September 1, 2004

By David McBride

“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."

- Ohno, 1988 -

Many companies today are working towards 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 everyday 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.

Imagine Company X that manufactures tractors. The line makes small, medium, and large tractors. The medium 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:

Company X might later decide to implement 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 they want to on their mixed model assembly line. The four benefits of leveling the schedule is:

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.

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