Practical Tips for Sheet Metal Design

Design guideline

Clearly mark the direction of raw edges

Sheet metal will produce R corners and burrs during blanking and punching. Especially after the mold is worn in the first stage of mass production, the burrs will become more serious and may even cut your fingers. Therefore, when making drawings and making molds, the direction of the burrs must be clearly marked according to the function.

Hole spacing and heat dissipation hole design

1. The shortest distance between two adjacent holes and the edge of the hole to the edge of the other hole shouldn'tbe less than 1.5 times the thickness of the material, otherwise the master mold will easily break and cause the production line to break. Broken lines, mold repairs, etc. are all the culprits that cause increased costs and reduced profits. If it has to be less than 1.5 times the thickness of the material, the tab method must be used.
2. In mold making, round holes are the strongest and easiest to manufacture and maintain, but the opening rate is low.
3. The square hole has the highest opening rate, but because it is at a 90-degree angle, the corners are prone to wear and collapse, causing the mold to be repaired and the line to be stopped. The hexagonal Honeycomb has a 120-degree angle greater than 90 degrees and is stronger than the square hole opening, but the opening rate is a little worse than the square hole at the edge.

The distance between the protrusion and the bent edge

When bending, the parts on the bottom edge such as studs or internal protrusions shouldn't be too close to the bending edge, preferably more than 10mm, otherwise the R angle of the corner under the protrusion will be larger than the R angles on the left and right sides without a male die stamping. The R angle is discontinuous, which will affect the appearance. The solution is to punch out an indentation of appropriate length on the folding line before bending, which will improve its appearance.

Distance between Hole and Bend Edge

When bending, the opening on the side wall shouldn't be too close to the bending edge, preferably more than 3mm, otherwise the opening will be deformed due to bending. The solution is to punch out a long hole that is as long as the opening and 1.5 times as wide as the material thickness before bending. This can cut off the pull without affecting the appearance of the opening.

(2) Punch through holes or drawing holes on the surface of the sheet metal parts, and then tap the M3 or M4 mechanical teeth with a screw tap.

If screws with a diameter of 3mm are used for locking, the hole diameter d before tapping should be 2.6mm.

If screws with a diameter of 4mm are used for locking, the hole diameter d before tapping should be 3.6mm.

If the material thickness is 1.0~1.2mm, it is recommended to use drawing holes instead of through holes. Due to the material thickness of 1.2mm, when tapping M3 teeth, there are only 2.5 teeth, so it is easier for the teeth to slide.

(3) Punch a through hole on the sheet metal surface and rivet the ready-made fixing nut (Self Clinching Nut). The hole diameter d of the riveted fixing nut is preferably the size specified by the manufacturer. However, you must be careful when riveting nuts (Self Clinching Nut). PEM (Penn Engineering & Manufacturing Corp.), a major stand-off/self clinching nut manufacturer, has a special machine for riveting. However, riveting is done one by one, which is labor-intensive, time-consuming and expensive. Therefore, almost all processing plants use general punch machines for riveting. If unfortunately the machine used is a traditional stamping machine, the problem of nut falling off may occur. The reason is that the stamping speed of the traditional stamping machine is too fast, and the workpiece material has no time to fill the nut or stand-off groove. There is no problem at all from the appearance, but some nuts will fall off when assembling the finished product. Therefore, it is best to use a machine that can adjust the stamping speed for riveting fixed nuts.

Generally, the materials we often use to prevent EMI shrapnel include: tinplate, corrugated copper, stainless steel, etc.

1. The surface of tinplate is tin-plated (Tin). Hand sweat left after touching it can easily cause rust. The cutting surface is prone to rust even if it isn'ttreated after cutting. Easy to stamp and form, lowest cost.

But the least elastic. Due to its low carbon content, even heat treatment can't increase its elasticity.

2. Titanium and copper have the best conductivity and the most expensive material cost. But it is the easiest to break and has structural direction problems. The directionality of the material must be paid attention to during production. If necessary, elastic qualitative treatment can be done to increase its elasticity.
3. Stainless steel is currently the most used. It doesn'trust and isn'teasy to break, but it isn't easy to stamp and form. The mold is easy to wear and the finished product is prone to burrs. To achieve good elasticity, it is necessary to perform elastic qualitative processing.

Otherwise, you will be overwhelmed and unable to reply. If you want the Cost Down to not perform elastic qualitative processing, it is best to make a Stopper in an appropriate place to prevent the shrapnel from being over-pressed and unable to rebound, thus losing its meaning.

4. Generally, after bending sheet metal parts, metal material will protrude on both sides of the folded corners due to extrusion. The resulting width is larger than the original size, and the protruding size is related to the thickness of the material used. The thicker the material, the larger the protruding point. In order to avoid this phenomenon, you can make a semicircle on both sides of the bending line in advance. The diameter of the semicircle is preferably more than 1.5 times the material thickness. When designing with edge material folded back, handle it in the same way.

Bending radius & height

When bending sheet metal parts, the internal R angle is preferably greater than or equal to 1/2 of the material thickness.

Because if the R angle isn't made, the right angle will gradually disappear after multiple stampings and the R angle will be formed naturally.

After this, the length of one or both sides of the R angle will become slightly longer.

The bending height should preferably be greater than 3mm. (t: 1.0~1.2mm) Otherwise, the size will be unstable due to too small clamping size.

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Punching & die dimensions

When sheet metal parts are blanked and punched, 1/3~2/5 of the cut section close to the male die punch is a flat cutting surface, while 3/5~2/3 close to the female die is an oblique tearing section. Therefore, the size of the hole diameter when making the mold or inspecting the size is based on the punch. The outer dimensions of the workpiece when blanking are based on the inner dimensions of the master mold.

Reinforcing ribs

At the corner of the sheet metal part, if there is no special requirement for a 90-degree angle, be sure to treat it as an appropriate R angle. Because the right angles on the edges of sheet metal parts can easily create sharp points that can cause cuts to workers.

The right-angled tip of the master mold is prone to cracking due to stress concentration. The male mold is prone to cracking at the tip, which requires mold repair and delays mass production.   Even if there is no cracking, an R-angle will be formed over time due to wear, causing burrs on the product and resulting in defective products.

Sheet metal parts are easily deformed under stress after bending. In order to avoid deformation, an appropriate amount of reinforcing ribs with a 45-degree angle can be added to the bending area, on the principle of not interfering with other parts, to increase its strength.

Generally, when sheet metal parts are long and narrow, it is difficult to maintain their straightness, and they are more likely to deform after being stressed.

Therefore, we can fold one edge into an L shape or fold two edges into a lip shape to maintain its strength and straightness. But often the L and mouth shape can't be connected from beginning to end and are interrupted due to some factors. What should I do?

An appropriate amount of ribs can be added to increase its strength.

Label markings on chassis

Before molding the chassis, it is best to design the location and size of the required labels. You can also mark the chassis first to facilitate alignment when labeling. There are two most common tags:

1. Make "L" shaped marks around the label, either on the upper and lower sides on the left, or on the left and right sides above. This method has cheaper mold costs, but the label protrudes from the surface of the chassis and is easily scratched.
2. Take the shape and size of the label and increase the size by 0.3mm, and make a mark at the place where the label is to be attached.

0.2~0.3mm dent.

No matter which method is used, you can choose an appropriate corner from the four corners to make a 45-degree leading angle. The relative positions of the markings on the chassis are at the same 45-degree angle. For foolproof purposes. Avoid labeling in different directions at different times or by different staff.

1. When the server chassis is placed on the rack, there are slide rails on the left and right sides to support it, so there is no doubt about the dent in the depth direction. But in the horizontal aspect, the width of the rack is 450mm, deducting the left and right slide rails of 10.mmX2, the width of the chassis is about 430mm. It is also difficult to prevent the central part of such a wide and 1.2mm thick sheet metal from sinking. The chassis itself includes front and rear walls. If a longer chassis is designed with a center wall, the problem of denting can be avoided. The middle wall is best designed to have a C-shaped steel structure and is closely integrated with the side walls and the bottom of the chassis. The strength of the entire system will be greatly improved. Even if it can'tbe continued along a straight line, it is better to design a break than to cut it off halfway.
2. In addition to increasing the strength of the chassis and fixing the fans and air ducts, the middle wall can effectively prevent EMl if it is in perfect contact with the inside of the upper cover, greatly preventing the motherboard noise from radiating from the front. Therefore, it is best to avoid placing plastic parts on the middle wall, blocking contact with the upper surface.
3. Avoid sharp angles where there are gaps, and don’t forget to design a large R angle. This prevents the sharp angle from pressing against the upper cover to cause a bulge that affects the appearance when the upper cover is pressed heavily.

   Bump positioning

   1. In the chassis assembly design, there are often two pieces, or 3 or 4 or more pieces that are combined with each other. Common fixing methods include locking screws, rivets, hole riveting or spot welding. When spot welding, the spot welding machine must have a positioning point or positioning pin or jig to ensure the correct position. If there are already corresponding screw holes or rivet holes when using screws or rivets, it is often not necessary to add a few more positioning holes for positioning. However, the diameter of the screw hole rivet hole is generally designed to be larger for easy assembly.

   Therefore, the relative positions between parts are also prone to errors.

   2. It is recommended to use positioning bumps with smaller gaps in this case. When doing tolerance analysis, it is more accurate to use positioning points with smaller tolerances for datum calculations.

   Crack relief

   It is best to have crack relief grooves in the bend between the flat surface and the bending surface, or to retreat the opening edge behind the bend. Otherwise, burrs will occur, and the width of the narrow hole is preferably greater than or equal to 1.5 times the material thickness. Also, when drawing, don’t forget or be lazy and fail to mark the R angle. Male and female molds with right or acute angles are prone to cracking, which will cause additional losses in future line stops and mold repairs.