Production Scheduling – Schedule at One Point
This post is part of our series discussing the 7 lean manufacturing guidelines. In this post we’ll discuss supermarkets as a way to design a lean future state.
The fourth of the seven guidelines for designing a lean future state is to try to send the customer schedule to only one production process.
Our previous guidelines called for developing as much flow as possible and then introducing supermarkets with pull-based replenishment where we could not flow from an upstream process. Everything downstream from the process that initiates flow is automatically scheduled based on one-piece flow from the prior process or flow through a FIFO lane.
Upstream from the initial flow process are supermarkets that hold buffer stock. Each of these supermarkets is replenished by a process that runs exclusively in order to maintain the targeted stock level for that supermarket. When the supermarket is drawn down below its reorder point, the replenishment process is triggered with a kanban or pull signal to build more, but only enough to replace what was used. So all of these replenishment processes are likewise scheduled.
Moving from Pull to Flow
The process where we move from pull to flow is called the pacemaker process, also known as the schedule point. The pacemaker production process is controlled by the customer’s orders and is the single point of scheduling for the entire value stream. Work gets launched at the schedule point and the rest of the value stream responds in flow downstream and pull upstream.
Optimizing the Pacemaker Process
The design guideline is to try to release customer scheduled work at a single point. In simple operations that’s not hard to do. In high mix operations, finding a single pacemaker can be challenging. There may not be a single process through which all work flows. You may end up with pacemaker processes by product family. There may be multiple independent production lines in order to build the total volume required each of which has a pacemaker process.
Scheduling Pacemaker Processes
Just how do we schedule our pacemaker processes? If we go back to our first principle of production scheduling, while we launch work at the pacemaker or schedule point, that process may not be our overall capacity constraint (or bottleneck) and we may have material shortages that limit how much we can build. That means that we should not release work to the pacemaker processes without considering the capacity limits anywhere in the value stream – including considering the material availability not just of end products, but of every sub-assembly and raw material component as well. In high mix manufacturers, here’s where lean manufacturing software is essential.
When You Have More Work Than Material
Let’s consider what happens if we release more work than we have material to build. Work will start and products will be partially built when we hit the last of a needed component. What’s the impact to the production line? One option is that all work stops until the part arrives. But in a high mix world, just because we have run out of a component for one product may not mean that the entire line has to stop. We may have plenty of components to build other products. In order to keep going, we may be tempted to pull semi-finished products off the line and hold them aside till later. As you can easily imagine, these unfinished products clutter up your operation and require you to keep track of where they all are, how far you got and what’s left to go on each unit. You also risk damage due to multiple movements. In addition, you are consuming material resources for unshippable inventory, which may cause a shortage of those common components that could be used to build other (shippable) products.
When You Release Too Much Work
Or what happens if we release more work than we have capacity? The first step is to identify the capacity bottleneck. Recognize that capacity constraints are not always at the same process and not all products may require the constrained process. In that case, when we hit the actual capacity constraint, the work ahead of that process must wait. On its own in one-piece flow or with FIFO, that will cause the upstream processes in flow to stop as well. And with supermarkets and pull, the constraint process will reduce the pace of withdrawals from each supermarket. Of course we still have to deal with where to put the work that is queueing up ahead of the capacity constraint. What can make this even more challenging is that the mix of products ordered can vary from period to period so the capacity bottleneck can move from one process to another.
Our future state design then must identify the pacemaker processes where work is launched into the value streams and identify the means to ensure that the scheduled work does not exceed available capacity or material across our entire operation. That can be a challenging order!
Stay tuned for the next segment on this series about leveling the production mix.
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Written By: Phil Coy