Building a capable manufacturing process

If you have worked at or done work for a large manufacturer, you are probably familiar with the terms C_pK, P_pK, P_p and C_p. These statistical calculations provide insight into the capability of the manufacturing process.

A capability study, as the name implies, gages the ability of a manufacturing process to reliably produce a product that meets customer requirements. Six Sigma continuous improvement tools and ideas work in conjunction with these statistical tools.

All images courtesy C. Tate

A machinist prepares a vertical machining center for a process capability study.

Before designing a manufacturing process, shops must understand the process requirements and have a solid definition of potential problems. It most cases, there are three responsibilities inherent in making a product: design, manufacturing and quality. These entities take on different names depending on the organization, but the functions are the same. Having them work together is critical for a capable process.

Design is responsible for providing a product definition that conforms to the customer’s requirements. Product definition is usually based on a drawing or written specification. It is the responsibility of all parties to ensure product definition, no matter the form, is clear and complete.

Manufacturing is responsible for reviewing, analyzing and understanding the customer’s requirements. It is common for manufacturing to blindly accept the product definition from the design team and find out later that some aspect of the requirement cannot be achieved. Having product requirements clearly defined, well understood and agreed upon is imperative because those requirements will drive the manufacturing process. A process that consistently and accurately conforms to design requirements is considered capable.

Manufacturing and quality should examine all of the requirements to ensure they are consistent with the organization’s capabilities, demonstrate best practices and fall within industry standards. Most importantly, the requirements must be achievable.

Once there is a clear understanding of the requirements, the team must determine how to verify or inspect the requirements. Large manufacturers often use process failure mode element analysis (PFMEA) to create a control plan. A control plan outlines the frequency and method of inspection for each of the requirements and eventually becomes the basis for statistical process control. The use of these tools requires a team composed of engineers and quality personnel, so they are typically used only by very large manufacturers.

When the PFMEA is created, each of the requirements is reviewed with the intent of developing the inspection methods and frequency, based on the criticality of the requirement.

For example, in one case the shaft of a power steering gear was inspected for surface cracks that could be caused by heat treatment. All of the parts were inspected because crack propagation could have caused a catastrophic failure, an obvious safety concern. Conversely, the wrench flats on the same shaft, used in the assembly process, were only inspected three times a shift because they posed little risk of failure.