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“The slower but consistent tortoise causes less waste and is much more desirable than the speedy hare that races ahead and then stops occasionally to doze. The Toyota Production System can be realized only when all the workers become tortoises.
- Taiichi Ohno, 1988
Many companies today are working toward the ultimate lean goal of continuous or one-piece flow. They want to be able to make just what the customer wants when they want it. Instead, what we often see is a “hurry up, then slow down” build-to-order approach. Customers’ orders vary from month to month, creating uneven production scheduling. Build-to-order companies will be building huge quantities, paying overtime, and stressing their people and equipment one week, but then sending them home the next due to light orders. This environment can also create large amounts of inventory, hidden problems and poorer quality. What many organizations fail to do is the difficult process of creating a true balanced lean workflow. This is the Toyota concept of heijunka, leveling out the work schedule.
Heijunka is the leveling of production by both volume and product mix. This system does not build products according to the actual flow of customer orders. Heijunka takes the total volume of orders in a period and levels them out so the same amount and mix are being made each day. In a true build-to-order system, you build products A and B in the production sequence of customer orders (e.g., A, A, B, A, B, B, B, A …).
This causes you to build product irregularly. If your orders are twice as much on Monday compared to Tuesday, you end up paying overtime on Monday and sending employees home on Tuesday. The answer is to build a level schedule every day by taking the actual customer demand, determine the pattern of volume and mix, and building your level schedule. If you know you are making five A’s and five B’s, you create a level schedule of ABABABAB. This is called leveled, mixed-model production.
Figure 1 gives an example of traditional unleveled production for Company X, which manufactures tractors. The line makes small, medium and large tractors. The medium tractors are the big sellers and are made early in the week, Monday through part of Wednesday. There is a changeover and the small tractors are made Wednesday through Friday morning. After another changeover, the largest tractors, which are in smallest demand, are made Friday afternoon. This typical unleveled method creates four problems:
Customers usually do not buy products predictably. If the customer decides to buy the large tractors early in the week the plant is in trouble.
The risk of unsold goods that must be kept in inventory.
The use of resources is unbalanced.
There is an uneven demand on upstream processes.
Figure 2 represents an example of mixed-model leveled production. By reducing the changeover time and employing other lean methods, the plant is able to build the tractors in any order it wants to on its mixed-model assembly line. The four benefits of leveling the schedule are:
Flexibility to make what the customer wants when they want it.
Reduced risk of unsold goods.
Balanced use of labor and machines.
Smoothed demand on the upstream processes and suppliers.
Figure 1. Traditional Production
Figure 2. Leveled Production
To achieve the benefits of continuous flow, companies must level out the workload. Heijunka will eliminate waste by leveling your product volume and mix, but most importantly, will level out the demand on your people, equipment and suppliers. Without leveling, waste will increase as people are driven to work like mad and then stop and wait, just like the hare.
About the author:
David McBride is co-founder of EMS Consulting Group, a Carlsbad, Calif.-based engineering and management consulting firm. David has a bachelors of science degree in mechanical engineering from Ohio State University. He has a successful track record in the development and implementation of FMEA and Design for Manufacturability programs at several organizations and has greatly reduced manufacturing costs through the utilization of lean manufacturing, kaizen events and manufacturing system analysis. He has also been highly successful at developing and executing new product introduction processes, and staffing and capital equipment plans. For more information, visit www.emsstrategies.com or call 866-559-5598.