The word “flexible” is defined many different ways by different manufacturers. That inconsistency can make it difficult to measure and monitor the effectiveness of flexible assembly lines.

“Reduction in changeover time of a manufacturing system is a good way to measure flexibility, since it also implies equipment reconfigurability,” says Frank Chen, Ph.D., director of the Center for Advanced Manufacturing & Lean Systems at the University of Texas at San Antonio. “But, the most critical measure of flexibility would have to be the number of product types that the system can produce with moderate or little changeover efforts.”

To get a realistic handle on cost, changeover time should be measured from good part to good part. “A good part is a saleable part or package that meets all the customer requirements and parameters,” says Rick Harris, president of Harris Lean Systems Inc. (Murrells Inlet, SC). “The physical changeover may take 20 minutes, but the good-part-to-good-part time may be 2 hours. So, in that instance, the real changeover time would be 2 hours instead of the 20 minutes that most people think about.”

Two factors should be included when measuring and monitoring changeover speed: The mean and the variation of the changeover speed, measured from the last good part of one run to the first good part of the next run. “There are several reasons for this,” notes Jamie Flinchbaugh, a principal at the Lean Learning Center (Novi, MI). “If I measure the trend of the mean, it will tell me if I’m getting better. However, one of the major problems in changeover is lack of standardization, and the variation will tell me that.

“We want to see that variation moving closer and closer to zero,” adds Flinchbaugh. “It is also important, for the sake of making improvements, to measure the time spent in different elements of the process, because it will tell if you have to work on coordination issues, technical issues or communication issues.”

Another good way to measure flexibility is to examine takt time. For instance, consider the ability of a flexible line to meet customer takt time with as little disruption in flow as possible.

“This might require measuring changeover time and other process changes,” notes Mark Dinges, product marketing manager for linear motion and assembly technologies at Bosch Rexroth Corp. (Buchanan, MI). “If so, it’s important to establish baseline metrics for the items you’ve defined as key indicators of flexibility and seek to improve the performance constantly. The ultimate metric, however, is the ability to meet customer demand.”

To measure flexibility, it’s important to identify key metrics that are relevant to your company. “Manufacturers need to tie it back to how the flexibility enables a competitive advantage,” explains Lou Mello, global segment business manager for assembly, manufacturing and alternative energy OEMs at Rockwell Automation Inc. (Milwaukee). “Then, manufacturers need to make sure the equipment can produce that data and feed it up to the information system.”