Toyota shrank every batch until defects had nowhere to hide

By the 1950s the world's dominant manufacturing doctrine was Ford-style mass production: huge batches, dedicated tooling, and changeovers so slow that swapping a die on a stamping press could tie up a line for hours. Toyota, rebuilding after the war with a fraction of Detroit's capital and demand for only a handful of units per model per day, could not afford the giant production runs the American model assumed.

Large batches were the obvious answer because every textbook and every competitor used them: spreading a slow, expensive die change across thousands of identical parts minimized the changeover cost per unit. Idle-machine time during setup was treated as an unavoidable tax on production, not a variable anyone expected to shrink — so nobody optimized for it.

Starting around 1969, Shigeo Shingo worked with Toyota's press shops to separate 'internal' setup steps, which require the machine to be stopped, from 'external' ones that can be done while it keeps running, and to convert as many internal steps to external as possible. This turned die changes that had taken roughly one to several hours into changes measured in single-digit minutes — the method he later named Single-Minute Exchange of Die (SMED).

Split die changes into internal steps needing the machine stopped and external ones done while running, cutting changeover to minutes so small batches surface defects.

Splitting setup into 'internal' steps (machine must be stopped) and 'external' steps (can be prepared while the machine still runs) and moving as much as possible into the external category means the

The method only pays off where changeover cost is genuinely the binding constraint on batch size and where a real internal/external split exists to exploit — a process with almost no external-izable s

Any operation — a factory press shop, a hospital OR, a restaurant kitchen, a software deploy pipeline — that treats setup or changeover time as a fixed cost of doing business rather than a variable it

By later Toyota Production System accounts, press die-changeover times fell from hours to an average of around three minutes by the early 1970s, which let Toyota run batches small enough that a defect surfaced within the same shift instead of being buried in a week's worth of parts. The precise plant-by-plant sequence is disputed — historians differ on how much credit belongs to Shingo personally versus Toyota's own engineers — but the order-of-magnitude time reduction and its role in enabling just-in-time, small-lot production are well corroborated across Toyota Production System literature.