How do we perform Process capability study for Batch Production

Dear Frind:

In general, the answer to your question is that, you cannot do it, unless certain conditions are met. If the batch is a relatively large one of say 300 pieces, and if the process had previously shown to be statistically stable, one could cull out a sample size of at least 100 pieces, subsets of 5, taken from different points within the substantial run of 300,and perform a Cpk calculation.

The purpose of a capability study is to predict how your process would perform in the future based on the data. So, to get good data for such a calculation, it is necessary to allow the process to run for sufficient time to observe the many sources of variation that could develop. This just cannot be done with small batch sizes of 20, 30, 40, or 50 parts, etc. My advice would be to perform a Ppk study. This is a process performance study, but at least it would tell you how likely it is that your process would perform successfully over a batch of product that size. The weakness of such a study is that it is not a reliable predictor of what your process would do into the future, long term. Hope this helps.

Hi,

for calculating process capability for a batch process is calculated in a same way as process capability for mass production the only difference in this is that the extrapolation of the value is high as the batch process runs for short term only but we have to project it as 1 million transaction.

regards

Hello,

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Best of luck,

John

Hi,
If you explain a bit more about what you want to achieve, I think I could help. Have you considered a batch as a unique ouput unit with multiple opportunities for a defect? Or have you considered a sampling plan that would sample from each batch, and therefore allow you to calculate a process capability?
Happy to explore it more with you.

Hello,
recently whilst browsing the web for information on R&R maths I came across a PhD thesis which treats exactly the problem you mention : capability analysis and SPC for small batch productions. Now, the only drawback may be that it is the work of a frenchman, and therefore is written in french. The professor is A Haurat, from Annecy laboratory LLP(Laboratoire de Logiciels pour la Productique), the thesis dates back to march 2004. It is possible to download a pdf version.