In our workshop, we argued for quite long about the acute bending in air formation as well as its relation to the least radii bending. Do they differ or what do they differ in? Could you consider this subject to provide us with credible information on it and its appliance within our tasks?

Answer: Sometimes there is a need to extend or clarify the definition of a certain thing. So, this is just the case. Having looked into the concerning matters like K-factors, I happen to find out that changing the specification of various kinds of bending radius is a must.

There are 3 usual kinds of a radius in air formation: the least and the profound radii. However, to represent the whole survey in the bend of sheet metals done recently, it is necessary to apply accurate terms. 

5 Disciplines of Bending Radius

Fig. 1

There exist 5 inner bending radii disciplines. The whole accuracy is based on Ir value, which is used to estimate bending tolerance or allowance (Ba) as well as bending deduction (Bd). These are the 5 disciplines:

  1. Acute radii bending
  2. The least bending radii
  3. Perfect radii bending
  4. Superficies and radii bending
  5. Deep bending radii

Sharp Radius Bending

The acute radii bending implies creasing at the central area of bending. The crease formation occurs due to the pressure exposed to the limited space, where the used load increases the sheet strength-resistance, creating a possibility for the punching nose to puncture the sheet face. 

The crease at the radii center brings about changes in sheet thicknesses, yielding force, extension force, granule directions. All the mentioned changes cause angular changes in the finished bending and bending deductions (Bd).  In the worst case, acute bending creates a faint spot in sheets and lead to failure of the bending in finished products. It is the sheet material that determines the bending sharpness, not the sharp punching noses at your workshop.  In case the punching tips are extremely little, compared to the loading demanded for formation, the loading concentrates upon a very limited space enabling punches to puncture the sheet face.

Now there are 2 options for you to choose. First, apply acute bending and determine the Ba, outer setbacks (OSSB), then Bd making use of the values of natural floating radii. In case punching nose radii should stay unchanged, you have to carefully monitor angular bending in the producing process. Once more, as acute bending punctures the sheet face, it reinforces such changes as granule directions, thicknesses, extension and yielding force.

The 2nd choice implies calculating Ba, OSSB as well as Bd making use of the natural floating inner radii, just in this case the punching noses should be changed to radii closest to the natural floating radii not increasing the increasing radii values. In case punching noses surpasses the floating radii values, your sheet acquires renewed, greater radii, once more causing changes to the Bd value as well as plane work piece. Maintaining punching nose radii closest, yet, smaller compared to the floating Ir can provide you with angular stability and consistence, as a result, linear dimensional stability.

Fig.2

The Least Bending Radius

The least radii bending isn`t the effect of the most acute punching nose. This generally causes operators to make mistakes. The least bending radii might be the description of 2 points.

First of all, it serves as the space at which bending gets sharpened, which is followed by the nose penetrating into the sheet face. Name this specification “the least boundary”. (Fig.1) Second, it means the least air bent inner radii achievable protecting the bending outer face.

As for the 2nd specification, the caterers of materials mostly make the list of the least inner radii in material thickness multiples, for example 1Mt, 2Mt. If clearer the least bending radii could be calculated through the material extension decrease.

To keep the process on its go bend with the least radii through acute punching nose, which will puncture (1st specification) as well as will shape cracking upon the outer radii. Theses 2 specifications are in close relation, for they depend on the extension force of sheets. The more the extension force, the bigger the punching noses have to be to achieve crackless outer bending. This is the case for toughness as well. Tougher sheets require bigger radii.

No matter you will crease the central area of bending or not, the two types of the least radii will risk the material integrality and hardness. What is the reason of this? Acute and the least radii bending bring on extreme extension strain. The mentioned point changes the radii forms, which in its turn changes the bending extension. 

In case of accurate metal sheets, each of the parts, each of the bends, each kind of materials possesses particular features causing each of them to carry their certain least inner bending radii. This has to be taken into consideration if you design work pieces out of metal sheets. To achieve consistent products, design work pieces of inner radii closest to the material thicknesses. At this point we come to the following radii type, i.e. perfect bending.

Perfect Radius Bending.

This bending implies one-to-one relation of inner radii to material thickness, which means inner radius and material thickness are not different, besides it includes a small set of values starting with the least radii reaching to 125 % of the material thickness.

At one-to-one relation, bending possesses the most stability possible, which allows producing radii with no or fewest changes among bends. It enables you to gain angular and dimensional consistency with the minimal back spring amounts. 

This one-to-one ratio turns out the one value at which the 8-fold rule applies, which means the stamp breadth must be eight times as great as material thickness. The following point will not work as soon as the 1 to 1 relation gets bigger or less.

Superficies and radius bending. Deep Bending Radius

This bending implies inner radii from 125 % to 12 times as much as material thickness. This is a rough calculation. A more accurate top bound of the bending radii is connected with the sheet performance.

When the inner radius and material thickness relation gets higher, the back spring also rises. In case of excessive relation of inner radius and material thickness, the materials become less versatile, even under lower extensile force, which might cause many breakages. (Fig.2) More often in materials with lower extension force and rare in materials with high force, multi-fracturing is manifested in the fact that the inner radii of materials is separated off punches. Multiple rupture occurs if the relation of inner radius (Ir) and material thickness (Mt) rises 12: 1, while in the correct conditions it may reach 30: 1. 

Fig. 3

Now, what is the point at which radii bending turns into the deep radii bending? It happens when materials get separated from punch radii. Once more it appears the effect of the inner radius and material thickness relation increase up to 12:1, while there might be cases when it comes to 30:1.  

Properties of materials are decisive in the gained result. Considerable changes in chemical composition, processing, nature of materials will change, making it too difficult to predict the precise points at which variations might emerge.

In case of no more than the outer 90-degree bending, materials can go along punching radii outlines. Next, the penetration into die area as well as back spring take actions. When the outer bending angles exceed, the proportionate extent of back spring also rises. Then the back spring must be compensated for, if the disconnection of Ir and Rp happens to be larger, Ir gets less in relation to the punching radii. The deep radii bending gets compensated or pushed backwards to hold materials contacted with punching radii. (Fig.3)

By the way, these can later get separated through bend methods: air, bottom, coin, fold, wipe formations. Still, this is an issue for some other day or article. However, in case of air formation the mentioned 5 disciplines are helpful for all at the workshop enabling them to solve various problems relating to the bend process.