Just In Time Manufacturing

• Get what is needed at the time it is needed, and in the amount needed. Every workstation acquires the required materials precisely as needed, just in time.

• The benefits of JIT can be attributed to the use of WIP caps.

• The seven zeros serve as the goal of JIT. The north star is towards zero inventories — a metaphor for a high level of excellence and innovation.

  • Zero Defects - parts must be of good quality. Defects cause delays.
  • Zero Excess lot size - stock must be replenished as it is taken. We maximize responsiveness if workstations replace parts one at a time.
  • Zero Setups - because setups cause large batch sizes, we eliminate setup times to account for frequent setups.
  • Zero breakdowns - no machine failures, all operators must be available. Breakdowns cause delays.
  • Zero handling - material is given at the time required and in the amount required. The system will run smoothly if material is fed with no intermediate pauses. Handling causes delays.
  • Zero lead time - when a workstation requests parts, they are provided immediately.
  • Zero surging - sudden changes in quantities of products mean delays because the system has to respond. We want level production plans and uniform product mixes.

• No matter how well a manufacturing system is running, there is always room for improvement

• The key lessons of JIT to the broader field of manufacturing management are as follows

  • The production environment is, itself, a control
  • Operational details matter strategically.
  • Controlling WIP is important (small cycle times)
  • Flexibility is an asset. The inflexibility of JIT is compensated for by continuously improving on flexibility
  • Quality can come first (and is more profitable)
  • Continual improvement is a condition for survival. Manufacturing as a field is dynamic.

Implementation §

• JIT requires a precise and smooth operating system — small disruptions easily cascade throughout the system.

• Autonomation is used to prevent disruptions

  • Machines are automated so one worker can operate many machines
  • Machines are foolproofed so they automatically detect problems.

• We use a Master Production Schedule to specify which products are to be produced in each time interval. The sequence of produced products need not match customer orders.

  • We want the MPS to be level over time.
  • To mitigate surges, we make use of a Final Assembly Schedule to specify requirements over the short term. This requires
    • Smoothing aggregate production requirements.
      • This demands a repetitive manufacturing environment where the flow rate is kept constant and significant deviations are compensated for
    • Sequencing final assembly. We do this by translating product specific requirements to a production sequence.
      • The sequence on the line should maintain the proportions of required product.
      • This requires mixed model production where lines can support multiple products.

• JIT requires mitigating machine failures which may cause delays.

  • Capacity buffers involve scheduling for the facility to run less than 24 hours a day. If there is a delay, the factory can catch up. If there is excess, workers are allocated other tasks.
  • Two shifting - two shifts are scheduled per day separated by a down period for maintenance or catch up.
  • We reduce WIP so production is JIT, but we maintain excess capacity just in case

• JIT necessitates reducing setup times. This is especially needed for our uniform final assembly sequence.

  • We distinguish between internal setups - those when the machine is stopped, and external setups - those when the machine can keep producing product
  • Internal setups are disruptive to production.
  • We have four concepts for setup reduction
    • Separate internal and external setup. Ask what tasks must be done with the machine stopped.
    • Convert as much as possible of the internal setup to external setup.
    • Eliminate adjustment process.
    • Abolish the setup itself — use uniform product design or parallelization.

• JIT works best if we have multifunctional workers who can move wherever in the plant.

  • We can use a worker rotation system where workers are rotated through various jobs and then cross-trained. This:
    • Keeps multiple skills sharp
    • Reduces fatigue and boredom
    • Fosters systems thinking for everyone
    • Increases innovation
    • Introduces flexibility
  • The layout of the factory is changed from linear to U shaped cells.
    • One worker can attend to all the machines with minimum walking.
    • Many workers can occupy the cell depending on production requirements.
    • Workers can monitor inflows and outflows, facilitating JIT flow.
    • Workers can easily smooth out unbalanced operations.

• JIT requires a high level of quality to function. JIT highlights detects problems and identifies their sources.

• Quality is not checked in separate stations. Workers check quality at every step of the way. This gives better feedback.

• There are seven principles of quality

  • Process control - workers themselves make sure the production processes operated properly.
  • Easy to see quality - use visual displays of quality measures. They give better feedback to customers and inspectors.
  • Insistence on compliance - demand compliance with quality standards at every level of the system. Quality first, output second.
  • Line stop - each worker had the authority to stop the line to correct quality problems.
  • Correcting one’s own errors - no rework lines. Workers that produced the defective items fix them.
  • 100% check - inspect every part not just a random sample. Automate inspection if necessary.
  • Continual improvement - aim for zero defects.

• The kanban system works as follows.

  • When a part is removed from the final inventory point, the last workstation in line is given authorization to replace the part.
  • Workstations send authorization signals upstream to replace the part just used.
  • Toyota has formulated six rules for kanban
    • Each process issues requests (via move cards) to its suppliers when it consumes its supplies
    • Each process produces (via production cards) according to the quantity and sequence of incoming requests.
    • No items are made or transported without a request
    • The request associated with an item is always attached to it.
    • Processes must not send out defective items.
    • Limiting the number of pending requests (i.e., WIP) makes the process more sensitive and reveals inefficiencies.
  • The number of card counts per station dictate the level of WIP
  • The mechanics of the kanban system is similar to the base stock model. Every time a part is consumed, a request is made for that many parts to be produced (in effect, a level base stock is kept).

Limitations §

• JIT is more of a philosophy that may be difficult to transport. It is more of an idea than a toolbox and firms implement it differently.
• Because it has a myriad of implementations, it is less structured than Material Requirements Planning. It requires discretion on choosing what method to use for the business, and even then it also requires deviating from the status quo.
• JIT does not guarantee immediate improvements. It is neither simple nor inexpensive
• JIT’s objectives can often compete with each other, which requires balancing.
• While quality is an important metric, it is far from the only important metric.
• Kanban does not scale well with a high number of parts produced infrequently.
  • Kanban is typically only applicable in repetitive manufacturing environments where material flows along fixed paths at steady rates.
  • Kanban is not robust against swings in product mix, product complexity and volumes.
  • Optimal card count allocation is a function of mix. Hence, to achieve high throughput with low WIP, we may need to dynamically vary the card counts over time
• Kanban induces more operator stress because they must replenish voids as quickly as possible to prevent starvation.

Links §

• Hopp and Spearman - Ch. 4

• System Science - JIT is systems-focused.