JIT is acronym for Just in Time. It is created in Japan at 1900’s when Japanese tried to compete with Americans. Japanese labor productivity was only one-ninth of the Americans so they had to do something differently. Most of the JIT practices comes from Toyota.
Toyota wanted to compete with the Americans but could not do it by mass production because domestic demand wasn’t high enough.Toyota decided to focus on two main pilars:
- Just-in-time, produce only what is needed
- Autonomination, automation with a human touch
The idea for JIT philosophy was invented from supermarket. In supermarket customer gets what he needs in time when he needs and the amount he needs. This was then transformed to model where workstation is customer and it is served from previous workstation when needed. Of course in supermarket shelves are filled in bigger lots than 1 and shelves are not fulfilled after every client. In factory this kind of action needs very smooth operations.
Flawless and smooth actions are driven by automation. It was meant to create automated machines that can independently detect problems and one worker could operate multiple machines.
JIT philosophy can be summarized as seven zeros:
- Zero defects. Production process must go on so there can be no faulty parts. Because there is no excess inventory all errors means delays. Product can’t wait for end of the line to be checked, every part leaving the workstation must meet the quality standards.
- Zero (excess) lot size. In JIT philosophy lot size 1 is the goal.
- Zero setups. Biggest reason for big lot sizes is time spent for setups. In order to make lot sizes of 1 happen we must try to reduce setup times.
- Zero breakdowns. Because of no excess work in process (WIP) unplanned breakdowns or operator unavailability can not be tolerated.
- Zero handling. When parts are made just in time just for need excess handling is not required. If we will do more handling than necessary it will cost us time and we need more inventory and sooner. In optimum circumstances parts go directly from workstation to another.
- Zero lead time. In perfect circumstances workstation gets its parts just when needed. This means zero lead time for previous station.
- Zero surging. When JIT is working perfectly and parts are produced only as needed product flow is smooth as long as the production plan is smooth. If there are sudden changes (surge) in quantities etc. process is forced to stop. Because no extra capacity or WIP there is nothing else to do.
In practice all of these are impossible to happen but they still are the backbone of the JIT philosophy. They describe the idea, conception behind the JIT. They are the goal that is tried to catch.
As Americans were trying to optimize lot sizes between setup and storage costs Japanese were trying to eliminate setup costs and therefore pass the whole problem. Correspondingly in purchasing Americans accepted what vendors offered when Japanese were trying to negotiate long term contracts and operated with fewer vendors in order to minimize lot sizes and deliver more frequently. Also as stated before Americans had inspection procedures when Japanese demanded quality from the vendors and own workers.
JIT in practice
As stated in zero surging JIT needs smooth production plan. Demand must be smoothed. Going back to supermarket if all the customers would come in the same day and wanted to buy same products would these products be sold out? Yes they would. Fortunately demand is distributed for different days and products. So in practice there is almost always those products that customer wants to buy.
In production plant this means that if we know that demand is 1000 units in certain period of time. 500 of them is A product, 250 B product and 250 C product. Then we should schedule our production plan to be like this:
This requires a lot from the production line as next product is always different. That’s why setup times are not allowed.
When minimizing the setup times we must first isolate internal and external setup times. Internal setup times are those that can be made when machine is stopped. Surprisingly many things can be made or prepared when machine is running and these are called external setups. Also good question is can we prepare something so that it could reduce the idle time.
Toyota also invented job rotation that was intended to produce multi-skilled workers in order to ensure smooth production and generate development ideas. First workers were rotated in different jobs and when they learned multiple jobs daily rotation were started. Rotation was meant to keep multi-skills alive and reduce workers boredom. This job rotation played large part lifting the Japanese labor productivity in the same level as Americans had.
In order to work JIT need good quality. Quality control changed from inspection procedures for controlling all parts in the production. If workstation detected errors the whole line was stopped because there were no excess inventory from where to take the next part. Also no one else is repairing errors in Japanese production. Product is sent back to the original worker for repair.
This made overall quality rise into the whole new level. Detecting errors is much easier when they are interfered immediately and there is now possibility to produce plenty of faulty products.
Here is summarized seven quality practices in JIT:
- Process control. Workers themselves control quality and make necessary changes.
- Easy to see quality. Quality can be displayed with boards, lights and other gauges.
- Insistence on compliance. Workers were encouraged to demand quality. If supplier delivered faulty parts they were sent back. Likewise faulty parts were not tolerated in the production line either. Attitude was that quality comes before throughput.
- Line stop. Because quality came first every worker had the authority to stop the line. In some plants there were board in visible location where whole production line was shown in lights and everyone could see what was the status of the line.
- Correcting own errors. As opposed to the Americans rework lines Japanese required everyone to fix their own errors. Workers had full responsibility of the quality.
- 100 % check. It was intended to check all products, not just some random samples. Inspections were made automated and simple. If 100 % inspection was not possible Japanese use the method of n = 2 where first and last part of the production batch were inspected. If they were fine it could be assumed that the machine was not out of the adjustments and the whole batch were fine.
- Continual improvement. Japanese looked to the future and their goal was zero defects. It means that there is always room for improvements.
The ideal JIT production works only on perfect conditions. To be able to survive from real world errors (machine breakdowns, faulty products) we should have backup capacity. This can be achieved so that we don’t plan to run factory 24 hours a day. Example we have only two shifts and we are prepared to do overwork.
One way or another we should be able to increase production. Because our line is already as smooth and quick as possible there is not any capacity left and the only way to add production is to add production hours. Also we should schedule some time to do planned maintenance and repairs which means we can’t run our factory 24/7.