Have operators who strain their backs lifting large panels during the press brake forming cycle? It doesn’t have to be that way.
Part I. Press brakes
Horizontal bending tools for the press brake generally are available in two varieties: the rotary bending tool (top) and the wiping tool (bottom). A press brake may have sufficient safeguarding, including light curtains on the sides and cameras or lasers detecting objects just below the punch tip. But all the safeguarding technology in the world can’t make the press brake operator’s back feel better.
Historically it’s been common to see two press brake operators straining to lift a large panel as it whips up during the forming cycle. Working with thin panels, they take care to support the work to ensure gravity’s effect on the portion of the workpiece whipping upward doesn’t distort the resulting bend. They also take care to get out of the workpiece’s way. Being hit by a workpiece whipping up during the bending cycle isn’t very pleasant. A press brake that is lifting a large unsupported workpiece during the forming cycle also can increase the forming tonnage required.
Fabricators usually tackle this problem in several ways. At the very least, the press brake has supports in the front that help operators position the workpiece against the backgauges. They also may have something other than a human lift to support the part as it arcs upward during the press brake’s bending cycle. This could include some type of crane or hoist, the most common choice for thick plate.
For thin sheet, fabricators have options, including a sheet follower. The sheet follower does just what it says. Positioned in front of the press brake bed, the device supports and follows a large sheet as it whips upward during bending.
Another approach involves changing how bending occurs. Instead of both the short and long legs of the bend whipping upward as the punch descends into the V die, the long leg stays horizontal while the short leg bends upward or downward. For making edge flanges in large panels, this makes all the sense in the world.
Fabricators have several ways to accomplish this: with special tools on a press brake or a different machine altogether, such as a panel bender or folder. As with any technology, each machine and tooling choice has its limitations. But for the majority of sheet metal applications, fabricators have a plethora of options.
Horizontal Bending on the Press Brake
The press brake can use a range of special tools, but when it comes to horizontal bending—that is, where the sheet in front of the tooling remains horizontal during the bending cycle—two categories of tools are available: wiping and rotary-style.
Wiping tools work as they sound: A punch essentially wipes the sheet against the die. Wiping could be described as a bottoming operation, and each die set is made for a specific material type, thickness, bend angle, and radius. As such, wiping requires significant tonnage, which is why you rarely see wiping operations for mild steel thicker than about 0.125 in. Most wiping tools are made to form 90-degree bends, but some custom wiping tools can be made to form workpieces to other angles.
Because the tool is designed for a specific material thickness, slight variations in material thickness can cause quality problems. The gap between the forming punch and stationary die is critical. If the material is a little thicker than what the gap is designed to handle, the wiping action tends to mark or gall the material. If the material is thinner than the gap, you’ll end up with an underbent angle. In traditional air bending, you’d simply descend the punch a little farther to achieve the angle you need. Also,springback sometimes can be difficult to control. The wiping punch wraps the sheet metal around the die, but once the cycle completes and the pressure is released, the bend angle can relax slightly.
In panel bending, hold-down tools clamp the workpiece in place, while bending blades form the workpiece in the positive and negative direction. Rotary-style horizontal bending tools can form edge flanges in a much more controlled manner. Rotary tools consist of a Pac-Man-shaped cam that sits in a saddle, basically a circular pocket in the tool. The operator slides the sheet to the back of the tool. As the press brake ram descends, the cam wraps the material around the die, which is called the anvil. The anvil is recessed, so you can over-bend a few degrees to account for springback.
You can also bend different angles by adjusting the stroke of the ram. A rotary tool can bend included angles anywhere between 75 and 135 degrees, while some tools can form open angles up to 145 degrees. They’re usually used for material that’s 0.25 in. and thinner, also custom tools sometimes can be designed to handle thicker stock. This is accomplished by building a larger cam, which gives the rotary tool a larger opening.
The rotary tool can account for variations in material thickness. If, say, the tool underbends, and the included angle ends up being slightly more open than it needs to be, the operator can adjust the ram position to add a few more degrees for over-bending.
Horizontal bending tools allow for multiple-bend geometries like return flanges consisting of two or more bends. But these tools do have flange depth limitations. The tool size governs the flange depths you can achieve. Of course, as the tool size grows, so does the expense.
Horizontal bending large workpieces on a press brake still requires operators to lift and support the sheet in some way. Two or more workers may hold the sheet in place, or they may use a support table to position the large panel for bending.
Alternative technologies, though, take another approach to bending altogether, which in turn changes how operators handle the workpieces—as well as the number of operators (often just one) needed to handle large sheets. We will be talking about them in the next parts of this article, which is going to be published on Italian Machinery Association very soon.
BY: TIM HESTON, THE FABRICATOR
- Bending is a broad subject with many issues and challenges. Italian Machinery Association is here to assist you with bending processes and press brakes in many different ways.
This article consists of three parts. Please read the other two:
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