Kanban replaces schedules and expediting with simple signals that make each process produce only what the next one has used. How to size a two-bin or card system, set the rules and avoid the mistakes that break it.
A component store held three weeks of fasteners, labels and packaging, and the assembly line still stopped twice a week because something ran out. The problem was not the amount of stock. It was the replenishment system: a weekly count, a spreadsheet and a buyer who ordered based on memory. When the team replaced it with a two-bin kanban, so an empty bin was the reorder signal, stock on hand fell by about half and the stoppages stopped.
That is kanban at its simplest: a visual signal that triggers replenishment of exactly what has been used.
Push versus pull
In a push system, a schedule tells each process what to make, based on a forecast. When the forecast is wrong, which it usually is, inventory piles up in some places and runs out in others. In a pull system, each process replaces only what its customer, the next process or the end customer, has actually consumed.
Kanban is the most common way to create pull.
Two common forms
- Two-bin kanban: two containers of a part at the point of use. When the first bin is empty, it is sent for refilling and the second is used. Ideal for low-cost, high-use items such as fasteners, labels and consumables.
- Card kanban: each container carries a card stating the part, quantity, supplier and location. When the container is used, the card goes back to the supplying process or supplier as a production or purchase order.
Sizing the system
A simple starting formula for the number of kanbans is:
number of kanbans = (average demand × replenishment lead time × (1 + safety factor)) ÷ container quantity
For example, if a line uses 400 parts a day, replenishment takes two days, the safety factor is 20% and each container holds 200 parts, you need (400 × 2 × 1.2) ÷ 200 = 4.8, so five kanbans.
Start conservatively, then reduce the number of kanbans as replenishment becomes faster and more reliable.
The rules that make it work
- Produce or order only when there is a kanban signal.
- Produce or order only the quantity on the kanban.
- Never send defective parts to the next process.
- The kanban always travels with the material.
- Reduce the number of kanbans over time to expose and solve problems.
Common mistakes
- Starting with the most complicated, highest-value parts. Start with simple, frequently used items.
- Ignoring demand changes. Review kanban quantities when demand shifts significantly.
- Letting people bypass the system "just this once" with expedited orders.
- Not fixing the reasons behind long or unreliable lead times, such as long changeovers. That is why SMED and kanban so often go together.
- Treating kanban as a stock-control system rather than an improvement tool.
Where to start
Pick a store or supermarket feeding one line, with a family of simple parts. Set up two-bin or card kanbans, train the people involved, run it for a few weeks and measure stockouts, inventory and time spent counting and expediting. Then extend it upstream, towards suppliers and feeding processes, step by step, following the future state on your value stream map.
