Some technical points and experience sharing in sheet metal design and processing
In the field of sheet metal design and processing, sharing technical points and experience is particularly important. By in-depth discussion and sharing of these technical insights, we can continuously overcome technical problems and promote the progress of the industry. At the same time, disseminating attitude-based news and powerful information is also an important way for us to let more people understand the details of the automotive industry.
When designing sheet metal parts, the first task is to identify the purpose of the various bending knives. During the bending process of the metal sheet through the bending machine, the sheet first undergoes elastic deformation and then enters the plastic deformation stage. In the early stages of plastic bending, the sheet can bend freely. However, as the upper or lower mold exerts pressure on the sheet, the sheet gradually comes closer to the inner surface of the lower die groove, and at the same time, the radius of curvature and the bending arm gradually decrease. Continue to pressurize until the end of the stroke, so that the upper and lower molds and the plate can achieve full three-point contact, thus completing a bending process. Therefore, when designing sheet metal parts, you must carefully consider which bending knife is suitable for forming the designed sheet metal parts. Next, we’ll dive into the main types of bending knives.
2. Main application scenarios of various types of bending knives.
Due to its design characteristics, straight knives are often preferred when they only need to be bent once and do not need to consider position avoidance, as shown in Figure (2):
However, in some cases, where multiple bends are required or where avoidance must be considered, a fixed blade may no longer be suitable. At this time, you need to consider choosing other types of bending knives, as shown in Figure (3):
As can be seen from Figure (3), when the bending is obviously interfered with, it is particularly important to choose other types of bending knives for bending. In the situation shown on the left side of Figure (3), a small scimitar can be used first to perform the avoidance operation according to the actual situation, as shown in Figure (4).
For the situation shown on the right side of Figure (3), it is more appropriate to use a sharp knife for avoidance. However, it should be noted that the strength of the sharp knife is relatively weak due to its sharp head design. Therefore, it is not suitable for bending operations of thick plates. Generally, the use of sharp knives should be avoided when the plate thickness exceeds 1.5mm. At this time, we need to find other solutions in process planning, such as arranging the riveting nut process after the bending process to ensure smooth processing.
When a small scimitar cannot meet the avoidance requirements, we need to consider using a large scimitar for avoidance processing, as shown in Figure (6).
But sometimes even using a machete may still not be able to meet the need for avoidance. At this time, we can consider using a special process to solve this problem. First, make a mark on the bending line, as shown in Figure (7). The purpose of this is to perform the bending operation more smoothly and avoid collision with the blade. Next, bend the edge to 90 degrees, as shown in Figure (8). Typically, the embossing process is suitable for bending situations that cannot be completed in one go. In this way, when the second bending is performed, the bending line can be formed more accurately along the original bending mark, thereby avoiding the offset problem caused by the second bending or the bending angle control. Adverse consequences caused by improper use.
During the design process, we need to consider avoiding the use of this special process, because although it can solve the avoidance problem, it is less effective than direct bending, and there may be size and angle deviations. At the same time, we should also pay attention to other applications of the bending machine, such as the dead edge of the workpiece. Usually, the workpiece is bent to 30 degrees first, and then pressed with a flat knife. However, it should be noted that if an ordinary punch machine can meet the demand, try not to use a bending machine to ensure a better effect on the dead edge.
During the bending process, not only may the workpiece interfere with the bending tool, but also interference with the bending machine may occur when the workpiece is large in size. Therefore, when arranging the process, we must fully consider the ease and feasibility of bending. Bending is a key process, and its operation difficulty directly affects product quality. Many quality problems stem from improper control of bending dimensions. Therefore, special attention must be paid to bending issues during the design stage to ensure quality stability during mass production.
In addition, the bending size of different plate thicknesses is limited and cannot be too small. Usually, the selection of the lower die is related to the groove that is 6 times of the plate thickness, so as to ensure that the bending coefficient is consistent with the empirical value. The classification of tool slots is mainly based on their outer width, as shown in Figure 9. The most common slots are No. 6 and No. 10.
但当选用过小的刀槽进行折弯时，往往会产生显著的压痕，特别是在6号槽以下，这种压痕问题尤为明显。若工件表面未经处理，且折弯边尺寸较小，那么这种压痕不仅影响外观质量，还可能引发锈蚀问题。因此，在设计时必须对此类问题给予充分考虑。
In actual bending operations, you will face many challenges. We not only need to pay attention to the effect of the bending itself, but also consider its impact on other process links. For different situations, we need to flexibly adjust the process arrangements to ensure the quality of the final product.
Next, we’ll look at common bending knife sizes and their applications.
Sheet metal design experience sharing
1: It is important to clearly mark the direction of the burrs.
During the sheet metal processing process, the occurrence of R corners and burrs is inevitable. Especially in the mass production stage, the wear and tear of the mold may aggravate the burr problem, and in severe cases, the operator may even be cut. Therefore, when drawing and making molds, we must clearly mark the direction of the burrs according to the actual functions to ensure safety and quality during the production process.
Sheet metal design experience sharing
2: Hole spacing and heat dissipation hole design points
In sheet metal design, the design of hole spacing is crucial. The shortest distance from the edge of two adjacent holes to the edge of the other hole should be at least 1.5 times the material thickness to prevent the master mold from breaking and causing production line interruption. Unexpected situations such as wire breakage and mold repair will increase costs and reduce profits. If the distance needs to be shortened due to special circumstances, a skip design must be used to deal with it.
In addition, in mold making, round holes are favored because of their robustness and ease of manufacturing and maintenance, but the opening rate is relatively low. Although the square hole has the highest opening rate, its 90-degree angle is prone to wear and collapse, which may cause the mold repair line to stop. And the 120-degree angles of the hexagonal Honeycomb are sturdier than the square holes, although their edges are slightly less open.
Sheet metal design experience sharing
3: The appropriate distance between the protrusion and the bent edge
In sheet metal design, attention needs to be paid to controlling the distance between the protrusion and the bent edge. When performing bending operations, parts on the bottom edge, such as studs or internal protrusions, should be kept at least 10mm apart. If these protrusions are too close to the bending edge, the R angle at the corner below the protrusion may be larger than the R angles on the left and right sides due to the lack of male die stamping. This discontinuity in R angle will affect the appearance of the product. In order to avoid this situation, you can punch out an indentation of appropriate length on the fold line before bending, which can effectively improve the appearance.
Sheet metal design experience sharing
4: Distance control between hole and bending edge
In sheet metal design, the location of the hole and the distance from the bent edge are also factors that need to be carefully considered. In order to prevent the hole from being pulled and deformed during the bending process, it is usually recommended to keep the distance between the hole and the bending edge at least 3mm. To ensure that the appearance of the hole is not affected, you can pre-punch a long hole with the same length as the opening and 1.5 times the material thickness before bending. This can effectively cut off the pulling force during bending.
Sheet metal design experience sharing
5: Key points in screw hole design
In sheet metal design, the design of screw holes is crucial. There are three common screw fixing methods: directly punching or drawing holes on the sheet metal plane, and using self-tapping screws for fixation. Among them, triangular self-tapping screws are highly recommended because of their better locking performance, which can effectively reduce the occurrence of tooth slipping. However, it should be noted that the driving force will be slightly heavier when using triangular self-tapping screws.
When selecting screws with a diameter of 3mm for locking, the recommended hole diameter range should be between 2.42.5mm; and if screws with a diameter of 4mm are selected, the hole diameter d should be controlled between 3.43.5mm. This design ensures that the screws can be locked in smoothly while avoiding problems caused by oversized holes.
(2) Punch or drill holes on the sheet metal surface, and then use a screw tap to tap to produce M3 or M4 mechanical teeth. If screws with a diameter of 3mm are selected for locking, the hole diameter d should be controlled at about 2.6mm before tapping; and if screws with a diameter of 4mm are selected, the hole diameter d should be preset to 3.6mm. In addition, when the thickness of the material used is between 1.0 and 1.2mm, it is recommended to use the drilling process instead of through holes, because when tapping M3 teeth with a 1.2mm thick material, only 2.5 teeth can be formed, which can easily lead to tooth slipping.
(3) Punch holes on the sheet metal plane first, and then rivet the ready-made fixing nuts. For the hole diameter d of the riveted fixing nut, it is recommended to refer to the dimensions provided by the manufacturer. It should be noted that when riveting the nut, if a traditional punching machine is used, the nut may fall off due to the punching speed being too fast. Therefore, it is recommended to use a machine with a stamping speed adjustment function to ensure that the nut can be riveted firmly.
Sheet metal design experience summary
6: EMI shrapnel material
When making anti-EMI shrapnel, the materials we often choose include tinplate, corrugated copper and stainless steel. The following is an analysis of the characteristics and applicability of these materials:
Tinplate: Its surface is tin-plated, but sweat from your hands can easily cause it to rust. After cutting, if no treatment is done, the cutting surface will easily rust. Although it is easy and cheapest to stamp and form, it has poor elasticity and cannot be increased by heat treatment.
Titanium Copper: Excellent conductivity, but expensive material. However, it also has the problem of being easily broken and having structural directionality. During production, special attention must be paid to the directionality of the material. If necessary, qualitative elastic processing can be performed to enhance elasticity.
Stainless steel: widely used because it does not rust and is not easy to break. However, stamping is relatively difficult, and the mold is prone to wear, resulting in burrs in the finished product. In order to obtain good elasticity, qualitative elastic processing is required, otherwise the original shape may not be restored due to excessive stamping. To reduce costs without this treatment, it is recommended to set a stopper at an appropriate position during design to prevent the shrapnel from being over-stamped and losing its ability to rebound.
The problem of metal material protruding after bending: After the sheet metal part is bent, due to the extrusion phenomenon, metal material will protrude on both sides of the folded corner, causing the width to be larger than the original size. The degree of protrusion is related to the thickness of the material. The thicker the material, the more obvious the protrusion is. To avoid this problem, semicircular grooves can be pre-designed on both sides of the bending line, with a diameter at least 1.5 times the material thickness. Similarly, the same method should be used when designing the edge folding.
Sheet metal design experience summary
7: Bending radius
During the bending process of sheet metal parts, the design of the internal R angle is crucial, and it is usually recommended that its size should be at least half of the material thickness or more. This is because if there is no R-angle design, the original right angle will gradually wear out after multiple stampings, and eventually an R-angle will naturally form. This change will not only affect the aesthetics of the sheet metal parts, but may also have a negative impact on their functionality and durability. At the same time, it is worth noting that after the R angle is formed, its length on one side or both sides will inevitably change slightly. Therefore, this factor should be fully considered in the design to ensure that the size and shape of the sheet metal parts after bending can meet actual needs.
Sheet metal design experience summary
8: Bending height
In sheet metal design, bend height is a key parameter. In order to ensure dimensional stability after bending, it is recommended that the bending height be at least 3mm, especially for thin sheet metal parts (such as thickness between 1.0~1.2mm). If the bending height is too small, the clamping size may be insufficient, thereby affecting the dimensional stability. Therefore, reasonable selection of bending height in design is crucial to ensure the quality and performance of sheet metal parts.
Sheet metal design experience summary
9: Punching and mold dimensions
During the blanking and punching process of sheet metal parts, the cutting sections will show different characteristics. Specifically, the 1/3 to 2/5 part close to the male die punch has a flat cutting surface; while the 3/5 to 2/3 part close to the female die has an oblique tearing surface. Therefore, when making or testing molds, the size of the hole diameter should be based on the punch. At the same time, the outer dimensions of the workpiece when blanking need to be based on the inner dimensions of the master mold.
Sheet metal design experience summary
10: Corner R corner processing
At the corner of the sheet metal part, unless there are special requirements to specify a 90-degree angle, it should be appropriately treated as an R angle. This is to prevent the right angles of the sheet metal edges from creating sharp points that could injure workers. At the same time, the right-angled tip of the female mold is prone to cracks due to stress concentration, while the tip of the male mold may crack, causing the mold to need to be repaired, thus affecting the mass production progress. Even if the tip of the male mold does not break immediately, long-term use will form an R-angle due to wear and tear, which will lead to burrs on the product and result in defective products.
Sheet metal design experience summary
11: Application of bending reinforcing ribs
After sheet metal parts are bent, force deformation is a common problem. To prevent this, we can add an appropriate amount of 45-degree reinforcing ribs in the bending area while ensuring that they do not interfere with other parts, thereby effectively improving the strength of the area.
Sheet metal design experience summary
12: Convex rib reinforcement strategy
在钣金设计中，狭长形部件往往难以维持其直线度，受力后变形问题更为突出。为增强其强度和稳定性，我们可以采用L型或口型折弯设计。然而，在实际操作中，由于各种因素，L型和口型可能无法实现连续的从头到尾的连接。针对这一问题，我们可以巧妙地添加适量的凸肋，从而显著提升该部位的强度。
Sheet metal design experience summary
13: How to mark labels on the chassis
Before molding the chassis, the location and size of the required labels should be pre-designed and determined. To facilitate alignment when labeling, the corresponding locations can be pre-marked on the chassis. There are two common marking methods:
Put "L" shaped marks around the label, either on the upper and lower ends of the left side of the label, or on the left and right sides of the top. This method has relatively low mold costs, but the label will slightly protrude from the surface of the chassis, and there is a risk of being scratched.
According to the shape and size of the label, add a 0.3mm margin and make an indentation of 0.2~0.3mm at the location where the label is to be attached. No matter which method is used, you can choose an appropriate corner on the chassis and make a 45-degree lead angle. At the same time, the same 45-degree lead angle is also made at the corresponding marked position on the chassis to prevent fooling and ensure that the label will not be oriented incorrectly when attached.
Sheet metal design experience summary
14: Partition wall design in server chassis
在服务器机箱的设计中，当其放置在机架上时，由于滑轨的支撑，机箱在纵深方向上通常不会出现凹陷问题。然而，在横向方面，由于机架宽度为450mm，减去两侧滑轨各占用的10mm，机箱壳体的实际宽度大约为430mm。这样的宽度，再加上钣金件的厚度仅为1.2mm，中央部分难免会出现凹陷。为解决这一问题，机箱设计时可以考虑增加一堵中墙。这堵中墙的设计应类似于C型钢的结构，并与两侧墙及机箱底部紧密结合，从而显著提升整个系统的强度。即使中墙无法直线延伸，采用断差设计也优于中途截断。
2、中墙不仅增强了机箱的稳固性，还能用于固定风扇和导风管。若其与上盖内部紧密结合，还能有效防止电磁干扰，从而显著减少主板噪声从前方的散发。因此，在设计时应避免将塑料部件置于中墙上，以免影响与上盖的完美接触。
3. Where there are gaps, be sure to avoid sharp angles. Don’t forget to design a large R-angle to ensure that when the upper cover is pressed heavily, the sharp angle will not press against the upper cover, thereby affecting the aesthetic appearance.
Sheet metal design experience summary
15: Bump positioning
In the chassis assembly design, two or more components are often involved. These assemblies are usually fixed using locking screws, rivets, riveting or spot welding. When spot welding, precise positioning must be ensured with the help of locating points, locating pins or jigs. Although there are corresponding screw holes or rivet holes for screw or rivet fixation, these hole diameters are often designed to be slightly larger to facilitate assembly, which also leads to possible position errors between parts.
In this case, it is recommended to use positioning bumps with smaller gaps for positioning. During tolerance analysis, using the positioning point with smaller tolerance as the benchmark can greatly improve the accuracy of the calculation.
Sheet metal design experience summary
16: The importance of crack relief grooves
In sheet metal design, the bending operation at the intersection of the flat surface and the bending surface is crucial. To prevent cracks, it is recommended to set crack relief grooves at the bends or ensure that the edge of the opening is behind the bends. In addition, the width of the narrow hole should also be designed with attention. The width should be at least equal to 1.5 times the material thickness to ensure structural strength. At the same time, the marking of the R angle cannot be ignored during the drawing process. The male and female molds at right or acute angles are prone to cracking, which will lead to unnecessary line stops and mold repair losses.