As with most press brake operations, hemming can be both challenging and time-consuming without the right tooling. Dave Bishop, business development manager for the Western region,
Wila USA, says there’s nothing to worry about, though. Wila has an entire series of hemming solutions to fit any budget and any level of production. The trick is figuring out which ones to use and how to use them. Fortunately, Bishop and the team at Wila can help.
Not familiar with the term hemming? Look down. The bottom of your blue jeans or skirt was hemmed at some factory in China or Bangladesh. For a sheet metal example, look no further than the kitchen, where various parts of the refrigerator and microwave oven have all been hemmed. Other examples can be found on the toolbox in your garage, the doors on your pickup truck and the metal shelving in the storage shed that you’ve been meaning to clean out all year.
As any press brake or turret punch operator knows, sheet metal edges are sharp. They’re also a bit wimpy, prone to bending and kinking, and can be ugly besides. Hemming cures all of these problems. All that’s required is to fold the metal over and then flatten the resulting bend.
The result is greater thickness – at least twice as much, actually – providing greater edge strength with very little increase in product weight. The edge is now smooth and rounded as well, ready for painting, powder coating, plating or, in the case of stainless steel, nothing at all. The only downside is that the inside of the hem can retain moisture, which is why some hems are left “open” by creating a standard tear-drop shaped hem or by inserting a piece of shim stock before the flattening operation, thus giving the hem a little breathing room.
Hammer or U-shape
One of the lowest cost ways of hemming, at least from a tooling perspective, is with a so-called hammer or flattening tool. As its name implies, this is nothing more than a flat-faced punch and die set, which serves to smash the metal edge down after it’s been bent to an approximate 30 degree angle. Unfortunately, this approach requires two separate sets of tooling – one to create the initial bend, the other to flatten the hem – leading to additional setup time, work in process and expense. For occasional hemming or where budgets are tight, however, Bishop suggests this is an effective way to go.
Next on the solutions list is a U-shaped combination hemming tool. It’s designed for more frequent hemming than its hammer-like cousin. It is limited in the range of material thicknesses that it can form, although it does complete the hem in a single setup.
Here, a 28-degree acute angle punch is used to air bend the metal. The operator then pulls the workpiece back temporarily so the punch can slide down into the die’s U-shaped opening (hence the name). The operator then pushes the bent blank against the top tool’s face, using it as a backstop. “Flattening jaws” located a little higher up on the punch completes the operation by forcing the workpiece against the top surface of the U, thus flattening the pre-bent metal edge to a tear-drop shape, as shown, or as flat as a pancake if that is what’s preferred.
Spring loaded or Dutch
One of Bishop’s favorites is the spring-loaded hemming tool. The forming action is similar to that of a U-shaped combo tool, but the die contains a special spring-loaded upper piece that performs double duty. The V-shaped opening in the upper surface of this movable piece, together with a standard acute angle punch, serves to create the initial bend on the workpiece. The ram is then made to retract slightly and the springs push the workpiece upward, thus opening the flattening jaws below.
As with the U-shaped tool, the vertical face of the flattening jaws serves as a backstop. As the ram comes down again, the metal is captured deep within the flattening jaws to prevent it from slipping out, and the flattening operation is performed. Voila! A hem is created, but faster than when formed when using an acute angle punch and die and a
separate hammer tool, and more reliably than when using the U-shaped combination hemming tool.
Lastly, there’s the hydraulic Dutch bending table, or hemming table. Here again, the tool uses a three-piece arrangement, but instead of springs, hydraulic pressure moves the middle section upward and performs the flattening operation.
This hemming solution uses standard acute angle punches and dies that can be changed as necessary to meet the material requirements, thus greatly increasing its flexibility. It also has the greatest range, able to bend mild steel from 0.020 in. to 0.118 in. thick. It’s admittedly the most expensive one of the bunch – and also requires installation of a hydraulic power pack to drive the flattening section – but it is fast, versatile and an easy investment for anyone engaged in high-production hemming.
But wait, there’s more
There’s plenty more to this hemming story. Bishop listed off a host of material- and application-dependent recommendations and operating parameters, information that he and his team are more than happy to discuss further. Bishop closed with one final observation, something that is critical to hemming success.
“During any flattening operation, the magic occurs in two places,” he says. “The first is having adequate depth to the flattening jaws, something that many two-stage hemming systems do not have. Secondly, the leading corner on the flattening jaws must be hardened and have a radius that’s sharp enough to capture the metal. If not, the workpiece can slip out before the hemming operation is complete. And as with any Wila tooling, the
use of proprietary high-alloy steel along with CNC deep-hardened and precision-ground work surfaces assures long life and the greatest return on investment.”