Just in Time (JIT): produce only what is needed, when it is needed

In one line

Just in Time (JIT) means producing and moving the exact quantity of material or product at the exact moment it is required, neither earlier nor in a larger quantity.

What it is

The Universidad Icesi paper defines it like this: producing and moving the precise quantity of materials and products at the moment they are required. It is implemented with kanban, a way of controlling the transformation and movement of materials through visual devices (cards, containers, signals), so that the quantity and location of material in the process are controlled by design.

Bellon Álvarez adds the origin and the motive: JIT was developed to increase production without increasing inventories, and out of the need to produce at low cost while saving on labor and equipment. It is tied to the other pillar of the Japanese model, autonomation (jidoka).

How it works: pull instead of push

Niebel and Freivalds explain that JIT is based on controlling production with a pull system, in which the demand for parts comes from the station further downstream in the flow, instead of a push system, in which parts are produced regardless of the needs of the system, which creates long queues and bottlenecks. The difference is developed in the article on push and pull systems.

What it takes to make it work

• Quick changeovers: Niebel points to single-minute exchange of dies (SMED) as a necessary component: Shingo's techniques for changing over machinery in under 10 minutes. Without that, producing in small batches is not viable.
• Defect-free quality: instead of the acceptable quality level typical of mass production, lean manufacturing demands a zero-defect policy for parts, because with no reserve stock a single defect stops everything.
• Leveled production (heijunka): repeating the product mix in the smallest possible interval, with the ideal of producing one unit at a time, as the Universidad Icesi paper describes. This means more material movements, so material handling has to be adapted.
• Reliable suppliers: Toyota's keiretsu, which delivers quality parts on time.

Real example

The origin of JIT at Toyota is a consequence of the 1949 crisis described in the Universidad Icesi paper: when the company came close to bankruptcy, the banks financing it required it to cut staff and to adjust the number of cars produced to the number actually sold. Producing what is sold, and only when it is sold, stopped being a theory and became a condition for survival.

In a more recent case, the article on Lean Six Sigma from Ciudad Juárez used JIT, together with line balancing and takt time, to solve missed deliveries to a customer that had gone from ordering weekly shipments to three per day.

Benefits

• Less inventory, less space and less tied-up capital.
• Quality problems become visible quickly: with no stock, a defect cannot hide.
• Shorter lead times.

Limitations to keep in mind

• It is fragile in the face of stoppages: with no buffer inventory, a machine failure or a supplier failure halts the flow; that is why it goes hand in hand with TPM.
• It requires relatively stable demand, or at least a production leveling system.
• Implementing it piecemeal (cutting inventory without cutting changeover times or defects) usually ends in shortages.

In summary

JIT is a promise between workstations: "I hand you what you need, when you need it." Keeping it takes quality, quick changeovers, reliable maintenance and a leveled flow. It is one of the two pillars of the TPS.