In a large power electronics manufacturing facility, the manufacturing system is complicated by the combination of both high and low volumes and both standard and custom products. Visual controls are one of many countermeasures that have been implemented to reduce complexity, eliminate non-value added activity and reduce manufacturing intervals. Visual controls are part of a continually improving Total Quality Management System. Visual controls are a system of signs, information displays, layouts, material storage and handling tools, color-coding, and poka-yoke or mistake proofing devices. These controls fulfill the old fashioned adage: a place for everything and everything in its place. The visual control system makes product flow, operations standards, schedules and problems instantly identifiable to even the casual observer. The system has built in checking and corrective action to maintain performance and identify areas with room for improvement1.

The experience in introducing visual controls into one business unit of a major manufacturing company as a model which other manufacturers can follow. The examples are particularly applicable to electronics manufacturing but the ideas are applicable to all manufacturing firms.

Introduction of visual controls began with the clean up and relayout of each work cell. Even though the factory had been completely renovated in 1988 and is widely known for setting a standard of excellence for housekeeping, many changes to individual work stations were made as the visual control concept took hold. Now every work position has signs posted describing process operations and assignments of products. Higher volume products are dedicated to particular work positions to simplify the availability of tools, documentation and materials. Tool lists for each operation make it easy to determine, before a job is begun, whether everything that will be needed is available. Many tools, fixtures and documents traditionally stored in drawers are now mounted within easy reach of the Production Specialist using that work station. Specially designed pull out drawing holders, mounted at eye level, make the large drawings needed for frame and cabinet assembly instantly accessible. Work in process (WIP) moves beside the work station. Large bulky material is stored directly behind the Production Specialist. Small parts and tools are kept in bins hung in front of the work position.

Production scheduling boards provide simple conversion of the customer order and resulting master schedule into clear signals for the Production Specialist to follow. The boards indicate how many of each product or subassembly to produce. These boards were a natural extension of the JIT manufacturing system installed a number of years ago2. In this JIT or pull system, Kanban cards link what has been consumed with what should be built next. For example, for a unit or module that is used as a subassembly of a complete finished power system, the master scheduler determines the number of Kanban cards to be released onto the production boards based on an analysis of the typical and peak volumes ordered. A typical Kanban card is shown in Figure 1. This card is for the L10 building block or module that is used on the J85500K1 and is also separately orderable as the J85801A L2, L31, LP, LWA. The card also shows the product code number that is used for electronic transactions concerning this product.

Figure 1. Typical Kanban card for the L10 building block used on the J85500K1 frame.

These cards are placed onto the production board shown in Figure 2 in order to call for production of this unit. The board has separate zones for different priorities of operations. The highest priorities occur infrequently. Emergency shortages of a unit for a frame that is due that day are placed in the red zone and receive first priority. Low volume or special custom orders are placed in the orange or blue zones, respectively. High volume products are generally scheduled using the yellow zone. Note that on this particular production board three different units are built regularly and each has its own yellow zone container. The green zone is a predetermined buffer used to balance the different rates of construction of different codes and operations. The last green zone is always labeled STOP BUILDING KANBANS ARE FULL. When this last unit is built, the Production Specialist can build one of the other units for which they have cards or if they have none, switch to another work station.

After building a unit, the Kanban card, which is magnetic, is placed on the unit. This identifies it to the successive operations who will use the unit. When they use the unit, the card is returned to the Production Board signaling that another unit must be built. The Master Production Scheduler can adjust for changing customer demand by adjusting the number of Kanban cards in the shop.

Figure 2. An example Kanban Production Board showing various priority zones.

Material is provided to the work stations via a two bin system. After using up the material in the first bin, the Production Specialist places the bar coded reorder card from that bin in a special holder for the material handler. The material handler replenishes material at regular intervals, which depend on the size and volumes involved, such that the first bin is restocked before the second bin is used up. Low volume, infrequently built products do not have all of their material stored at the work cell. Reorder sheets have numbers, descriptions, and bar codes for the unique material used on that product. Wanding in each of the bar codes on the sheet automatically requests the delivery of one set of items from the stockroom. Large items and WIP are stored in Kanban squares marked on the floor with tape and labeled to indicate what material is to be placed there. Small items are stored in color coded bins:

Note that the breadman system is a JIT delivery arrangement with a local supplier in which the supplier weekly refills the bins throughout the shop to predetermined levels without any orders being placed. An annual agreement establishes the process and the Purchase Order number against which the supplier invoices for the parts he delivers. All bins are labeled with identifying information and where practical, information identifying the end products on which that part is used.

One of the critical elements of any visual control system is built in maintenance. Twice a year a Red Tag system is used to identify excess materials and facilities. The Red Tags shown in Figure 3 are placed in each bin.

Figure 3. A Red Tag used in a semiannual clean up process to eliminate excess material, tools and machines.

When material is withdrawn from that bin or a tool is used, that Red Tag is discarded. After a designated period, most of the shops use 30 or 60 days, any material that still has a Red Tag must be assessed for disposition. A similar system is used for all tools, fixtures and machines. Master Production Schedulers review material and engineers review tools and machines to determine the appropriate disposition. Most items are sold to other potential users; some are put in long term storage or scrapped.

Process control boards provide real time feedback to production specialists concerning the in-process and outgoing quality levels of their products. In high volume shops, traditional control charts are used as the heart of the process control board. Attribute charts such as p charts are used to monitor end product defect levels, while upstream variable charts are used on the key processes to maintain optimum control. In the low volume shops, a modified U chart is used to provide quick feedback to the shop as shown in Figure 4.

Figure 4. Process Control Board for a low volume shop using a modified control chart.

In this example for a unit shop, the major operations are listed such as unit mounting, wiring and marking. Defects found anywhere within the process are marked by an X in the appropriate area. The yellow line in each area indicates the average defect level as calculated by the quality engineer for that shop. Due to the limited sample size, control limits are only calculated for the total defect count shown at the bottom. Exceeding the average for a specific area requires a discussion with the responsible engineer. The line is shut down if the total exceeds the control limit until corrective action can be taken. Descriptions of the defects and corrective actions taken for either the total or a specific area are listed on the board and permanently recorded by the quality engineer. Photos are displayed of defects to alert the Production Specialists to take extra care when performing that operation.

Poka-yoke is a Japanese word that means mistake proofing. Poka-yoke devices are generally simple tools that eliminate the possibility of making a mistake or make the mistake obvious. Poka-yoke is based on the concept that all human beings are bound to make mistakes but it is possible for a manufacturing system to be so robust that none of these inevitable mistakes turn into defects in your products or services.3 Several engineers and trainers were certified as poka-yoke trainers and hundreds of designers, engineers, and Production Specialists received poka-yoke training. The poka-yoke class is required for all engineers. The entire factory now has thousands of these devices and hundreds of applications have been developed in the office and design areas. A few examples with a visual control element follow.

To prevent misinsertion in the field, many telecommunications circuit packs are designed with a unique key. The key consists of an array of holes, each of which can either contain a plug or not. The backplane contains pins that correspond to the hole positions of the array, thus insuring that improper circuit packs are not inserted. The system only works properly, however, if all of the key plugs which are used to fill the designated holes are inserted properly in the factory. Their small size coupled with the many possible combinations of key plug patterns resulted in an infrequent, but recurring quality problem.

A very inexpensive set of templates has completely eliminated the problem. Two templates were made for each code of circuit pack. One is used by the Production Specialist who inserts the key plugs. This template, shown in Figure 5, provides true mistake proofing.

Figure 5. An example key plug template and its associated circuit pack

The template has the circuit code engraved on it and is placed over the circuit pack connector so that only the correct holes are exposed. The Production Specialist can only put the plugs in the exposed correct holes. The second template is used at final product inspection as a successive check ensuring that the key plugs are correct.

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Figure 6. Poka-yoke fixture used for cable wiring.

Power Systems has a recycling program that now recycles nearly 100% of the recyclable material previously disposed as waste. To minimize transportation costs associated with recycling Styrofoam used by suppliers to package their components, the used Styrofoam is collected until a complete trailer load is obtained. Because of its odd sizes it was difficult to store and to estimate when a complete trailer load had been accumulated. Two members of the recycling committee suggested a Poka-yoke device that solved both problems: a chain link fence was installed in the recycling area that was exactly the size of a trailer load. Now they know exactly when to call for a trailer to transport the Styrofoam to the recycler and no time is wasted straightening the awkward bags of Styrofoam.

This visual approach has even extended into the manager's office where the daily work management metrics of the TQM system are visually displayed on the wall. Each accountability has a metric with a target plotted for each period. Actual results are plotted regularly with color coded indicators (green for targets that have been met and red for results that have missed the target) for each of the manager's accountabilities. Anyone just walking by the manager's office can get an instant impression of their position relative to the targets, without needing to know any of the details. Although no additional information is being collected over the previous system of keeping the data in notebooks or files, many managers have commented on the psychological impact of red indicators appearing on a chart that every visitor to your office sees and perhaps more importantly, you must look at every day.

The result of the implementation of this system in one business unit has been to reduce the total manufacturing cycle of the complex frame level products by a factor of three in only one year to a world class level of less than three days. Manufacturing cycle time is measured from the time the first operation begins to the time the frame is packed and ready to ship

Total productivity and outgoing quality levels have continued to improve during the same period due to the introduction of visual controls and the other improvements taking place under the TQM system. In addition, Production Specialists in the areas most affected by the visual controls have expressed their appreciation for the simplicity of the system. Several have become familiar enough with the approach to make significant additional improvements in their own areas.

The visual control system in a major power manufacturing plant uses control devices, color-coding, layouts, signs, and poka-yoke or mistake proofing devices to establish a common language throughout the workplace. This system is used to establish control, simplify assembly information at the point of use, make irregularities and non-value added activities obvious to both the production specialists and managers, and constantly uncover improvement opportunities. This system minimizes the time lost waiting or searching for materials, tools or documentation. It helps in the overall reduction in manufacturing cycle time. It eases the time necessary to train a new or reassigned employee. Inventories are controlled to predetermined levels by clear indicators that make it obvious if excess is present. Finally quality levels are improved since simplification, poka-yoke devices, dedicated tools and materials minimize the opportunity for mistakes.

This system can be adapted to any manufacturer and has been demonstrated to be an important element of a Total Quality Management system for a world class company.

The author would like to thank the Production Specialists and Managers of the Energy Systems Business Unit who have all contributed to this work through their ideas, efforts and cooperation. I would like to particularly thank Jeanne Patterson, the manager who spearheaded much of the visual control implementation and Durand Wine and Mike Blackburn who were the engineers on the initial implementation of visual controls. Special thanks also go to Mary McNamee, Bill Sims, and Wanda Karasek who actually made and installed many of the visual controls described in this paper.

[1] H. Hiranno, JIT Factory Revolution, Productivity Press, 1988.

[2] M. P. Cassidy & S. P. Sharma, "1990's Revolution in Power Systems Manufacturing," APEC, 2/25/92, pp. 160-166.

[3] Shigeo Shingo, Zero Quality Control: Source Inspection and the Poka-yoke System, English Translation 1986, Productivity Press.