Critical Chain - Eliyahu M. Goldratt [55]
"Don't forget that in practice, machines are not working smoothly," I'm making sure that he doesn't turn it into one of those artificial examples Johnny likes so much to use in his articles. "And don't assume any neat pattern. A worker may slow down, tools can break, materials jam. You never know; you only know that it happens."
Johnny smiles at me. "Precisely." He behaves as if I'm trying to help him. "Under the realistic conditions that Rick so vividly described, how can we guarantee that our X machine will always have enough material so that it will be able to work constantly?"
"Put enough stock in front of it," Jim collaborates.
"Good idea." And Johnny draws, in front the circle with the X, a hump that is supposed to represent a pile of stock. "Now, as Rick told us, Murphy hits; one of the work centers upstream has a problem. The flow of material to the X machine stops. But not to worry, due to Jim's suggestion there's a pile of material ready. We can continue to utilize our X machine to one hundred percent."
I like it. It's simple. And Johnny will soon admit that he is wrong. There is no problem having as many bottlenecks as we want, the only price is some inventory. I'm not going to let him give a baloney speech on the cost of holding inventory as an excuse for having just one bottleneck. Oh, no.
"So far, so good," he smiles. "But what, unavoidably, happens during this time? The flow of material to X stops and X continues to work, pulling from its pile. The amount of inventory in the pile must go down."
Johnny stops, turns to me and asks, "Should we agree that Murphy does not strike only once? That sooner or later one of the feeding machines will, once again, stop?"
Even though I see where he is leading, I must agree.
"And if so, from time to time, the pile will continue to be drained. Can we afford to let the initial pile be drained to zero?"
I don't answer. I don't like people asking trivial questions. I hate it even more when they bother to answer their own trivial questions. Which Johnny does. "If we do, the next time Murphy hits one of the feeding machines, our X machine will be starved, it won't work for one hundred percent of its time. So, if we don't want this initial pile to be drained to zero, when the problem in the feeding machines was corrected and the flow resumed, what did we have to do?"
"Rebuild the pile in front of X," Jim plays Johnny's trivial question and answer game.
"But to do that," Johnny says triumphantly, "the feeding machines have not only to supply the ongoing rate of X, at the same time they also have to rebuild the stock. Quickly, before Murphy hits again. Which means . . ." he looks directly at me, "which means that each one of them must have more capacity than X."
He goes back to his seat. "Conclusion. If we want to utilize even one resource to one hundred percent, all its feeding work centers have to have more capacity. Since Murphy is not negligible, and the feeding machines have limited time to rebuild the stock, they must have more than infinitesimal excess capacity. QED."
I stare at the board. Johnny's proof is surprisingly elegant. I can't find any crack in it. But if he is right on this point, I have to accept the resulting five focusing steps.
So what? What's bad about it?
In the background I hear Jim asking, "How much excess capacity do the feeding work centers need to have?"
And Johnny answers, "It depends on the magnitude of the breakdowns, not less on the frequency of the breakdowns, and, of course, the amount of the stock you allow or want to build in front of X."
I know what's bad about it. Since the minute I heard it the first time, I knew that these five focusing steps are the key to solving all the riddles of project management, and I wanted to try and give it a shot. But now, now that B.J. puts it as a condition to prove myself