Die bending
In die bending (often referred to as folding ), the unbent or pre-bent workpiece (the sheet metal part) is placed on a die with a V-shaped opening. The back gauges shown make it possible to position the workpiece quickly and precisely on the die in a repeatable manner. In modern CNC machines, two computer-controlled back gauges are used. The sheet metal forming takes place through the controlled downward movement of the upper tool, which is known as a punch or sword.
Free bending, embossing bending and three-point bending
A distinction is made between free bending (also: air bending , free bending, free bending in the die) , embossing bending and three-point bending .
As Free bending the method shown in figure a) and b) is referred to. A characteristic of this is that the processed sheet only touches the die on both edges. To produce bends with sufficient accuracy, precise control of the punch is required, for which a CNC machine is used. A particular advantage of free bending is the possibility of producing different angles without changing tools. For this reason it is used more often than embossing bending or three-point bending (see below). The resulting radius at the bending edge is essentially determined by the opening width of the die ("V-opening"). The radius at the lower edge of the stamp has little influence on the radius.
During embossing , the sheet metal is embossed with high pressure between the punch and die. The opening angles of the punch and die are close to 90 °. Embossing bending provides somewhat more precise results, but does not require the control system to be so precise, but is less flexible because the tool set is designed precisely for one bending angle.
With three-point bending , the punch presses the workpiece down to the die ("U-opening") so that the workpiece touches the die at three points. The bending result is more precise than with free bending, the opening angle is determined by the position of the three workpiece support points (more precisely: three support lines, one on each upper inner edge of the die and one on the die base acting as a counter holder). If the die is equipped with a height-adjustable counter-holder, different bending angles can be produced without changing tools. However, the control by this additional controlled axis (that is, by the movable counterholder) and the need to match the lower end point of the punch precisely to the position of the movable counterholder, is more complex. In addition, the tools are more expensive. Only a few machine manufacturers produce bending machines for this process.
Bending an envelope
Bending an envelope , also known as a fold or doubling, takes place in two steps:
- As described above, pre-bending takes place as free bending in the die.
- Before inserting the sheet, the die is moved forward. Modern bending machines offer their own numerically controlled axis for this purpose . The punch is lowered and serves as a back gauge (3rd picture). Then it is pressed shut.