Which is best for a lean environment, a U-shaped cell or assembly line? Why?
Dear Gemba Coach
Which is best for a lean environment, a U-shaped cell or assembly line? Why?
This is a question that comes up quite often – and the pros and cons of cells versus lines have been discussed at length, so let’s look at this from another angle: before you kaizen your existing operation, what is the problem you’re trying to solve?
Whether in a straight line (I-cell) or a U-shaped cell, lean thinking highlights two fundamental problems: ease of use and flexibility. The first issue is that operators have to be able to work safely, intuitively, and keep to quality and takt time requirements. The second is that operators have to be able to deal with continual change in terms of mix, volume, and new product introduction.
Issue number one is to keep the operator working in a strike zone, to use a baseball expression. Imagine a window in front of each worker where all the work happens. To visualize this, start by looking at feet movement (steps and rotations), then hand movements (stretching, turning), and finally eye movements. The operator should be able to do the work without ever moving his or her hands away from this strike zone.
Typically, in assembly, operators have to deal with a number of components they’re putting together, usually with some mechanical help. The problem is then to fit all components in front of them within the strike zone. Keeping takt time stable often involves stability of volume, but with great flexibility in mix (fractioning and mixing orders for heijunka). In this case, the shelf of components in front of the operator can expand rapidly if different products use different components. Since the strike zone cannot be expanded, this means using very small containers to hold all components in front of the operator. The team member is autonomous if he or she can build product A or product B according to how the kanban cards pull, and without needing any outside help to change over his or her workstation.
Small containers of parts (they start being small enough when you can’t buy the containers in industrial catalogues but have to go to the supermarket for Tupperware instead) impose frequent resupply, and hence a “small train” or tugger. Very quickly, you’ll discover that the effectiveness of your workstation is directly driven by the tugger’s regularity, as any deviation will create parts shortages on the line. Consequently, one key problem to consider when organizing the cell is the train route.
In assembly, according to work content, operators will typically work within a defined area (defined by the components supply system) in front of them. If volume goes down, you might take a person out, and the zone will be larger (more work content). If volume goes up, the team leader might want to step in, making the operators’ work area smaller. In any case, having them work in a “U” doesn’t necessarily add any value – foot movement is essentially defined by the component shelf, and operators have to be able to pass on sub-assemblies from one hand to the next, as in a relay race. All in all, in assembly work, it usually makes sense to have a straight assembly line rather than a U-shaped cell. It’s also a cell – five team members, one team leader, etc. – but shaped as a straight “I” in order to make it easy for the tugger to supply work stations frontally by passing at the back of the station. This also makes it easy for operators to work laterally without having to ever turn around (definitely out of the strike zone).
In machining, however, operators deal with equipment that has its own run time. A typical cell will have several consecutive processes, and, to keep work intuitive and safe, we don’t want to have the operator tied to the machine as it operates. In order to avoid wasting the operator’s time, he or she needs to be able to place the component, start the machine working and then move on to the next task while the machine processes. In this configuration, the operator will be walking somewhat around the cell, and it then makes sense to have short circuits. These kinds of operations usually entail less components, so the tugger is not so much of a worry, and we can start thinking in terms of U-cells rather than lines.
The key here is that in order to work continuously, the operator should be able to place a part, move on to the next machine and withdraw the machined part and seamlessly place it in the next process. Automatic ejection of parts is a key component to make this work. The second issue is that, as with the assembly line, the cell should be changeable autonomously (without requiring outside help) and easily. So the next big problem is how to design machines that can be changed, by the operator(s) alone (without specialized changers) and without creating greater walking distances in the cell. This tends to be quite a challenge and requires many PDCA loops to get it better. Last week I was in a factory where manufacturing engineers are working on new ideas for protective casing to let parts and people flow freely – and safely – and, well, it’s not easy. The biggest room remains room for improvement.
A U-cell is therefore better adapted to using in-out automated equipment. First, the operator should be able simply to push parts from one process to the other (no carrying). Second, the cell should be able to handle well the product mix (easy to change). Finally, the cell should be designed to handle volume changes too. If the cell is composed of two rows of machines face to face separated by about 1,20 meters, when demand is high, each operator can take care of one workstation, and when volume is low, one operator can take care of the entire cell by walking from one process to the other in a standard circuit.
Understanding the Problem
With these generic problems/generic solutions in mind, we can now look into the real problem: how are you going to get your people to figure it out. Just the other week I came across a fascinating example of kaizen at the Gemba. The plant manager of a plants making car seats just-in-time, used the “pull” concept to improve the labor effectiveness of his assembly line by about 10%. Now, just-in-time plants are incredibly demanding as the OEM assembler expects seats to arrive for assembly in sequence throughout the day with a total lead-time of something between three to six hours. The pressure is relentless (if the seat is late the automobile line will stop) and the variety of seats is huge, so they need the information in order to build. There is a small buffer of finished seats waiting to go into the trucks for the five-minute drive across to the OEM’s line. Historically, just-in-time sites consider they don’t need to pull more rigorously because they already are pulled by the customer to a manic degree. But what this plant manager realized is that his frontline management kept trying to keep buffers filled at all times, and so created stop-and-go on the production line: early stoppages when a customer variation meant no more work, or rushes the moment the line started pulling again. This, in turn created overburden on employees (during the rushes) and quality issues with the seats, which generated more instability.
Working with the company’s sensei, he started a series of workshops and then kaizen efforts to pull more rigorously on his final buffer. In the end, they placed a mechanical stop on the buffer’s conveyor that pulled seats EXACTLY at takt time. This simple device stabilized the entire flow downstream, with all related benefits. But this is not the point. What the plant manager highlighted is the work he did with his middle-managers and frontline managers so that they themselves understood the problem. First they had to change their mind about what they measured and start worrying about in-process seats (between supplier and customer) rather than just trucks (if the truck is not complete, the customer line automatically blocks unloading). Then they had to understand the impact of their decisions on the stability of the assembly lines. Finally they had to work with team leaders to manage takt more rigorously at line level.
He is definite that if his engineers had come up with the technical solution, implemented it and he’d have to then get everyone to work accordingly, he’d have failed. He feels strongly that his site is already under enough pressure from the just-in-time situation without needing further tension from unnecessary conflict. In the end, his investment in time and effort has paid back further than he anticipated, and he is now thinking about working further with his own suppliers.
So, rather than worry yourself about what is the best solution in general, how about investigating how you’ll explain the problem to your line managers and team leaders so that they can come up with their own solutions. Line or cells are responses to technical drivers and there can be no once-forever right solution, as the situation changes all the time. However, if people have understood what they’re trying to get right - creating an environment where the operators can work safely and seamlessly at takt time whie changing all the time – they will start exploring and discover better and better technical solutions.
Believe that lean is about kaizen + respect. kaizen is about improving the lines and respect is about creating an environment where people can succeed at their work, work safely, and have an input on how their work environment is organized. Technical solutions about how to have the “best” line have been around for decades, and I’m certain your engineers will know many of them. The issue here is respect. How do you get your group leaders, team leaders, team members and manufacturing engineers to work together on a shared problem in order to design the lines that best satisfy them all. That’s the real challenge.