Our customers require that we follow the ANSI Z1.4 standard for attribute sampling plans; however, it is not feasible to wait until lots are completed to perform inspections. Lots can be large and run for many days and waiting until lot completion to determine the sample size, based on the finished lot size, is too late because we will have missed our chance to correct any production issues that may result in defective parts. Another limitation is a lack of space to stage product while waiting for the final inspection of the completed production lots. Product is made as orders are received, and not typically stored as inventory, so our on-time delivery demands also hinder our ability to hold product for final inspections of completed production lots. Therefore, we are seeking guidance on a practical way to implement a in-process inspection during production that follows the ANSI Z1.4 standard.
Yes, you can sample as you produce to get to the sample size. It is important that you keep track of your accept/reject items. Since you know how long you are running the product, you can project the approximate lot size to get the sample size. Work with your scheduler before the product starts so you can take samples early and continue on in the process.
In addition, if you have material changes as the product is running, I am sure that you are sampling then to make sure everything is set correctly, you can use those samples also. As an example, let’s say your product is running 4 days and based on the projected lot size, you have a sample of 28 to take, you could take 7 samples each day spaced throughout the day or you take 10 samples the first day, 7 the second day, 6 the third day and 5 the last day of production. You need to figure the right sequence that fits your history of the product.