Just in time manufacturing is a strategy for eliminating the deadly waste of over production by delivering a product or service just in time. This means to deliver just the right quantity at just the right time–no more and no less.
This concept is responsible for a variety of popular lean manufacturing tools and principles including kanban, pull systems, continuous flow, takt time, quick changeover (SMED), work cells, value stream mapping, and more. So in this article, we overview this big idea of just in time manufacturing and how all these concepts come together.
- The History of Just in Time Manufacturing
- Continuous Flow
- Pull Systems and Kanban
- Takt Time
- Quick Changeover (SMED)
The History of Just In Time Manufacturing
What we know today as “lean” was previously called just in time manufacturing until The Machine that Changed the World was published in 1990. But just in time concepts haven’t been replaced or obsoleted. They remain as a subset of practices within lean manufacturing philosophy.
In fact, the original Toyota Production System house includes “Just In Time” as one of two pillars.
Arguably the most popular of just in time concepts is the idea of continuous flow, or what is also called one-piece flow.
In traditional batch-and-queue manufacturing, material and information does not flow. In fact, some have come to call it “batch-and-stagnate” manufacturing because of how work in process (WIP) inventory tends to build up in queues and stagnate within the system.
But as the batch size is reduced to an ideal size of one, some extraordinary things happen.
To see what we mean, checkout our most popular YouTube video on converting batch-and-queue manufacturing to a continuous one-piece flow operation.
Let the numbers speak for themselves!
Keeping all things constant and a process time of 1 minute, as you reduce the batch size the lead time decreases dramatically. The total processing time for the entire build drops, too. In this way, many traditional batch-and-queue processes can be converted to continuous flow operations and reduce lead times by as much as 95%.
Even for those processes that cannot reach one-piece flow immediately, reducing the batch size by any degree makes for significant improvement.
Typical benefits to creating flow:
- Massive lead time reduction
- Considerable space savings
- Higher labor productivity
- Increased quality
- Fewer defects
Pulls Systems and Kanban
Although continuous flow is beautiful and represents an ideal state to strive for, it is not immediately possible or practical to introduce flow everywhere. In these situations, we apply another just in time manufacturing principle: pull.
A pull system is used to “pull” products or services into a process from a supplier process. This is preferred over “push” manufacturing when a supplier process pushes a large batch of something onto the customer process whether or not the customer process is ready for it. That’s over production and a big no-no! It’s the antithesis of just in time manufacturing.
Instead, we use pull systems–commonly called kanban–to trigger the movement of material so it is created and delivered just in time and still in small quantities.
In this way, equipment like presses and other short-cycle or batch-oriented processes can be decoupled from continuous flow operations and the over production is kept to a minimum.
Common types of pull systems include:
- 1-card kanban
- 2-card or “2 bin” kanban
- Kanban supermarket
- First In First Out (FIFO) lane
Each of these systems limit the total amount of work in process (WIP) in the system and also trigger production activities to occur just in time.
Combining Pull Systems with Continuous Flow
With these two concepts working together, you can effectively design a process for maximum flow. The goal is to create continuous flow everywhere, but in the places you cannot, you can use pull systems instead.
This follows the Lean Enterprise Institute’s five-step recommendation for achieving just in time manufacturing:
- Identify value
- Map the value stream
- Create flow
- Establish pull
- Seek perfection
The pace of production activities has not yet been discussed but is a critical component for optimal flow. If the system flows well but produces faster or slower than customer demand, you’ll still end up with either massive over production or shortages.
So to regulate the speed of all activities in the system, a standard pace is determined called takt time. Takt is the German word for “meter.” If you’re a musician and familiar with a metronome, that’s precisely how takt time works.
Takt time is equal to available production time divided by customer demand. This describes how much time for each widget or service being produced.
For example, if you had 7.5 hours of production time and 600 widgets to produce, you’d calculate the takt time as follows:
How Takt Time Relates to Continuous Flow and Work Cells
Takt time becomes obviously important when considering the design and management of a work cell.
A well-designed work cell will allow for flexible manpower that is matched to takt time.
- If customer demand decreases (i.e. takt time increases), the number of people working in the cell is decreased.
- If customer demand increases (i.e. takt time decreases), the number of people working in the cell is increased.
By designing and managing the cell in this way, maximum labor productivity can be preserved even if customer demand changes over time.
Dealing with Time-Consuming Changeovers
A common hindrance to establishing flow and pull for just in time manufacturing is long changeovers.
Once of the primary reasons why organizations manufacture in large batches is because of the costly downtime incurred when changing over a machine from one product to the next. This downtime can be significant.
The common management and accounting workaround is to increase the batch size. This makes the financials look better.
For example, if a changeover takes 60 minutes, instead of doing a batch of 60 widgets, the financials look better when doing a larger batch… let’s say 3,600. Now, instead of wasting 1 minute per widget doing a changeover, it is reduced to 1 second per widget.
This changeover time is more obvious as a cost of production than the less tangible and harder-to-quantify costs of over production. Therefore, many organizations resist reducing their batch sizes.
There is a simple solution: Quick Changeovers!
The simple solution to this problem is to reduce the amount of time required to perform a changeover. If the changeover previously took 60 minutes to perform and now takes 30 minutes, you can now changeover twice as often with zero added cost.
By changing over twice as often, your batch size is effectively cut in half.
Quick changeover is a synonym for SMED, which stands for single minute exchange of die. Word-class organizations apply SMED and quick changeover so well that they can changeover equipment in mere minutes–9 minutes or less!
The outcome is a far more flexible and nimble production system, and one that allows for manufacturing to be performed just in time!
Additional Resources for Just In Time Manufacturing
- Course: The Fundamentals of Lean
- Course (FREE): Lean Manufacturing Mini-Course
- Video: Single Minute Exchange of DIAPER
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