Why does lean treat stock as waste?
Inventory is one of the eight wastes of lean. Stock ties up cash and space, ages, gets damaged and lost, and, most importantly, hides problems. Large buffers let a machine breakdown, a late supplier or a high scrap rate go unnoticed because there is always something on the shelf. Lean stock lowers the level on purpose so those problems surface and get solved.
How do you make stock lean?
- Map where stock sits. A value stream map shows every pile of raw material, work in progress and finished goods, and how many days each represents.
- Pull instead of push. Replenish from consumption, not from a forecast. A kanban card or empty bin tells the supplier process what to make or move.
- Pace to demand. Use takt time to set the rate, and move to smaller, more frequent batches and deliveries.
- Set a minimum, maximum and reorder point for each item, sized from usage and replenishment lead time, and put it where people can see it.
- Organise the store. 5S with marked locations, first-in-first-out lanes and min/max markers makes shortages and excess visible at a glance.
- Lower it step by step. Reduce stock gradually and fix whatever the lower level exposes before reducing again.
Lean stock vs safety stock
Lean does not mean zero stock. Just-in-time means the minimum a reliable process needs. Where supply or demand is genuinely unstable, a deliberate, named buffer is better than an accidental one, with an owner and a plan to shrink it as the cause is fixed.
Example
A fabrication shop held about six weeks of fasteners and fittings, yet still stopped lines for missing parts. The team counted the stock, split it into fast and slow movers and moved the fast movers to two-bin kanban stations at the point of use. Each bin held a reorder quantity sized from weekly usage and supplier lead time. Slow movers went to a single marked shelf with a maximum level. Within a couple of months stock fell by roughly a third and stoppages for missing parts dropped, because empty bins now triggered orders before a shortage. (Illustrative example: the figures are typical, not from one named site.)
Measures to watch
- Days of inventory on hand, by category
- Stock-outs and line stoppages for missing parts
- Stock value against the agreed minimum and maximum
- Slow-moving and obsolete stock