This article is an excerpt from our “Ask the Expert Colum” featured in our March 2013 newsletter
Q: I’ve created flow through many area of my operation, but the one area that’s proved difficult is the upstream, shared processes in my facility. There are so many parts passing through the machines there, and scheduling is always a challenge. What do you recommend when it comes to creating flow through shared resources?
A: First off, your thinking is already on the right track, since you indicated that you’re going to try to implement flow, not pull, through your shared resources. Supermarkets are often used to connect shared resources, both to one another and the downstream processes where flow more commonly takes over.
While supermarkets are a better option than uncontrolled inventory, flow is superior to them both because it is the method by which we can connect processes that involves the least amount of waste (and therefore also offers shorter lead times, faster throughput, and higher inventory turns, too).
When dealing with processes that produce parts for multiple product families (also known as shared resources), creating flow can indeed prove difficult. Central to creating flow through shared resources is the use of multiple FIFO lanes, not just one single FIFO lane. Using a single FIFO lane will likely lead to many of the same scheduling issues with which you are presently dealing because a single FIFO lane feeding a shared resource provides limited visibility into the time by which each part will be processed.
Additionally, as soon as one of the processes supplying the shared resource’s FIFO lane goes offline for some reason, our tendency will be to move that machine’s parts to the front of the FIFO lane once it comes back online, but this destroys the standard work and rules of FIFO we’ve developed, and soon we’ll find ourselves back to scheduling and managing our processes, our previous gain from flow reversed.
Instead of using a single FIFO lane to feed shared resources, multiple FIFO lanes offer a great way to create and sustain flow at these processes. Below is a simple example of how multiple FIFO lanes would work. Each process supplying the heat treat would produce parts for its product family and then place those parts into their own distinct FIFO lane feeding into the shared resource. The shared resource would then empty and process each FIFO lane according to a standard sequence.
A mix indicator, such as a sign with the words “Next Job” written on it, would tell everyone where the shared resource is in cycling through the FIFO lanes and which FIFIO lane is next to be emptied. And, since each FIFO lane holds one product family, everyone will know which product family is being processed, too.
At batch processes, we probably will not be able to run a batch through the machine unless a certain amount of parts has accumulated first. In each FIFO lane, then, we can put down a marker or identify a zone indicating that if parts have accumulated up to this point, we can safely run a batch through the machine.
The red lines in the diagram above would be the marker in this example (the thresholds can be different for different FIFO lanes). So, once product has accumulated past this point in a given FIFO lane, the heat treat can process that batch. For non-batch processes, it may be possible to simply process the entire contents of each FIFO lane every time we cycle through it, though we would need to develop standard work for this.
Perhaps the most important benefit of a multiple FIFO lane system is that it provides regularity and repeatability to shared resources by creating a guaranteed turnaround time (GTT) for them. This means that everyone will know the time by which each FIFO lane will be emptied. Also, it means that if parts are in a FIFO lane by a certain time, then they are guaranteed to be processed by the shared resource by a certain time, and managers and supervisors will no longer have to spend their time chasing down parts, schedules, and priority lists to ensure demand is met.
The exact timing of the GTT involved will depend on the interval of the shared resource. Coupled together with multiple FIFO lanes, it represents an excellent way to create flow through this challenging part of the operation.