Milk Run and Mizusumashi: Supplying the Line on a Fixed Cycle
In the future state the line no longer fetches its own material, and the forklift no longer drops off a whole day’s supply. A tugger train runs a fixed route on a fixed cycle: it brings material from the supermarket, takes empties and kanban cards back and often delivers the next heijunka card to the pacemaker. In the Toyota Production System the driver is called mizusumashi, a water strider insect, known in English as the water spider.
External and internal milk run
The term comes from the milk truck that visits fixed farms in a fixed order. Externally, a truck on a milk run picks up from several suppliers or delivers to several customers; on the value stream map it is a shipment with a frequency, such as “daily, milk run” at the customer in the steering bracket example.
Internally, the milk run is a tugger train route between supermarket and line. It replaces trips on request with a timetable: every stop is visited on every round, even if little is needed there at the moment. This makes material supply as predictable as production itself.
What the mizusumashi does
- Picks material from the supermarket, with withdrawal kanban as the order.
- Presents material at the point of use in small containers, within easy reach and in a fixed quantity.
- Takes empties and kanban cards back and puts the cards in the kanban post at the supermarket.
- Hands the next card from the heijunka box to the pacemaker and picks up the finished quantity.
- Reports problems: missing parts, full buffers, breakdowns. The driver sees the whole route and notices problems early.
Route cycle time and utilization
The route cycle time is usually the pitch, the time in which one container quantity is consumed at takt time. A round consists of travel time and handling at each stop. It has to fit into the cycle time with a reserve, otherwise the next stop is served too late.
How much utilization is right?
Plan for 70 to 85%. The reserve absorbs waiting at stops, breakdowns and occasional extra trips. Above 100% the driver cannot complete the round and the line waits. The fixes are fewer stops per route, shorter handling (containers within easy reach, cards in fixed slots) or a second tugger train.
Below 50% it pays to combine routes or give the mizusumashi other tasks, such as preparing changeovers. Do not simply lengthen the route cycle time for this: every extra minute of cycle time means more inventory at the line.
Line-side inventory and kanban
The point of use only needs enough material to last until the next round, plus a reserve. A card that is freed shortly after the stop waits until the next round and comes back filled one round later. Replenishment lead time through the tugger train is therefore up to two route cycles.
In the editor
In Pro mode you find the symbol “Route train (mizusumashi)” in the palette under Material. Enter route cycle time, stops, travel time, handling per stop and containers per round. The symbol shows utilization, orange from 85% and red above 100%; the checks on the right report the same. Connect the supermarket to the route train with pull and the route train to the consumer with a delivery. Lead time and timeline do not change, because the route train sits alongside the material flow.
Practice in the editor
Open the template “Steering bracket (future state)”, switch to Pro mode and click the route train. Increase the stops to 5: at what number does the check report utilization that is too high?
Open the template in the editor Start blankFrequently asked questions
What is a mizusumashi?
The driver of a tugger train in the Toyota Production System, literally a water strider, in English the water spider. The driver supplies the line from the supermarket on a fixed cycle, takes empties and kanban cards back and hands heijunka cards to the pacemaker.
What is the difference between a milk run and a tugger train?
Milk run describes the principle: fixed route, fixed cycle, several stops. Externally a truck runs it between suppliers or customers; internally a tugger train runs it between supermarket and line.
How long should the route cycle time be?
Usually as long as the pitch, 20 min in the steering bracket example. Shorter cycles reduce inventory at the line but increase the number of trips and therefore the driver’s utilization.
When do I need a second tugger train?
When utilization is above about 85% and handling or stops cannot be reduced further. Then split the stops across two routes instead of lengthening the route cycle time.