A water spider or ‘mizusumashi‘ in Japanese (see our listing of Japanese Lean terms), is a person who has a prescribed set of tasks to keep materials in stock at the point of use in production areas. (Note that the water spider is alternately called a water strider.)
This differs from a material handler in that the sequence of operations and the way the tasks are performed are specified.
A water spider’s purpose is to handle the replenishment tasks so that the production personnel can focus on the value-added tasks that create products.
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Remember, a water spider has a standardized process. They should make their rounds the same way, and perform replenishment tasks identically each and every time. They take finished goods from the work area, drop kanban cards, refill bins from central locations, and remove waste materials. All of these tasks keep the operators in their work areas, and keep production flowing.
To best use a water spider, the proper systems are critical Operators must have a location for placing empty parts bins, visual controls for when they have work for the water spider, and a designated location (5S) for finished goods.
Mizusumashi help reduce variation and improve flow. When operations become increasingly Lean, there is little spare time, and thus little room for variation in work sequences. Simple intermittent tasks like refilling parts bins, emptying wastebaskets, and taking kanban cards to a kanban post can be disruptive to an operator’s standard work.
There is a bit of waste involved in a water spider’s rounds. They often go out of their way to check a work area that doesn’t have any shortages. The net benefit, though, is that the buffer time needed for all the other operators goes down. When an operator handles his or her own parts resupply, he needs a bit of extra time built into the process, even though it is not needed on every cycle.
Depending on the size of the work area and the material demands, a water strider may not perform that role full time. Their rounds should still be made at regular intervals, though, to keep operators from running out of parts.
In larger operations, water spiders frequently support multiple work areas. This makes standardization even more critical, as they will have multiple leaders to coordinate with. Standardization will help keep the water spider from being caught in the middle of turf wars.
During their rounds, a water spider should do only their prescribed tasks. There is a tendency for managers to see the role of a water spider as secondary to production. This often leads to bosses assigning additional tasks to the water spider, creating a risk for stock-outs on production lines. Don’t treat the water strider as a floater, or as an excess person. When buffer time is removed from line operators, they will not be able to maintain proper material levels in their cell. The timely support of the water spider becomes critical.