TWO THINGS TO TAKE INTO ACCOUNT IN THE BENDING OF LARGE RADIUS

To bend large radii in metal sheet in a press brake, there are two points that we must handle: the first, the punch, since we must choose a suitable radius so that the radius of the metal sheet is as we want. The second point is the die, this point is even more important than the first, since depending on the radius of the punch we must choose the appropriate V and above all we must pay close attention to its angle.

CHOOSE THE PROPER RADIUS OR DIAMETER OF THE PUNCH

For bending metal sheet with a large radius, there are standard punches available on the market for this purpose. One of the most used is for example in the Promecam System, but there are models for all moorings (Ajial, Wila-Trumpf, Beyeler, etc.), it is the punch holder mod. 40.05, in which the punch mod. 40.09 is fastened, as we see in the attached photos:

The punch holder and the punch are coupled by screws and special washers (supplied with the punch holder 40.05).

WHAT SHOULD WE CONSIDER FOR CHOOSING THE APPROPRIATE RADIO OR DIAMETER?

What we must take into account is that there are standard available diameters on the market, quite economic, with measures already specified that are usually Diameters of: 20, 25, 30, 35, 40, 50, 60, 80 and 100 m/m. Obviously other measures can be manufactured, but then their price and lead time will be higher.

Another thing that we must take into account and it is extremely important is that when metal sheets are bent in a press brake with large radii, the fibers of the metal sheet are NOT broken, but deformed. This is very important, since due to this deformation the metal sheet has a recoil after the bending operation and makes that its angle “opens” and as remaining more open, its internal radius also increases.

The angle increase must be compensated with a more closed die, but we will see this later.

The increase of the radius is important to our calculation. To give an example if someone asks us that in a 1,5 m/m metal sheet they want to make an outer Radius of 15 m/m, that is, an inner radius of 13,5 m/m (Outside radius minus sheet thickness), we should start the test with a punch with a radius of 10 m/m (diameter of 20 m/m), since after bending the metal sheet it can open by recoil to an interior radius of between 12 and 13 m/m. The difficulty of this type of bends is that when the fiber of the material is deformed, the return depends on the thickness of the sheet (the thicker the more return), the type of material (INOX has much more return than iron), the diameter of the punch etc. Therefore, the best thing, in the absence of an exact formula, is to do a small test and verify that the measurements obtained are those that we require.

Aumento del Radio al plegar con Punzón con radio grande
Radius increase when bending with large radius Punch

CHOOSE A DIE WITH SUITABLE V

Here are also two important points to keep in mind; first the opening or V of the die and the second its angle.

The choice of the size of the V is simple, it must be somewhat larger than the diameter of the punch plus twice the thickness of the sheet. To give an example, if we are going to bend a metal sheet of 1,5 m/m with a diameter of 20 m/m, we will have to put a die larger than 23 m/m (20 of the diameter plus 2 x 1,5 of the thickness ), therefore, and to go leftover, it is advisable to put a V of 32 m/m, as shown in the attached sketch, and thus ensure that the diameter enters enough into the die to be able to bend the sheet.

Escoger la matriz para doblar con radios grandes
Choosing the die to bend with a large radii

The second point is the choice of the angle of the die. There are no conclusive studies here either, but we should know that the thicker the metal sheet, the more it will open after bending. Therefore, if you want to make as in the previous example and bend a metal sheet of 1,5 m/m INOX with a diameter of 20 m/m at 90º, the V of the die simply has to be 85º, this will be enough to prevent it from opening beyond 90º. Since we will bend it at 85º and with the return it will remain at 90º. As we see in the attached sketch.

Retorno de una chapa de 1,5 m/m INOX al plegarla con radio grande
Return of a 1,5 m/m INOX sheet when bending it with a large radius

Keep in mind that it is Not the same to bend iron sheet (which will have less return), than if the sheet is INOX (which will have much more return).

In the case of wanting to bend a sheet of 3 m/m in INOX at 90º, with a punch with a diameter of 20 m/m, the die will not be enough at 85º, but we must put it with less degrees. Since the return, once the sheet is bended, can be 10 or 15º, therefore, we must bend it more, so that when the process is finished, the sheet recovers its position and leaves us at the desired 90º. See the attached sketch as an example.

Retorno de una chapa de 3 m/m INOX al plegarla con radio grande
Return of a sheet of 3 m/m INOX when folding it with a large radius

A VIDEO AS A BEST EXAMPLE

In the following video we will see how a 1,5 m/m sheet is bent in INOX with a diameter of 30 m/m, with different types of dies and obviously with different results;

1st TEST

It is bent with a 35 m/m wide die at 85º (V included in the standard die mod. 20.09) and it can be seen that it bends at 85º and once the sheet returns it remains at 90º. That is to say, it has had a return of 5º.

2nd TEST

It is bended with a 40 m/m wide die at 45º (standard die mod. 343), it is bended to the bottom of the die and once the sheet returns it remains at about 52º. In other words, it has had a return of 7º.

3rd TEST

In this last bend an Adiprene Block is used, with dimensions of 75 x 50 m/m and with a central hole (Standard Adiprene Block mod. 62.4), its Container is also used (also standard mod. 60.4) and we see how it can be bend further and the result is a 25º fold.

In this last test, the die also does not cause any marks on the sheet during the bending operation, since when using Adiprene, we do not have drag as it usually happens in conventional dies.

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