Overall equipment efficiency (OEE) is just as it sounds: the measure of how efficiently a stamping press, CNC machine tool or other piece of industrial machinery operates. The term was coined by Seiichi Nakajima, an engineer at the Japan Institute of Plant Maintenance and father of the Total Productive Maintenance system, who shared his important concepts with the world in the 1984 book, Introduction to TPM.
OEE is based on three values, Nakajima told us: availability, performance and quality. The first of these is determined by dividing the planned production time (i.e., shift length) by the amount of time the equipment actually made parts. If a machine was down for 45 min. during a 10-hour shift due to unplanned tool maintenance, availability falls to 93 percent. Quality is equally straightforward. When 63 of the 1,850 parts produced during that shift fail to pass inspection, the quality level is just 97 percent; not abysmal, but nothing to write home about, either.
Now that we know how many parts the machine delivered that day, it’s possible to calculate performance, which is the total count divided by the number of parts that should have been made based on the ideal cycle time. Put another way, performance considers slowdowns due to common events like minor tool adjustments, replacing a spent coil and pausing the press long enough to clear a slug. In this example, the ideal cycle time is 15 sec. for a performance level of 83 percent. Again, nothing to brag about.
Here’s the formula for any readers that would like to check it for themselves:
Availability: (Operating Time / Planned Production Time) X 100
Availability = (555 / 600) X 100 = 93 percent
Performance: (Ideal Cycle Time × Total Pieces) / Operating Time X 100
Performance = (0.25 X 1850) / X 100 = 83 percent
Quality: (Good Pieces / Total Pieces) X 100
Quality = (63 / 1850) X 100 = 97 percent
OEE: Availability X Performance X Quality
OEE = 93 percent X 83 percent X 97 percent = 74 percent
It might seem unfair that these otherwise decent productivity values equate to a less-than-stellar OEE score, but that’s how it works. Ironically, a manufacturer must hit 95 percent on each of these metrics in order to achieve 85 percent OEE, a value considered “world class” by most experts.
That said, acceptable OEE levels are highly dependent on the type of work, production quantities, changeover frequency, process stability and a host of other factors. What’s more important than targeting an arbitrary value, therefore, is to establish a baseline for the shop and then engage in continuous improvement efforts to raise it as much as possible.
Higher performance
René Zwahlen, president of Sangiacomo Presses Americas LLC and operations manager for Bruderer Inc. (co-located in the same Huntsville, Ala., facility), agrees with the importance of realistic OEE targets, but takes exception with one of the values used to calculate it – performance – although not for the reasons one might suspect.
“How does one determine the ideal cycle time, and with that, acceptable productivity levels?” he says. “In my experience, there’s a great deal of misconception across the industry. Visit most any stamping house and management will tell you they’re running at a specific efficiency level – let’s say 90 percent – and they’re quite content with that. I counter by saying, ‘But how much higher would your productivity be if you had the optimal press?’”
Zwahlen has been quoted in this magazine and elsewhere expounding on the benefits of adjustable stroke stamping presses. These include greater throughput, longer tool life, less downtime and better part quality. And while his enthusiasm for the decades-old technology continues unabated, that’s not what he’s talking about here; instead, he’s talking about the stamping industry’s tendency to sacrifice speed for increased flexibility.
“Most everyone does it to some extent, but I notice that job shops in particular like to invest in machines with higher tonnage and longer stroke lengths than the majority of their work requires, just so they can accommodate the occasional tall part or thick material,” Zwahlen says. “The result? The press is slower and the majority of production is far less efficient than it could be had they only bought equipment better suited to those requirements.”
Adjustable but…
Again, Zwahlen isn’t necessarily advocating for adjustable stroke presses, although these certainly address much of this problem. Rather, he’s talking about selecting machinery with stroke lengths at or near optimal for the work a shop excels at (or could excel at, given the right press).
“At some point, you as a press buyer have to make a decision,” he says. “If you’re stamping a few hundred of this and a thousand of that, you might be willing to accept a relatively high level of inefficiency in order to handle a wider range of work. But when your job quantities are in the tens or hundreds of thousands, you need to make them as quickly as possible. This warrants investment in a press with the correct stroke length and tonnage for that part, allowing you to focus on and maximize the subset of work at which you are highly efficient.”
Granted, this could mean making the strategic (and usually unpleasant) business decision to walk away from jobs that don’t fit, to no-quote or outsource work that doesn’t fit the shop’s capabilities or, as Zwahlen is more than happy to discuss, ask whether it finally makes sense to invest in a far more flexible machine – an adjustable stroke press.
“It’s different from company to company, situation to situation, but sadly, these questions are rarely asked,” he notes. “There’s little dialogue about optimizing processes or why things are done a specific way, let alone whether a different machine would be more efficient with certain part sizes, materials and geometries. More often, you’ll just hear, ‘That’s as fast as the job can run’ or ‘‘It’s always been like this and there’s no reason to change it.’ Discussion closed.”
Making changes
Unfortunately, many shops don’t even realize the immense improvement opportunities available to them. Everyone sees that the press is going up and down during the day and they call it good, neglecting to quantify the actual OEE levels by engaging in the necessary data collection and calculations described earlier.
Doing so requires (or is at least made easier by) some small investment in analysis software, and perhaps some integration with the press controller. Worst case, it might mean installing sensors on older equipment, although this is a rarity. As discussed in Stamping Productivity’s September feature, “Monitoring Motion,” Wintriss Controls Group and other press automation providers offer “very comprehensive monitoring and control of a metal stamping press” and have been doing so for decades.
“The optimal stroke is the shortest stroke, period,” Zwahlen says. “The shops that understand and use this approach are seeing 30 percent and greater efficiency gains. But unless you’re willing to ask the tough questions and decide how much inefficiency the shop (and your customers, ultimately) are willing to accept, you’ll never know how much it’s truly costing in terms of OEE, part quality, tool life and lost opportunity.
“However, the fact is too few manufacturers in this country are willing to embrace the change needed to remain competitive in our rapidly changing world,” he concludes. “We as an industry need to look at stamping efficiency as more than, ‘Is the ram moving up and down?’ Until that happens, we’ll continue to see more and more work go overseas.”
Sangiacomo Presses Americas LLC
