The Mizusumashi (みずすまし), translated as “small Train” or “waterSpider”, literally means in Japanese ” Make clean water ” or ” purify Water”. In its function of water purifier (in Japan, the ” Mizusumashi ” is a floating water purification system that helps to avoid the development of algae), the small train allows:
- Better to drain the flow by removing stops for lack of parts.
- Does not allow the arrival of stock.
- Removes tasks from the value-added streams.
This concept, developed in the years 50 by Toyota at the beginning between factories1, consists in bringing together in a function all the supplies to the positions avoiding the movements of the operational to fetch its parts. This function is done via a ” small train “. Its role is to ensure the proper functioning of the workshop by supplying the requested parts, where they are requested, and when they are needed.
The requested parts
The train is not a deported stock that would contain all the references potentially used by the production line. The train must have only the parts requested in the previous ” lap ” via most often a Kanban system. In other words, the train must unload at 100% the components transported at each cycle.
Where they are requested
The Mizushumashi allows to concentrate the transports Muda and part of the Muda of movements carried out by the operators. It is the driver who performs all the tasks “unproductive”: Opening of cartons, installation of the parts as close to the operator, recovery of empty cartons… And thus reduce the variability generated by these Muda at the operator level. The challenge is not to move the problems to another function but to concentrate them to master and improve them.
When they are needed
The objective of the small train is to deliver the lines via a service rate of 100%. This does not indicate that the stock of the workstation must be permanently full, but simply that the Mizusumashi supplies the necessary fair supply with regularity.
The loop of the little train
In its design, the train connects in the same loop a set of suppliers (shop, injection shop, welding workshop…) and Customers (final assembly, painting…). The train then functions as a metronome, regularly and previously defined (30mn, 2h…) All the client needs to pass it on to the suppliers.
This loop consists of the following steps2 :
- Wait for instructions and start time at the station
- Take the Parts
- Take the parts to each production cell
- Take the finished parts of each production cell
- Take the finished parts to the customer stations
- Return to the station
It should be noted that the loop should plan to take the finished products regularly to the preparation area for loading the trucks. You should not use the last workstation as a customer storage.
We are talking about Chorobiki : This principle consists of unloading the last Position regularly to fuel the area of preparation of the delivery trucks rather than to deliver in a single shot just before the departure of it.
The working standards of the small train
The small train is a function like any other and requires the setting up of standard work to improve the performance.
Like a real transport truck, the train breaks down into several trolleys designed specifically according to the products and a loading plan is found:
- The different cars numbered according to their hanging
- The references to transport, their layout and their UC
A standard document covers all the tasks to be taken from the parts, during circulation and in the delivery of the parts. We will find in particular the tasks to be performed at each stop, the instructions of the packaging units (stacking…), the speed of the train…
Circuit or loops
The train continuously follows the same delivery/loading path. This circuit requires a dedicated circulation space to avoid incidents, and must be readable on the factory plane. We find:
- The direction of circulation, designed according to the factory and the other interactors of the factory (forklift operators, operator…)
- Train station, place of train parking
- The stops of the train: Each stop is materialized on the Gemba by a panel and must be located closest to the need
Like a real train, the Mizusumashi has a precise schedule of departure times of the station. Placed on a steering board of the train at the departure station or on the Heijunka Box, it indicates the departure times and a space must be left free to share a problem (delay…).
This schedule must also take into account the related elements:
- Driver Break Time
- Scheduled Maintenance
- Recharging Batteries
- At the time of departure indicated in the schedule, the train starts and goes to the suppliers to recover the needs requested in the previous round (empty boxes/cartons).
- At the supplier, the train collects the necessary parts by making an empty/full exchange and then goes to his client.
- At his client, the train supplies the workstation with the supplier’s parts and recovers empty boxes/cartons, thus giving it the ” need ” for the next round.
- Once the customer (s) are delivered, the Mizusumashi returns to the station and waits for the next round.
- This raises the question of the autonomy of a line. Between each delivery there is a complete cycle time. During this time, the line must have the necessary stock to continue to produce. Therefore, it is understood that at the most the duration of the cycle is low at the most one limits the risk of failure of stock and at least there is stock of stocks.
- In the case of small rooms where the size of a CU is much higher than the demand (hardware…), the train can be used as a rolling shop. At each pass the driver will be able to supply the line stock himself.
- In the case of very large parts, where the necessary space on the line does not guarantee the autonomy, one must have at all times the equivalent of the consumption of a cycle on the train.
The flow of information
To ensure the operation described above, the information standards must be structured and durable. In particular, the information of the containers to be supplied on the customer line must be preserved until loading at the supplier. For this, several solutions are possible:
- The empty container itself is the information. It is so designed to easily understand what the product is and what the quantity is. A design via Poka-Yoké Here is widely advised. This also requires that unloading of empty elements and loading of solids be in the same place at the supplier to avoid errors.
- The information is transcribed via a Kanban system. The client returns Kanbans at the same time as the empty items. During the exchange at the supplier, the Kanbans are then repositioned on the full CUs.
Train and circuit design
If the project is validated and using product-related data (reference, hourly consumption, CU size…), we can determine:
- Identify the lines that need a small train.
- Identify the parts to be delivered on each of the positions.
- Identify the traffic plan and validate the number of small trains according to the distances, the number of parts and the supply cycle times.
- Secure spaces, set up line edges and supermarkets
- Size the small trains (length, loading plan…) According to the handling times, the cycle times of the small train.
- Design the different standards: circuit, loading plan…
Depending on the objectives and the means, we can improve the ergonomics and reduce the wastes with the following principles:
- At the store, emptying is done in the same place as the loading of the full
- Do vacuum tests with the train to check the turns and improve traffic.
- The heaviest containers are at median height on the cars.
- The loading plan is optimized to reduce travel.
- The picking is arranged according to the loading plan adopted.
- The defective dynamics of the customer lines (Return of empty bins) are repaired.
It is also possible to design automatic Mizusumashi (called AGV for Automated Guided vehicles):
Several indicators are used to track the performance of a Mizusumashi. We first find the service rate. This must be the closest to 100%.
Then we find:
- Performance related to handling = handling time/cycle time. This indicator makes it possible to know the ratio of the handlings in relation to the cycle time, the challenge being to minimise the handlings.
- The performance related to the movement of the train = travel time/cycle time. This monitoring will be able to highlight in particular the variability in the movements and therefore understand if problems related to the circuit are present.
- The filling rate of the train = average volume transported by cycle/volume available on the train. Although the target is 100%, the train is considered to be properly used from 50%. Shorter, it will probably be necessary to lengthen the cycle times to increase the volume used.
Generally, it is noticed that the train carries a stock of each component reference and at each crossing on the line, it supplies a bit of everything. The operating problems are:
- Disorganization of empty/full operation causing variability in cycle times…
- Increases the volume transported by the train generating: additional wagons, ergonomics problem…
- Loss of control of the stock and therefore potentially increase of it.
1 – J. Nomura, S. Takakuwa (2006) – Optimization of number of containers for assembly lines: The fixed-course pick-up system
2 – M. D. Rossetti, R. R. Hill, B. Johansson, A. Dunkin, R. G. Ingalls (2009) – Simulating and applied model to optimize cell production and parts supply for laptop assembly
C. D. Lewis (1970) – Scientific Inventory Control
M. Baudin (2004) – Lean Logistics: The nuts and bolts of delivering materials and goods
T. Sakikawa (2012) – Transforming Japanese Workplaces