In our fast-paced, results-driven world there never seems to be enough time to do the things that need to be done. Companies are faced with continually increasing customer demands, ever-decreasing leadtimes, tighter margins and insufficiently qualified people.
Recognizing the necessity of doing more with less, we must take steps to be as productive as possible. We improve our CNC programs to minimize non-cut time, find better tools, introduce innovative workholding devices and maximize machine speeds and feeds, all with the goal of getting parts made as quickly as possible. This is all well and good, and we have come a long way in reducing piece-to-piece cycle time on our modern, high speed equipment. Unfortunately, even when we do this, there are too many times when finished parts just sit there. I refer to this phenomenon as the “hurry up and wait” syndrome.
What causes this syndrome? The answers are varied but boil down to not looking at the overall manufacturing plan (also known as “the big picture”) and instead focusing on the efficiency of individual machines. Many people believe that as long as a machine is running (especially a machine that costs a great deal of capital to acquire), things are good. They are driven to keep these expensive machines running, and that drives their production schedule.
Compare a manufacturing process to sand running through an hourglass. When we turn the hourglass over, a lot of sand sits in the top. A small portion of the sand then runs through the narrow center section (referred to as the bottleneck). Finally, the sand sits in the wide bottom section of the hourglass (this section being large enough for sand to sit and wait until the hourglass is turned over once more). Now substitute the sand to manufactured parts and change the hourglass to a manufacturing process. High speed, highly productive machines in the first part of the process (the top of the hourglass) feed slower running, or bottleneck equipment (the narrow part of the hourglass), which then feed either more high speed machines or racks of inventory at the end of the process (the bottom of the hourglass). It does not matter how fast the equipment runs at the beginning or end of the process, throughput is controlled by the capacity of the middle or bottleneck process. If we do not increase the capacity of the bottleneck, but instead focus on pushing parts through the other processes, parts will sit and wait. The waiting can be short, perhaps minutes or hours, or it can be long, perhaps days, weeks or months. This waiting leads to excessively long throughput times for our parts.
There are ways of getting around the “hurry up and wait” syndrome. We can implement manufacturing cells in which one machine immediately feeds another to keep the part moving. Where this is not feasible, we can employ pull scheduling. In a pull system, we limit the amount of parts that can sit between steps in the process by instituting some type of physical barrier. For example, we can limit the floor space or rack space allowed for incoming material. If there is no place for excess parts, they cannot be delivered (and should not be made). We can employ visual signals, such as lights that direct whether a machine should produce parts for the next operation. An example might be multi-colored lights attached to a machine. A green light indicates that the machine is running and the supplying process can complete and deliver the next job. A yellow light indicates the machine is temporarily stopped, perhaps due to a changeover or tooling issue. The supplying process can complete the job in the machine but not start another job for that specific downstream process. A red light indicates the machine is stopped and will be for some time, and the supplying process should stop making parts for that downstream process. The logic behind pull scheduling is that sending parts to a process that cannot use them does no good; in fact, it is just a waste of time and resources.
We can also focus our production schedules on shorter intervals of time, say a day or two, and make every effort to assure that material, tools, equipment and people are available to work on the parts that need to be produced in that short time span. This serves to give full attention to what is really important here and now.
Look around your organization. Are there partially completed parts waiting to start the next process? Think of the time you have invested in making these parts, then think of what you could have accomplished with that lost time.