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Structural Design Knowledge of Sheet Metal Parts (二): Structural Design Criteria

October 24, 2022

When designing product parts, the problem of easy manufacturing must be considered. Try to think of some methods that can not only make processing easy, but also save materials, and increase strength without waste. For this reason, designers should pay attention to the following manufacturing issues.

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The manufacturability of sheet metal parts refers to the difficulty of parts in punching, bending and stretching. Good process shall ensure less material consumption, fewer processes, simple mold structure, long service life and stable product quality. In general, the performance of materials, geometric shape, size and accuracy requirements of parts have the greatest impact on the processability of sheet metal parts.
How to fully consider the requirements and characteristics of processing technology in the structural design of thin plate components, several design criteria are recommended here.

1. Simple shape criteria
The simpler the geometric shape of the cutting surface, the more convenient and simple the cutting and blanking, the shorter the cutting path, and the smaller the cutting amount. For example, straight lines are simpler than curves, circles are simpler than ellipses and other higher order curves, and regular graphs are simpler than irregular graphs.
The structure in Figure 2a is meaningful only when the batch is large, otherwise, it will be troublesome to cut during blanking. Therefore, the structure shown in Figure b should be used for small batch production.
2. Material saving criteria (configuration criteria for blanking parts)
Saving raw materials means reducing manufacturing costs. The scraps of leftover materials are often treated as waste materials, so in the design of thin plate components, the leftover materials should be reduced as much as possible. The blanking waste shall be minimized to reduce material waste. Especially, the effect is remarkable when cutting off components in large batches. The ways to reduce cutting off are as follows:

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3. Criterion of sufficient strength and stiffness
⑴ The bending edge with beveled edge shall avoid the deformation area
(2) If the distance between two holes is too small, cracks may occur during cutting.
For punching design of parts, proper hole edge distance and hole spacing shall be reserved to avoid punching cracks. The minimum distance between the punching edge of the part and the shape of the hole has certain restrictions depending on the shape of the part and the hole. When the punching edge is not parallel to the contour edge of the part, the minimum distance shall not be less than the material thickness t; In parallel, it shall not be less than 1.5t. The minimum hole edge distance and hole spacing are shown in the table.
The round hole is the most solid and easy to manufacture and maintain, but the opening rate is low.
The square hole has the highest opening rate, but because it is a 90 degree angle, the corners are easy to wear and collapse, resulting in mold repair and line stop The hexagonal opening has a 120 degree angle greater than 90 degrees, which is stronger than the square opening, but the opening rate at the edge is a little lower than the square opening.
(3) The thin and long lath has low rigidity and is easy to crack during cutting, especially the tool is seriously worn.
In general, the depth and width of the convex or concave part of the blanking part should not be less than 1.5t (t is the material thickness). At the same time, narrow and long cuts and too narrow grooves should be avoided to increase the strength of the cutting edge at the corresponding parts of the mold.

4. Reliable blanking criterion
The semicircle tangent structure shown in Fig. 9a is difficult to cut. Because this requires accurate determination of the relative position between the tool and the workpiece. Accurate measurement and positioning is not only time-consuming, but more importantly, the accuracy of the tool usually cannot meet such high requirements due to wear and installation errors. Once there is a slight deviation in the processing of such structures, it is difficult to ensure the quality, and the cutting appearance is poor. Therefore, the structure shown in Figure b shall be adopted, which can ensure reliable blanking processing quality.

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5. Criteria for avoiding sticking (configuration criteria for piercing parts)
When punching and cutting in the middle of the component, there will be a problem that the cutter and the component are bonded tightly. Solutions: (1) Reserve a certain slope; (2) Connection of cutting surface
When the lapping is made into 90 ° bending edge by punching in one process, the material should not be too hard, otherwise it is easy to crack at the right angle bend. The process cut shall be designed at the bending position to prevent cracking at the corner.

6. Bending edge vertical cutting surface criterion
After cutting, the sheet metal shall be further formed, such as bending. The bending edge shall be perpendicular to the cutting surface, otherwise the risk of cracks at the intersection will increase. If the vertical requirements cannot be met due to other restrictions, a fillet shall be designed at the intersection of the cutting surface and the curved edge, with the radius greater than twice the plate thickness.

7. Gentle bending criterion
Steep bends require special tools and are expensive. In addition, too small bending radius is easy to produce cracks and wrinkles on the inner side
8. Guidelines for avoiding small round curling
Edges of thin plate members are often crimped, which has several advantages. (1) Stiffness is strengthened; (2) Sharp edges are avoided; (3) Beautiful. However, two points should be paid attention to when crimping. One is that the radius should be greater than 1.5 times the plate thickness; The other is not completely round, which is difficult to process. The curling shown in Fig. 18b is easier to process than that shown in respective a.

9. Non bending criterion of groove edge
A certain distance shall be kept between the bending edge and the slot edge. The recommended value is the bending radius plus 2 times of the wall thickness. The stress state of bending zone is complex and the strength is low. Slots with notch effect should also be excluded from this area. Not only can the entire slot be far away from the curved edge, but also can the slot span the entire curved edge (see Figure 19).
10. Manufacturing criteria for complex structure combination
It is difficult to form components with too complex spatial structures by bending. Therefore, the structure should be designed as simple as possible. When it is not complicated, composite members can be used, that is, multiple simple thin plate members can be combined by welding, bolting, etc. The structure of Figure 20b is easier to machine than that of Figure 20a.

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11. Criteria for avoiding straight line penetration
Thin plate structure has the disadvantage of poor lateral bending stiffness. Large plate structures are prone to buckling and instability. Further bending and fracture will occur. Generally, the groove is used to improve its rigidity. The arrangement of pressure grooves has a great influence on the effect of improving the stiffness. The basic principle of pressure groove arrangement is to avoid straight line penetration in the non pressure groove area. The through narrow band with low stiffness is easy to become the inertial axis of buckling instability of the whole plate surface. Instability always revolves around an inertial axis. Therefore, the arrangement of pressure grooves should cut off this inertial axis and make it as short as possible. In the structure shown in Figure 21a, the non pressure groove area forms a plurality of through narrow strips. Around these axes, the bending stiffness of the entire plate is not improved. The structure shown in Figure 21b has no potential connected unstable inertial axis. Figure 22 lists the common groove shapes and arrangements. The effect of stiffness enhancement increases gradually from left to right. Irregular arrangement is an effective way to avoid straight line penetration.

12. Connected Arrangement Criteria of Pressure Groove
The weak link is the low end fatigue strength of the groove. If the groove is connected, some of its end points will be eliminated. Figure 23 shows the battery box on a truck, which is subject to dynamic load. The structure in Figure 23a has fatigue damage at the groove end. The structure in Figure 23b does not have this problem. Steep groove end face shall be avoided, and the groove shall extend to the boundary if possible (see Figure 24). The penetration of the pressing groove eliminates the weak end. However, there should be enough space at the intersection of the grooves to reduce the interaction between the grooves

13. Space notching criterion
The instability of spatial structures is not limited to one aspect. Therefore, setting pressure grooves on only one plane cannot improve the stability resistance of the whole structure. For example, the U-shaped and Z-shaped structures shown in Figure 26 will lose stability near the edges. The solution to this problem is to design the pressure groove as a space
14. Local relaxation criterion
Wrinkling occurs when local deformation on the sheet is seriously obstructed. The solution is to set a few small pressure grooves near the folds, so as to reduce the local stiffness and deformation resistance.

15. Configuration criteria of blanking parts
⑴ Minimum punching diameter or minimum side length of square hole
When punching, it shall be limited by the strength of the punch, and the size of the punch shall not be too small, otherwise the punch may be easily damaged. The minimum punching diameter and minimum side length are shown in the table.
*T is the material thickness, and the minimum punching size is generally not less than 0.3mm.
(2) Principle of punching notch
Sharp corners shall be avoided for punching notches, as shown in Fig. a. Sharp corners are easy to shorten the service life of the mold, and cracks are easy to occur at the sharp corners.