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Short Run SPC

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Lately I’ve been hearing more and more interest in Short Run SPC (Statistical Process Control). Maybe it’s because of the economy, or maybe it’s because more manufacturers are adopting lean production techniques with goals for Zero Setup Time. Whatever the cause, more manufacturers seem to be shifting to shorter and shorter production runs.

Some people jump to the conclusion that they can’t apply SPC with runs of any one product that are shorter than, say, eight or ten hours. In my experience, this is because they’re thinking only of Variable SPC, or because they’re thinking the “P” in SPC stands for Product instead of Process.

The problem, of course, is that when you base a control chart on the product, you may plot a series of points for a couple hours, and then put the chart away for a couple months until you run that product again. When you pick up the chart again, the first set of points you plotted have almost nothing to do with  what you’re doing today. You end up with a stack of meaningless charts with mostly meaningless and disconnected data on them.

In most situations you can eliminate this problem using what I like to call “clever coding” techniques. I’ll describe these techniques in more detail in another post. Right now I want to explain when you can and can’t use clever coding.

The basic rules are that you can use clever coding whenever you can explain all the expected variation between any data you intend to place on the chart, and the process that produces the product is essentially the same.

By these definitions, clever coding is perfect for:

  • Processes that use the same tool. For example, the overall length for several products where the length is set by adjusting a stop on the machine.
  • Many electronic tests
  • Fill weights
  • Hardness, durometer, density or some other physical property

You won’t be able to use clever coding to mix features on a control chart if the process is producing very different key characteristics. You can’t use a Short Run Variables Control Chart to mix data from, say, an outside diameter followed on the same chart with a weight, and then a cycle time. The chart has to have some consistency.

If you have short production runs making unique, one-of-a-kind products then you probably need to shift to defect (attribute) data.

In an upcoming post I’ll look at the coding methods in more detail. Until then, use the ShareThis button below to mark this page, or leave a comment, tweet me, schedule a conversation, or call 800-958-2709.

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