Many manufacturers believe in traditional “long runs” for manufacture because it is supposed to be more efficient to run a big batch than to run several shorter batches that include changeovers. But the manufacturers that are leading their industries have found that just the opposite is true.
If changeover times are drastically reduced and simplified, we can changeover more often and please more customers. Today’s customers do not want to hear that they have to wait for long runs of “other” products before you will get around to making their product. Quicker changeovers on smaller and more flexible equipment makes it easier to please many customers while reducing the overall cost of holding large quantities of inventory that is waiting for production opportunity.
Traditionally, we have identified the approaching end of a long run and, when finished, turned off the equipment. We then started to clear the machine of old products and went to get the necessary equipment and tools to do the next changeover. We pulled out the old dies and tooling, installed the new dies and tool, and set the adjustments needed to produce the new product. We cleaned up after the changeover and then started up the equipment to begin the new run.
Typically this process took anywhere from 1.5 to 6 hours. The actual changeover of dies and tooling took only 30 minutes to an hour. What were we doing all the rest of the time? To answer this question we have to record the actual changeover very carefully and separate the external activities from the internal ones.
Many of the activities described could have been done “externally,” while the equipment was still producing the last pieces of the first run. The only “internal” activities are the ones that require “Power Off.” All of the other activities can be done before or after the actual changeover and do not require Power On.
The first step to drastically reducing total changeover time is to have as many changeover tasks performed “externally” as possible, while the end of the current run was still finishing with Power On. An additional person (or the operator who has the time, in many cases) can prepare by collecting the required dies and tooling, specifications, and needed transfer equipment ready without shutting off the equipment or ending the first run.
The same is true after the dies and tooling are in and adjustments have been made: the equipment can be turned On to start producing new parts while the old dies, tooling, tools, and transfer equipment are returned to their proper locations. These first improvement steps cost almost nothing to change, but are sometimes the hardest to do because of years of old habits and resistance to change.
Once many of the old “internal” activities have been moved to external and are done before or after Power Off, the next step is to reduce the time required for the remaining internal activities. A valuable resource available on the subject is A Revolution in Manufacturing: The SMED System by Shigeo Shingo. His referral to SMED stands for Single Minute Exchange of Dies. (He believes the target for all changeovers should be 9 minutes or less. Pretty fast, huh?)
There are many ways to reduce changeover times. Installing and removing dies or tooling can go faster with roller tables or conveyors. Often, transfer equipment can be modified to also serve as installation and removal devices. Hydraulic clamps can be used in the place of screws or bolts along with guide pins and hard stops for alignment. Connectors can be ganged together and hoses can be joined using a manifold to reduce the number of connections to take off and reconnect. Set screws that require specific tools and are used for tightening can be replaced with knobs and fasteners that can be tightened quickly by hand.
And, of course, the handling of dies, tooling, tools, and spec sheets can be improved in most facilities. All of the dies and tooling should have their status, i.e. serviceable, broken or being modified by engineering, clearly marked and tagged using visual control techniques. Likewise, all of the tools and the spec sheets should have specific homes that are clearly marked and should be returned to their homes after use. These 5S practices make the overall process of completing a changeover less stressful for everyone involved.
If you put together a cross-functional team from maintenance, operations, quality assurance, and the tooling department (if it is separate from maintenance), the results can be amazing. These people have many ideas on how to improve changeovers and reduce the time required. They need to be empowered to suggest, plan, and implement these improvements.
So now, we begin to see how all of these tools, together in concert with people who “get it,” who buy into the culture and improved way of conducting business, really do make a difference. In cases like these, it’s the sum of all the little tools, and all of the people using them, that can overturn the old idea that “long runs are better.” There’s a whole lot of ideas like this that a good organization can overturn.