WORKING WITH THE PRESS BRAKE: TYPES OF FOLDING

Before starting to work with a Press brake machine, it is essential to know some fundamental concepts of this equipment and the work they perform. One of these concepts is the different types of folding that we can perform with our Press brake.

Types of sheet bending that we can perform with press brakes

Air folding

It is the folding carried out on the die without the sheet folded with the punch reaching the bottom of it. In other words, when the punch reaches its lowest position there is a light or an air space below the sheet, between it and the die. This makes it possible to obtain different bending angles using the same tools, which requires working with extreme precision to guarantee a perfect result.
Since the introduction of CNC to hydraulic press brakes, the most frequently used form of bending is air bending or partial bending. During any bending process, the sheet undergoes an elastic return as soon as we stop making pressure on it. Thanks to the CNC air bending technique, it estimates how many degrees this return will be and lower the punch taking this factor into account.

There are two fundamental aspects that we must control when carrying out this type of folding: the position of the beam when lowering and the pressure on the material, these two concepts are essential to be able to use this folding technique correctly.
Some of the advantages of air folding are:

• Greater range of bending angles: This technique allows to obtain a greater range of angles in the folded pieces. With a tool kit (punch and die) at 30 °, any graduation from 30 ° to 180 ° can be obtained by air bending.
• Faster in execution: Bending in the air is faster, since to obtain a sharper angle, it is enough to lower the beam a few millimeters. In other techniques, however, more pressure must be applied or the rest time of the beam must be increased.
• Lower pressure: The necessary pressure is lower than in other working ways.
• Less friction: The metal sheet is less marked due to the low friction surface with the die.
• Less attrition: Punches and dies increase their useful life.
• Lower tonnage: A press brake with lower tonnage is needed than would be necessary for the same folding with another technique.

Like the advantages of this type of folding, it is also necessary to know the disadvantages:

• Lower folding precision: The precision of the folding angle is lower than in other types of folding. Accuracy is approximately +/- 1 degree.
• Elastic return: This technique works with the elastic return, but does not eliminate it. Thus it is a difficult technique to work large radii or pieces with high elastic return.

Deep folding

It is the folding carried out on the die, pressing the sheet with the punch until the lower part of it is reached. The resulting bend angle depends on the punch and die used.

With the deep folding technique, the sheet comes to rest on the bottom of the V of the lower die. This technique is preferable when seeking greater precision in the angle and radius of the profile to be bent.

The precision increases since the sheet is formed with a lower support that allows to obtain a more homogeneous radius at the tip of the fold and a greater breakage of the elastic return. This ultimately results in a more precise folding operation.

For this kind of folding it is essential to choose the right punches and dies. The angle and opening that the tools must have to obtain the desired profile with the elastic return that said sheet will present must be precisely calculated.
The punch and die must have the same angle, to obtain a better distribution of the force on the sheet between both sides.

The advantages of deep folding are:

• Lower pressure: Good precision with lower machine pressure.
• Mass production: Greater repeatability of the pieces in case of large production series.
• Less elastic return
• Less deformation in punched sheets: In the case of holes close to the bending area, these are less deformed. This deformation is corrected, in part, when the sheet is pressed between the tools.
• More accuracy: Approximate tolerance of 1/2 degree.

The disadvantages of deep folding are:

• It does not allow corrections: It is impossible to correct errors in the result of the angle, since the tools are at the maximum of their possibility. The punch cannot be lowered further.
• Small range of angles: This technique is used only to obtain angles between 80 ° and 90 °.
• Need for more tools: Specific tools are needed for each particular bend.
• Higher friction: Poor aesthetic quality of the folded parts due to greater friction between the sheet and the die.

Stamped

The stamping allows the sheet to be folded on itself to provide more resistance to certain areas of it.

The stamping technique gets its name from the process by which coins are stamped. This is due to the fact that the stamping process is the one that allows to obtain pieces that are perfectly equal to each other, such as coins.

The stamping process, however, is one that requires a very high pressure relative to the thickness of the sheet. This technique requires a force of 4 or 5 times the pressure used in air bending. The tools used in the stamping process must have the exact angle that you want to obtain in the piece of sheet. In fact, the force that the process requires is the necessary to break the elastic return of the sheet, and thus "print" or "stamp" the shape of the tools on the sheet.
Obviously, it is a constant process, but limited to certain thicknesses and some folders in particular.

Some of the advantages of metal sheet stamping are:

• Constant result.
• Tolerance of 1/4 degree
• Possibility of obtaining minimum radii in the folded piece

On the other hand, the disadvantages are:

• Shorter useful life of the press brake and tools
• Higher friction: Poor aesthetic result of the folded sheet due to the high degree of friction.
• Need for more tools: Each sheet thickness must use a different punch and die, respecting the angle, radius, resistance and the opening of the die.
• Only for small sheet thicknesses: It is impossible to use this technique in thicknesses greater than 2mm since the necessary pressure would be too high.

Sheet metal bending or calendering
This type of bending is a variant of air bending that allows different radii of curvature of a sheet to be obtained from multiple sequential bends made on it. The result of this type of bending is an angle with a larger or smaller radius.
Thanks to the CNC of the sheet metal folding machines, we can program this type of folding by entering the radius of curvature, the distance between the bends and the angle. We choose this distance according to the type of finish we want, the shorter the distance between the folds, the better the definition of the curve. Once these data have been entered, the CNC will  carry out the necessary calculations to determine the degrees of bending in each one of them so that the final result is a curve.

Some of the advantages of sheet metal bending are:

• Possibility of bending: Although sheet metal folding machines are not the best choice for bending sheet metal, this type of folding allows us to make bends very close to a fold that would not be possible in any other way.
• Wide range of bending radii: This technique allows to obtain a wide range of bending radii in the folded pieces. With a tool kit (punch and die) at 30 ° it is possible to obtain, by means of these bending sequences, any curve from 0° to 90°.
• Lower pressure: The necessary pressure is lower than in other working ways since when making multiple bends, the degrees of bending are small and do not require much pressure.
• Less friction: The sheet is less marked due to the low friction surface with the die and the low pressure of the punch.
• Less attrition: Punches and dies have a longer useful life.

Like the advantages of this type of folding, it is also necessary to know the disadvantages:

• Perception of folds: Unlike sheet metal bending, calendering using multiple bends does not provide the same finish in the final result. In the sheet curved using this technique you can see the folds.
• Elastic return: This technique works with the elastic return, but does not eliminate it. Thus, it is a difficult technique to work large radii or pieces with high elastic return.

To learn more about types of bending, how to choose the punches and dies, how to program the CNC, interpret the bending tables, the factors to consider to achieve optimal bending and a lot more, you can see the video tutorial on bending. Nargesa MP3003CNC hydraulic press brake.