their method correct for this situation?
This customer generates 70% of our sales, and I have a hard time with telling them that this may not be an appropriate sampling method for the situation. This is starting to cause a slight animosity between us.
Can anyone help me with this situation or at least guide me to a publication or articles that discuss this in more detail?