Rooting out Root Causes

All too often, root cause analysis is treated as an ex post facto requirement when dispositioning a defect tag, completed only so that inspectors can show internal and external auditors that an issue has been taken care of. Rarely is the true root cause identified, and there is almost never a follow-up to make sure the implemented corrective action was actually effective.

Timely intervention upon discovery of a defect is vital to finding the root cause. If a company has a fully implemented SPC system and has set up suitable control limits, then this will act as an effective early warning system, especially if the root cause behind the out-of-tolerance measurement is something like tool wear in machining. In such a case, one robust corrective action would be to adjust the tool life management system to take machining tools offline more quickly, before they wear down to the point of creating an out-of-tolerance condition. It might also be appropriate to optimize the feed and speed of the CNC machine. These are frequently set at excessively aggressive levels, so backing them off can dramatically improve tool life, especially when working with hard materials like titanium

Sometimes, root causes are buried so deep that an SPC system cannot provide early warning. One case I remember from the days when I owned a machining company was a real brainteaser.

We had two cells, each having two Mori Seiki SH 630's with 16 pallets. Everything about these cells was identical, so a part run on one should have always gone the same on the other. But when we moved a specific part number from Cell A to Cell B, a critical hole dimension was out of tolerance; when we moved it back to the original cell, it was back in tolerance. This was a tight-tolerance hole that required a precision boring bar and an air gauge, both of which were connected to our SPC data collection software (an early version of Net-Inspect).

At first, no one could understand how two identical machine processes could produce different results. We thought the fixture may have been the problem, so we decided to move the entire pallet with the fixtures still on it. Again, Cell B produced an oversized hole. We then tried moving other parts from Cell A to Cell B but encountered no problems: the parts were virtually identical and easily within tolerance. Our SPC charts confirmed that except for the troublesome part number, the machines were indistinguishable from each other.

After many days of research, we ruled out the machine, the machine program, the fixture, and the operator as root causes. What was left? Very little. So I posed a question to the engineering team: If I wanted to make a bigger hole using the same tool running on the same machine that I used to make the original hole, how would I do it?

I received an immediate response, that it could be done by heating the tool. Even disregarding the coefficient of expansion making the tool larger, the extra heat from the tool would make the metal of the part easier to remove. We next investigated why tools running on different cells might have different temperatures, and as soon as we watched Cell B in action, the answer became obvious. What was different between Cell A and Cell B was not anything about the machines or process, but rather the part that had been manufactured just before the part in question. As it turned out, the run prior to our "problem part" on Cell B belonged to a complex part with many precision holes which, by sheer coincidence, used the same tool as the problem part. And to compound the issue even further, the precision hole on the problem part was immediately next in sequence after the previous part's multiple holes were produced.

The problem was easily solved when we placed a second, identical tool in Cell B and made the program call the cold tool to make the part rather than the hot tool. Problem and root cause resolved.

For more than 20 years, Net-Inspect has not only helped companies implement SPC and non-conformance tracking software, but has also shared experiences like this to help companies think differently about the challenges they are facing and maximize the value of our cloud-based solution. Please contact our team at [email protected] or +1 425-233-6176 if you are interested in talking in more detail about how Net-Inspect can be used to discover and track even the most well-hidden root causes at your manufacturing facility.