Inspect only when needed: Turning Performance

Inspection is a normal part of the manufacturing process, and many machine shops dedicate people to verifying that parts meet the required specifications. Unfortunately, inspection is also a nonvalue-adding activity when it is not a specific requirement.

So why do manufacturers continue to have dedicated inspectors, cumbersome inspection procedures and gather reams of inspection data? In some cases, it is mandated by customers or certifying bodies, such as the Federal Aviation Administration, or there is a statutory requirement. In these cases, inspection can generate revenue, making it a value-adding activity. One primary reason aerospace components cost more than similar nonaerospace components is these inspection and documentation requirements.

When inspection adds no value, a manufacturer doesn’t get paid for it. Therefore, eliminating or minimizing inspection achieves significant efficiency gains and cost reductions.

The first and best way to drive out inefficiencies associated with inspection is to stop inspecting when a customer doesn’t require it. Habitual inspection, however, is common—especially in older plants. Engineers and shop personnel get in the habit of processing parts the same way each time, which means potentially copying unnecessary inspection steps.

An aerospace company I worked at would commonly inspect parts after each major machining operation. Frequently, a machinist had already completed and documented the inspection, so the subsequent inspection added only an unrecoverable cost.

Frequency of inspection should always be dictated by risk. This is a fundamental precept of a process failure mode element analysis (PFMEA). This analysis evaluates the engineering specifications that govern a component’s manufacture, the likelihood that the component does not meet the specification and the significance of failing to meet the specification. PFMEA is used to create a control plan governing the inspection processes.

When building power steering components, Mitsubishi performs magnetic particle inspection to find cracks caused by the hardening process. In this case, 100 percent of the components are inspected because crack propagation could cause catastrophic failure in the steering system and loss of vehicle control. This requirement involves more than two million inspections per year. Conversely, some part dimensions have little or no impact on the functionality of the steering system, and many are checked only once a shift. PFMEA quantifies the risks and the control plan dictates all inspection decisions, so we don’t inspect more than needed while still ensuring critical features are maintained.