JUST IN TIME MANUFACTURING

Just-In-time manufacturing, or JIT, is a management philosophy aimed at eliminating manufacturing wastes by producing only the right amount and combination of parts at the right place at the right time. This is based on the fact that wastes result from any activity that adds cost without adding value to the product, such as transferring of inventories from one place to another or even the mere act of storing them. 

The goal of JIT, therefore, is to minimize the presence of non-value-adding operations and non-moving inventories in the production line. This will result in shorter throughput times, better on-time delivery performance, higher equipment utilization, lesser space requirement, lower dpm’s, lower costs, and greater profits. 

JIT finds its origin in Japan, where it has been in practice since the early 1970’s. It was developed and perfected by Taiichi Ohno of Toyota, who is now referred to as the father of JIT. Taiichi Ohno developed this philosophy as a means of meeting customer demands with minimum delays. Thus, in the olden days, JIT is used not to reduce manufacturing wastage, but primarily to produce goods so that customer orders are met exactly when they need the products.

JIT is also known as lean production or stockless production, since the key behind a successful implementation of JIT is the reduction of inventory levels at the various stations of the production line to the absolute minimum. This necessitates good coordination between stations such that every station produces only the exact volume that the next station needs. On the other hand, a station pulls in only the exact volume that it needs from the preceding station.

The JIT system consists of defining the production flow and setting up the production floor such that the flow of materials as they get manufactured through the line is smooth and unimpeded, thereby reducing material waiting time.  This requires that the capacities of the various work stations that the materials pass through are very evenly matched and balanced, such that bottle necks in the production line are eliminated. This set-up ensures that the materials will undergo manufacturing without queueing or stoppage. 

Another important aspect of JIT is the use of a 'pull' system to move inventories through the production line. Under such a system, the requirements of the next station is what modulates the production of a particular station.  It is therefore necessary under JIT to define a process by which the pulling of lots from one station to the next is facilitated.

JIT is most applicable to operations or production flows that do not change, i.e., those that are simply repeated over and over again. An example of this would be an automobile assembly line, wherein every car undergoes the same production process as the one before it.

JIT has likewise been practiced successfully by some semiconductor companies. Still, there are some semiconductor companies that don’t practice JIT for the simple reason that their operations are too complex for JIT application. On the other hand, that’s precisely the challenge of JIT – creation of a production set-up that is simple enough to allow JIT.

Some Key Elements of JIT

1. Stabilize and level the MPS with uniform plant loading (heijunka in Japanese): create a uniform load on all work centers through constant daily production (establish freeze windows to prevent changes in the production plan for some period of time) and mixed model assembly (produce roughly the same mix of products each day, using a repeating sequence if several products are produced on the same line).  Meet demand fluctuations through end‑item inventory rather than through fluctuations in production level.  Use of a stable production schedule also permits the use of backflushing to manage inventory: an end item’s bill of materials is periodically exploded to calculate the usage quantities of the various components that were used to make the item, eliminating the need to collect detailed usage information on the shop floor.

2. Reduce or eliminate setup times: aim for single digit setup times (less than 10 minutes) or "one‑touch" setup ‑‑ this can be done through better planning, process redesign, and product redesign.  A good example of the potential for improved setup times can be found in auto racing, where a Formula One pit crew can change all four tires and put gas in the tank in under 20 seconds.  (How long would it take you to change just one tire on your car?)  The pit crew’s efficiency is the result of a team effort using specialized equipment and a coordinated, well-rehearsed process.

3. Reduce lot sizes (manufacturing and purchase): reducing setup times allows economical production of smaller lots; close cooperation with suppliers is necessary to achieve reductions in order lot sizes for purchased items, since this will require more frequent deliveries.

4. Reduce lead times (production and delivery): production lead times can be reduced by moving work stations closer together, applying group technology and cellular manufacturing concepts, reducing queue length (reducing the number of jobs waiting to be processed at a given machine), and improving the coordination and cooperation between successive processes; delivery lead times can be reduced through close cooperation with suppliers, possibly by inducing suppliers to locate closer to the factory.

5. Preventive maintenance: use machine and worker idle time to maintain equipment and prevent breakdowns.

6. Flexible work force: workers should be trained to operate several machines, to perform maintenance tasks, and to perform quality inspections.  In general, JIT requires teams of competent, empowered employees who have more responsibility for their own work.  The Toyota Production System concept of “respect for people” contributes to a good relationship between workers and management.

7. Require supplier quality assurance and implement a zero defects quality program: errors leading to defective items must be eliminated, since there are no buffers of excess parts.  A quality at the source (jidoka) program must be implemented to give workers the personal responsibility for the quality of the work they do, and the authority to stop production when something goes wrong.  Techniques such as "JIT lights" (to indicate line slowdowns or stoppages) and "tally boards" (to record and analyze causes of production stoppages and slowdowns to facilitate correcting them later) may be used.

8. Small‑lot (single unit) conveyance: use a control system such as a kanban (card) system (or other signaling system) to convey parts between work stations in small quantities (ideally, one unit at a time).  In its largest sense, JIT is not the same thing as a kanban system, and a kanban system is not required to implement JIT (some companies have instituted a JIT program along with a MRP system), although JIT is required to implement a kanban system and the two concepts are frequently equated with one another.