Brandon posted a question in the comment section of an article about hourly production control boards, asking:
“I have an automotive assembly line and it contains six different stations. The vehicles are on a automated line that has the speed controlled according to the fixed takt time. If an andon is hit the whole line stops. How do I use an hour by hour chart to expose the reasons for the line stoppages????”
Great question, and one that gives us an opportunity to clear up a common misconception about the stop-the-line concept by introducing the fixed position stop system. One of the common misconceptions is that when a worker pulls the Andon cord at Toyota, the whole line stops. That would be a huge problem with hundreds of people stopped from working many times each day. Toyota uses what is called the fixed position stop system to avoid this. When a worker pulls the andon cord, a light on the andon board for that station will light up yellow. The line will keep moving until the automobile reaches the fixed position.
Unless stops are very minor and quickly addressed, we recommend using the fixed position stop system in combination with andons and hour by hour charts rather than a pull-and-stop type of andon that shuts down the whole line immediately. Within the assembly line each assembler must perform the work sequence defined within takt time. In a fixed position stop system when there is a problem and the work can not be completed by takt time the entire line does not stop. The line will keep on moving until it reaches the fixed position, and if the problem has not been resolved by that time, the line will stop.
In a robust management system such as lean, many things should be “fixed” or in other words clearly defined and set. This includes location, quantity, sequence, timing, space and such. The more specific one can be, the less ambiguity there is and this improves speed and quality. For automotive lines or linked automatic conveyance lines the fixed space through which the vehicle travels represents a fixed amount of time and also the specific work sequence or problem solving activity that can be done in that space.
When there is a problem on the line the assembler is required to push the stop button. The yellow andon lamp lights up and the team leader or supervisor comes running to help. At this time the line continues to run. If the worker is behind the team leader can help the person catch up. If there is a problem that can not be solved or if the team leader does not come by the time the fixed position is reached the line stops.
The team leaders and supervisors need to be well trained in the standard procedure for responding to line stoppages. If the problem can be fixed before the automobile reaches the fixed position, then the yellow andon will be switched off and the line will keep moving. If not, the line will stop. The problem will not be passed on downstream.
In very long lines where it is not possible at a glance to see the location of the stop on the line, visual management of the problems as well as the overall progress of production is managed by a combination of individual lights at the stations as well as a large andon board displayed above the assembly line, usually in several locations so that it is visible from any point on the assembly line. This screen shot from the Toyota website shows an example of such an andon system.
The main advantage of the fixed position stop system is that it allows workers to find problems and pull the andon cord without stopping the line and causing production delays. This reduces reluctance by workers to pull the cord and so smaller problems are identified more frequently, thus promoting earlier corrective action and a stronger spirit of kaizen. Of course when there is a safety problem the line should stop immediately regardless of where the work piece is relative to the fixed position.
The fixed position stop system is also insures that operations are not left half-finished. Workers do not go to break in the middle of a work sequence, returning later to remember exactly where they were. The fixed position stop requires the work to continue until it reaches the position. This is error-proofs the process so that critical bolt-tightening tasks half done, risking a the human error of forgetting when restarting after break. Start ups after breaks and lunches will be smoother as well since you are at a logical starting place (beginning of a work sequence).
The stop-the-line concept is a wonderful discipline to build quality into the process and speed up the flow of information between people. Yet people are naturally reluctant to stop for every minor problem or question, if this stops the entire line. When people feel pressure to keep the line moving and meet production targets they will avoid asking so-called dumb questions or for clarification when work instructions are vague or the quality is borderline. Having a fixed position and the time to solve the problem (between discovery of the problem and the arrival at the fixed position) relieves some of this pressure.
It is a pun of the English language, but one bit of wisdom from TPS is that “the more things are fixed (set), the less things need to be fixed (corrected).”