Production Scheduling – Develop Flow
This is the third post in our series discussing the 7 lean manufacturing guidelines.
In this post, we’ll cover why it’s important to design your lean future state to continuously flow wherever possible.
What is flow?
Flow is manufacturing production that is done one piece at a time from process to process without stopping. The lean manufacturing ideal state is to have a one piece flow with a batch size of one.
Using the wrong manufacturing performance metrics
CFOs often want to look at asset utilization and labor efficiency rates. However, in a manufacturing operation, optimizing for these two metrics can be a mistake because you have to:
- Process large batches that reduce the downtime required for changeovers.
- Isolate each process so it can run independently.
- Create an inventory buffer so there’s always work for the operator and machine to do.
Optimizing for efficiency frees the operator from any upstream interruptions but at quite a cost.
What’s the downside of large batches?
When you are running large batches, instead of focusing on flow, you will experience:
- Increased inventory – You not only have more inventory holding costs and cash flow tied up, but you increase the risk of inventory obsolescence.
- Longer lead times – To hold all the inventory buffers needed between processes, lead time is extended. Longer lead time makes it harder to move to make-to-order (MTO).
- More product quality problems – When large batches are produced, problems are also produced in large batches. Often defects aren’t discovered until later on in the overall process, which results in more scrap and/or rework.
- Process problems remain hidden – When every process in flow must stop for any issue, the visibility is immediately raised.
Moving to flow instead
Moving to flow reduces inventory and lead time and can contribute to reductions in scrap and rework. Flow forces a “horizontal” view of the interrelationships between manufacturing processes rather than a more silo-ed vertical view. That’s why flow exposes all kinds of process problems, highlighting every issue that forces a stoppage anywhere in the flow line. Equipment breakdowns, changeovers, yield issues, even bill of material inaccuracies will stop a line from producing. Production is also impacted by these same issues in a traditional manufacturing environment, but the issues remain hidden behind big piles of inventory.
Using FIFO for flow
In situations where flow is not strictly possible due to machine limitations of differing cycle times or changeover requirements, flow can be implemented by using a FIFO lane to accommodate small amounts of buffer inventory in order to smooth the process. So long as FIFO lanes strictly observe the first in, first out rule, they can be classified as flow.
Production scheduling using flow
From a production scheduling perspective, flow is very simple. You just launch the work for the first flow process. The rest of the processes in flow are automatically scheduled based on the work started. Production automatically stops when the upstream process stops. When using FIFO lanes, the process stops when the inbound FIFO lane is empty or when the outbound FIFO lane is full. This process is easy for operators to understand. Production scheduling just got a whole lot easier!
As we develop a future state, we should implement as much flow as possible but then continuously look at what causes stoppages and eliminate those stoppages. Doing so will reduce inventory, reduce lead time, and increase profitability.
Stay tuned for the next segment on this series about supermarkets and pull.
Written By: Phil Coy