Ensuring Enough Flexibility for high OEE
How to implement lean manufacturing? Can we reduce in-process inventory and maintain a smooth flow? On the one hand, the system needs some flexibility to be robust to changing daily order mixes but on the other hand, too much flexibility leads to traffic jams, increased inspections, and more complex part prove-out. In our experience, the sweet spot is when about 20% of the machine-hours are flexible; this provides enough flexibility to run lean with high efficiency, but not too much flexibility that the daily operation becomes overwhelming.
Part, Pallet, and Machine Assignment
To build the flexibility plan, we iteratively decide on an assignment of parts to pallets and machines. There are two approaches. First, we can start with every pallet visiting every machine and gradually reduce flexibility, ensuring that the machines stay busy. Alternatively, we can slowly grow the system by gradually adding parts and adding pallets to machines until the machines are fully loaded.
This is where our simulation-based tools SeedTactic: Planning and SeedTactic: Designer excel. We can easily edit the part, pallet, and machine assignments and then run a simulation to quickly see the resulting cell operation. Each system is different, and this simulation-based approach allows us to learn about the specifics of the system and through iterative development eventually decide on a flexibility plan.
One complication is variations in daily part mix; we might find a flexibility plan that works well with a given collection of part quantities but fails with a different part mix. Also, if the time frame is too long, the simulation might show that on average the machines are busy but day-to-day flow problems might result. For that reason, we encourage building a collection of daily part mixes. Each mix should be a collection of part quantities that might be seen in a typical day, with different mixes covering all possible variations. Each mix can be tested against the proposed flexibility plan to evaluate how the cell will operate over a single day.
Another major component of the flexibility plan is how multiple processes will be handled and tracked. In our experience, using the cell controller to track each processes individually provides enormous benefit. When the cell controller knows about each process, the system can run lean by scheduling a specific quantity since the cell controller can properly count completed parts. Instructions for the load/unload operator are accurate because the system knows exactly what should happen at the load station. Individual material can be tracked via a scribed or printed barcode across multiple processes. By using SeedTactic: OrderLink, the data on parts and processes is automatically sent into the cell controller, eliminating tedious manual data entry.
FMS Insight provides helpful assistance in tracking material as it transitions between processes. FMS Insight assigns each piece of material a unique serial and records a log of events as that piece of material flows through the entire manufacturing process. FMS Insight supports all types of multiple process configurations: multiple processes on separate pallets, multiple processes in separate cells, multiple processes on a single tombstone-type fixture, and more. Within FMS Insight, you define a collection of queues to hold in-process material. FMS Insight tracks material between the pallets and the queues, displays the contents of the queues, and provides instructions to the operator on moving material between the pallets and queues. (The full details on FMS Insight's material handling assistance is explained in the material tracking documentation.)
For example, consider the common scenario where a part has process 1 on pallet 5 and process 2 on pallet 6. When a completed process 1 part arrives at the load station on pallet 5, the part is unloaded onto a transfer stand. Pallet 6 is then brought over to the load station and the part is loaded from the transfer stand onto pallet 6. In this scenario, FMS Insight and the cell controller can work together to only bring pallet 6 to the load station if there is a partially completed part on the transfer stand. That is, FMS Insight will track if the transfer stand queue is empty or not and store the serial of the in-process part on the transfer stand queue. If the transfer stand queue is empty, FMS Insight will inform the cell controller and no pallet will be brought over to the load station. Once a process 1 part on pallet 5 is unloaded, FMS Insight automatically adds it to the transfer queue and informs the cell controller to bring over pallet 6. All of this happens without manual operator control, and in addition FMS Insight can provide the correct load and unload instructions to the operator.
As another example, consider the situation where multiple processes are on a single tombstone-type fixture on a single pallet. By programming each process separately, the cell controller and FMS Insight can track each face of the fixture with unique serials for each piece of material. The cell controller will know if the face is empty or full, properly count completed parts, and run the correct programs based on material that is loaded. When the pallet visits the load station, FMS Insight can properly track serials of parts as they are moved from the process-1 face of the tombstone to the process-2 face of the tombstone. The system can run lean with daily part quantities since the cell controller can count completed parts and properly run a pallet empty by not loading process 1 onto the tombstone but still running the remaining processes. For this to work, each process must be programmed individually. For standalone manually controlled machines, it is typical to write a single program which machines all faces of the tombstone. In our experience, whenever part-programmers attempt to use a single program for the entire tombstone in an FMS, the automation is prevented from working properly. The operators must constantly fiddle with and override the automation by manually controlling pallets which drastically reduces the OEE of the system. Programming each process individually might require more tool changes and a slightly longer machine time, but the overall OEE of the system is improved since the automation and part tracking can work properly.
The flexibility plan describes how the parts and their processes are assigned to pallets and machines. A stable flexibility plan provides high machine utilization across the entire range of possible daily part mixes. The flexibility plan includes the strategy for handling in-process material between processes, and describes a technique which empowers the automation to work without manual operator interaction. The flexibility plan can be developed using our SeedTactic: Planning and SeedTactic: Designer software and implemented on the factory floor with our SeedTactic: OrderLink and FMS Insight software.