Lesson #1: Clearly Mark the Direction of Burr
Sheet metal will produce rounded corners and burrs during cutting and punching. Burrs become more severe during mass production, especially after mold wear and tear, and can even cause finger cuts. Therefore, when designing and producing the mold, the direction of burrs must be clearly marked according to the function.
Image
Lesson #2: Hole Spacing and Heat Dissipation Hole Design
1. The shortest distance between the edge of two adjacent holes should ideally be no less than 1.5 times the material thickness. Otherwise, the master mold will easily break, causing production line interruptions. Wire breakage and mold repairs are the main culprits for increased costs and reduced profits. If it is absolutely necessary to have a distance less than 1.5 times the material thickness, a skipping method should be used.
2. Round holes are the most durable and easy to manufacture and maintain, but they have a lower aperture ratio.
3. Square holes have the highest aperture ratio, but because they have 90-degree angles, the corners are prone to wear and collapse, resulting in production line interruptions requiring mold repairs. The hexagonal Honeycomb, with its 120-degree angle greater than 90 degrees, is stronger than a square hole, but the aperture ratio is slightly lower at the edges.
Image
Lesson #3: Distance between Protrusions and Bend Edge
When bending, parts on the bottom edge, such as studs or internal protrusions, should not be too close to the bend edge. Ideally, they should be at least 10mm away. Otherwise, the corner below the protrusion, without a die, will have a larger radius than the corners on the left and right sides. This discontinuous radius will affect the appearance. A solution is to stamp an indentation of appropriate length along the bend line before bending; this will improve the appearance.
Image
Lesson #4: Distance between Holes and Bend Edge
When bending, openings on the side walls should not be too close to the bend edge. Ideally, they should be at least 3mm away. Otherwise, the openings will be deformed by the strain of the bend. The solution is to punch out a long hole with a length equal to the opening and a width 1.5 times the thickness of the material before bending. This can cut off the traction without affecting the appearance of the opening.
Picture
Experience Summary #5: Key Points in Screw Hole Design
Generally, there are three ways to fix screws
(1) Punch a hole (through hole) or draw a hole (drawing hole) directly on the plane of the sheet metal and use a self-tapping screw. Triangular self-tapping screws are the best self-tapping screws as they are less likely to cause thread slippage. However, the driving force is slightly heavier than that of non-triangular self-tapping screws.
If a 3mm diameter screw is used for locking, the hole diameter d should be between 2.4 and 2.5mm. If a 4mm diameter screw is used for locking, the hole diameter d should be between 3.4 and 3.5mm.
Picture
(2) Punch a hole (through hole) or draw a hole (drawing hole) on the plane of the sheet metal and then tap the holes with a screw tap, using M3 or M4 machine threads.
If a 3mm diameter screw is used for locking, the hole diameter d should be 2.6mm before tapping. If a 4mm diameter screw is used for locking, the hole diameter d should be 3.6mm before tapping. If the material thickness is 1.0~1.2mm, it is recommended to use a drawing hole instead of a through hole. Because when tapping M3 threads with a 1.2mm thickness, there are only 2.5 threads, which is more likely to slip. (3) Punch a through hole on the flat surface of the sheet metal and then rivet the ready-made fixing nut (self-clinching nut). The hole diameter d of the riveted fixing nut is preferably the size recommended by the manufacturer. However, when riveting the nut (self-clinching nut), it must be noted that PEM (Penn Engineering & Manufacturing Corp.), a major manufacturer of stand-off/self-clinching nuts, has a dedicated riveting machine, but it is processed and riveted one by one, which is labor-intensive, time-consuming and expensive. Therefore, almost all fabrication plants use conventional punch presses for riveting. Unfortunately, if a traditional press is used, the nut may fall off. This occurs because the punching speed of a traditional press is too high, preventing the workpiece material from filling the nut or standoff grooves before the process is complete. While the problem may not be apparent from the outside, some nuts may fall off during assembly. Therefore, it is best to use a machine that allows for adjustable punching speeds when riveting nuts.
Image
Experience Summary #6: EMI Shrapnel Materials
Typically, materials commonly used for EMI shrapnel include tinplate, corrugated copper, and stainless steel.
1. Tinplate is tin-plated, but hand sweat from handling can easily cause rust. Rust is also common if the cut surface is not treated after machining. It is easy to stamp and form, and is the cheapest.
However, it has the lowest elasticity. Due to its low carbon content, even heat treatment cannot increase its elasticity.
2. Titanium copper offers the best conductivity but is also the most expensive. However, it is most susceptible to breakage and presents structural directional issues. Material orientation must be factored in during production. If necessary, elasticity treatment can be applied to increase its elasticity.
3. Stainless steel is currently the most commonly used material. It is rust-resistant and resistant to breakage, but it is difficult to stamp and form. Molds are prone to wear, resulting in burrs on the finished product. For optimal elasticity, elasticity treatment is essential.
Otherwise, if over-pressed, the spring will not return. If cost reduction is desired without elasticity treatment, it is best to install a stopper at an appropriate location to prevent the spring from being over-pressed and unable to return, rendering it useless.
4. After bending sheet metal parts, metal protrudes on both sides of the bend due to material extrusion. This causes the width to be larger than the original size. The extent of the protrusion is related to the thickness of the material used; the thicker the material, the larger the protrusion. To prevent this, pre-form a semicircle on either side of the bend line. The diameter of the semicircle should ideally be at least 1.5 times the material thickness. The same approach should be used when designing the edge foldback.
Image
Lesson #8: Bend Radius
When bending sheet metal parts, the internal radius (R) should ideally be greater than or equal to 1/2 the material thickness.
If the radius is not formed, the right angle will gradually disappear after repeated punching, resulting in a naturally formed radius.
After this, the length of one or both sides of the radius will slightly increase.
Image
Lesson #9: Bend Height
The bend height should ideally be greater than 3mm (t: 1.0-1.2mm). Otherwise, the dimensions will be unstable due to insufficient clamping clearance.
Image
Lesson #10: Punching and Die Dimensions
When punching a sheet metal part, the cut surface near the punch tip has a smooth cut surface for 1/3 to 2/5 of the material, while the cut surface near the die tip has an oblique tearing surface for 3/5 to 2/3 of the material. Therefore, when manufacturing or inspecting the die, the hole diameter should be based on the punch tip. The outer dimensions of the workpiece during blanking should be based on the inner dimensions of the die.
Image
Lesson #11: Corner Radius
At the corners of sheet metal parts, unless a 90-degree angle is specifically required, ensure that the angle is appropriately angled. Right angles on sheet metal edges can easily create sharp points that could cause injuries to workers.
In female molds, the sharp edges of right angles are prone to cracking due to stress concentration. Male molds are prone to cracking at the tips, requiring mold repairs and delaying mass production. Even if cracks don't occur, wear and tear can cause the angle to form over time, resulting in burrs and defective parts.
Image
Lesson #12: Bend Reinforcement Ribs
Sheet metal parts are prone to deformation after being bent. To prevent deformation, add appropriate 45-degree reinforcement ribs to the bend, ensuring they do not interfere with other parts and increase strength.
Image
Lesson #13: Rib Reinforcement
Narrow and long sheet metal parts generally have difficulty maintaining straightness and are more susceptible to deformation under stress.
Therefore, we can fold one side into an L-shape or two sides into a lip to maintain strength and straightness. However, if the L-shape or lip is often not completely connected and is interrupted due to some factors, what should we do?
You can add appropriate ribs to increase strength.
Image
Lesson 14: Label Marking on the Chassis
Before making the chassis mold, it is best to design the required label location and size. Marking the chassis beforehand can facilitate alignment when applying the label. There are two most common marking methods:
1. Make "L"-shaped marks around the label, either on the top and bottom of the left side or on the left and right sides of the top. This method is less expensive, but the label protrudes from the chassis surface and is easily scratched.
2. Make a 0.2-0.3mm indentation at the location where the label is to be applied, 0.3mm larger than the label shape.
Regardless of which method is used, select an appropriate 45-degree chamfer at one of the four corners. Apply the same 45-degree chamfer to the corresponding position on the chassis. This serves as a foolproof method. Avoid labels being applied in different orientations at different times or by different personnel.
Image
Lesson #15: Server Chassis Center Wall
1. When the server chassis is mounted on the rack, it is supported by slide rails on both sides, so there's no concern about it sagging in the vertical direction. However, horizontally, the rack is 450mm wide, minus the 10mm x 2 slide rails on each side, leaving the chassis approximately 430mm wide. Preventing center sagging on such a wide, 1.2mm thick sheet metal would be difficult. The chassis itself has front and back walls. Adding a center wall to a deeper chassis can avoid sagging concerns. It's best to design the center wall as a C-shaped steel structure, tightly integrated with the side walls and chassis bottom. This will greatly enhance the overall system's strength. Even when a straight line isn't possible, creating a gap is better than cutting it off midway.
Image
2. In addition to increasing chassis strength and securing fans and air ducts, the center wall, if in perfect contact with the interior of the top cover, effectively prevents EML and significantly reduces motherboard noise from escaping from the front. Therefore, it's best to avoid placing plastic parts on the center wall, which would block contact with the top cover.
3. Avoid sharp corners where there are gaps, and don't forget to design a large radius. This prevents the top cover from being pressed against by the sharp corners, causing a bump that affects the appearance.
Image
Lesson #16: Bump Positioning
1. Chassis assembly design often involves assembling two or more components. Common fixing methods include screws, rivets, riveting, or spot welding. When spot welding, always use a spot welder with locating points, dowel pins, or a jig to ensure correct positioning. If screws or rivets are used, corresponding screw and rivet holes are already present, adding additional locating holes is often unnecessary. However, screw and rivet holes are generally designed with a larger diameter for easier assembly. Therefore, the relative positioning of parts is prone to error.
2. In this case, it is recommended to use locating bumps with smaller clearances. Using locating points with smaller tolerances as reference points during tolerance analysis also results in more accurate calculations.
Image
Lesson #17: Crack Relief Grooves
Bends between flat and bent surfaces should preferably have crack relief grooves, or the opening edge should be set back beyond the bend. Otherwise, burrs will form. The width of narrow holes should ideally be greater than or equal to 1.5 times the material thickness. Also, don't forget or be lazy when drawing the planar drawing to indicate the radius (R) angle. Molds with right or sharp angles are prone to cracking, resulting in additional losses from subsequent production stops and mold repairs.
Image
Image
