The targets of the Just-In-Time are called the seven zeroes, which aim to reach ‘zero inventories’ within a manufacturing plant.
a) Zero defects: To avoid the disruption of the production process in a JIT environment where parts are pulled by workstations only as they are demanded, it is essential that parts are OK in terms of quality. A defect will always cause a delay, therefore, it is very important that every part is perfectly manufactured at the first time.
The source of quality is the machine operator never the quality inspector.
Self-controls are widely used to assure this fact.
b) Tend-to-zero lot size: The objective is to replenish stock taken by a
downstream workstation as it is used. Since the downstream workstations is supplied by parts of many kinds, maximum responsiveness is maintained if each workstation is capable of replacing parts one at a time. On the other hand, if the workstation can produce parts only in large batches, then it may not be possible to replenish the stocks of all the parts quickly enough to avoid delays.
c) Zero setup: The reason for large batch sizes in manufacturing industries is the existence of long-lasting changeovers to new formats. Small batch sizes would lead to more frequent setups and, hence, decreasing machine capacity. That is why, eliminating setups is a condition for achieving lot sizes of one.
d) Zero breakdowns: Without excess of WIP in the system to buffer machines against breakdowns or others, will rapidly stop production throughout the line. Thus, an ideal JIT environment cannot tolerate unplanned machine failures. This is obviously the ideal situation which will never occur but that it is a reference to which companies should aim.
e) Zero handling: the more a product or material is handled, the more probability to create defects and scrap. No extra moves can be tolerated than the ones strictly needed. The ideal situation is to feed the material directly from workstation to workstation with no intermediate operations or movements. Any additional handling will be considered as additional waste.
f) Zero lead time: the perfect and ideal situation is when a downstream workstation requests parts and they are provided immediately with no inventory at all. The goal of zero lead time is very close to the core of the zero inventories objective.
g) Zero surging: In a JIT process, where parts are manufactured only when needed, the material flow through the workfloor will be smooth as long as the production plan is smooth. If there are changes (surges) in the quantities to be produced or new product mix, consequently, as there is not any excess of WIP which can be used to level it, the system will be forced to respond automatically.
Obviously, these seven zeroes are not achievable in practice but they are the perfect guide for designing a process without waste and optimizing lead times and inventories.
Source: Wallace J. Hopp. Origins of JIT, 1996.