Statistical process control, SPC, is a set of tools to enable monitoring the stability of a process. SPC is also the first step to checking process capability with measures such as Cpk. Many consider SPC a quality or manufacturing tool. Yet, having and maintaining a stable process is also essential to creating a reliable product. Let me explain why.
SPC to Reduce Manufacturing Scrap
As a manufacturing engineer, years ago, we learned about and began using control charts on our manufacturing lines. We assessed our processing equipment and installed measures at critical points in order to first establish control limits, then monitored for ‘out of control’ signals.
The intent was to improve the stability or the consistency of the product we were making, primarily to reduce the scrap created by out of spec products. We found that the variability of our processes leads to the majority of the scrap created. Therefore, working to identify what is causing the variability helps us to improve our processes and improve stability. Over time we also learned how to reduce the span of the control limits and center our process to minimize the chance of out-of-spec results.
This is a common application of SPC.
SPC to Determine Process Capability
In a few cases, we found the natural variation of our materials and processes did not present ways to reduce the variability even after establishing stability. Some manufacturing processes just have too much variability when compared to the desired specifications.
The key to keep in mind is variability will occur in every process. Some of this variability we can eliminate or reduce, yet some will take a significant investment in new equipment, materials, etc to further reduce the variability. Sometimes, we just are unable to practically reduce variability.
It is in comparison to the specification that we can judge if the variability is good or bad. If all of the variability results in products within the expected specification, that would ok or good variability. If the variability leads to the product falling outside the specification, well, that is undesirable.
We learned that in some cases the easiest thing to change is the specification. If we can still create a product with the features and performance desired by the customer, then change the specification to accommodate the process variability.
A common measure for this application of SPC is Cpk, for example.
SPC to Establish Tolerance Specifications
As we continued to learn about SPC and creating better products, one of the design engineers ask about the variability of critical processes that impacted the product design for a new product. Not knowing the specifications, we just provided the control charts along with associated limits which indicates the range of variability to expect for the specified processes.
The design engineer then used that knowledge of process variability to establish the design tolerances and set of manufacturing specifications. She used the information as input to the design process in order to accommodate the variability of the manufacturing process directly within the design.
Making the tolerances fit the variability reduced scrap, improve process capability, and actually improved the reliability performance of the product.
SPC Improves Reliability
If a designer sets a very tight tolerance for some critical aspect of a product, that does not always translate that the manufacturing of the product will always be able to achieve that tight tolerance. That depends on the capability of the process, which is driven by process variability.
A tight tolerance doesn’t mean a design is better or worse, it really has to account for the ability of the process to achieve the tolerance. Even if we are able to inspect out all out of tolerance products we still have products that are very near the tolerance and thus less able to continue to function well over time if part features change with time, going out of tolerance, or in many cases have degraded performance when compared to parts centered within the tolerance.
The idea here is that no two manufactured items are identical. The same part, feature, equipment, process, material, etc will result in some difference between the two items. The goal is to create parts and products that are similar enough that all the products perform well overtime for the customer. Reducing scrap is also a good benefit, as well.
First, get the manufacturing processes under control and stable. Then, relay the process variability information to the associated design teams and encourage them to create designs that accommodate the expected variability.
Improvements in reducing process variability provide a touch more flexibility for the establishment of tolerances within designs. Appropriate tolerances allow for higher yields, less testing, and better products.
While SPC is often considered a quality or manufacturing tool, it is a vital and useful reliability tool as well. Stable variability improves product reliability. Process capability, tolerance, and reliability – all good.