When things go off the rails quality professionals start a root cause analysis routine to determine what went wrong and why. This is a logical process of elimination until the culprit or culprits are identified. The hard part of the process is defining the cause when an instrument or gage is the primary suspect. The ‘fix’ part of it all comes easy, if it’s possible, but too often ‘fixes’ are tried before the cause has been accurately nailed down. When the analysis indicates gages or measuring instruments are at fault, sometimes basic assumptions are wrong—not the tools that provided the data. I’ll leave the overall concepts of the analysis to my colleagues on this publication that specialize in this area and focus on those ‘basic assumptions’ I mentioned.
The ‘assumed quality level’ regularly pops up when a new gage or instrument is used and starts rejecting parts. The first approach is to blame the new gage or instrument. Why? Because ‘we’ve been making these parts for years without any problems.’ The new device is often better than what it replaced and it is essentially indicating that while production may have been trouble-free the parts may have rarely, if ever, met the tolerances on the drawings. If the device these components are part of functions well, it means the specs need to be brought into alignment with reality. This usually means the drawing tolerances need to be opened up.