Heijunka, production levelling
through an example

Last update: March 3rd, 2010

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Production levelling, an example

A workshops works 7 hours a day, 5 days a week, 20 days per month, manufacturing a product in one hour, product available in six colors. The market average monthly demand has following distribution:

RED	BLUE	GREEN	ORANGE	YELLOW	MAUVE	TOTAL
28	60	18	18	10	6	140

The market demand can be managed, as our load-capacity check confirmes.
Changeovers and setups are negligible.

First approach, "traditional planning"

A relatively common approach in mass production is to optimize the usage of resources, seek economies of scale with huge production batches, hence minimizing the time spent for changeovers. Another common practice is to start the month with the longest series, letting the shorter ones for the end of the month.
This happens because of the belief that bigger series are important and do not have to suffer any disruption. Frequently however, this way of doing has no other reason then to save managers some period of relief with planning complexity, while longer series are running.

The table hereunder shows the usable hours in each day (columns) for the whole month of 20 days.
Each hour (table cell) a product can be manufacured, the table allows making production planning visual.

In such a case, the client ordering a unit of each type, 6 in total, and ordering the first day of the month, has to wait the month's end (20 working days) before being delivered. All that waiting for an accumulated production time of 6 hours. The ratio usable time / total time is:

6 / 20 x 7 = 4%

A first improvement

A first improvement could be reversing the order of series; starting the month with shorter ones.

The client can then be delivered within 12 days (8 if one Red unit can be shipped as soon as manufactured), which is already a significant gain. The ratio usable time / total time is;

6 / 12 x 7 = 7% Progress +43%

To sustain it, the next month should not restart with a long Blue serie to save a changeover, otherwise the same client, with the same order will see his service rate fall again.

Reading material
HC online

Batch size reduction
Files, FIFO, LIFO and this kind of things