3. Metal Bending
3.1 The minimum bending radius of a metal bending part
When the material is bent, the outer layer is stretched and the inner layer is compressed.When the material thickness is constant, the smaller the inner R is, the more serious the tension and compression of the material are.When the tensile stress of the outer fillet exceeds the ultimate strength of the material, cracks and fractures will occur.Therefore, too small bending radius should be avoided in the structural design for bending parts.The minimum bending radius of the commonly used materials is shown in the table below.
Table 5 List of minimum bending radius for commonly used metal materials
|No.||Material||Minimum bending radius|
● The bending radius refers to the inner radius of the bending part, and t is the thickness of the material.
● T is material thickness, M is annealed state, Y is hard state, Y2 is 1/2 hard state.
3.2 The straight side height of a metal bending part
3.2.1 General requirements for minimum height of straight side
The height of the straight side of the metal bending parts should not be too small, and the minimum height is required as (Figure 126.96.36.199): h＞2t.
Figure 188.8.131.52 The minimum straight side height of a bending part
3.2.2 Height of straight edge with special requirements
If the straight side height (h) of the bending part is as h≤2t due to the design needs, the first step is to increase the bending height, then process it to the desired height with other ways.Or make groove in deformation zone before metal bending,and then bend it to design shape.(as shown below Figure 184.108.40.206)
Figure 220.127.116.11 Requirements for height of straight side under special circumstances
3.2.3 Height of a straight side with a oblique angle
When making a metal bending parts with a oblique angle on side of the curved bend (Fig. 18.104.22.168), the minimum height on the side is: h=(2～4) t＞3mm
Figure 22.214.171.124 The straight-side height with a oblique angle on the side of a curved bend
3.3 Hole margin on a metal bending part
Hole margin: when bending after hole punching, the position of the hole should be outside the bending deformation zone, so that avoiding deformation of holes during metal bending.The distance from the hole wall to the curved edge is seen below the table:
Table5 Hole margin on a metal bending part
3.4 Technical cut of local (partial) bending
3.4.1 The bending line of metal bending parts should avoid dimention-changing position
When the edge is bent at a certain section, in order to prevent the crack in the stress concentration at the sharp corner, the bending line can be moved to a certain distance to avoid the dimention-changing position (Figure 126.96.36.199 a), or make groove for bending(Figure 188.8.131.52 b), or punch holes for bending (Figure 184.108.40.206 c). Note the size requirements in the drawing: S≥R; groove width k≥t; groove depth L≥ t+R+k/2.
Fig.220.127.116.11 The design method of local (partial) bending
3.4.2 Forms of cut that are adopted when the hole is in the metal bending deformation area.
When the hole is in the bending deformation zone, the example of the form of cut is adopted as following: (Fig. 18.104.22.168).
Figure 22.214.171.124 Example of form of cut
3.5 The metal bending with oblique edges should be avoided in the deformation zone
Figure 4.5.1 The metal bending with oblique edges should be avoided in the deformation zone
3.6 Design requirements of returned bends
The length of the returned edge (L) is related to the thickness of the material. As shown in the figure below, the minimum length of returned edge: L≥3.5t+R
T is the material wall thickness, and R is the minimum internal bending radius before process of returned bending (as shown below).
Figure 4.6.1 The minimum length L of the returned edge
3.7 Positioning hole added at design time
In order to ensure the accurate positioning of bending blank in the mold and prevent the blank from offset during the bending process, the positioning holes for processing should be added at the time of design, as shown in the following figure. In particular, multiple bending parts must be based on positioning holes, so as to reduce accumulated errors and ensure product quality.
Figure 4.7.1 Positioning holes for processing added at the time of designing multiple metal bending
3.8 The process craft should be considered when dimensioning the metal bending parts.
Figure 4.8.1 Dimensioning example for bending parts
As shown in the figure above, a) punch holes first and bend, L dimensional accuracy is easy to guarantee, processing is convenient. b) and c) if the accuracy of dimension L is high, then it is necessary to bend first then do holes processing, processing is a little bit troublesome.
3.9 Springback of a metal bending part
There are many factors that affect springback, including mechanical properties, wall thickness, bending radius and positive pressure during bending.
3.9.1 The greater the ratio of the fillet radius to the plate thickness, the greater the springback will be.
3.9.2 An example of a method of suppressing a springback from a design
The springback of metal bending parts is mainly taken by manufacturers with certain measures when designing the mould. At the same time, some structural improvements are designed to reduce the springback angle as shown below: make strengthening ribs in the bending zone can not only improve the stiffness of the workpiece, but also help to suppress the rebound.
Figure 126.96.36.199 Design method for restraining springback