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Traceless Bending Technology of Sheet Metal

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Sheet metal processing technology continues to improve, especially in some applications such as precision stainless steel bending, stainless steel decorative parts bending, aluminum alloy bending, aircraft parts bending, copper plate bending and so on, further put forward higher requirements for the surface quality of the formed workpiece. The traditional bending process is easy to damage the surface of the workpiece, and the surface contacted with the die will form an obvious indentation or scratch, which will affect the aesthetics of the final product and reduce the value judgement of the product by users.

Reasons for Bending Indentation

This paper takes bending a V-shaped part as an example to discuss. Sheet metal bending is a process in which sheet metal is first elastically deformed and then plastic deformed under the pressure of punch or die of bending machine. In the initial stage of plastic bending, sheet metal is freely bended. With the pressure of punch or die on the plate, the plate and the inner surface of V-groove of the die gradually close, while the curvature radius and bending force arm gradually become smaller, and continue to press until the end of the stroke, so that the die and the plate close to the full contact of three points, at this time complete a V-bending. When bending, because the sheet metal will be extruded by the bending die to produce elastic deformation, the contact point between the sheet metal and the die will slip along with the bending process. In the bending process, sheet metal will undergo two distinct stages of elastic deformation and plastic deformation. In the bending process, there will be a process of holding pressure (three-point contact between die and sheet metal), so after the bending process is completed, three indentation lines will be formed. These indentation lines are usually produced by the extrusion friction between the sheet and the shoulder of the V groove of the die, so they are called shoulder indentation. As shown in Fig. 1 and Fig. 2, the main causes of shoulder indentation can be simply classified as the following.

 

Bending method

Since the shoulder indentation is related to the contact between the plate and the V groove shoulder of the die, the gap between the punch and the die affects the compressive stress of the plate during the bending process, and the probability and degree of indentation are also different, as shown in Figure 3. Under the same V-groove condition, the larger the bending angle of the workpiece, the larger the deformation of the sheet metal under tension, and the longer the friction distance between the sheet metal and the shoulder of the V-groove; furthermore, the longer the bending angle is, the longer the holding time of the pressure exerted by the punch on the sheet metal will be, and the more obvious the indentation caused by the combination of these two factors will be.

Structure of V-groove in concave die
 
The width of V groove is also different when metal sheets with different thickness are bent. Under the same punch condition, the larger the size of V groove, the larger the indentation width. Correspondingly, the smaller the friction force between metal sheet and V groove shoulder, the smaller the indentation depth. On the contrary, the thinner the plate thickness, the narrower the V groove and the more obvious the indentation.
 
When it comes to friction, another factor that we consider is the coefficient of friction. The R angle of the V-groove shoulder of the die is different, and the friction on the sheet metal is also different in the process of sheet metal bending. On the other hand, from the point of view of the pressure exerted by V groove on the plate, the bigger the R angle of V groove, the smaller the pressure between the plate and V groove shoulder, the lighter the indentation, and vice versa.
 
Lubrication Degree of V-groove in Die
 
As mentioned earlier, the V groove surface of the die will contact with the sheet metal and cause friction. When the die is worn out, the contact part between V groove and sheet metal will be rougher and rougher, and the friction coefficient will be larger and larger. When the sheet slips on the surface of V groove, the contact between V groove and sheet metal is actually the contact between innumerable rough bumps and surface points, so the pressure acting on the surface of sheet metal will increase correspondingly, and the indentation will become more obvious. On the other hand, if the workpiece does not clean the V groove of the die before bending, it will often cause obvious indentation due to the residual debris on the V groove, which usually occurs when the equipment bends the galvanized sheet, carbon steel sheet and other workpieces.
 
Application of Traceless Bending Technology
 
Since it is known that the main cause of the bending indentation is the friction between the plate and the V-groove shoulder part of the die, the friction between the plate and the V-groove shoulder part of the die can be reduced by means of the process technology based on the reason-oriented thinking. According to the friction formula f=u.N, the factors affecting the friction force are friction coefficient and pressure N, which are proportional to the friction force. Correspondingly, the following technological schemes can be formulated.
 
Non-metallic material for V-groove shoulder of die
 
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