To date, sheet metal bending is carried out in various ways. The most widely used machines are the Press-brake type. The popularity of using this technology is due to the following features.
On the same equipment, you can make parts of different configurations from different materials and different thicknesses.
High production flexibility — most parts are manufactured without changing equipment or changing tools. Moreover, often the tool does not change at all, while in production there may be a large number of parts that differ both in configuration and in material/thickness.
- High performance.
- Low cost of equipment and cost of production.
- Applicability of process automation.
At the same time, the possibilities of such a technological process cannot be unlimited. The main limiting factors, or a combination of them, are:
- material properties;
- tool features;
- knowledge and skills of technologists and operators;
Let’s consider the main, most important issues of sheet metal bending on press-brake machines.
Deformation of the metal
Bending on sheet bending presses is based on the 3-point principle. The sheet is based on 2 matrix points. The punch presses on the sheet between the 2 points of the matrix, forming a third, central point. When the punch is lowered, the central point of the sheet is lowered along with it, and the lower side surfaces of the sheet slide along the radii of the V-shaped hole of the matrix. The material is somewhat compressed at the place of the punch pressure and is significantly stretched from the lower side of the sheet. Also, the material is deformed at the point of sliding along the matrix — there are visible or invisible traces of deformation (indentation).
Schematic representation of metal deformation during bending
The length of the sweep in the direction perpendicular to the bend line is always increased. In this regard, the scan length is made less than the sum of all the sides. The elongation of the workpiece at each bend depends on:
- thickness and type of material, bending angle,
- bending radius (width of the V-shaped hole of the die and the radius of the punch)
The theoretical calculation will always be approximate. The most accurate result can be obtained experimentally. To do this, you need to take several blanks, for example, 100×100. To celebrate the rolling direction. The bend is equal to the number of workpieces along with the rolling and across. Make measurements of the received boards. For each blank, add the length of the sides and subtract 100. The resulting difference will represent the relative elongation for the specified bending conditions. Comparing the results obtained, we can evaluate the following:
- stability of the results,
- influence of the rolling direction.
In most cases, the difference in elongation along with the rolling and across can be ignored. However, if the accuracy requirements for the resulting dimensions are very high and/or the number of bends is large, then this difference should be taken into account when creating the scan and placing it on the sheet.
Separately, it should be noted that the more you need to deform the metal (reducing the minimum side, angle, and radius of bending), the greater the impact will be required. Here, the impact is directly related to the pressure and the moment of force. The pressure is the ratio of force to the area over which it is applied. So to increase the impact, you need to apply more force to a smaller area. The moment of force, in turn, is the product of the acting force and the length of the lever of the force application. Reducing the minimum side or bending radius requires the use of a die with a smaller V-shaped hole and, as a result, a smaller lever for applying force. Accordingly, all other things being equal, bending on a die with a smaller hole require s more force to be applied.