Things Change Even When They Remain the Same
The key to maintaining the quality of long-run parts is regular spot-checking.
In manufacturing, we mostly deal with new parts, part revisions, new material, new machining processes or new operators. All these challenges are relatively short-lived, and before you know it, a part run is over and you’re on to the next challenge.
But there are some parts and processes that remain virtually unchanged for decades. Sure, there might be a tweak here or there, but all in all, the same family of parts just keeps running on a machine that was probably purchased to make the first part.
Such long-run parts can provide great benefits to a shop for several reasons, but there are also dangers that lurk in the apparent “comfort” and “stability” of long-run processes. These dangers include complacency, wear and, oddly enough, inexperience.
If there is to be a long run, it’s not unusual for specific machines to be purchased just to make that particular part. But because of the longevity of the process, it’s also not unusual for maintenance and characterization of these part-specific machines to slip over time, allowing error to creep into parts.
The same can happen to gages dedicated to measuring these parts. Because the process is so stable and routine, complacency and wear can take their toll on the part-inspection process. Gages are not mastered as frequently as
necessary or inspected for wear as often as they should be. Measurements, too, can become sloppy: parts are not sufficiently cleaned, contacts are not carefully aligned, critical dimensions are skipped or not measured in appropriate locations. Sometimes, measurements are even skipped and data entries faked. After all, the complacent mindset says, “We make these parts all the time. What could go wrong?”
The biggest change that takes place with these long-run parts is a change in personnel. In only a very few cases will the same operator or manufacturing team be in place 20 years after the process has started. In fact, because the process becomes so “stable” and “reliable,” usually the most inexperienced operators are assigned to it—or operators who for whatever reason may not be as capable or as productive as the “A Team” that tackles the shop’s toughest challenges.
One can only imagine how much history about the process and its creation can be lost over time, and how many errors can recur that need to be diagnosed and corrected over and over again. Or how many undetected errors can creep into the process and ultimately put the entire job at risk.
Take something as simple as a diameter in a bore or a shaft with a difficult tolerance requirement. When such processes are first set up, the usual close monitoring steps are taken. Gaging is purchased for the diameter, the process is run, parts are checked, and, if it’s a tight-tolerance part, usually form and surface-finish gaging is employed to really understand the geometry. It is not uncommon for gaging to be changed to fit the challenging part characteristics. Good gages hit the scrap heap because of things tried, learned and moved on from.
With tight tolerances, we know the manufacturing process can introduce form or surface-finish errors into the part, which may influence part diameters and how they interact with each other. Once the process is understood, a selection of two- or three-point measuring tools are chosen to account for even or odd lobbing conditions in the parts, based on form-testing results. Over the years then, you may accrue a collection of gages that were used either in the early stages of setting up the process or during troubleshooting of some issue that may have come up, or even as replacements for the original gages, which were then not removed from service. With personnel coming and going over the years, and all these gages lying around, no one may remember why one type of tooling was used in the past over another.
The key to keeping these long-run measuring processes on track and to continuing to produce good parts year after year is to spot-check them on a regular basis. Do a “zero-based” review of the entire process—almost from the ground up—to ensure that things that are changing can be corrected before something goes wrong. If this review becomes a regular event rather than a reaction after something bad happens, not only can the quality of the process be maintained, but people can be trained and kept sharp. And they will learn that things can change, even when they remain the same.
Thus, on a regular basis—weekly, monthly or quarterly—a sample part should be given the once-over for total process control. Check every dimension, along with surface finish and form, and record this data so that a long-term history of the part can be collected and referred to whenever an error or change comes about. This check is very similar to the blood test your doctor conducts annually. It sets a baseline against which all future work is compared.
The same is true with your long-run manufacturing processes. If you don’t know what you did in the past, the future may become the past all over again.
Gages can conform to either standard. Which are right for your application?
Tailor your procedures, but conduct them regularly, to keep your measuring equipment working properly.
It doesn’t take a complex, expensive computer system, so even small shops can reap the benefits of statistical process control.