The Mighty Water Spider

Hi colleagues and welcome back to Week-6 of the Weekly Lean Series. Today we are going to discuss a mighty hero, the Water Spider of the shopfloors (a.k.a Mizusumashi).

If you recall the Value Stream Maps, we had two distinct flows in a Value Stream : "The Information Flow" and "The Material Flow" . Without the material flow, no one will add any value for the customer no matter how skilled they are or how productive they are. Availability of the necessary parts and materials is a key factor in efficiency, throughput and undisrupted value creation for the customer. The water spider, as the term originates from, is a capable spider that can hang between the water and the air while quickly going from place to place, keeping both of the environments in their sight simultaneously. Hence the term water spider in lean terminology is the responsible leaders and operators who carry on this flow in a shop floor moving parts and materials to required points of use.

It may seem a bit odd to add a person specifically to this role in the shop floor. First question can be wouldn't it be more labour efficient if everybody did their own part of material flow , they will also know parts better than one dedicated personnel? As it turns out, it is not. The rationale behind having such a person is to allow the rest of the personnel to devote their full attention to tasks that add value to the process. This also highlights how much transportation waste and inefficiency exists in the process by isolating it all into one or more positions.

The water spider role is very much linked to the Milkrun concept. The mikrun in a shopfloor is the frequent supply of materials from your warehouse to the point of use. This process can be initiated either as a push strategy ,as a pull strategy or a mix model. Milkrun as a push strategy will take into account the production plan, calculate required materials through BOM, check the shelf and space availablity at the lines , define frequency and execute material flow in a short interval plan setting. However, as you may understand many of the parts in a manufacturing process are common for different products they will always be required in the shopfloor. These materials can have a defined stock level and be replenished as used if the frequency of the milkrun is compatible with inventory depletion time. So some of the materials are managed via pull strategy , with established seperate or shelf supermarkets and some materials are fed to milk run as required.

Now, let's think. The milk run is a scheduled supply with defined stops at the shopfloor , but just as a bus cannot stop at every house, milk-run cannot deliver to every single point of use in a shopfloor. .If you had a bus stop at every house, this would immensely slow down the bus and it would be very inconvenient for all the passengers. Instead you can leave the bus stops at close proximity and a small distance can be covered by the residents. Point of use provider takes action in this close proximity.

At each station the worker's cyclic work is the work they are adding value for the customer; like assembly.Part of this work includes grabbing the part from a shelf or box to use for the process. The non-cyclic part of this work starts when the box or shelf is depleted. At this point, the worker has to go and pick up material, or boxes to the point-of use. ssemblers are required to step away from their station to gather a new batch of raw materials. This causes a delay in the process and is a waste as movement. The water spider can reduce this by carrying around a mini shopping cart and either with a small kanban system , an electronic trigger or via visualizing requirements; supply the materials needed by all the assemblers. This prevents assembly line workers from having to perform the non-value work, the required materials are delivered to them in a way that does not disrupt their workflows. A water spider is not limited only to this task, they can do a variety of other things too, e.g.update status boards, pack materials to be taken away, transport finished products, move Kanban cards, supervise job execution and since he/she has the time to continously monitor the whole system, is a valuable member of Kaizen activities.

How chaotic it may look the water spider is not free from standard work concept. Water spider's job is to minimize variation for everybody else on the production floor or within the process, so their task should follow a standard. The route must be fixed, they should visit stations and operators in the same order and at the same interval. The frequency should reflect the depletion times of inventories at the point of use. Neither the routes must be too excessive for the water-spider; nor the value stream be interrupted. The standard of how material is retrieved and supplied (kanban, electronic etc.) should be included in the work instruction of the water spider as well. You can also add a quick time study to understand the utilization of the water spider. With this setting you provide a professional waiter to all stations that can supply the requirements.

If you are using electronic signals to show requirements vs availability, it is more easier to route the water spider . If not , do not create an additional kanban loop between the milkrun stop and the Point of use, it may not be as beneficial as you may think. Rather use visual controls like red, green, yellow levels at point of use shelves or simple demonstrations showing the total amount that the line should have (10 boxes, 8 pallets etc.) and the water spider can quickly do the math based on existing material. It is also common to divide the milkrun materials via symbols or labels to differentiate between routine kanban material vs. specific requirements. With this way the water spider will immediately supply the non-stock items first and then focus on refill of the stations for the kanban items. Whatever your milk-run logic is, your water spider should work in the same manner.

The more non-cyclic work you add to a station the more variability you will observe in your cycle time, so even if you think you can leave this work to the operator after a comparison with your takt time, think again; the variability may cause disruptions in your process.