|
|
The Impact of Takt Time on Cell Design
In any lean manufacturing environment, time, that is time well spent, is a critical component of manufacturing efficiency--so it only makes sense that the first step in the design of manufacturing cells involves the determination of the customer demand rate or takt time. As shown in the following excerpt from "Lean Manufacturing: A Plant Floor Guide", a new book scheduled for release by SME in the fall, this measure of time influences everything from asset allocation to the physical layout of the cell.
Takt time is determined by dividing customer demand for a specific period by available manufacturing time during that period. Consider a manufacturing firm that must produce 38,400 widgets for the next month on a processing line that operates continuously 16 hours a day. Twenty work days per month equals 320 hours per month, and 38,400 divided by 320 equals 120 widgets per hour, two pieces per minute, or one every 30 seconds. The takt time is thus 30 seconds, unless the line only runs only one shift, in which case it is half that or 15 seconds. Most processes do not operate continuously, though, so break times and lunch must be deducted from the available time.
 Cycle time of operator three exceeds the takt time
With takt time identified, the next step is to determine asset allocation for the process. Say, for example, that a small hydraulic press required for an operation can produce 60 pieces per minute, yet the takt time for that process is 30 seconds or two pieces per minute. One-piece flow requires that the press produce only two pieces per minute. The options are to buy a press that produces at the takt rate or to use the overly capable press at a much slower pace. If the latter option is chosen, the press operator should have additional responsibilities to consume the balance of the takt time.
One goal of the cell design stage, therefore, should be to incorporate right-sized equipment that meets the takt time whenever possible. If the manufacturing cell will also include overly capable equipment, the next step will be to balance operations. In Figure 1a, for instance, the cycle time of operator three exceeds the takt time. With no changes to the balance, overtime will be necessary to satisfy customer demand. With one-piece flow, operator three controls the pace of processing, so output falls below the customer demand level. Before moving to a cell operation, the implementer must eliminate the waste in operation three or determine how to assign some of that operator's job steps to the other operators in the prospective cell.
 Balance load across operations so cycle time is just below takt time
Figure 1b illustrates the results of balancing. The key to balancing a process lies in always loading the operations so that the cycle time is just below the takt time. The normal result will be a final operation not fully loaded. In this case, operator five then has time to assist the other operators, clean up, or perform maintenance. The ultimate goal, though, should be to reduce the cycle times of the first four operations enough so that the fifth operator can be shifted to another cell.
Takt time also plays a role in physical cell layout. Of the various options, the U-shaped cell generally uses space more effectively and reduces material handling time. S-shaped cells will work when the process includes a large number of operations, like an automobile assembly line. I-shaped cells are effective when deploying a small number of operations. L-shaped cells are the least effective design because space is not used effectively.
For practical purposes, assume a cell will have a U shape. When developing the most efficient layout of the cell, the cell designers should strive to do the following:
1. Minimize the square-footage for each operation,
2. Allow space for small containers of detail parts,
3. Keep the length of conveyors to a minimum,
4. Implement counterclockwise process flow (most people are right-handed),
5. Locate cell operators inside the U.
In the final layout of the U-shaped cell, the process will thus flow from right to left, with the operators positioned close together to facilitate efficient communications. Throughout the analysis phase, engineers also should have balanced the cycle time for each operation to make it just below the takt time.
John Allen, Charlie Robinson and David Stewart
John Allen and Charlie Robinson are principals, and David Stewart is research director of Total Systems Development, Inc., a lean engineering consulting company based in Lexington, KY. For more information, please see the company's Web site at www.leanmanufacturing.com.
Copyright © 2010 Society of Manufacturing Engineers
SME Customer Service: 1-800-733-4763
|
|
|
|
 |
| May 9, 2001 Issue: |
|
|
