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Tom Pyzdek

Six Sigma

Developing a Standardized Approach to Work, Part 2

Efficiency enhancements of work cells

Published: Wednesday, June 30, 2010 - 08:23

In this four-part series, we take an in-depth look at how to design an effective work environment. Part one discusses the elements of continuous-flow work cells. Part two considers how to enhance the efficiency of such work cells. Part three explores the 5S methodology. In part four of the series, we look at single-minute exchange of die (SMED).

 

In part one of this series, I discussed the elements of continuous-flow work cells. In this part I will discuss efficiency enhancements. If the production capacity is less than the quantity needed per day, we would have a bottleneck, which we need to address so we can meet the required demand. In lean Six Sigma, a bottleneck is any process that has a cycle time that’s greater than takt time. It is possible to have multiple bottleneck operations. There are several ways of breaking bottlenecks:

Improve its cycle time. Use lean Six Sigma, kaizen, and other methods. You should review the production capacity table for the process and focus your attention on the operations with the lowest production capacity. Look at the basic time and tool change time for ideas about improving the operation.

Improve quality. It is vital that the units produced by the bottleneck conform to requirements. Take extra care that only acceptable quality materials are delivered to the bottleneck. Aggressively address any issues with the quality of production created by the bottleneck.

Supplement bottleneck production with purchased materials. Use a supplier to fill the gap temporarily until the bottleneck is brought up to capacity.

Work the bottleneck longer hours. This option will require returning to batch-and-queue until a better option, such as one of the approaches above, can be implemented. The real-time output from the bottleneck can be supplemented with inventory produced earlier by the bottleneck. Try to keep this additional inventory to an absolute minimum.

Add capacity. Purchase additional equipment to allow additional production. This is usually the least desirable option because it tends to lock in the expense. If you need to do this, look into leasing equipment.

What materials do we need to have on hand to produce the items (standard stock)?

“Standard stock” refers to materials that are needed to begin work within a process, such as work-in-process inventory (WIP). The design of the work cell will influence the WIP requirements. Ideally, one piece will start at the beginning of the work cell and progress through each process step without the need to stop. However, there are circumstances that may require additional stock. For example, if a part is welded at one step and needs to cool before it can be processed through the next step. Or if there is a need to perform an inspection before the part is placed in a subassembly where it can’t be accessed afterward. The bottleneck situation described above may require some amount of additional WIP.

What equipment do we need to produce the needed items?

Equipment in work cells tends to be smaller and more flexible than the equipment used for mass production batch-and-queue operations. The machines are also often slower than those used for batch-and-queue systems. There are several reasons for this:

• Smaller machines can be placed closer together. This reduces the travel distance required by workers. Because WIP inventory is small or nonexistent, we don’t need much space between machines for storage.

• Equipment used in lean work cells can be slower, so “fast enough” is good enough. Unlike mass-production equipment, the goal isn’t to produce a large batch quickly; it is to produce at the pace of customer demand, or takt. This means that a machine running at a rate faster than the required rate is wasteful.

• Smaller machines save space. Lean work areas often produce triple the value per square foot compared with their nonlean counterparts.

• Small machines can be moved more easily. A work cell can be quickly reconfigured by rearranging equipment to produce a variety of different parts.

• Flexible machines must be easy to set up fast. If changeover and setup times are low, it is easier to produce a variety of parts in small quantities.

• Small, slower, and more flexible machines are less expensive, easier to operate, and easier to maintain.

 

Equipment is also used in office environments. Computers, printers, fax machines, file systems, desks, tables, and mail carts are needed to perform work in an office. The lean equipment principles also apply here.

Autonomation (jidoka)

“Autonomation” is Taiichi Ohno of Toytota’s word to describe a production system that mimics the human autonomic nervous system, i.e., it automatically adjusts to external and internal conditions. For example, when we get too hot, our body automatically reacts to cool us down; we don’t have to think about it. Similarly, production systems should react to customer demands, increasing production when demand goes up, or decreasing production when demand goes down. They should react to WIP inventory buildup by producing less or producing on a different schedule. Lean mechanisms to accomplish this include takt time, visual controls, pull systems, and exploiting constraints.

Actually, this concept was embodied in the very first product made by Toyota, a loom. From an early age Sakichi Toyoda worked on improving looms. In 1891, he obtained his first patent for the Toyoda wooden hand loom. Among the innovations was the feature that the loom stopped if a thread broke. It can be argued that this invention led directly to the formation of the Toyota automobile company. A company was founded in 1926 as Toyoda Automatic Loom Works Ltd. by Sakichi Toyoda. In 1933, the company established its automobile department, led by Kiichiro Toyoda, the eldest son of Sakichi Toyoda. This department was spun out as Toyota Motor Co. Ltd. in 1937 and is now the well-known Toyota Motor Corp.

Autonomation within a work cell is used to eliminate the need for an operator to stand and watch a machine as it does its work. Work cell equipment is intelligent in the sense that it stops and signals when an operation is complete, or if there is a problem. Although operators often load the machines, the machines usually unload automatically. More important, after the operators load and start the machines, they move on to other activities in the cell rather than watching or tending to the machines.

Modifying equipment to use jikoda is usually quite simple and inexpensive. Microswitches, simple sensors, a light beam, and other mechanisms are commonly used for the purpose. Poka-yoke is Japanese for mistake proofing. These devices are used either to prevent the special causes that result in defects, or to inexpensively inspect each item that is produced to determine whether it is acceptable or defective. A poka-yoke device is any mechanism that either prevents a mistake from being made or makes the mistake obvious at a glance. This YouTube video illustrates the concept.

Consideration

What methods should be used to accomplish the goal using the resources we have?

What standard work procedures should everyone follow?

Designing continuous-flow work cells involves developing standard operations. These are descriptions of work that combine people, equipment, and materials to create value as effectively as possible. The components of standard operations are takt time, the work procedure, and the parts and materials essential to start work within the process, including parts or materials at the machines. If all three of these components are not present, standard operations cannot exist.

At companies such as Toyota, standard operations are determined by the foreman. Toyota foremen are masters of the work being done by their employees. Foremen are able to demonstrate the standards to their people. If an impartial observer agrees that the pace of work being done by the foreman following the standard is appropriate, then Toyota expects workers to adhere to the standards.

In many companies, however, things are done differently. It is my experience that few foremen are able to do all the work being done by their employees. The history of management in the United States and most European countries has led to a system where work is designed by experts in work design (industrial engineers), supervised by experts in management (foremen and supervisors), and performed by blue-collar specialists at a given trade (machinists, assemblers, welders, assembly-line workers). This model worked adequately in the past for batch-and-queue systems where specialization was the norm, but it is no longer possible to remain competitive using this approach.

However, it raises a question: If we don’t have a well-rounded supervisor to design work, how can it be done? The answer arrived at by many companies is to use teams. Teams designing and analyzing work are provided with basic training in the principles of work design, inventory control, and lean. The team must include at least one member who is highly skilled at the work done at each process in the subproject loop. Preferably, this will be the same person, but if there is no such person available, the team may have more than one skilled worker. Once the work of several people/machines is combined into a work cell, the company will need to cross-train all the workers in the cell so that they may help one another.

It may be that no one person on the team knows as much about work design as the industrial engineer, or as much about management as the business school graduate, or as much about any operation as the blue-collar journeyman, but experience has shown that tremendous improvements are possible using the team approach—if the company leadership truly embraces the lean philosophy.

Standard operations are the sum total of all of the ways that people, materials, equipment, and information combine to create value. There are three components to standard operations:

  • Cycle time
  • Work procedure (work sequence)
  • Standard stock on hand (WIP inventory)

 

Each of these items is required to standardize the operations.

Are there any tips that people should know about doing the work?

Knowledge of work is often a collection of insights. Experienced people have developed tricks that make it easy to do the task at hand or help them avoid problems that might cause injuries or quality issues. When designing the work you should create a document that can be given to workers showing them what to look for. This will include such things as the work sequence, how to handle the items, how to set up the tools needed for the job, and other relevant work details. The emphasis is on specific, concrete directions. Avoid abstractions. Include drawings, videos, pictures, animations, and other media to help show the proper way to do the work. Flowcharts are often helpful.

A good model for work instructions are those provided to customers for assembling products. Instructions for assembling a baby’s crib include step-by-step directions, drawings, safety warnings, and tips on how to put the crib together more easily. It also includes tips on maintenance, cleaning, and storage; guidance on how to test your work to ensure that it’s done correctly; instructions on changing the crib into a toddler bed, etc. Think of work instructions as the workplace counterpart to assembly instructions for consumers, and you’ll have a good idea of what’s needed. The idea is to provide simple, easy-to-understand, and complete instructions.

Manual of work directions

The manual of work directions tells how to perform the standard operations properly. It is based on the production capacity table (see figure 1) and the standard operations routine sheet (see figure 5). It describes the work to be done by each person in the work cell. In addition, the manual includes:

  • Safety and quality items for each step in the work sequence
  • Illustrations of machine placement for work performed by each individual worker
  • Cycle time for each operation
  • Work sequence
  • Standard stock on hand
  • Instructions for checking quality

Figure 1: Production capacity for machining process

Machine placement is shown on a separate sheet of 11 x 17 paper (A3 paper). The sheet will have columns showing work sequence, standard stock on hand, cycle time, net operating time, and safety and quality checks. The completed sheet is called a “standard operations bulletin” (see figure 2). Standard operations bulletins are displayed at the line and show the workers what is expected of them. In addition, supervisors can use the bulletins as visual control tools to audit compliance with work instructions. Managers and process improvement teams can study the bulletins for ideas on improving the work.


std-operations-exmpl

Figure 2: Standard operations bulletin [image courtesy of Kanban Just-in-Time at Toyota (Productivity Press, 1989) p. 114]

Which nonvalue-added activities can be eliminated immediately?

Of course, the standard work description, including the work element times, is just a starting point. Constant improvement is expected in the way the work is done and the time it takes to do it. For the long term, consider creating an opportunity map for the work within the cell and pursuing the opportunities over time. It’s a good idea to do this even before the original work cell design is complete. You can use the work-element time data you just collected for this purpose. Create a stacked bar chart showing all of the work elements in the work cell. One of the bars is for the work elements used currently, and the other will show only the work elements that are actually included in the work cell design. In particular, you need to focus on eliminating nonvalue-added work. Work is considered value added only if all of the following are true:

• It is done right the first time. Rework does not count as value-added work.

• The customer is willing to pay for it.

• It changes the thing being worked on. Moves, inspection, or storage are not value-added.

 

Figure 3 shows a stacked bar chart of the initial improvements that can be made to the weld and inspection process when changing the design from batch-and-queue to a continuous-flow cell. The team focused on nonvalue work elements, mainly waiting and moving, and made the improvements shown.

Figure 3: Stacked bar chart of initial improvements

How should the workplace be arranged?

Cell design is performed in two phases:

Phase 1: Document the current state. At this point in your lean Six Sigma project, you should have already created a lean value stream at the process level. Cell design begins from there.

Phase 2: Convert to a process-based layout. Cause-and-effect diagrams are a useful tool here.

A cause-and-effect diagram can be used to identify the causes of a problem you are trying to solve. Here the problem is one of achieving continuous flow. When creating a cause-and-effect diagram use the “Four Ms” as a starting point: Men (and women,) Methods, Machines, and Materials. How should these elements be combined to achieve maximum flow?

How should we lay out the equipment so movement of people and materials is efficient?

Continuous-flow work cells are nearly always shaped either like a U or C to minimize walking. The equipment and workstations are arranged close together in the sequence of the work steps. This arrangement reduces walking distance to a minimum and results in the worker being near the start point of the next work cycle when she completes the work cycle. It is different than many traditional operation-based work layouts where a worker sits or stands in one position and does a simple repetitive task all day. The traditional work arrangement leads to psychological issues such as boredom or mental fatigue, as well as physical problems from repetitive stress injuries.

Where will work in process be stored?

Standard stock refers to the materials needed to begin work within a process, such as work-in-process (WIP) inventory. The design of the work cell will influence the WIP requirements; conversely, WIP requirements will influence the design of the work cell. Ideally, one piece will start at the beginning of the work cell and progress through each process step without the need to stop. However, there are circumstances that may require additional stock—for example, if a part is welded at one step and needs to cool before it can be processed through the next step. Or if there is a need to perform an inspection before the part is placed in a subassembly where it can’t be accessed afterward. Bottlenecks, by definition, can’t produce enough to meet takt time requirements. The bottleneck problem is sometimes solved by additional WIP to supplement the bottleneck’s output.

How can we rearrange the workplace quickly when we need to make a different item?

As discussed earlier, the equipment used for lean production tends to be smaller and more mobile. It is usually possible to rearrange the equipment in a work cell quickly so different parts can be made using the same equipment. Work cell design should make this as simple to do as possible. Also consider where equipment, fixtures, WIP, and other items will be placed when not needed for the item currently being produced. Storage areas should be nearby and clearly marked so workers know where to store unneeded resources, and where to find them when they are needed again. It should be easy to physically move the equipment and, if necessary, reconnect to power, plumbing, etc.

How many people are needed?

Consider the process described in figure 1: knuckle machining. The requirement for this process is shown to be 255 units. The team’s idea for the process was to set it up as shown in the work cell layout in figure 4. The U shape will minimize the distance that people need to move. Figure 5 shows the standard operations routine sheet for the process, which graphically shows a timeline for each operation, the manual time, the automated machine time, and the travel time. The total time to complete a full cycle for this process is 113 seconds (see either figure 5 or figure 6). Although some machines continue to operate automatically at this time, the operator can return to the beginning of the process and start another cycle. By the time the operator reaches each operation on the next cycle, the machines will have completed their cycle and, due to autonomation (jikoda) they will have automatically stopped.

This process can be operated by one person. A full day’s production can be produced from this process in 28,815 seconds (quantity of 255 × 113 seconds cycle time). Because a full work day is 51,600 seconds, the worker will have time to perform other duties each day, including keeping the workplace safe, clean, and organized—as in a visual workplace (which we’ll discuss in Part 3 of this series).


work-cell-layout

Figure 4: Work cell layout



standard-operations-routine

Figure 5: Standard operations routine sheet


calculate-cycle-time

Figure 6: Process cycle time calculation

What skills do people need?

The lean Six Sigma cell layout requires that workers be multiskilled and able to handle any of the tasks in the work cell. This differs from an operations-based layout where, for example, all drill presses are placed together, and a person would only need to know how to operate a drill press. In contrast, a work cell might have drill presses, grinding machines, mills, and a deburring station, and all workers need to know how to operate all of this equipment competently.

How can we keep track of cross-training?

It’s a good idea to keep track of which operator has which skill in a work cell, as shown in figure 7.

cross training chart

Figure 7: Cross-training chart

An additional advantage to the continuous cell layout is that workers don’t waste time standing around waiting for the machine to finish a task. Instead, they load one machine, start it, and move to the next operation in the cell while the machine completes its automatic cycle.

How can we arrange the workplace and assign workers so those working in it can easily help one another?

Yet another efficiency enhancement is that when more than one person is working in the cell, the workers can help each other if one of them falls behind. Quality also improves as workers are able to check on each other’s work and often catch quality problems right away. Work cell layout should deliberately take this into account, making it easy for a more experienced worker to help a fellow worker. Worker assignment should also consider this. Try to assign workers so new or slower people have more experienced and faster workers on either side, or immediately after them in the flow.

Coming up

In Part 3 of this series, we’ll discuss “5S” which represents the five disciplines for a visual workplace: sort, set in order, shine, standardize, and sustain.  

Discuss

About The Author

Tom Pyzdek’s picture

Tom Pyzdek

Thomas Pyzdek’s career in business process improvement spans more than 50 years. He is the author more than 50 copyrighted works including The Six Sigma Handbook (McGraw-Hill, 2003). Through the Pyzdek Institute, he provides online certification and training in Six Sigma and Lean.