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What is JIT manufacturing

Introduction

This chapter will acquaint the reader with manufacturing management practices used to improve quality and productivity. People who use SPC should become familiar with just-in-time (JIT) and total quality control (TQC) concepts, because they provide a framework for addressing quality issues on a plant level. In addition, supplier certification programs can help address quality issues before they even reach the plant floor.

With SPC, the task of reducing variation in a process soon takes us beyond the process itself. Concerns about proper machine maintenance, adjustment, and operation turn into other concerns, like simpler methods, better raw material uniformity, and more efficient parts handling. Further improvement in quality and productivity soon becomes an environmental issue; that is, SPC can document a need for improvement in just about any manufacturing environment, but there are some environments where the actual improvement can occur faster than others. This chapter is a look at those types of environments.

JIT, TQC, and supplier certification practices can optimize the use of SPC. The different methods of JIT and TQC reflect the broad nature of manufacturing itself and people with different cultural backgrounds (the Japanese with JIT, for instance).

Just-in-time Manufacturing

There are many articles and books on the subject of JIT. The most notable source for the information found in this section is Japanese Manufacturing Techniques, Nine Hidden Lessons in Simplicity, by Richard J. Schonberger (Macmillan Publishing Co., Inc., 1982).

Just-in-time is a manufacturing strategy that is intended to increase profit and competitive position. It is not simply a way to reduce inventories or force suppliers to deliver goods “just in time” for use in manufacturing. JIT is based on the idea that a company should buy or produce only what is needed and only when it is needed.

JIT aims at the timely delivery of materials and tools to each work station, and the many benefits that occur from working toward this goal. Below are some of the quality and productivity benefits:

  • Workers become responsible for making defect-free parts, resulting in less scrap, rework, material waste, and wasted effort. 
  • There is an increased awareness of the sources of delay and error. 
  • There are higher levels of worker motivation.
  • There is a fertile environment for plant-wide quality improvement. 
  • There is greater productivity and lower cost — which fuels a continuous quality effort.

JIT was developed in Japan for companies that did repetitive manufacturing, which is both capital-intensive and laborintensive in Japan and the United States. JIT is a radical departure from traditional U.S.-style manufacturing, with its shoporiented plant layout, large inventories, elaborate material handling, and computer-controlled production management systems. Instead of focusing on factory automation and elaborate control systems, JIT aims at simplifying and streamlining the flow of goods and labor.

JIT and Quality

JIT manufacturing represents a commitment to quality. In a sense, a company using JIT bets its whole manufacturing process on the quality of its goods. If there are no defects, the process runs smoothly; however, if there are defects, the process grinds to a halt. Defects become diseases from which the whole body suffers, and they are dealt with quickly and efficiently. This is unlike traditional batch-mode production methods, where a certain level of defects is assumed as unavoidable; in order to keep the process running, batches of parts are made so that there are always enough good parts produced.

With reduced inventories and movement from batch-mode production, producing defect-free parts becomes much more important. A part with a defect creates a break in the chain that can cause work to stop all along the production process. With batch-mode production, a bad part simply gets thrown into a scrap pile or shipped to a customer, and production does not stop because a worker can simply pick another piece from the bin.

Not coincidentally, JIT practices apply wherever SPC can be used to improve quality. JIT requires each worker to be aware of the quality requirements of his work and to assure that these requirements are met. Every worker must know what quality looks like at every work station, and workers must feel responsible for it. The feeling of responsibility is not a dictated feeling, either. It is a personal motivation continually reinforced by relationships with other production workers in the plant.

Although it is difficult to apply in principle, an ideal JIT practice is to have each worker physically hand his finished piece to the next worker in line. This tends to create a strong feeling of pride, social responsibility, and teamwork. Each worker knows that the next worker depends on that piece to be well-made and done on time in order to do his work. This motivation is lost in the case of a worker who simply puts the piece in a finished parts bin, and the bin moves by conveyor or lift-truck to an unknown destination on the other side of the plant.

JIT Practices 

The JIT commitment to quality is part of a central goal of reducing waste and, therefore, reducing manufacturing costs. 

JIT can be started in a plant using these practices: 

  • Cutting lot sizes 
  • Cutting setup times 
  • Implementing total quality control 
  • Implementing a pull system 
  • Organizing the plant for continuous flow manufacturing 
  • Withdrawing buffer inventories 
  • Simplifying buying practices 

Each of these practices will be briefly described. The biggest misunderstanding about JIT is that it should start with one’s vendors — reducing carrying costs by forcing vendors to deliver just in time. Although JIT does take on that appearance, as a strategy it should start at the other end of the plant, in the final assembly area, and work backward.

Cutting Lot Sizes

Lot sizes are usually determined by the cost and time it takes to set up a machine and the cost of producing a batch of parts. As machine tools become more performance-oriented and more expensive, larger lot sizes occur. The negative ramifications of large lots are not examined as closely as machine setup and inventory carrying costs; large lot problems include the following:

  • They hide defects.
  • They create waste. 
  • They create a “hurry up and wait” mentality, where a fast machine produces a lot of parts that sit around and wait for the next manufacturing stage. 
  • They require elaborate shop floor control and material handling to schedule and move the parts to and from the machining center

Many of the costs associated with large lots are assumed to be fixed costs. Cutting lot sizes may initially create more costs because we are forced to confront fixed costs, such as setup times, production scheduling, and material handling.

Effort put into simplifying and streamlining the operation can eventually create many benefits, not the least of which is a more flexible manufacturing operation capable of responding faster to market demand.

With a lot size of 1 (an ideal with JIT), an order to ship is all the documentation necessary. Successive manufacturing stages are positioned right next to each other. The material is handled one piece at a time, station to station. Setup time is reduced to seconds, and work is apportioned so that each step takes an equal amount of time. Teamwork occurs because each worker knows that work will halt and quotas will not be met if there are defective parts or problems at any one station.

A lot size of 1 is impractical in most instances, but the goal of reducing lot sizes is not. Reducing lot sizes exposes quality problems and sources of delay. It is easier to trace a problem to the time it occurred and the circumstances behind it. Smaller lot sizes force us to think of ways to create efficiencies between manufacturing steps, rather than relying on the internal efficiencies of some high-speed machine tool to make up the difference in waste and carrying costs. 

Cutting Setup Times 

Machine setup times must be cut in order to justify cutting lot sizes. Japanese companies expend tremendous effort cutting the setup times of machinery. Some companies make special fixtures and conveyors for dies and jigs, and have teams of workers from neighboring machines join in and set up a particularly cumbersome machine when needed.

When setup times are no longer viewed as fixed, a whole new type of plant engineering can take place: that of developing manufacturing systems with minimal setup, responsive to more instantaneous demand. Much of the setup time caused by general purpose machine tools is engineered out. The resulting more-dedicated machinery can produce better quality in smaller lot sizes.

Implementing Total Quality Control 

Total quality control will be explained later in this chapter. Total quality control allows JIT manufacturing to function smoothly. JIT and TQC are self-perpetuating because JIT exposes defects and TQC serves to eliminate them, which allows lotless production to continue.

Implementing a Pull System 

Most manufacturing systems are push systems. Work piles, or queues, up in front of each machine with the idea that this guarantees continuous production. Work begins at the next station when there is a large enough accumulation of unfinished parts. To smooth over the whole production process, buffer inventories and safety stocks are put in front of machines that operate faster than others, so they won’t be idle.

A pull system responds to final assembly, which in turn is geared to customer orders. Rather than pushing materials through a factory, a part is made when that part is needed at the next stage in the process. Parts are thus delivered just in time to the next station. In the Japanese Kanban (card) shop order system, when a station needs a quantity of parts, a card is placed at the previous station to signal production of those parts. Idle labor is not as big a concern with a pull system. Productivity eventually increases as management and workers divide up the work more equitably, and the system smooths itself out.

Organizing the Plant for Continuous Flow Manufacturing 

Just-in-time manufacturing requires plants to abandon the job-shop layout in favor of a cellular layout. In a cellular layout, successive stages of a production cycle are physically situated right next to each other — a punch press next to a grinding/deburring station next to a drill press, and so on. It is much more important to have people work on the same part side-by-side and understand successive steps in the making of the part than to group like machinery in separate shops. A cellular layout removes much of the need for material handling apparatus, such as fork lifts. With less buffer inventories between stations, there is more floor space.

Withdrawing Buffer Inventories

To expose problem sources, Japanese managers use the offensive strategy of removing buffer stock between machinery. This stimulates activity toward removing sources of delay, removing potential quality problems, and continuing to perfect the process. Once the process smooths out, the managers remove a bit more buffer to expose the workers to other problems.

Simplifying Buying Practices 

Buying practices for JIT are aimed at dock-to-assembly-line movement of parts, rather than dock-to-inventory movement. Arrangements are made for more frequent deliveries of less parts without a lot of the formal paperwork. The vendor becomes more responsive to the needs of the plant and is coached to develop his own resources so that he can play the role profitably. Long-term relationships and quality are stressed over competing for a low bid.

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