Supply Chain Process Control and Capability

If you buy more than one of an item used in your product, you will have to deal with variability. In general, the variability from part to part is minimal and expected. Occasionally, the variability is large and causes reliability problems.

According to O’Connor and Kleyner, “The main cause of production-induced unreliability, as well as rework and scrap, is the variability inherent in production processes.” O’Connor, Patrick D. T. and Andre Kleyner. 2012. Practical Reliability Engineering. Chicester: John Wiley and Sons. Web.

The old premise that we are not able to inspect in quality applies to reliability as well. The key to minimizing product induced failures is to identify and control the processes that create the parts and assembled equipment. Plus, we need to create designs that take into account the naturally occurring variability with proper tolerances.

The match between what a process is capable of producing and the designs accommodation of the expected variation goes a long way to improving quality and reliability.

Your Supply Chain’s Role in Process Control

Ayers, in the Handbook of Supply Chain Management, Second Edition (Resource Management) defines statistical process control, SPC, as

A set of techniques and tools that help characterize patterns of variation. By understanding these patterns, a business can determine sources of variation and minimize them, resulting in a more consistent product or service. Many customers are demanding consistency as a measure of high quality. The proper use of SPC provides a powerful way to ensure that the customer gets the desired consistency time after time.

Many suppliers understand the need to minimize variability. Many are not very good at implementing SPC. One step in the selection of vendors in your supply chain is to evaluate their ability concerning creating consistent parts.

You should ask the following questions expecting substantive and meaningful responses.

1. What causes the most variation in the produced parts? What evidence do you have to support this information?
2. What specific elements of your process do you monitor and why?
3. What parts of your supply chain do you monitor and why?
4. What should we control as we integrate your part into our product and why?

One of the key’s to an effective SPC program is the monitoring and control the key factors causing unwanted variation.

If there isn’t a solid technical connection between what is monitored and the resulting ability to control variability, it may signal a poor process control program.

Capability and Tolerances

If a vendor does have their processes well controlled, then ask for the process capability given the expected design tolerances required. Connect your design intent with the manufacturing induced variability.

A good measure to use to gauge the capability is Cpk as it accounts for the centering of the variability within the tolerance range. Cpk is

$$ \large\displaystyle {{C}_{pk}}=\min \left\{ \frac{USL-\mu }{3\sigma },\frac{\mu -LSL}{3\sigma } \right\}$$

Where USL and LSL are the upper and lower specification limits or the tolerance limits. The mean, $\mu$, and standard deviation, $\sigma$, are from the process measurements of interest.

If the Cpk value equals 1 it implies the process will produce parts that are within the tolerance approximately 99.7% of the time. This assumes a normal distribution and no process variation of the mean or standard deviation. When evaluating Cpk values number above 2 indicate less than 1 part per million defect rates. Plus the low defect rate will continue event with the normal shifting of the mean and standard deviation of a stable process.

Conclusion

See Juran’s Quality Handbook: The Complete Guide to Performance Excellence 6/e for details concerning the calculation and interoperation of process capability indices. Statistical Quality Control Handbook. 1958. Writ. Bonnie B. Small by. Western Electric Co., Inc., Web. The NIST Engineering Statistics Handbook is another practical reference for process control and capability calculations. NIST/SEMATECH e-Handbook of Statistical Methods, http://www.itl.nist.gov/div898/handbook/, 16 August 2015.

As you work with your supply chain, first understand what is critical for your product’s success, this includes part consistency. Mastering process control and capability techniques allows you to evaluate your vendor’s ability to monitor and control their process.

The communication across your supply chain about the amount of part variability will enhance your team’s ability to establish robust design and produce reliable products.