Changeover time represents the elapsed time to changeover from the production/processing of one product or service to a different product or service. Admittedly, there is little math involved here. Some readers may scratch their heads and wonder why one would need a math entry at all! However, changeover time is important in the overall scheme of Lean Math and, with it, the principle of flow and pull. More specifically, internal changeover time (T∆i) is a critical driver of batch sizes, lead time, and intervals.
Changeovers are really a subset of setups, see nearby figure. However, the two terms are often used interchangeably. For the purpose of this post, we'll treat the terms as the same.
The legendary Shigeo Shingo was ostensibly the first to formally differentiate between internal and external changeovers. This became the foundation of his set-up reduction approach known as single minute exchange of die (SMED), exchanging dies in less than 10 minutes, and enabled revolutionary elimination of, or at least reduction, in batch sizes.
Internal changeover represents the elapsed time between the production/processing of the last piece (part, transaction, or service) of the prior run and the first good piece of the next run. External changeover represents the elapsed time during which workers conduct the preparatory activities for the changeover BEFORE the last piece of the prior run and/or the elapsed time for any changeover related activities AFTER the first good piece of the next run. Changeover reduction strategy is summarized in the internal changeover post. This all leads to some straighforward math:
T∆ = total changeover time, typically in minutes or seconds
T∆e = external changeover time, in the same unit of measure as T∆
T∆i = internal changeover time, in the same unit of measure as T∆
Example. Mathtronix stamps metal parts for use in their assemblies. One of the company’s workhorses for smaller parts is a 60-ton press. Prior to their changeover reduction activity, one service operator would review the schedule to determine the next run and then locate and pre-stage the die for the next product prior to the changeover. Depending upon fork lift availability, this external changeover activity could take up to 17 minutes. The operator would then, after completing the last part of the prior run, shut down the press, locate the necessary tools, remove the old die, position and attach the new die, run one or (many) more pieces, make adjustments, and ultimately conduct a first-piece inspection when the adjustments were deemed successful. This internal changeover time routinely took approximately 73 minutes. The service operator would later pick up the old die and return it to its home location. This particular external changeover time took about 14 minutes.
T∆= 17 minutes + 14 minutes + 73 minutes = 104 minutes