Mar 5 2014
When One-Piece Flow Restricts Capacity
Philip Marris told me of the case of a machining cell in an auto parts plant where management was ready to buy more machines because it was “lacking capacity,” but he was able to find a cheaper way to increase capacity by 17% in 15 minutes.
Unlike manual assembly cells, in which work can be balanced among stations, cells that involve machines always have one that is slower than all others, and, reallocating work among machines with different capabilities is not an option. In particular, almost all machining cells have a bottleneck, and the situation Philip described involved this bottleneck and the machine feeding it. The cell practiced one-piece flow. Therefore, if the feeder machine had worked perfectly, the timelines of the Feeder and the Bottleneck would have been as follows:
The Feeder would have started one piece at the beginning of each takt interval, and, since it is faster than the Bottleneck, it would have finished the piece before the end of the interval. The Feeder then would have waited for the bottleneck to pick up the piece before starting the next one. The Bottleneck would have been working 100% of the time; the Feeder would not.
But what Philip discovered by observing operations was that the Feeder had microstoppages. When the Feeder was hit by a microstoppage, the delay it caused passed to the bottleneck, which was prevented from working 100% of the time, as shown below:
This reduced the capacity of the entire cell. In the actual case, even with its microstoppages, the Feeder had enough capacity to feed the Bottleneck, on the average, just not on a takt basis. The microstoppages caused the output of the Feeder to fluctuate and disrupt the operation of the Bottleneck.
To anyone trained in Lean, the only appropriate solution was to eliminate the microstoppages… But it was easier said than done. Sometimes, all it takes is slowing down the machine, or changing a maintenance policy from “clean for one minute” to “clean until it is clean.” But it is not always that simple.
Microstoppages are often unreported because they are fixed on the fly by production operators. To understand microstoppages, you need to monitor the machine to observe when they occur and trace their causes. Eliminating them may require you to modify chutes, fixtures, jigs or dies, or even the basic process, and it can take time, but you need to do it if you want one-piece flow to work.
In the meantime, what do you do? Buying more equipment is an expensive solution, especially when you don’t expect to need it once you are rid of the microstoppages. A cheaper countermeasure is to protect the supply of parts to the bottleneck against fluctuations by decoupling the two machines with a buffer of WIP. You can set the size of this buffer by trial and error, knowing that it is not a long-term solution.
Of course, manufacturing engineers understand that you cannot have one-piece flow with microstoppages. So why did they ignore their own wisdom? The most likely explanation is a demand from a corporate “Lean group” to implement one-piece flow everywhere and “damn the torpedoes!” These engineers had complied not because they thought it was a good idea, but because it was required to keep their jobs.
Technically, Philip sees this story as a case study in the addition of Theory of Constraints (TOC) thinking to Lean; I just see it as due consideration of equipment issues in cell design, as I was taught it more than 25 years ago. From a management standpoint, I see it as an example of the local consequences of half-baked corporate mandates.
Mar 7 2014
Shortage of skills, not yet – but very soon – a wake up call (part 2) | Wiegand’s Watch
This is a translation of the bulk of Bodo Wiegand’s latest newsletter, about Lean in Germany, followed by my comments:
As many discussions of the “Lean office” do, Wiegand’s lumps together all activities other than production. Much of his letter is devoted to the standardization of office work, which he presents as essential to avoiding a skill shortage by increasing productivity. While a case can be made for the value of following documented procedures in transaction processing like rental car issue and return, it is far-fetched for creative knowledge work like R&D.
In product development, it helps to have some discipline in managing the flow of projects through phases, with appropriate validation at various checkpoints, but there is little evidence that it is essential. The history of product development is replete with cases where all the procedures were in place but the products failed, and, on the contrary, of cases of product developers who broke the rules and succeeded.
Wiegand describes the transition from craft control to controlled, documented processes in production as a battle fought won in the past 20 to 30 years. I view it instead as a struggle that started with the industrial revolution about 1750 and is still going on, with the Lean approach to it being only the last of a long list. And it does not involve standardizing everything. If you have machines with controls that are visually obvious and mistake-proof, you don’t need instructions.
Another theme of Wiegand’s letter is the change from organization by function, where employees are in departments focused on one operation, to organization by process, where they are in teams in charge of all the operations needed to generate a finished output. It is like the change from a machining job-shop with departments for turning, milling, heat treatment, grinding, etc. to a flow shop with lines or cells that machine blanks from start to finish.
Wiegand asserts that only 10% of companies still have functional organizations in production. It is a number I have a hard time believing. I don’t believe it’s true even in Japan. In fact, the functional, or job-shop, organization is not wrong for everything. Once you have done your Runner/Repeater/Stranger analysis, it is actually what you need for Strangers. And it is not always wrong in office work either. Product development at Toyota, for example, is done by functional departments.
I am also puzzled by his description of “break-even analysis” as the last great innovation in accounting. It does not strike me as particularly advanced. What about discounted cash flows, internal rates of return, activity-based costing, and other concepts that shine a light on different aspects of operations than just break-even points?
One last comment is that Wiegand mentions “optimization” six times and “improvement” never. One of my pet peeves is that, in Lean, you always improve but never optimize, because it is, by definition, the end of improvement. I have been assured both in Germany and France, that they mean “improvement” when they say “optimization,” which begs the question of what they use when they actually mean “optimization.”
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By Michel Baudin • Blog reviews 0 • Tags: Bodo Wiegand, Germany, Lean Office