December 1996 € Volume 6 € Number 12

Virtual Cells:
A Good Tool For Quick-Response Manufacturing

By Dietrich L. Petersen, Ph.D., CPIM

In today's demanding business environment, shorter delivery cycles are frequently a key competitive weapon. Delivery cycles often can be improved in job shop facilities by speeding up order entry processing, reducing machine set-up times, and utilizing equipment arrangements that are known as cells.

A traditional cell is an area in the shop where dissimilar machines are located close to each other so that all productive work on a specific part can be done in that one location. Significant cycle time savings are possible because parts do not need to be queued up behind various machines waiting their turn to run, little movement of material is required, and the machines in the cell are dedicated to producing a limited number of items. Also, setups can be eliminated or drastically reduced by dedicating machines in the cell to producing a small variety of parts.

In many manufacturing situations, however, establishing traditional cells proves to be difficult or impossible for various reasons. There may be uncertainty about which machines and part numbers to use, plant floor space may be limited or unavailable, or some of the equipment required for a cell may be "monuments"&emdash;machines that are too difficult or too expensive to relocate because they require heavy foundations or need complicated "tethering" central systems for dust collection, cutting oil filtration, etc.

As a result, even though cells could be a positive factor in improving delivery cycles, they often are unachievable.


Virtual cell solution
Fortunately, a solution to this dilemma can be found in the use of a "virtual cell." It is created&emdash;without moving equipment from functional departments&emdash;by temporarily dedicating one or more machines in different departments to simultaneously run sequential operations on the same shop order. Then, by moving small quantities of parts continuously, lead times are reduced just as they are with traditional cells, but without rearrangement costs or any implementation delays. Departmental machines not used in a virtual cell continue to be assigned batch work per the production schedule.

The virtual cell approach was developed in a recent consulting assignment to help achieve substantial reductions in cycle times to meet competition. The client's manufacturing facility included a number of monuments in the process flow, and machines were grouped in traditional functional departments such as mills, lathes, and so on.

This plant also had a number of manpower, material systems and machine problems that were being solved over time with various corrective action programs. However, there was an immediate, critical need to start shipping "on time" to prevent further erosion in new customer order levels. The virtual cell concept was developed and implemented to deal with this "life or death" issue before the longer term action programs were completed.


Cut cycle times in half
The cycle time reductions can be seen by comparing a traditional job shop schedule for functional departments to one utilizing the virtual cell concept.

In Table 1, parts are run in batches of 100, then moved to the succeeding departments and queued up for their turn in the production cycle. In Table 2, parts are run simultaneously in more than one department after being moved in small quantities. Significant lead time savings are achieved because parts do not wait in order-size batches while setups are being performed or other parts are run.

In actual practice, because the mix and volumes of part numbers produced varied widely, up to 44 percent of the weekly load in key departments was scheduled with virtual cell volumes. Total machine loads frequently ranged from 50 percent to 72 percent of utilized capacity. The number of virtual cells running at one time ranged from zero to three. Sometimes cells were put together for a single order, then dismantled and the dedicated machines returned to job shop operations. If there was only a short interval, direct labor productivity may decline if the planned work flows do not materialize. The inevitable disparities in run times lead to queues building up or to incoming material drying up. These may be handled by working overtime or, if necessary, by reassigning manpower temporarily until in-bound material starts to show up.

Using virtual cells in functional departments eliminates the foreman's capability to assign the dedicated cell machines to regular job shop orders. During periods of high machine utilization, if relatively large amounts of virtual cell work coincide with major volumes of critical job shop work, the resultant loss of flexibility may prove unacceptable. Ultimately, plant management must determine when it makes sense to reduce increments of flexible job shop capacity and dedicate them to simultaneously running single orders in virtual cells.


Powerful operations technique
Implementation of virtual cells can provide substantial reductions in job shop cycle times. They can be installed in a very short period of time with minimum implementation expenditures. However, during periods of high machine utilization, the benefits of cycle time reductions may be outweighed by the loss of individual machine capability and scheduling flexibility. In the end, judicious use of the virtual cell concept can be a powerful operations technique in job shop operations.

Contributors note: Material submitted for this column must be original and no more than 650 words in length. Include a proposed title, short statement establishing your credentials, your name, company, business address and telephone and fax numbers. Send to: Henry H. Jordan, APICS Consultants Forum, 900 Secret Cove