Most companies have a hard time getting their arms around one-piece-flow. Not just logistically; the concept is the exact opposite of their traditional approach.
The basic idea behind one-piece-flow is fairly self-explanatory. Take a traditional manufacturing example to illustrate. Let’s say that it takes 10 machines to make Product B. All the machines are in their traditional functional departments. A company that ‘batches’ production in this manner will also have a traditional finance mindset—putting pressure on making the absorption hours so that the results for the month will look good.
Now, say that one of the machines is broken and can’t make any parts. So the decision is made to go ahead and run the other nine machines at full blast to get as many absorption hours as possible. Never mind that because one machine is broken we can’t complete a single thing today that we can sell to a customer. In addition, because the machines all run at different speeds we will be piling up parts in uneven amounts. Maybe we make 2,000 covers but only 300 bases, for example. The inventory buildup is, of course, a huge form of waste by itself.
Even worse is the fact that I tied up my equipment making inventory that I can only sell at a later date when I could have used that equipment to make something that I can sell today. The whole approach is nothing but a big pile of waste. And yet, at the end of the day the finance guy will still say, “you had a good day because you were still able to produce 95 percent of the absorption hours.”
One-piece-flow operates on a completely different approach. What you want to do is go get one of each of the machines that it takes to make Product A and lay them out next to each other in a cellular layout. Get the machines as close to each other as possible and leave no room for inventory other than the standard amount of work in process that is required (normally one piece per machine). We normally think of a cell as U-shaped, but a couple of parallel lines work just as well as long as they are near each other (close enough to stand in the middle and reach either side). Once you have a flow cell, if one of the machines is broken, you can no longer run the other nine. The whole cell shuts down and you put an intense focus on fixing the one machine that is broken. This is the exact opposite of the traditional approach.
Some folks will find that shutting down the whole line is pretty severe. But it’s worth it since making the shift to one-piece-flow will uncover a wide range of problems. The fact that you have excess machine capacity becomes clear right away, for example. This problem has always existed but was hidden by your batch state and six-week lead times. Now, when you run the line at takt time to meet customer demand, this will jump out at you. The finance guy may have an especially hard time with this. Don’t let it bother you. Having excess capacity is in fact normal and actually a good thing.
The second thing that will become very clear right away is that, in their batch state with long lead times, these machines were not well maintained. They break down all the time and stop the entire line. Being in one-piece-flow will force you to fix the problem immediately. Moreover, to be able to do this, you will have to develop a total productive maintenance (TPM) approach to prevent any breakdowns from occurring in the future. You have to commit to the mentality that unplanned downtime is simply no longer acceptable. This won’t happen overnight of course but you need to take the steps to put in a daily/monthly/yearly maintenance program that will eliminate future downtime.
One-piece-flow will also immediately highlight any problems you have with your vendors. These issues could concern delivery or quality; they will also shut down the line until you fix them. On the plus side, one-piece-flow allows you to easily observe what is going on—bottlenecks for example—and put fixes in place that lead to future gains.
Above all, one-piece-flow is the key to quality improvements. In my experience it is pretty common to get a 10x or better gain in quality once you are in a one-piece-flow. This will occur naturally and is something that you get for free.
Here’s why. In a batch configuration with a six-week lead time, when something goes wrong it may be six weeks before you find you have a problem, when you go to assemble the final product. At this point it is very hard to determine what went wrong. Let's imagine that you get lucky and can narrow down the problem to something that happened in department number 5. So now you go to department 5 and you find that the part could have been made on any of 10 different machines. It could have been made on one of two different shifts by a total of 20 different operators. In addition, you could have been using raw material that came from one of three different vendors but by the time you discover the problem you can no longer determine whose material we were using. Pretty tough problem to solve at all let alone get a permanent solution. Oh, and of course you have six weeks of bad inventory that you have to deal with in some fashion.
Once you have the product in a one-piece-flow you can see the work much easier and more immediately. Now when you discover the problem you know whose raw material you are using. You know which machine caused the problem and who the operator is. You only have ten pieces of work in process to deal with. So not only do you have all the information you need to solve the problem, but you now can get a permanent solution.
One of the key ideas in lean in any setting, manufacturing or non-manufacturing, and which one-piece-flow supports, is that you build quality into the product at every station in the line as opposed to trying to inspect quality at the end. It’s a simple and powerful shift in how people think about their work. Never pass a defect down the line.