Run is to Milk as Spider is to…

A pair of questions about lean logistics over the past few weeks prompted this post.

Santosh from India asked “what is the milk run method?”

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 milkman 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 quantity-fixed and 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 due to changeovers or mixed models.

Even within a facility where conveyance distances are relatively short, time-fixed milk run style 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 keeps 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

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.”

“I never quite exactly figured out why they call it a water spider. My guess is back in the history of assembly line jobs, the water spider would be the person who would bring people on the line water to drink. Nobody seemed to know!”

Onni from Finland asked for clarification about the role of the water spider:

I am trying to focus our waterspiders to do only the material replenishment and to cut out everything else. […] In our case the water spider is really busy doing the replenishment and there is no time to do any filling for the operators. I can’t see the point in any filling for the operators. What I mean is that the really important material providing work interrupts too much if the WS does stuff like that.

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 in 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 (does not 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 choosing to ask them not to get to know their patients, talk 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 time up close and personal with the people and processes.

This same thinking can apply to any process that is team-based and has a work flow depending on minimizing interruptions. The reasons 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 non value 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 towards 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 can to maintain takt time, the water spider gains insights 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 problem that arise such 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 anytime we water down a lean system it creates the risk of “fake lean”, the failure of lean and the conclusion that lean doesn’t work or isn’t respectful to people. Just as painting the floors is not 5S, lines on the floor marking storage space is not a kanban system and a red lamp without an escalation system and supporting local staff is not an andon system, a water spider is much more than a material handler. We should call things by their right names.


  1. Michael Lombard

    March 15, 2010 - 10:35 am

    Just an offshoot thought that your post provoked….
    The consolidation of waste into a single entity (person or group) is a powerful strategy that is often underutilized in organizations.
    It is a powerful strategy for at least two major reasons that I can see:
    First, by consolidating waste, we make it highly visible. Instead of imperceptible levels of waste being spread out evenly among team members, a big chunk of waste is embodied by the presence of a single individual/group. Like 5S, the consolidation of waste makes waste highly visible, which makes it susceptible to problem-solving and continuous improvement. Like with kanban, we can systematically whittle down the waste because it has been contained and controlled.
    Second, by consolidating waste, we make value highly visible. Once waste is stripped from a process, we start to see and understand value. In other words, we move past complexity to elegant simplicity, and develop a more intrinsic understanding of value. This enhanced perception of value is invaluable to an organization, as it allows for innovation and ultimately improved customer value.
    But, waste consolidation is not always understood by organizations. I think this is because it requires a long-term view (to justify the presence of a person/group who only performs wasteful activities) and an outlet for waste-reducing and/or value-adding ideas and suggestions. Some organizations only think short-term, and can not justify these long-term tactics.
    Just a few random thoughts anyways. Have a great day!

  2. Karl Arps

    March 16, 2010 - 3:41 am

    As anyone who has been canoeing in northern Wisconsin knows, water spiders run around on the surface tension of calm waters, flitting to and fro in a pattern that seems irrational but does, in fact, meet some natural plan. Thus, the “water spider” of Lean appears to the outsider to work chaotically, but is actually reacting to natural and varying needs of the value-adding team members.