Tooling Article



Assembly Automation Think Out of the Box, and In the Die

By: Brad Kuvin

Friday, October 1, 2010
Suppliers to European automotive OEMs have long understood that welding nuts, studs, bushings and other components to stamped parts is out, and that fastening them mechanically in the die is in. Simply, if you want to be competitive in that market space, in-die fastening is required.

Now, U.S. automotive OEMs are taking the same approach. “General Motors, Ford and Chrysler all have fasteners in their current catalogs suitable for in-die assembly,” I was told by Victor Lanni, CEO of MDS Fastening Systems, Romeo, MI. “And, they all have been open to new designs in their quest

Parts being assembled
Want to see just how versatile and complex in-die assembly processes can get? Check out this sample of parts being assembled in-die at various metalforming facilities. From top to bottom: a seat part with pilot pin, processed at 35 strokes/min. with an installation load of 3 metric tons; a ball-pin application that runs at 45 strokes/min.; and a door-hinge bracket  with a floating nut bracket, which runs two-out at 40 strokes.min.
for better and more cost-effective assembly,” Lanni adds. “The Tier One suppliers have been right there with them, and the trend also is growing amongst OEMs in the trucking industry as well as in the white-goods industry. So, if you’re not doing it (assembling in the die) today, you will be required to do it tomorrow, or risk losing your customers.”

MDS Fastening Systems supplies the equipment and the engineering support (fastener selection and die design) metalformers need to automate their fastening operations, so we asked Lanni to establish the lay of the land in the application of this important and growing technology trend.

“We’re seeing a big pickup in interest in the technology from stampers because their customers are requiring that they move in this direction,” Lanni says. “There’s initial concern about possible press slowdowns when in-die automation is added, as well as concern for added setup time and a possible reduction in run time. But the reality is, if a system is properly engineered and planned for, our customers typically only experience an additional 5 min. of setup time related to the in-die assembly automation equipment. And, the systems can feed and assemble at the same run rate as before the press was automated—special high-speed feeders are rated at 800 parts/min., or as many as eight different assembly heads receiving fasteners in the die at a maximum press speed of 70 strokes/min., depending on the size and shape of the part being fed.

“Bottom line,” Lanni stresses: “In-die automation will not slow you down, and nearly every system installed nowadays will pay for itself in less than one year.”

Why In-Die?

The benefits of assembling parts in the die are numerous—one operator runs the press and monitors fastener installation, with no need for secondary operations and the floor space, energy and manpower required. Logistics costs drop, as bins and cartons of in-process parts inventory no longer need to travel the shop floor to and from secondary operations. And, shops that assemble mechanically rather than using welding to attach nuts and other fasteners avoid harmful weld fumes and spatter, and the associated cleanup costs. Lastly, parts can be assembled without concern for health or technical issues surrounding the fabrication of precoated and prepainted materials.

Of course, careful consideration of the type of fastener used is critical, as is the design of the fastener-installation equipment developed to feed the fasteners to the die, insert them in the die and ultimately onto the stamped part. Otherwise metalformers may not obtain the desired results from their capital investment.

“All of the companies involved in a project must function as a team,” stresses Lanni, calling out the stamper itself (‘Can the press-line equipment take instructions from the fastener feed unit and its control, and send signals back to the feeder?” he asks), as well as the die shop (“Is the die design robust enough to handle the fastener feeder and installation heads?”) and the supplier of the fastening system (“Is the feed system robust enough?”)

Productivity Hurdles are Low, But Take Care to Clear Them

Asked about common hurdles that can cause an in-die fastening system to stumble, Lanni starts by focusing on fastener selection.

“The fastener selected for a given application must be able to be robustly set in the die hit after hit,” Lanni says. “Choose the wrong style of fastener and it can be crushed or the threads destroyed. In short, you need an automation-grade fastener that’s been designed to be set in a stamping process and capable of meeting the torque-out and push-out performance in the sheet with a given thickness and hardness.”

Metalformers select from three different styles of fasteners—a self-piercing nut, a clinch fastener and a riveting-style fastener. Lanni suggests stampers avoid the use of self-piercing nuts at all costs, because “you can’t guarantee that the slug will be pulled out. We do, howevertime,” he says, “particularly in dies where space is at a premium and no other option is suitable. Also, the fact that you can pierce and install in one hit it can make self-piercing nuts a preferred choice, particularly in transfer dies. In those cases, the stamper must ensure it has included sufficient in-die sensoring to check for slug removal after each hit. And, even with a sound sensor setup, it’s still risky—I’ve never seen an application with in-die installation of self-piercing nuts where the stamper did not ship at least some parts with a slug, unless they’re inspecting and sorting 100 percent of the parts.”

The riveting-style nut, on the other hand, is growing in use for in-die automation, Lanni says, particularly in applications involving advanced high-strength steels (AHSS) and when working with very thin sheet. These nuts feature a collar that protrudes completely though the material and extends out the opposite side. The collar then gets folded back over on top of the nut in the press to create a joint strong enough to satisfy the design requirements of AHSS stampings.

Other trends identified by Lanni include development of new styles of fastener feeders that generate less noise and are less likely to jam; and development of system components that fit together with quick-connect fittings to allow stampers to quickly move the equipment among setups. Also, systems can be designed to feed not only nuts and studs, but special fasteners such as positioning pins, spacers and ball pins.

Other Concerns (Hurdles) that Block Productivity

Working with the die designer on implementing an in-die fastening solution, Lanni strongly suggests that the stamper also bring the supplier of the feeder and head and fastener into the process from project inception. This will help ensure that the heads and fastener-delivery mechanism do not impede the functionality of the die.

“We urge designers to set fasteners near the end of the die,” Lanni says. “This if something does go wrong during production, it only affects the back end of the die. A common mistake I see is that the die punches the hole for the fastener too early in the process, and then subsequent forming operations make it nearly impossible to repeatedly track hole alignment and maintain hole geometry.”

Another common mistake noted by Lanni: locating the fastener heads right next to where the die lubricant is being applied. “I’ve seen setups where the heads become filled with lubricant,” he says. “This is the type of error that occurs when there’s a lack of communication between parties.”

Yet another concern Lanni expresses relates to operator training. “Often, we’ll perform training at PPAP, and then the stamper won’t run the die for another 6 to 8 months. By that time the operators have forgotten everything that we taught them. If they forget to follow the proper sequence of operations, it can take a lot longer to set up the system and, at worst case, the heads can crash.

“In particular,” continues Lanni, “a common error we see is a stamper installing a head that already contains a nut left over from a previous run. In this case, when the die is set and the press begins to stroke, the nut may already have been set on the lower die before the strip gets there. Then, when the strip arrives and the ram comes down and makes a hit, you’ve already set a nut. Then a second nut could be fed to the same spot. Result: a double-hit and a blowout.” The moral to the story: Don’t thread the sheet until the heads have been inspected and installed.

Lastly, Lanni suggests that in-die fastening installations employ guided strippers, and that the die shop design the pilots in the web, to optimally control the part throughout the process. “This will help ensure a robust, repeatable process,” he says. MF


See also: AUTORIV

Related Enterprise Zones: Tool & Die

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