For u-bend,
The springback decreases with the increase of die depth and the decrease of die gap. If the bending accuracy is high, you can
Take the bending unilateral clearance value z = t；; If higher bending accuracy is required, slightly thinner bending will be more helpful to reduce springback. Due to zero or negative gap bending, the stress state of the sheet can be changed from ordinary bending to bending with tensile bending performance, and the inner side of the neutral layer of the defective material is in a compressive stress state, so that the whole cross section of the blank is in a tensile stress state in the tangential direction, and the springback of the inner and outer fibers can cancel each other after unloading, which can reduce the springback (Figure 4). Therefore, it is very beneficial to control springback by using stretch bending technology and die with adjustable gap. The compensation method of controlling springback is to control the geometric shape and size of the working part of the die according to the springback direction and springback amount of the bending part, so that the springback of the workpiece after bending can be compensated. For example, for a U-shaped part with large bending, the end face or top surface of the punch can be made into an arc shape, or the punch and the die can be made into an inclined plane with a certain angle, so that the curved arc shape or the deformation at the inclined plane during unloading can compensate the rebound deformation of the two fillets. (Figure 6)
From this extension, the workpiece that is difficult to bend by conventional methods can be bent by bending compensation method. For the shape shown in Figure 8, the two sides are close together and there is no molding space. The curve can be changed by means of deformation springback and correction compensation.
As shown in figs. 7 and 8,
The first step is to make the bottom of the convex-concave die into an arc with equal arc length.
The straight line length at the bottom of the workpiece. In the second step, the flat-bottomed punch can be used for leveling. Correction and compensation can be realized through the inclined wedge, hinge or die structure, which makes the compensation easier to realize. For example, the male die and the female die shown in fig. 6a are both made to a certain size.
Inclined surface, when the workpiece is made into inclined surface, it is difficult to demould.
Surface, the workpiece is difficult to demould, when making the hinge bending die shown in figure 9, there is no such difficulty.
For workpieces with thin blanks, polyurethane rubber dies can be used for bending to control springback.
The effect is good. Because the bending die of polyurethane rubber can not only achieve seamless bending, but also achieve bending similar to deep drawing, so the bending quality is high.
For example, the neoprene bending die shown in Figure 10. The blank is subjected to impact pressure P from top to bottom in the die, and pressure F and friction FU are applied on both sides respectively. FU is caused by the friction between the blank and polyurethane rubber. During bending, the pressure f increases with the deepening of workpiece pressing, and so does FU. It is because of Fu that the stress distribution in the blank is changed. As shown in figure 1 1, figure 165438+, it is obvious that σF changes the internal stress distribution law of the blank, moves the position of the stress neutral layer to the inner layer, obviously increases the tensile stress distribution area of the outer layer and reduces the layered stress distribution area of the inner layer, so the springback is smaller than that of the steel die. Generally speaking, high quality bending parts can be obtained by using extrusion method to correct bending in inclined wedge bending die.
Figure 12 is a bending die with an internal inclined wedge. As can be seen from the figure, at the end of the bending of the two movable punches, the corner of the U-shaped bending part is squeezed and corrected by the action of the internal inclined wedge, so the accuracy is high. There are many bending dies with similar structures. For example, the outer inclined wedge can be used to extrude and correct the bending angle.
Figure 13 is. In the open state, the female dies 2 and 5 are opened under the action of the spring 4, and the gap z between the male die 1 and the female die is equal to the thickness of the sheet. The punch 1 is lowered to bend the blank between female die 2 and 5. It is worth noting that: ① the inclination between the mold and the mold base is about 20; ② The reaction force of the spring 4 is greater than the bending force required by the workpiece. When the two shoulders of the male die are connected with the upper plane of the female die, the female die will slide down along the inclined plane of the die seat and close to the middle, and then the workpiece will be squeezed and corrected. Because the punch and die have rebound compensation, the right-angled bending parts can be obtained after the workpiece rebounds. For U-shaped bending, Xia Hua et al. recently thought that the corner of the bending part should be thinned and bent with a large fillet female die and a small fillet male die as shown in Figure 14, so as to make it completely plastic.
In addition,. These measures can reduce the springback of bending parts and improve the surface quality.
Li Wendong and others recently designed a model? As shown in fig. 15, the bent parts of the die are straight, and the shape and size accuracy are also high. The key to success is to reduce the bending resistance, that is, the α angle in the drawing should be small and the radius of the die fillet should be large. As shown in the figure, the punch consists of a fixed punch 1 and a floating punch 4. The larger the floating distance So S of the floating punch is, the smaller the α angle is, which is more beneficial to bending. However, it should be noted that at the moment of initial bending, it is necessary to ensure that the part slightly exceeds the E point of the fixed punch after turning over.
Common v-type, u-type, z-type and? It is best to use a full-forming upsetting bending die for Class I bending parts (Figure 16). Whether its stress can be regarded as pure plastic bending with a large normal stress superimposed on it, it can be considered that the bending stress after full forming and upsetting is composed of all single stresses, so there is almost no rebound phenomenon, and high-quality bending workpieces can be obtained. There are four common bending methods: pressure bending, rolling bending and extrusion bending. In elbow, besides solving the problems of outer edge crack, inner circle wrinkling and pipe diameter flattening, there is also a very important problem, that is, overcoming the rebound problem in elbow.
In order to ensure the quality of the bent pipe, it is necessary to estimate the springback value in advance in the design of the variable pipe die, and then apply an appropriate amount of pre-springback to ensure that the bending radius and bending angle of the bent part meet the design requirements after unloading, thus avoiding the trouble of artificial shaping.
Therefore, the method to overcome the rebound of elbow is the same as that of sheet metal bending. The first step is to find a way to estimate the curvature springback value △P and the angle springback value △ α of the pipe as the basis for designing the die for compensation, and the second step is to make final correction through die trial. For example, when bending, the curvature rebound value △P and the angle rebound value △α can be calculated as follows:
△P=(σs*Sx/E*Jx)+(D/E*ρ)
△α=(σs*Sx/E*Jx+D/E)*α
Where σ s refers to the yield limit of the material approximate to the actual stress curve;
D- strain modulus of material;
Sx-static moment of cross-sectional area relative to X axis;
JX- moment of inertia of section area relative to X axis;
α —— Bending angle before rebound;
ρ —— the radius of curvature before springback.