Improve Processes to Keep Inspection Costs in Check

Most shops rely on their shop rate to set pricing or cost (if you are in a cost center). Inspection and inspectors are part of the fixed expenses that make up the cost to operate the shop. Unfortunately, inspection is a non-value-adding activity, which for today’s conversation means it adds cost without adding revenue to offset the expense. So why do we spend time inspecting if it adds cost to the parts?

Simple. Because we don’t want bad parts. Bad parts also cost money, and inspection is the way we mitigate the risk of bad parts. It is also an activity that is so ingrained in machinists that it travels from generation to generation like a genetic mutation.

Probably because we are taught from the beginning to measure twice and cut once. Perhaps also because our old habits have yet to catch up with modern machining processes that are far more capable than those in use around the time the genetic mutation occurred. Therefore, we are conditioned to inspect away process defects that should be designed out. Under coercion, I have been baptized in the corporate continuous improvement Kool-Aid.

Even after several forced immersions I remain a lean manufacturing heathen and haven’t given myself over to all of the cult’s beliefs. However, I have become a devout believer in process capability, sometimes known as statistical process control (SPC). Process capability and associated calculations would fill a text book, though most shops don’t need to know all the statistical calculations. They only need to understand the concepts that are intended to drive accuracy and repeatability.

Accuracy in machining is like accuracy in archery. A very accurate process delivers part dimensions that are always very close to the bullseye or nominal dimension. Repeatable machining processes create parts with dimensions that are always the same. When the process is accurate and repeatable, then it is capable.

Creating machining processes that are aimed at delivering accuracy and repeatability is the foundation of reducing inspection and associated costs. Look around the shop and examine what people are doing. Most likely, you have machinists standing at machines, unloading parts and then checking them with various gauges. Sometimes they find an anomaly with a gauge and make an adjustment like changing a tool offset or adjusting a boring head to keep things on size.

Have you considered what happens in these normal man-machine interactions and how they impact the bottom-line? The first thing I think about are machine tool crashes. It has been my experience that machine tool crashes are almost always a result of someone interacting with the machine in an attempt to resolve a dimensional problem. Iam sure you have seen this play out in your shop.

A machinist will measure something and determine an adjustment is needed. They go to the offset page in the control and enter 0.100 instead of 0.001, an honest mistake.

The next thing you know the tool is plunging into the work piece at full rapid. In the best case you have a broken piece of carbide and scrap part. In the worst-case scenario you end up replacing a spindle. The other scenario is one of lost productivity.

Less skilled operators running parts may run into a dimensional problem they cannot resolve. Whether it’s from a lack of confidence or training, they don’t attempt to resolve the problem. Instead, they wait for a more senior person to do so. The result is lost productivity and increased costs.