Work Cells: The Heart of Lean

The “work cell” can be a pretty mysterious beast to anyone working outside of the Lean Six Sigma arena.

But the simple fact is that most people already know how to design a work cell, whether they know it or not. It really is a matter of taking some rather common sense principles, and using them in every day life.

Think of your kitchen at home. Often, it is designed as a very efficient, or “Lean” work cell. Usually designed in a U-shape, with a sink and refrigerator on one side, a stove and range on the other, food flows around the U-shape ultimately to the kitchen table where it is consumed. One meal is prepared at a time – and food flows in one end of the “U” as groceries, and out the other as a prepared meal.

This two-minute video explains the Lean Work Cell very nicely!

Taking the level of complexity up a notch or two, supply chain practitioners normally think of work cells in the context of manufacturing, but the approach can be used in a variety of commercial applications, which can include but is certainly not limited to office work, distribution centres,  print shops, and – as we have seen in the video – food services.

Cellular manufacturing systems emerged through the 1990′s  in response to the intensification of worldwide competition, when just-in-time (JIT) methods became enabled on the factory shop floor through computer integrated manufacturing (CIM).  The concept of the work cell was born, which in turn allowed forward-thinking factories to improve the efficiency of the flow of production.

According to the classic textbook Manufacturing Planning and Control Systems (4th ed.) by Thomas Vollman, William Berry, and D. Clay Whybark, “cellular manufacturing systems are designed to process part families in dedicated production areas, referred to as manufacturing cells. Benefits associated with the use of manufacturing cells include reduced order flow time, less work-in-process inventory, smaller setup times, lower material handling costs, improved quality and productivity, improved job satisfaction and status, and simplified planning and control procedures.

All of this, of course, is completely consistent with 21st Century concepts of Lean, and work cells have become Best Manufacturing Practice.

In the April 2012 edition of Plant Magazine, Richard Kunst, President and CEO of Kunst Solutions Corp., brings us up to date by  presenting a wonderfully concise summary of how to design and engineer a workcell that embraces Lean principles:

Workcells are at the heart of lean. They increase productivity and quality, while simplifying material flow, management and accounting systems. How well they work depends on subtle interactions of people and equipment. Each element must fit with the others in a smoothly functioning, self-regulating and self-improving operation.

A cell’s design is an engineering concern that proceeds through a logical sequence of steps where compromises between conflicting requirements or technical limitations are made. Doing it well requires a comprehensive knowledge of the elements of a workcell, their functions and interactions. The following elements are key to successful workcell design:

1. Select the products. Find compatible families of products that a group of machines can process without creating other difficulties. Many problems arise from attempting to combine too much variety. Important tools are enterprise value stream mapping and group technology. Which products belong together in a workcell? What is the design production rate for the cell? Is reserve capacity needed?

2. Engineer the process. Understand every process event and the times required for setup, personnel activities and machine cycles. Calculate the number of people, machines or workstations needed and in what sequence? What equipment should be employed and how much of it? How many people are needed? What lot size is appropriate?

3. Define the infrastructure. Its elements support the process but do not touch the product. They include containers, scheduling, balance methods and motivation. Consider the following:

• Methods for material handling.
• How the workload will be balanced.
• Production scheduling.
• How much work in process is necessary.
• How to motivate people.
• How to assure quality.

4. Laying out the cell. Task procedure diagrams can be simplified. Start with the process chart and move directly to a layout.

• What is the best physical arrangement?
• How do we handle external constraints?
• How do we integrate with the overall layout?

Design flow and work breakdown structure by calculating takt time, the maximum time allowed to produce a product to meet demand. In a lean manufacturing environment, the pace time is set equal to the takt time.

Takt time is defined as T= Ta/Td, where:

Ta = Net available time to work [minutes of work/day]
Td = Total demand [units produced/day]
T = Takt time [minutes of work/unit produced]

The net available time for work to be done excludes break times and any expected stoppage time, such as scheduled maintenance or team briefings.

If you have a total of eight hours in a shift less 30 minutes lunch, 30 minutes for breaks, 10 minutes for a team briefing and 10 minutes for basic operator maintenance checks, then net available time is 400 minutes.

If customer demand is 400 units a day and you run one shift, your line would be required to spend a maximum of one minute to make a part to keep up. Realistically, people never maintain 100% efficiency and there may also be stoppages for other reasons, so allowances are needed to set up your line and run at a proportionally faster rate.

Takt time has direct implications concerning the allowable time for completing individual steps in a production process, especially those that modify the product and observe/control the process. Similarly steps that require a part be placed in an accurately fixtured position must be completed in less than the total takt time so additional time is allowed for loading and unloading or positioning. A quicker measurement or test step places less constraint on product motion between steps. For example, a measurement process that captures all the information about a part in one go will permit shorter total takt time and a higher pace of production flow

Using takt time reorganizes work packages. If worker one performs actions A1 through A5 and worker two performs actions A6 through A8 then a reduction in takt time would mean three work packages to fit the new shorter/faster pace. They might be package 1 (A1 to A4), package 2 (A5 to A6) and package 3 (A7 to A8). Now three people are doing the work that was achieved by two.

Subdividing work-packages rather than working in parallel on unchanged packages of actions is new to many people. This way of working requires:

• a flexible workforce willing to accept changes in routines;
• a multi-skilled workforce, since people may be asked to pick up actions performed by others;
• flexible work cells – what two people do today may need three people tomorrow;
• more hand-offs, so no significant overhead;
• simple workflow, so what the process delivers is clear to all;
• speeding up production steps because the new context of each action encourages innovation.

How do you get lean buy-in from cell team members? Follow these steps:

1. Treat each of them like a customer.
2. Create a set of experiences that walks them from current to desired belief.
3. Overcome the valid “no.” Most people don’t say “no” to be difficult. Maybe it’s risk; so mitigate it. If it’s time; find the time.
4. Call in reinforcements. Would someone else influence team members more? Engage that person’s help.
Don’t rely on hope. Own it. And do the hard work. After all, an idea unrealized is worth very little at all.

Richard Kunst is president and CEO of Kunst Solutions Corp., which publishes the “Lean Thoughts” e-newsletter.

Comments? E-mail jterrett@plant.ca.

This article appears in the April 2012 edition of PLANT.

Sumber : 

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