I received pair of questions about lean logistics over the past few weeks that prompted this article. The questions were “What is the milk run method?” and “What is the role of the water spider?” 

The milk run

The milk run is an example of time-fixed, quantity-variable replenishment of materials. This means that delivery is based on a timed schedule regardless of actual usage. The milk man comes every morning and replaces empty bottles with full bottles. The timing in this case is fixed at daily, but the quantity may vary; if no empty bottles are returned, there will be no milk bottles dropped off. The time-fixed milk run is used when distances between processes make frequent material delivery impractical.

The opposite method—quantity-fixed, time-variable—works best when conveyance distances are short and processes are clearly connected by pull (not connected by flow). The reason for quantity-fixed conveyance may also be due to large lot size production upstream caused by changeovers or mixed models.

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