I think most would agree that the Toyota Production System (TPS) has revolutionized business around the world, particularly manufacturing. What makes the TPS so revolutionary?
I worked in management at TMMK (Toyota Motor Manufacturing – Kentucky) for 15 years. My experience convinced me that what makes the Toyota Production System (TPS) special is that the elements—the tools, methods, and concepts of the TPS—work together as a system. Excepting standard work, these elements were developed/adopted to support the pull method of production. This is the production method Toyota adopted after Taiichi Ohno’s 1956 visit to America.
What is the pull method? The pull method means that products are pulled from the upstream process to the next downstream process in the correct quantity at the correct time. Toyota adopted this production method because it reduced inventory (waste), and it paced and synchronized production.
This pull method of production evolved into Toyota’s Just-In-Time system through the adoption of the TPS elements and the expansion of the pull method to its suppliers. It’s one of the pillars of the Toyota House. Yasuhiro Monden, author of Toyota Production System: An Integrated Approach to Just-in-Time, has said that the Just-In-Time concept is the most important feature and the central part of the Toyota Production System. I agree.
Over the years I’ve witnessed dissatisfaction with the adoption of various TPS elements because of unsatisfactory results. There may have been other contributing factors to this dissatisfaction but one thing is certain: The TPS elements do not work in push systems. Why? Because they weren’t developed for push systems. For the elements of the TPS to work, and to achieve a Just-In-Time system, the pull method must be adopted.
The obvious question, then, is “How do we implement a pull method?” Based on my experience with the TPS, these are the major steps to take.
Determine TAKT Time – We must understand the required rate of production.
First, we need to know demand/day to satisfy customers. TAKT time is the # of products required per day/production time.
Continuous Flow – We must align processes and equipment in the order of product processing flow.
Why is this important? Because this makes pulling in one-piece flow possible. There are a couple of ways to accomplish this. A plant layout where processes are close to one another and in order is the best way. Secondly, the use of transport devices like rollers and conveyors can be used when transportation is unavoidable. Both substantially reduce transportation man-hours. Keep in mind that the further the distance between processes will likely result in more WIP between them. Continuous Flow also shortens the production cycle. We know that making and moving one-at-a-time speeds up production. Additionally, quality at the source is possible which provides more immediate feedback and helps reduce defects.
A potential issue is with differing machine capacities in or between processes. Ideally, machines would have the same capacity but, unless designed this way, they generally don’t. The key is that the lower-capacity machines must meet demand. The higher-capacity machines are idled during portions of the production cycle so as not to overproduce.
Heijunka - We must level the workload.
Line balancing is required to minimize over-cycling and/or waiting between processes. We segment and standardize the work elements to balance the work and eliminate lost time associated with imbalance.
Pulling Orders – We must pull the actual orders through production.
Everyone has a daily schedule, including Toyota. Most companies use an MRP (Material Resource Planning) system. Using this system to determine daily run schedules results in materials being pushed through production, sacrificing pace and synchronization. The absence of pace and synchronization in production results in delays and excess inventory throughout production.
It’s ok to keep your MRP system. Use it to determine “what” to make but not “the order to make them.” To eliminate excess inventory and achieve pace and synchronization we pull orders through production. This is accomplished by arranging the daily schedule in the desired order of production. We then assign each job a sequence number (1, 2, 3…etc.). Then we pull jobs in sequential order.
Pull Subassemblies – We must pull sub-assemblies associated with each job in sequential order.
Each subassembly on the sub-assembly process’s schedule is given the same sequence number as the job it’s associated with (All sub-assemblies associated with a given job are assigned the same sequence #). The subassemblies are then built in sequential order (1, 2, 3…etc.). Building each subassembly in sequence aligns orders throughout the system in sequence which synchronizes production. The subassemblies flow through the plant synchronized so that they mate up appropriately.
Pull Component Parts – We pull component parts using a kanban system.
A kanban system is a Japanese manufacturing system in which the supply of components is regulated through the use of an instruction card sent along the production line. Component parts that make up the sub-assemblies are pulled based on usage with a kanban system.
SMED – Perform SMED events on machines that make parts in batches.
In preparation for pulling small lot sizes of batch-built parts, we must first perform SMED events on all machines that build parts in batches. When we pull batch-built parts in small lot sizes, the number of changeovers is increased. Performing SMED activities reduces the changeover times. We’ll need that time because we’ll increase the number of changeovers when producing in smaller lot sizes. Small lot production is a critical component in the pull method.
Pull Batch-Built Parts in Small Lot Sizes – We pull batch-built parts using a different type of kanban.
Parts that must be produced in batches are pulled using the Triangle Kanban method. The Triangle is a signal kanban that works similarly to the reorder slip in your checkbook. This kanban is responsive to changes in demand which makes it possible to produce these parts to actual demand. The Triangle method also enables demand satisfaction by determining the optimal small lot sizes. A nice additional benefit of the Triangle Kanban is that it eliminates schedules for batch-built parts. The Triangle self-schedules all parts based on actual demand.
In conclusion Taiichi Ohno, the Father of The Toyota Production System, believed that Toyota’s Just-In-Time system was a “manufacturing advantage” for Toyota over the competition. In fact, for many years, he didn’t allow anything to be recorded about it! It remains a manufacturing advantage for Toyota.
Implementing the pull method of production is the first step to developing your own Just-In-Time system that will give you this “manufacturing advantage” over your competition.
Phil Ledbetter worked in production leadership at Toyota Motor Manufacturing – Kentucky for 16 years. He’s the author of The Toyota Template, The Plan for Just-In-Time and Culture Change Beyond Lean Tools. Phil is the principal consultant at The Toyota Template.