The Goal

X -" Y

"How much of Y's 600 hours can be used productively here?" asks Jonah.

"Only 450 hours again," says Stacey.

"That's right," says Jonah. "If Y is depending exclusively upon X to feed it inventory, the maximum number of hours it can work is determined by the output of X. And 600 hours from X equates to 450 hours for Y. After working those hours, Y will be starved for inventory to process. Which, by the way, is quite acceptable."

"Wait a minute," I say. "We have bottlenecks feeding non- bottlenecks here in the plant. For instance, whatever leaves the NCX-10 will be processed by a non-bottleneck."

"From other non-bottlenecks you mean. And do you know what happens when you keep Y active that way?" asks Jonah. "Look at this."

He draws a third diagram on the floor with the chalk.

In this case, Jonah explains, some parts do not flow through a bottleneck; their processing is done only by a non-bottleneck and the flow is directly from Y to assembly. The other parts do flow through a bottleneck, and they are on the X route to assem- bly where they are mated to the Y parts into a finished product.

In a real situation, the Y route probably would consist of one non-bottleneck feeding another non-bottleneck, feeding yet an- other non-bottleneck, and so on, to final assembly. The X route might have a series of non-botjtlenecks feeding a bottleneck, which in turn feeds a chain of more non-bottlenecks. In our case, Jonah says, we've got a group of non-bottleneck machines down- stream from X which can process parts from either the X or the Y route.

"But to keep it simple, I've diagrammed the combination with the fewest number of elements-one X and one Y. No mat- ter how many non-bottlenecks are in the system, the result of

activating Y just to keep it busy is the same. So let's say you keep both X and Y working continuously for every available hour. How efficient would the system be?"

"Super efficient," says Bob.

"No, you're wrong," says Jonah. "Because what happens when all this inventory from Y reaches final assembly?"

Bob shrugs and says, "We build the orders and ship them."

"How can you?" asks Jonah. "Eighty percent of your prod- ucts require at least one part from a bottleneck. What are you going to substitute for the bottleneck part that hasn't shown up yet?"

Bob scratches his head and says, "Oh, yeah... I forgot." "So if we can't assemble," says Stacey, "we get piles of inven- tory again. Only this time the excess inventory doesn't accumu- late in front of a bottleneck; it stacks up in front of final assem- bly."

"Yeah," says Lou, "and another million bucks sits still just to keep the wheels turning."

And Jonah says, "You see? Once more, the non-bottleneck does not determine throughput, even if it works twenty-hour hours a day."

Bob asks, "Okay, but what about that twenty percent of products without any bottleneck parts? We can still get high effi- ciencies with them."

"You think so?" asks Jonah.

On the floor he diagrams it like this...

This time, he says, the X and Y operate independently of one another. They are each filling separate marketing demands.

"How much of Y's 600 hours can the system use here?" asks Jonah.

"All of 'em," says Bob.

"Absolutely not," says Jonah. "Sure, at first glance it looks as if we can use one hundred percent of Y, but think again."

"We can only use as much as the market demand can ab- sorb," I say.

"Correct. By definition, Y has excess capacity," says Jonah. "So if you work Y to the maximum, you once again get excess