I'm a shooter. Occasionally at the range, we have a contest. We set an 8-inch target 100 yards down range. At that distance, it looks like a period typed on a piece of paper. The contest is who can hit the target the most times with 20 shots, using their off hand and open sights. Loser buys dinner. Rifles today are dead accurate. The only real variable is the shooter's ability.
Now the rant:
When molding parts, the 'target' is making an acceptable part consistently. You get no extra points for making perfect parts where every dimension is dead on at the mean dimension (the bulls-eye). Like the targets on the range, the object is to not miss the target (put every part within the specified tolerance). You get bragging rights only if you hit the bulls-eye; but nothing more.
CpK is a very useful tool when CNC machining, because the CNC equipment looks at the center of the turret and assumes the tool is cutting a specified distance from it. As the tool wears, the depth/length of the cut will vary. CpK therefore tells the CNC operator when to change his cutters.
Asking for and insisting on a CpK in molded (not machined) parts is really an admission of a mistake from the designer because he doesn't know what will happen when two or more parts mate together at differing ends of the tolerance spectrum. His mistake is the fear that 'within tolerance' might produce a bad part. Really? Does it sound to you that someone didn't do their homework?
Lean, CpK, 5S, Quality Ninjas, JIT, PPM-defectives and all the other Statistical CorpSpeak are really designed to improve profits by avoiding waste. In reality, it has little to do with quality; it's all about money.
The iconic AK-47 rifle became legendary not because of advanced metallurgy, innovative design, or iconic thinking. Its reputation came from loose tolerancing. It is a highly reliable machine that will do what it is designed to do regardless of where or how any component is manufactured so long as it is within the design tolerances. Loose tolerances allow it to function under almost any conditions. Take your zillion dollar deer rifle and bury it in your back yard, let a thunderstorm come in a soak everything. Dig it up after a snow storm when the temperature is freezing. Do you think it would work? Probably not; because most rifles are built to very exacting tight tolerances at the foot of alter of Statistics. If we applied the statistical constraints of Lean, CpK, 5S, Quality Ninjas, JIT, PPM-defectives to Mr. Kalashnikov's design, it would scare the Black-belts white. The sucker just keeps working without all this stuff.
So, are these tools necessary? Here's the two-handed argument:
On the one hand: If the goal (lean) is to reduce waste (in the most general sense), improve productivity etc. these tools are excellent guidelines. They do not, however, improve quality. The only thing they improve is consistency. This means if the part will fail; it will do so with irritating regularity with a tight CpK, manufactured with Lean principles following 5S methods. Read: this is a design fault.
On the other hand: There are people who make their living and building corporate kingdoms generating charts, graphs, PowerPoint presentations, and holding meetings throwing about highly esoteric terms like Students T tests, probabilistic certainty, and robust arguments. Unfortunately you have to pay them and support their efforts. Let's ask the ultimate consulting question: "What did they bring to the party?" Does the cost of all this fluff and sizzle have a net impact on profits? Did their effort generate the infamous 3X or 5X in additional profits measured against their salary and overhead?
What if the designer did his job by finishing the project with a 'due diligence' stack tolerance study and extensive real world functional testing showing or simulating that any combination within his tolerances would produce a functional part that would last for its intended lifetime? Would we need all this statistical stuff? Nope, but they are still good tools to have when you need them.
These are interesting questions for management to ask. It's embarrassing to get the answers.
So, the next time you see your tolerancing chopped in thirds with a CpK, ask the question: "Does the cost of all this improve reliability to the point of you being willing to pay for it?" Then sit down and politely listen to the answer and consider the source. If this is being shoved down your throat because it is the Management Philosophy Flavor of the Month, do what everyone else does: Nod your head knowingly, charge them for the exorbitant amount of labor needed to generate the mound of paperwork they require, and get on with your life. Next month they'll be another philosophy.
If they want the Crown Jewels, they should be willing to pay for them.
End of Rant.
Consultant Bill Tobin is a regular contributor to Injection Molding and PlasticsToday, with real-world injection molding experience at GM, Mattel, and Hewlett-Packard, among other places. You can sign up for his newsletter at www.wjtassociates.com.