Inspect only when needed

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.

Often companies will inspect parts because a particular dimension or specification has a high occurrence of failures. However, inspecting parts for this reason probably means the manufacturing process is not capable. A process is capable when it provides predictable and reliable results each time it is performed.

Processes that are difficult to perform or that produce dimensional features with close tolerances tend to be less capable. For example, holding a ±0.015 " tolerance on a shaft diameter is much easier than holding a +0.0000 "/-0.0005 " tolerance on the same diameter. It may not be necessary to inspect the ±0.015 " dimension on a shaft produced on a high-quality CNC lathe, but a +0.0000 "/-0.0005 " tolerance is tricky and probably requires checking every part made on the same lathe. It may even require two measurements: one before the finish pass and one after. Why? Because a -0.0005 " tolerance can be difficult to hold and going undersize will scrap the part. A turning operation like this would not be considered capable, because a capable process would not require this level of inspection.

Like turning, milling or grinding, inspection is a process. Therefore, inspection is subject to the same inefficiencies as other operations. In cases where frequent inspection is mandated, ensure it is done efficiently. Be sure you are using the correct inspection tools and automate inspection whenever possible.

While in the aerospace industry, I automated a process that had an integral inspection step. It was for an ID bore with a +0.0005 "/-0.0005 " tolerance. The old process was to make two finish passes, measuring the diameter with a bore gage after the first pass and then adjusting the machine to ensure the second cut was correct. After purchasing a new lathe with a probing option, measuring with the bore gage was eliminated, cycle time was reduced 25 percent and the scrap rate went to zero.

Manufacturers often feel the need to inspect parts to mitigate financial risk, because if parts are incorrect, they may not get paid. However, profitability can also be adversely impacted by unnecessary and inefficient inspection operations. Getting it right means mitigating the risk of making bad parts, understanding the requirements of the manufacturing process and choosing the best inspection tools for the job. CTE

About the Author: Christopher Tate is engineering manager, combustion shop, for Mitsubishi Hitachi Power Systems Americas, Savannah (Ga.) Machinery Works. Email: chris23tate@gmail.com.