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?”
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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.
Even within a facility where conveyance distances are relatively short, the time-fixed milk-run conveyance can thrive with the help of water spiders. The water spider is a quirky piece of terminology that refers to a person who helps keep materials and processes flowing within a lean operating system by following standardized work, which involves material replenishment and other potential interruptions to the line. An article in Gizmodo titled “The Secret Lives of Amazon’s Elves” mentions water spiders who work at the fulfillment facility for Amazon.com:
“Cherie was mainly a packer, putting items in the box and scanning them. Chris, on the other hand, was a ‘water spider.’ He explains, ‘A water spider is responsible for keeping all the packers supplied, so ideally they’d never need to stand up and leave their station to get any other supplies like all the different sizes of boxes, plus making sure their tape machines and paper-spitter machines are operating.”
Role of the water spider
The main job of the water spider within a production facility may be material replenishment. It’s true that we should be careful not to treat the water spider like a “floater” or fill-in person, introducing too much variability into their cycle and making it impossible for them to follow standard work for material delivery. Instead, the tasks assigned are specific and time-based but may include quasi-supervisory tasks such as updating of status and visual metrics, keeping an eye on new workers and providing ad hoc training, and even doing pitch-based work such as sealing and palletizing a box containing multiple finished units. So long as these tasks do not keep the water spider from moving materials to and from the line (doesn’t break their standard work) their work can include whatever makes sense.
It is not a problem to use water spiders for material replenishment only if this is how their standard work is designed and this is what the process requires in order to maintain flow and stay on pace. However, limiting the work of the water spider to only material replenishment or removal of trash and containers is not a good idea. That would be like making doctors responsible for fixing illnesses but asking them not to get to know their patients, or talking about preventive medicine, or healthy living. The water spider needs to do whatever is required and is part of their timed standard work to maximize value-added work done by people on the line or working within the main process flow. This should include some up-close-and-personal time with the people and processes.
This same thinking can apply to any process that is team-based and has a workflow dependant on minimizing interruptions. The reason restaurants have waiters, hospitals have various roles in support of physicians, engineering teams have program managers, and race teams have pit crews is the same: The isolation of nonvalue-added work to one person or a specific support team boosts the overall effectiveness of the primary process flow.
Water spider as front-line supervisor in training
Within the lean production system, the water spider is a gateway position toward front-line supervision. As water spiders gain expertise in managing the movements of goods or information through the production system, they become capable of taking on more leadership abilities in terms of identifying and solving problems. After making a few cycles around a problematic line, while doing everything she or he can to maintain takt time, the water spider gains the insights that the engineer or manager sitting in the office can only dream of. In other words, people who perform the water spider tasks learn enough about the entire process, the people, and how to solve problems, that they become excellent candidates for team leader and group leader positions.
In my view, it’s a shame not to use the water spider position as a people development opportunity. In some cases, there may be no time within the cycle or prescribed standard work to do more than move materials in and out. In this case, the simple solution may be to call material handlers who do only parts replenishment just “material handlers” or “cell supporters” and not water spiders. This may seem like a minor point, but any time we water down a lean system we create the risk of having a “fake lean,” and contribute to the conclusion that lean has failed, doesn’t work, or isn’t respectful to people.
Just as painting the floors is not a 5S action, lines on the floor marking storage space aren’t part of a kanban system, and a red lamp without an escalation system and supporting local staff is not an andon system, a person locked into only material handling is not a water spider.
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