R&D Update: Press brake bending

Modern press brakes have various drives, controls, and sension-bent products. Press brake bending is used extensively in the fabrication industry because it offers flexibility for forming different alloys, sheet thicknesses, and bending angles.

For example, vertical-motion bending is performed primary on press brakes when a part’s bending length exceeds a specified amount, for example, when a part is 1 to 30 feet long. Vertical bending also is used if production runs are so small that making a tool for conventional press is uneconomical.

Both air and die bending can be conducted on a press brake. For air bending operations, a sheet is supported by two shoulders of a stationary die, which is designed to be deep enough so the punch and the workpiece do not bottom out. In die bending, the work piece contacts the bottom of the die at the end of a punch stroke.

Press Brake Designs

Many press brakes have a C-type frame with double columns for bending lengths approximately 6 to 20 ft. (2 to 6 meters), and in special cases up to 33 ft. (10 m). Depending on the application, press brakes are available with mechanical, hydraulic. hydraulic/servo, or AC/ servo drives.

Mechanical Drive. A mechanical press brake has a motor, flywheel, and crankshaft that coverts the rotary motion of the motor and flywheel into vertical or reciprocating motion of the ram. High-speed flywheel rotation produces a large number of cycles per minute. When the ram approaches the bottom of the stroke, or bottom dead center (BDC), it often has enough momentum to exert force beyond the press’s rated capacity. At BDC, a press’s theoretical tonnage capability reaches infinity. Applied force is limited only by a press frame’s load-carrying capacity, so if the press is not set up properly, high-tonnage applications such as coining may cause tool damage or locking of the press at BDC.

Hydraulic Drive. A hydraulic press uses a motor and a pump to move hydraulic oil to the top of the cylinders (usually two) to power the ram. Hydraulic press brake tonnage is directly proportional to oil pressure and the area of the pistons under pressure. Adjusting oil flow controls ram speed. As compared to mechanical presses, hydraulic presses have lower strokes per minute.

Hydraulic/Servo Drive. In a hydraulic/servo press brake, a single hydraulic cylinder powers a mechanical linkage system to bend, punch, or form material. This design provides full tonnage at any point across the bend, ensures parallel movement of the ram regardless of off-center loading, and yields punch position repeatability ini the range of ± 0.001 inch.

AC/Servo Drive. Stroke, slide speed, and forming pressure are programmable on AC/servo press brakes, which can provide BDC accuracy in microns. In an AC/servo press brake, the motor maintains constant torque and energy even when the speed is reduced. In contrast, a mechanical press brake’s energy (which is supplied by the rotating flywheel) decreases as the flywheel slows down. For average production runs, numeric control systems are used to set up punch stroke positions and speed to minimize production time per piece.

Bending Angle Measurement

One of the most important requirements for modern press brakes is producing accurate bending angles. Variation in mechanical properties or thickness of a workpiece may cause bending angle errors because of springback. Precise measurements and control of springback is key to obtaining tight tolerances in bent parts. Several systems are available that measure the bending angle, which can reduce scrap rates and inspections times.

Optical/ Laser Sensors. This contact-free angle measurement methods projects laser beans onto the contour of the sheet (see Figure 1 ). Closed caption decor (CCD) cameras (positioned at A and B in Figure 1) diagonally look up and measure the positions of the laser projection line on both sides of the bent sheet. The system calculates the bend angle (Ø) by measuring the angle () between laser projection lines and the fixed camera angle (ß). Bending accuracies up to ± 0.2 degree can be achieved.

Mechanical Disk Sensors. With this method, two sensor disk different diameters are embedded into a punch (see Figure 2a Figure 2b ). The bending angle is measured from the distance between the two centers of the disks to modify the press’s ram position.

For bending long workpieces, three sensors are used (see Figure 3a ). Two outer sensors regulate angle accuracy and parallelism while a middle sensor controls crowning. Accuracies up to ±0.3 degree are possible. A similar method has four built-in probes in the punch (see Figure 3b ). The four probes measure material thickness and spingback and then modify punch penetration accordingly.

The next column will discuss how sheet metal deforms during bending, causing tensile and compressive strength in the deforms during bending, causing tensile and compressive strength in the deformed sections of the bent part. By understanding deformation in press brake bending, it’s possible to predict the bending allowance and spingback. Thus, good parts with desirable tolerances can be produced.