Flow can be hard to see when products have a numerous options, variations in both lead and cycle times, and intertwining between multiple processes. A high variety of products may make it difficult to determine product families. It is hard to tell which equipment can be dedicated to a product family and which must be shared across families. A high mix of products that share the same line or equipment also makes it harder to schedule. Complex material requirements planning (MRP) schedules often lead to higher inventories, which in turn lead to longer lead times, missing parts, increased costs, lack of control, and constant firefighting. Visibility on the floor becomes impossible, and managers can’t tell how the factory is performing.
Visual Flow with VSM
One of the best tools to see flow in factories that have a high mix of products is value stream mapping. Value stream mapping is a visualization method that allows an operation to map the flow of value from raw material to the customer. It is performed by identifying product families, mapping the current state, then developing a future state map using the 10 questions for mixed model flow:
- Do we have the right product families?
- What is the takt time at the pacemaker?
- Can the equipment support takt time?
- What is the interval?
- What are the balance charts for the products?
- How will we balance flow for the mix?
- How will we create standard work for the mix?
- How will we set the pitch at the pacemaker?
- How will we schedule the mix at the pacemaker?
- How will we deal with changes in customer demand?
The Importance of Takt Time
While the key to planning mixed model manufacturing is selecting the right product family, takt time is the heartbeat of a lean system. Takt time is the customer demand rate, meaning, how often the customer requires one finished item. It is used to synchronize the pace of production with the pace of sales, particularly at the pacemaker process – the point in the value stream at which the operation schedules work. It guides an operation, telling it how quickly it should produce.
In mixed model manufacturing, the best situation is to have every product in the mix be balanced to a cycle time close to the takt time at each station. If this were the case (and an operation should strive to
obtain this situation), it would not matter which product the customer ordered. All products could be made in the same amount of time, at the same rate, using the same number of people. All products would march through the cell at the same rate.
No Balance = Bottleneck
However, this may not occur due to the work content for some products, making them difficult to balance with others. Remember that an operation should try to group products that are within 30 percent of each other in terms of work content. With more than this variation, some products may exceed the takt time needed. Some may also have machine cycles that are fixed, which may cause them to exceed takt time.
A process that cannot meet takt time is called a bottleneck.
Pictured: operator balance chart (OBC)
One method for balancing work for flow is to use an operator balance chart (OBC). This tool helps balance work content per operator to takt time and, as a result, create continuous one-piece flow between operators at the pacemaker.
To learn more about creating mixed model flow that will increase on-time delivery, download our free white paper 10 Questions to Answer Before Designing Mixed Model Value Streams.