Lean production is a term that embraces many of the topics that we have covered in earlier chapters, topics such as flexible manufacturing, minimizing work in process, "pull" systems of production control, and setup time reduction. The term itself was coined by MIT researchers to describe the collection of efficiency improvements that Toyota Motors undertook to survive in the Japanese automobile business after World War II . Because of its origins at Toyota Motors, the same collection of Improvements has also been called the "Toyota production system'.
Historical Notes: The person given credit for initiating many of the methods of lean production was Toyota chief engineer named Taiichi Ohno (l9121990). In the post world War II period, the Japanese autornotive industry had to basically start over. Ohno visited a U.S.auto plant to learn American production methods. At that time, the car market in Japan was much smaller than in the U.S., so a Japanese automotive plant could not afford the large production runs and huge work in process inventories that we had here. (As it turns out. OUr plants cannot afford them any longer cithcr.) Ohno knew that Toyota's plants needed to be more flexible. Also, space was (and is) very precious in Japan. These conditions. as well as Ohno's apparent aversion to waste in any furm (mllda, as the Japanese call it). motivated him to develop some of the basic ideas and procedures that have come \0 be known as lean production. Over the next several decades. he and his colleagues perfected tbcsc ideas and procedures, which included just in time production and the kanban system of production control. smoothed production, setup time reduction, quality circles, and dcdkated adherence to statistical quality control Ohno himself didnot coin the term "lean production" to describe the collection of actions taken at Toyota to improve production efficiency. In fact, he titled his book, The Toyota Production System: Beyond Large Scale Production. The term "lean production" was coined by researchers at MIT to describe the activities and programs that seemed to explain Toyota's success: the efficiency with which they produced cars and the qua1ity of the cars they produced.
The MIT research project came to he known as the International Motor Vehicle Program (IMVP). Included in the research was a survey of 87 automobile assembly plants throughout the world. The research was popularized by the book The Machine that Changed the World. In the subtitle of the book was the term "lean production"
Let us provide two definitions of lean production. Our first definition is a paraphrase of two of the authors of The Machine that Changed the World. Womack and Jones define lean as doing "more and more with less and less less human effort, less equipment, less time, and less space while coming closer and closer to providing customers with exactly what they want". We are taking some liberties in using this quote, It comes from their book titled Lean Thinking (p. 15), and they use these words to define "lean thinking," which is lean production but expanded in scope to include distribution and other functions beyond the factory.
The second definition is developed to introduce our discussion of the principles of lean production. Lean production can he defined as an adaptation of mass production in which workers and work cells are made more flexible and efficient by adopting methods that reduce waste in all forms. According to another author of The Machine that Changed the World, lean production is based on four principles:
perfect firsttime quality
flexible production lines
Let us explain these principles and at the same time compare Jean production with its predecessor, mass production. The comparison is summarized in Table 27.1.
Minimize Waste. All four principles of lean production are derived from the first principle: minimize waste. 'Taiichi Ohno's list of waste forms can be listed as follows:
(1) production of defective parts, (2) production of more than till: number uf items needed, (3) unnecessary inventories, (4) unnecessary processing steps, (5) unnecessary movement of people, (6) unnecessary transport of materials, and (7) workers waiting. The various procedures used in the Toyota plants were developed to minimize these forms of waste. A number of these procedures have been discussed in previous chapters. For example, lean principle 2 (perfect first time quality), discussed next, is directed at eliminating production of defective parts (waste form 1). The just in time production system (Section 26.7) was intended to produce no more than the minimum number of parts needed at the next workstation (waste form 2). This reduced unnecessary inventories (waste form 3). And so on, as we will see now.
Perfect First Time Quality. In the area of quality, the comparison between mass production and lean production provides a sharp contrast. In mass production, quality control is defined in terms of an acceptable quality level or AQT. (Section 22.2.1). This means that a certain level of fraction defects is sufficient, even satisfactory. In lean production, by contrast, perfect quality is required. The just in time delivery discipline (Section 26.7) used in lean production necessitates a zero defects level in parts quality, because if the part delivered to the downstream workstation is defective, production stops. There is minimum in
ventory in lean system to act as a buffer. In mass production, inventory buffers are used just in case these quality problems occur. The defective work units are simply taken off the line and replaced with acceptable units, However, the prohlem is that such a policy tends to perpetuate the cause of the poor quality. Therefore, defective parts continue to be produced. In lean production. a single defect draws attention to the quality problem, forcing corrective action and a permanent solution. Workers inspect their own production. minimizing the delivery of defects to the downstream production station
Flexible Production Systems. Mass production is predicated largely on the principles of Frederick W. Taylor. one of the leaders of the scientific management movement in the early 1901}s(Historical Note 2.1). According to Taylor, workers had to be told every detail of their work methods and were incapable of planning their own tasks. By comparison. Iean production makes use of worker teams to organize the tasks to be accomplished and worker involvement to solve technical problems. One of the findings reported in The Machine that Changed the World was that workers in Japanese "lean production" plants received many more hours of training than their U.S, counterparts (380 hours of training vs.46 hours. Another finding was the lower number of job classifications in Japanese lean plants. The study showed an average of 11.9 job classifications in Japanese plants versus an average of 67.1 in L:.S. plants. Fewer job classifications mean more cross training among workers and greater flexibility in the work force
In mass production, the goal is to maximize efficiency. This is achieved using long production runs of identical parts. Long production runs tolerate long setup changeovers, In lean production. procedures are designed to speed the changeover. Reduced setup times allow for smaller batch sizes. thus providing the production system with greater flexibility. Flexible production systems were needed in Toyota's comeback period because of the much smaller car market in Japan and the need to be as efficient as possible.
Continuous Improvement. In mass production, there is a tendency to set up the operation, and ifitts working, leave it alone. Mass production lives by the motto: “lf it ain't broke, don't fix it." By contrast. lean production supports the policy of continuous improvement. Called kaizen by the Japanese, continuous improvement means constantly searching for and implementing ways to reduce cost, improve quality, and increase productivity. The scope of continuous improvement goes beyond factory operations and involves design improvements as well. Continuous improvement is carried out one project at a time. The projects may be concerned with any of the following problem areas: cost reduction. quality improvement, productivity improvement, setup time reduction, cycle time reduction, manufacturing lead time and working process inventory reduction, and improvement of product design to increase performance and customer appeal. The procedure for carrying out a continuous improvement project in the quality area is outlined in Section 21.4.2, Similar procedures can be applied to other problem areas.