It’s been used for decades to reduce downtime on stamping presses and other fabricating equipment. The term SMED, short for single-minute exchange of die, was invented sometime during the late 1960s by Japanese industrial engineer Shigeo Shingo, although most experts agree that he did not invent the technology itself. That honor goes to coworker and mentor Taiichi Ohno as part of his efforts in developing the Toyota Production System.
Even here, however, it appears that Ohno built on his observations of American stamping equipment and the subsequent purchase of multiple machines from the press builder Danly, which was well on its way toward rapid die change technology as far back as 1955. The “single minute” part of SMED is a bit of a misnomer, as well, in that the actual goal is to change a die set in “single digits,” meaning 10 min. or less.
Exploring quick change
Perhaps your shop has achieved this important milestone. It’s more likely though, that you still have some work to do. Whatever the case, and whatever the origin of quick-change die sets, the most successful stamping houses are those that have taken the SMED ball and run with it.
“The decision to use quick-change stamping tooling is largely driven by how many parts you want to make in a lot, the complexity of the tooling and how accurate you need to be,” says George Munschauer, regional sales manager at AIDA-America. “There are extreme die changes that happen in 60 sec., but I’d say the most common range is 5 to 15 min. Then there’s the shop that uses a fork truck to jockey the die set into place and then bolts or toe-clamps it in place. Depending on the size of the tool, this can easily take a couple of hours.”
If this last example best describes your shop, you’re not alone. Munschauer estimates that the majority of 400-ton and larger stamping presses are equipped with some sort of quick-change system, primarily due to the challenges associated with handling massive die sets.
But, for everyone else – especially those performing multiple setups per day – he’s happy to recommend a range of other quick-change options, including hydraulic, electromechanical and magnetic clamping systems. Each has its pros and cons, but all will improve profitability, flexibility and machine uptime.
Beckwood Press’ ASRS uses a trolley cart to stage dies while the stamping press is running production.
Catching a train
There’s more to the SMED story than getting dies into and out of the machine quickly and accurately. The stamping press might be busy cranking out hundreds or even thousands of parts per hour, but substantial human effort is still wasted transporting and organizing die sets, not to mention the safety risk involved with moving dies that weigh tens of thousands of pounds.
Die carts and overhead hoists are often the preferred methods, but what if you could automate die storage? What if you could push a button and have the die return to its designated location, then pick up whatever is needed for the next job, all on its own. According to Josh Dixon, director of sales and marketing at custom machine manufacturer Beckwood Press Co., that’s exactly what smart shops are doing.
“In response to the increasing demand for quick die changeover and improved operator safety, Beckwood recently developed an Automated Storage & Retrieval System (ASRS),” Dixon says. “The ASRS uses a trolley cart and T-table to stage the dies while the press is running production, and when the current run ends, airbags lift the tool, allowing it to be shuttled from the T-table into the press. Once the tool is in position, hydraulic clamps lock it in place, completing the die change sequence. While the newly loaded tool is running in the press, the ASRS returns the previous tool to its storage location and begins to stage the next sequence.”
Beckwood’s ASRS can store, stage and shuttle up to 20 tools, each weighing up to 15,000 lbs. With the ASRS, one Beckwood aerospace customer reduced die changeover time from 2 hours to just 10 min. with no operator intervention.
To implement this technology, the stamping press must be equipped with an automated clamping system. In many cases, this is accomplished using hydraulic piston clamps or ledge clamps, although a variety of options exists, including magnetics.
“The viability of a magnetic clamping system is primarily based on the application, the amount of clamping force that’s required and the amount of available surface area,” Dixon says. “For example, if each tool has base plates that are commonly sized on the top and bottom, stationary hydraulic ledge clamps are an affordable and effective option. But if the dimensions of the clamping surface vary from tool to tool, you’ll need a more flexible option like chain-driven clamps or a magnetic system.”
Because of all the available technology and the variability of stamping presses and parts, Dixon strongly recommends that fabricators consult with a knowledgeable application engineer to help them navigate through the different options and find the best fit for their application.
Magnetic workholding, like the type from Beckwood Press, offers quick changeover and flexible clamping for a variety of stamping applications.
Jeremy Edson, stamping product manager at Wilson Tool International, adds that high-quality punches and dies are at least as important as the method used to locate and clamp a die set. What’s more, he suggests that the ability to quickly service the die – both in the machine during production as well as routine maintenance in the toolroom – is another excellent way to improve stamping profitability.
To this end, Wilson Tool has developed its HP Accu-Lock and ball-lock style punch retainer systems. According to design engineer Mark McFetridge, these allow you to avoid pulling the die set out of the stamping press for servicing.
“Both systems use a ball bearing held in place by a spring,” he says. “You press on the spring with a screwdriver-like removal tool, pop the punch out, sharpen it and you’re right back in production.”
Wilson Tool’s HP Accu-Lock and ball-lock style punch retainer systems allow stamping press operators to service die sets without removing them from the machine.
Unlike custom retainers, which McFetridge says are expensive and can take weeks to design and build – typically at a specialty tooling house – installing the Accu-Lock is a matter of milling or wire EDMing a hole, cutting a counterbore and dropping the retainer in place. “It takes a matter of hours, and any shop can do it,” he says.
The only caveat is that it’s best to go into the die design process with the ball-lock tooling in mind, McFetridge notes. “It’s not impossible to retrofit a quick-change punch retainer to existing tooling, but it’s definitely easier to do it up front. That being said, we have customers that use our ball-lock system to build tooling modules that can be swapped out very quickly.”
In one application, a customer had five similar parts with different hole spacings; the Accu-Lock allowed them to change from part to part without pulling the die set.
Edson says this level of flexibility is increasingly important in this day of shorter lead times and falling lot sizes. In that same breath, however, he says the biggest benefit of the Accu-Lock is time.
“By doing a little planning and a little investment, shops can greatly reduce their machine downtime,” Edson says. “SMED is great, but it’s the unexpected production disruptions that must be avoided. That’s where all the money is lost.”