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Tom Pyzdek

Six Sigma

Developing a Standardized Approach to Work, Part 1

The elements of continuous-flow work cells

Published: Tuesday, June 29, 2010 - 11:15

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).

 

I

n my training courses, I teach how to change a traditional batch-and-queue value stream into a lean value stream. Although that’s valuable information on its own, in this article I want to discuss the design of the actual work that will take place within the processes of the value stream.

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By going a level deeper, we will be able to improve the flow of work within the different processes in the value stream. Specifically, I will explain how to design continuous-flow work cells. The discussion here focuses mainly on manufacturing work cells, but the lean principles described apply to any work, including that done in administrative, transaction, or services such as health care, retail, and so on.

Selecting subprojects

The first step is to identify subprojects within the value stream. Subprojects, sometimes called project “loops,” are determined by looking at the future-state value-stream map and choosing groups of related processes in the value stream for improvement analysis. Each subproject will require a different team with its own set of knowledge, skills, and abilities.

However, it is desirable to have at least one member of the lean Six Sigma team who participates on all of the subproject teams.

Once subprojects are identified, the lean Six Sigma team must decide which to pursue first, second, and so on. It is a good idea to begin at the customer end of the value stream and work backward. This provides the customer with improved service that they can see and feel quickly.

Another criterion is that the pacemaker process should be improved early, because it sets the pace for the rest of the value stream. The inside-out rule should be observed: Get your own house in order before extending your improvement efforts to include the value streams of outside customers and suppliers. Of course, your decision regarding the starting point should also take into account the likelihood that the subproject will have a big effect on the business and its customers.

Elements of work

Figure 1 shows the relationship between value streams, processes, operations, workplaces, and procedures in creating value. The relationship is hierarchical. To implement lean all levels of the hierarchy are considered. In designing work cells we will go deeper than the process level and look at the design of operations, including the layout of workplaces and the standard procedures followed to perform the work in each operation. Such operations are known as “standard operations” because the way work is performed follows strict standard procedures.

Figure 1: Value-creation hierarchy

Processes are distinct sets of operations nested within a value stream. In the context of designing continuous-flow work cells in lean Six Sigma, we focus primarily on the things in a process that inhibit flow, such as:

• Nonvalue-added process steps on the opportunity map

• The distance people, materials, or work-in-process (WIP) travel between process steps

• Changeover, setup, and adjustment time (discussed below)

• The root causes that create quality issues that are responsible for scrap, rework, or problems downstream

 

In lean Six Sigma we design work cells that improve the process as well as the specific operations within a cell. We get into nitty-gritty details of the work itself, considering fixtures, workplace layout, how materials are handled and moved, and the movement of various workers. The transfer of work elements (i.e., small units of work) between workers is carefully considered. “Work” is the sum of all of the work elements required to create one complete unit through the entire value stream.

Principles of motion economy

The rigorous study of efficient work design by Ralph Barnes predates lean Six Sigma by several decades.1 As a lean Six Sigma Black Belt or Green Belt, you should take advantage of this by learning the principles discovered long ago. Here are those principles most relevant to the design of work cells. You will see that we draw on these principles heavily when we discuss specific recommendations for work cells. Knowing the principles on which lean Six Sigma is based will help you understand why the recommendations are made, and it will make it possible for you to go beyond lean Six Sigma to discover improvements of your own.

Use of the human body

• The two hands should begin as well as complete their motions at the same time.

• The two hands should not be idle at the same time except during rest periods.

• Motions of the arms should be made in opposite and symmetrical directions and should be made simultaneously.

• Hand and body motions should be confined to the lowest classification with which it is possible to perform the work satisfactorily.

• Momentum should be employed to assist the worker wherever possible, and it should be reduced to a minimum if it must be overcome by muscular effort.

• Smooth continuous motion of the hands is preferable to straight line motions involving sudden and sharp changes in direction.

• Ballistic movements are faster, easier, and more accurate than restricted (fixation) or controlled movements.

• Work should be arranged to permit an easy and natural rhythm wherever possible.

• Eye fixations should be as few and as close together as possible.

 

Arrangement of the workplace

• There should be a definite and fixed place for all tools and materials.

• Tools, materials, and controls should be located close to the point of use.

• Gravity feed bins and containers should be used to deliver material close to the point of use.

• Drop deliveries should be used wherever possible. 2

• Materials and tools should be located to permit the best sequence of motions.

• Provisions should be made for adequate conditions for seeing. Good illumination is the first requirement for satisfactory visual perception.

• The height of the work place and the chair should preferably be arranged so that alternate sitting and standing at work are easily possible.

• A chair of the type and height to permit good posture should be provided for every worker.

  

Design of tools and equipment

• The hands should be relieved of all work that can be done more advantageously by a jig, a fixture, or a foot-operated device.

• Two or more tools should be combined wherever possible.

• Tools and materials should be prepositioned whenever possible.

• Where each finger performs some specific movement, such as in typewriting, the load should be distributed in accordance with the inherent capacities of the fingers.

• Levers, hand wheels, and other controls should be located in such positions that the operator can manipulate them with the least change in body position and with the greatest speed and ease.

 

Considerations

What do we need to produce? How many and when?

How fast do we need to produce in order to meet customer demand (takt)? Goal: produce precisely this much just in time.

Takt time

Takt time is used here when developing the continuous flow cell. You will recall that takt time, which is synonymous with cycle time in lean, is calculated as takt time = effective daily operating time / required quantity per day. Once this has been calculated, the amount of work for each worker is determined so he can work at a constant cycle time. No extra margin or “fudge factor” is allowed. In addition to takt, the team also determines the speed, degree of skill, and other standards required. New workers are considered trainees until they are able to consistently produce quality work at the required rate.

As with value-stream design, the work cell uses takt time as the standard cycle time. Because no fudge factor or unplanned downtime is included in the calculation, waste becomes obvious. There will also be individual differences in ability that will emerge. Although all workers are required to match takt time, some people will be able to produce faster than others. Allowing them to do so in a work cell would be counterproductive; it would merely result in accumulated work-in-process inventory. The work cell must be designed to productively utilize the skills of the superior workers for the benefit of the team and the company. More about this later.

Considerations

How fast do we need to produce in order to meet customer demand (takt)? Goal: produce precisely this much just in time.

Identifying work elements

Decomposing a process into work elements helps you identify sources of waste and allocate work among people. This involves identifying and timing each work element. You will find that some of the work elements are repeated in every work cycle, while others are not. Examples of nonrepetitive work are replenishing supplies or getting tools. Work cell design considers only work that is involved in every cycle. Nonrepetitive work is either converted to repetitive work, eliminated, or done outside the cell.

To identify the work elements, begin by defining the scope of the work being evaluated. This will be a subset of the work done within the cell. Watch a qualified operator do the work several times. For a while, just observe the work being done to help you get a feel for what’s involved. Once you have a sense of the whole, break it down into specific elements. Write down a description of each element and have the team participate until you arrive at a description that everyone understands and agrees upon. Be sure each element description has a clear start and end point. Describe the sequence in which the elements are performed. Identify which activities are done by people and which are done by equipment. Finally, record any nonrepetitive work that must be done outside of the cell, or which can be eliminated.

What are the specific tasks required to complete the work?

Overall cycle time for the value stream and each process within it is determined by customer demand and the time available for work. This also applies to the rate at which parts are produced within a cell. Because parts are completed as work elements are performed, work elements must be timed. When collecting data on work elements, the time it takes a capable worker to complete each work element must be determined. Collect actual data from various people doing the work. Work element time won’t be based on the very fastest or slowest time; rather, you are looking for a representative time that can  be performed repeatedly over time. The descriptive statistic most useful for this purpose will be a measure of central tendency, such as the mean, the median, or the mode.

Process Work Element Person Time 1 Time 2 Time 3 Time 4 Time 5 Meon Medion Mode
Weld + Inspect Place in jig A 5 3 5 5 4 4.4 5 5
B 7 6 6 8 5 6.4 6 6
C 5 4 4 4 3 4.0 4 4
Figure 2: Work-element time data

Figure 2 shows an example of a process study that recorded the time it took three different people to place a part in a weld jig. Five times were observed for each person, the times recorded, and statistics calculated. Based on these data, the team will determine the standard time for this work element. (What would you choose to be the standard time? 3) I suggest that work elements be timed by recording several cycles with a camera and evaluating the recording off line. You can use the video timeline to identify precisely when an operation starts and stops, or you can get this information with the pause button and a stopwatch.

Take a look at figure 2 again. Note that Worker B takes longer than either Worker A or Worker C. Such person-to-person variability is to be expected; people are different, after all. In the work cell design for multiple workers, you want to arrange overlap in areas of responsibility so the faster workers can help whenever the slower workers fall behind. Lean recognizes it’s the team, not the individual worker that produces value. It makes no sense to have the faster workers in a cell producing at a rate that exceeds takt while slower workers produce at a rate less than takt. People must work together to help their company compete.

Once you have the cycle time data for all of the work elements, you can combine the data to determine the overall production capacity.

Figure 3 shows this analysis for a machining process. The production capacity (column I) is based on the net operating time per day, which does not include scheduled breaks or lunch (cell I7, 51,600 seconds) divided by the total time per piece. The production capacity for the machining process is the smallest production capacity for all required operations. In this case, the value is 506 units, the production capacity for the Bore 10 mm ID operation. Because this exceeds the quantity needed per day of 255 (cell I5,) this process has adequate capacity, assuming unplanned downtime, scrap, etc. are held in check.

Figure 3: Production capacity for machining process

Coming up

In Part 2 of this series we will discuss bottlenecks, autonomation, flexible machines, and workplace arrangement.

 

Footnotes

1. Barnes, Ralph M. Motion and Time Study Measurement of Work (1937; reprinted 1980 by John Wiley & Sons). This is the classic, seminal work on the subject, and these principles are still relevant today.

2. Gravity feed bins, gravity chutes, and other mechanisms that “drop” the needed parts and tools to the proper place for use by the worker or for moving the part(s) to the next operation.

3.  I would choose 5 seconds. It is close to the overall mean, and it’s the average of the three medians and also the average of the three modes.

4. Cost accounting systems are often designed to measure output from individual pieces of equipment rather than from value streams. These systems need to be modified to accurately measure the lean value stream; failure to do this can undermine the entire lean Six Sigma program.

 

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.