In case it is necessary to bend metal plates in straight, parallel lines, sheet bending machines offer the best solution. Possessing electrical or hydraulic drive unit (very rarely manual), these machines allow producing quick and precise bending along straight edges with full width of the sheet forming elements of different profiles and pieces: bends and lapels along edges, grooves and channels, stiffing ribs, bends with shell and box formations including an enclosed one. The main feature of sheet bending presses is the function of bending sheets without preheating through the formation of parallel edges.
If not to talk about the rolling method between cylinders, there are two technical approaches to the sheet bending and accordingly to the design of the bending machine: either the plate is placed on the die deeply in the form of a groove, then the punch with its straight edge descends into the die to a predetermined depth causing the sheet to bend at a designed angle (press brakes), or
the sheet is clamped between the work table and the pressure beam, and the free edge of the sheet with the help of a rotary beam bends around the end of the clamping beam (bending machine with a rotary beam).
The advantage of the first method (press brakes) is the significant speed and universality. It is possible to change bending angle easily just by changing the dipping deepness of die into the punch (i.e. the stroke of the upper beam of the press brake on which the punch is fixed). On the other hand, bending machines with a rotary beam more easily form an enclosed box, and also practically do not damage the surface of the work piece, which is especially important when working with polished or painted sheets.
Press Brakes
Press brakes are hydraulic double-cylinder C-type presses with vertically movable upper beam, on which the press brake punch is fixed. When lowering onto a sheet blank located on the die, the punch is immersed in the die groove and bends the sheet along its edge.
The deeper the plunger of the press brake falls into the die, the stronger the part bends.
The range of bending machines is quite wide. Examples of the main bends made by the machines of this type are presented in the figure to the left. Depending on the planned range of works, hydraulic press brakes (press brakes) are equipped with various tool sets, the choice of which is wide and is measured by many pages of catalogs. For convenience, switching between different materials and bending modes, the punch of the bending machine is made in the form of quick-detachable typesetting sections. It is also possible that, to speed up the process, tools of one bending type are placed on the left side of the bending beam and the other on the right (of course, provided that the total width of the bending machine is several times greater than the width of the work piece).
Efforts are not spared to make the sheet bending die as universal as possible: often it is a square beam, on each of the four sides of which there are several channels (“streams”) with different depths and widths of the aperture. Therefore, the change of working “streams” on a sheet bending machine – a press brake – also takes little time.In order for the bend line of the bending machine to be strictly parallel to the edge of the sheet and to be at a predetermined distance from it, the back stop is used in the design of hydraulic press brake. The position of the back stop at the bending machine is controlled by the numerical control system of the machine (as a rule, this is CNC). Depending on the advancement of the bending machine, the number of back stops and their degree of independence (CNC control axes) can be different.
During bending, the edge of the part facing the bend machine operator rises. Therefore, to work with dimensional details, the sheet bending machines can be understaffed with “escort” – special calipers that rise up synchronously with the bend of the part. Such lifting calipers are controlled by the CNC bending system.
At the very first press brakes, the bending angle was uniquely determined by the angular geometry of the “die + punch” pair, since the bending punch entered the die completely, to the stop. With the advent of servo valves, sensitive electronic sensors and technologically advanced hydraulic systems, the bending machines were able to adjust the bending angle without changing the tool by changing the degree (depth) of immersion of the punch in the bending die, which was made possible thanks to the high-precision control of the course of the upper beam of the press brake (this process received the name “air bending”). The use of hydraulic mechanisms in bending machines, in addition to accuracy, provides two more advantages: the high pressure force necessary for the bending machine to work with materials of great thickness and width, as well as the possibility of an emergency stop of the movable beam of the bending machine at any time, when the safety sensors are triggered. Ability to change the angle through the immersion depth of the punch in this type of bending machine determines not only the high requirements for the synchronization of the hydraulic cylinders and the accuracy of the positioning of the bending beam (which directly determines the accuracy of the bending angles), but also the high requirements for the design of the bending press to resist deformation. The fact is that the strength of the sheet bending frame is not infinite, therefore, at the time of bending, the middle of the desktop of the sheet bending machine is slightly springy down, as a result of which the central part of the part bends at an angle less than at the edges, where the resistance of the side posts of the press brake provides higher rigidity. The wider and thicker the bent sheet – the more significant the deflection in its middle.
To level these distortions, a “bombing system” is used: under the bending table there are one or more hydraulic cylinders, which, receiving commands from the CNC system, at the moment of lowering the upper beam, create a compensating counter-force of the required size, which prevents the matrix from warping and ensures a constant bending angle through the entire width of the part. In simpler press brakes, “bombing” is carried out not by a hydraulic drive, but by means of mechanically movable wedges with manual adjustment of force. When bending, not only does the desktop of the press brake spring, but also the part itself. The characteristic of springing, as a rule, is set in the CNC bending machine in the form of a correction parameter. In order to increase the accuracy of the angles being formed, the most modern bend machine possesses laser angle control system, which measures the angles on the part directly during bending and makes it possible to more finely control the sheet bending process based on real, measured here and now, and not averaged-statistical data. Thus, with laser-controlled sheet bending machines, the error in the corners is reduced, guaranteeing a higher quality product.
Bending Machine With a Rotary Beam
Plate bending machines with a rotary beam, as well as press brakes, are used for cold bending of metal sheet blanks into shells. Sheet bending machines with a rotary beam under the control of the CNC through a series of multiple equal-distance bend at small angles, it is possible to create a conical or other complexly curved surface by bend machines.
The order of operations in the bending machine with a rotary beam is as follows: first, a rectangular metal work piece is placed on the desktop, the rear clamp is automatically displayed at the desired distance and helps the work piece to lie parallel to the edge of the work table of the bending machine. Then the clamping beam is lowered onto the work piece.
Next, the protruding part of the sheet with the help of a rotary beam of the bending machine is bent upwards – around the front edge of the pressure beam. The clamping beam in the sheet bending machine has another application: it is often used for booster, i.e. the formation of a bend at the edge of the part with an angle of 180 °, this is done by simply pushing the partially curved edge back to the main sheet. The dough can also be used with beam bending machines in other cases: for closing planes, “closing” the box, correcting the angle of bending, etc.
As mentioned above, it is safer to bend pre-polished or painted sheets on a bending machine with a swing beam, as these bending machines provide minimal deformation and do not scratch the front surface of the products. When using a rotary beam bend machines, control of bending angles is simpler than with press brake, in addition, the rotary beam allows you to create quite complex shapes. Nevertheless, according to the criterion of maximum permissible thicknesses of bending materials, bending machines with a rotary beam are inferior to bending presses due to the simpler and more rigid construction of the latter.