The Goal

Y -" X

Product parts are what join the two in a relationship with each other, Jonah explains, and the arrow obviously indicates the flow of parts from one to the other. He adds that we can consider any non-bottleneck feeding parts to X, because no matter which one we choose, its inventory must be processed at some subse- quent point in time by X.

"By the definition of a non-bottleneck, we know that Y has extra capacity. Because of its extra capacity, we also know that Y will be faster in filling the demand than X," says Jonah. "Let's say both X and Y have 600 hours a month available for production. Because it is a bottleneck, you will need all 600 hours of the X machine to meet demand. But let's say you need only 450 hours a month, or 75 percent, of Y to keep the flow equal to demand. What happens when Y has worked its 450 hours? Do you let it sit idle?"

Bob says, "No, we'll find something else for it to do."

"But Y has already satisfied market demand," says Jonah.

Bob says, "Well, then we let it get a head start on next month's work."

"And if there is nothing for it to work on?" asks Jonah.

Bob says, "Then we'll have to release more materials."

"And that is the problem," says Jonah. "Because what hap- pens to those extra hours of production from Y? Well, that inven- tory has to go somewhere. Y is faster than X. And by keeping Y active, the flow of parts to X must be greater than the flow of parts leaving X. Which means..."

He walks over to the work-in-process mountain and makes a sweeping gesture.

"You end up with all this in front of the X machine," he says. "And when you're pushing in more material than the system can convert into throughput, what are you getting?"

"Excess inventory," says Stacey.

"Exactly," says Jonah. "But what about another combina- tion? What happens when X is feeding parts to Y?"

Jonah writes that on the floor with the chalk like this...