Knowledge centre

8. Bending

Transforming a flat sheet into a three-dimensional product is often done through bending. Bending is a highly complex technique because of the many factors that influence the final result: type of material, material thickness, rolling direction, thermal influences, tool wear, groove width, upper blade, shortening values, setting angles, internal radius, influence of holes, etc. This chapter looks at the most important aspects of bending.

Free bending

Kepser uses so-called free bending, Fig. 8.1 because it is the most flexible way to bend an angle. By moving the top die downwards, the sheet is pushed into the bottom die and bent. This angle this produces depends not on the tool,but on the pressing depth and the sheet thickness. As both the upper blades and the bottom dies consist of separate pieces, an arrangement can be created for the optimal setting of each product. For the setting of stainless steel, tape is used to prevent the material being damaged. Fig. 8.2

Kepser has four bending presses, all equipped with a graphic visualisation of the bending process on the screen and angle control by means of the so-called ACB system (more information on the ACB system later in this chapter).

The programs used on the Trumpf presses are interchangeable, subject to the product’s dimensions being allowed on another press. The maximum length of the product to be set is 4080 mm (information on the dimensions per bending press later in this chapter).

Ratio of groove width to sheet thickness

An important aspect of bending is the ratio of groove width to sheet thickness and the associated internal product radius. As a rule, we use the ratio: groove width = 8 x sheet thickness. However, it is possible to bend a larger or smaller groove. Different combinations have to be examined individually.

The following applies to the associated radius:

• Standard groove (8 x s) Ri = 1.2 x s
• Smaller groove Ri = s
• Larger groove Ri = 1.5 x s
   
• Do not specify the radius on the drawing if it is not important.
• Draw the radius in accordance with the rules on the previous page. Kepser mainly uses universal tools, which means that machining has to be done using a fixed radius. At Kepser, the radius is adjusted to obtain good sheet results for production.
• Make a specific note of the radius if its accuracy is very important. This can then be pushed into a product with the help of a circular shaft. A special upper tool often has to be made for this.

Bending force

The bending force is calculated with the following formula: F = C*Rm*B*S2/V.

F = bending force.
C = correction factor, Trumpf reckons this to be 1.33. Rm = tensile strength.
B = width of product. S = sheet thickness.
V = groove width.

The maximum bending force is 400 kN/m edge length.

Accuracy of angle

During setting, an angle accuracy of +/ 0.5 degrees can be guaranteed.

Accuracy of the ACB system

After a sheet has been bent to a certain angle, it always rebounds. To compensate for this rebound, the sheet is bent slightly further than the required angle. However, the exact size of the rebound is difficult to estimate; it can even be different each time the same materials are pressed.

To avoid this, the bench presses of Kepser have an ACB (Automatically Controlled Bending) system. Fig. 8.3 This system automatically calculates how much a sheet has to be overpressed to achieve the setting the angle, the upper blade lifts slightly, after which the rebound is measured. Fig. 8.4 Using this measurement, the upper blade is pushed into the angle again and the system automatically corrects the product to the desired angle. The ACB system guarantees an accuracy of 0.3 degrees.

Accuracy of leg length

desired angle. After Deviations in product length that are caused by the laser cutting are increased by the bending. At Kepser, deviations are always kept smaller than 0.2 mm per setting. For deviations in several settings, see Fig. 8.5

Large notch

It is often necessary or desirable to to take a piece out of a sheet before bending it. Any distortion of the bend in the angle can be partially absorbed if the hole is still easy to weld shut. Welding actually causes the material to contract and the set parts can still pull slightly towards each other, which makes the perpendicularity better than flat after setting.

• In this case, use round holes with a diameter of 1.5 x sheet thickness (regardless of the sheet thickness and type of material). Fig. 8.6

Minimum dimension from hole to bending line

To determine the distance from the hole to the bending line, different formulae are used for different hole shapes. We differentiate between round holes, square holes and slotted holes. Fig. 8.7

Xmin = minimum distance to the bottom of the hole s = sheet thickness
d = hole diameter
Ri = internal radius
b = hole width (slotted holes and square holes)

Distance Xmin

• Round holes
  Xmin = Ri + 2 x s
• Square holes
  Xmin = Ri + 3.3 x s
  b ≤ 25
• and slotted holes
  Xmin = Ri + 3.5 x s
  25 < b <50
  Xmin = Ri + 4 x s
  b ≥ 50

Note: it is important to apply the correct radius when using these formulae.

• Round holes (8 x s)
  Ri = 1.2 x s
• Square holes
  Ri = s
• and slotted holes
  Ri = 1.5 x s

Kepser uses groove 20 as the ideal groove width for 3 mm (24 is not available).

• Strictly apply the formulae on the previous page only if distortion of the hole is absolutely not permitted.
• If any distortion is admissible, use the formulae to obtain the critical dimensions and then make the dimensions a little larger where possible (due to fluctuations in the material).

Sometimes, holes have to be closer to the bending line than is possible according to the above formulae. A trick that can be used here involves making an incision on the bending line. (Fig. 8.10) This trick works not only for holes, but also for other shapes that lie close to the corner. However, the following questions have to be answered first:

- Could the hole distort?
- Will the opening created by the incision remain open after bending?

Welding shut actually requires additional pro- cessing, which means extra costs, and the product may end up being distorted. The largest possible opening, ideally over the entire bending zone, is the preference here.

• Use the following rules if the opening has to be as small as possible:
  - Length of opening = minimum length of hole. - Width of opening = minimum length of the shortening in the inside corner (depending on the sheet thickness and setting groove). This shortening is to be calculated with the CAD package.

Minimum leg length

The minimum length of the leg to be bent (setting of backstop) depends on the bottom tool used. (Fig. 8.11) The table below contains an overview of the minimum leg length for different types of bottom tool.

Narrow products are easier to set than wide products. It is not always possible to position wide products perfectly straight on the setting bench, which can give rise to inaccuracies.

Setting U-profile

When it comes to the minimum U-profiles that can be bent, start with the internal dimensions (Fig. 8.12) so that the sheet thickness has no effect in the table. When bending U-profiles, work with upper blade BIU033/2. (Fig. 8.13)

Minimum dimensions

The minimum dimension of the U-profile to be bent depends on the sheet thickness in combination with the associated groove width (8 x s) and the dimension of the upper blade. There are three ways to look at the minimum U:

1. Starting from a minimum B length and the maximum associated A length.
2. Starting from a minimum A length to be bent and the associated minimum B length for which the product still has to be bent.
3. Minimum U dimension A=C. For the maximum U dimension, C is of no importance.

Maximum dimensions

e maximum dimensions depend on the dimensions of the blade and the machine. By allowing the length of B to increase, it becomes clear what the maximum length of A may be (see table on this page; (Fig. 8.15) intermediate values can be interpolated). Up to a B length of 130, the shape of the U-profile depends on the blade, thereafter on the upper bar. The table is compiled on the basis of using an upper blade without a spacer and without an opening between the blade and the clamping bar. For large U shapes, it can be seen what the options are if spacers are used.

The table assumes 2 settings for the fabrication of a profile without the set side distorting.

• Smaller shapes are possible with counterpressing. Ask us for more information. Bear in mind that this results in a slight distortion. (Fig. 8.16)
• It is also possible, in order to set the product from one part, to choose to place additional settings in the product. This would, of course, give the product a different appearance. (Fig. 8.17)

Bending a small box

The minimum dimensions of a box depends on the minimum dimension of the U to be bent and the narrowest available blade. The narrowest gooseneck upper blade from Kepser is 20 mm.

• To calculate the most critical box in terms of footprint and height, assume the smallest box surface area: 25 x 25 mm. This gives a height of 57.5 mm (see the table for bending a maximum U).
• For the feasibility of other box formats (larger footprint), look up the associated height in the table for bending a maximum U.

Z shape bending

The minimum dimension of a Z shape depends on the dimensions of the lower beam and the die. The upper blade has no influence. The maximum working height is 1050 mm (height including bottom die). The bases of the V85 and V130 (Tab. 8A) are identical; only the maximum length of the product is different. The V170 (Tab. 8B) has a wider lower beam, meaning that other values for the minimum Z shape apply here.

The Z shape can be bent in two ways, depending where the most accurate dimension should be. (Fig. 8.18)

V85 en V130 (Tab. 8A)

V170 (Tab 8B)

V85 en V130

V170

Offset bending

Length X (Fig. 8.19) depends on the groove width used. This is related to the total width of the lower die.

Flattening

To stiffen thin sheet material in certain places or to avoid sharp edges, these places can be flattened or folded double. (Fig. 8.20) That is done in two steps. First, the flange is bent as far as possible. Next, it is flattened. After flattening, the total height is 2 to 2.5 times the original sheet thickness. The maximum length of the flange is not important because the material is crushed into the angle, which distorts the entire flange. The maximum sheet thickness that can be flattened is 3 mm, the maximum length of piece to be flattened is 3 metres.

Setting

• Do not let the bending line run from the other leg in the course of the contour. Bulking up and stretching are hindered in the bending zone, which causes tearing. To prevent this, make a cut in line with the outside contour of the leg to be set. (Fig. 8.21) Use 1.5 x s as minimum slot width.
• Avoid slanting edges on the edge of the bend. These distort during setting. The minimum height from which the slanting edge may begin is: 1⁄2 x groove width + sheet thickness = Lmin. (Fig. 8.22)
• Better than a short bent leg Y is to shorten the edge of the other leg by size X. The size of X must be at least 0.5 x s. It is also possible to place the short bent leg on the bending line, but then a notch will have to made at the transitional point. (Fig. 8.23)
• Make a cut-out in the area of the bending line to counteract the bulging caused by the bending. (Fig. 8.24)
• Better than a short bent leg Y is to let the cut- out run around the bent edge and to shorten the other leg by size X. The size of X must be at least 0.5 x s. (Fig. 8.25 and 8.26)
• Placing two sets of tools next to each other on a bending bench, clamping one set the traditional way and the other set upside down (punch at the bottom and die at the top) makes it possible to make several bends without turning the product.
• For a long product with a short end setting, cutting and welding from this side are preferable to setting. 
• If a certain product cannot be produced on the bending bench, there is also the option of bending it by hand. In this case, make small notches on the bending line to preserve the bending position and to reduce the bending force required. (Fig. 8.27)

Bending losses

The bending loss depends on many factors: type of material, material thickness, rolling direction, thermal influences, tool wear, groove width, upper blade, setting angles, internal radius, etc. At Kepser, the sheet results and associated bending losses are calculated with the 3D CAD package. The table below shows the bending losses for the most commonly used types of material, set on the standard groove at an angle of 90 degrees. Other bending losses on request.