Sheet metal processing is commonly referred to as sheet metal fabrication. It involves using sheet metal to create various products such as chimneys, barrels, oil tanks, ventilation ducts, elbows, tees, funnels, and more. The main processes involved in sheet metal fabrication include cutting, bending, edge forming, welding, and riveting, requiring a certain level of geometric knowledge. Sheet metal parts are thin metal components that can be processed through methods like stamping, bending, and stretching. A general definition of sheet metal parts is that their thickness remains unchanged during the manufacturing process. In contrast, castings, forgings, and machined parts are different types of components.
Selection of Materials for Sheet Metal Fabrication
Commonly used materials in sheet metal fabrication include cold-rolled steel (SPCC), hot-rolled steel (SHCC), galvanized steel (SECC, SGCC), copper (CU), brass, bronze, beryllium copper, aluminum (6061, 6063, hard aluminum), aluminum profiles, and stainless steel (mirror finish, brushed finish, matte finish). The choice of material depends on the specific product application and cost considerations. Here are some considerations for different materials:
1. Cold-rolled steel (SPCC) is mainly used for electroplating and painting parts. It has low cost, easy formability, and is suitable for material thicknesses ≤3.2mm.
2. Hot-rolled steel (SHCC) is used for electroplating and painting parts with material thicknesses ≥3.0mm. It has low cost but is more difficult to form, mainly used for flat parts.
3. Galvanized steel (SECC, SGCC) has different types: N material and P material. N material is mainly untreated, with higher cost, while P material is used for spray-painted parts.
4. Copper is mainly used for conductive components. Surface treatments include nickel plating, chrome plating, or no treatment, but the cost is higher.
5. Aluminum sheets are commonly treated with chromate (J11-A), oxidation (conductive oxidation, chemical oxidation), and can also be silver-plated or nickel-plated. They have higher cost.
6. Aluminum profiles are used for complex structural parts and are widely used in various enclosures. Surface treatments are similar to aluminum sheets.
7. Stainless steel is mainly used without any surface treatment and has a higher cost.
Considerations for Sheet Metal Unfolding
Unfolding diagrams are 2D representations of 3D part drawings. Here are some considerations for unfolding diagrams:
1. Choose an appropriate unfolding method to save material and facilitate processing.
2. Select suitable gap and edge forming methods. For T ≤ 2.0, the gap is 0.2; for T = 2-3, the gap is 0.5. Long edges should wrap around short edges (for door panels, for example).
3. Consider tolerance for outer dimensions: negative tolerance goes all the way, positive tolerance goes halfway; for hole dimensions: positive tolerance goes all the way, negative tolerance goes halfway.
4. Pay attention to burr direction.
5. Indicate locations for tapping, riveting, tearing, and punching (including contours) by drawing sectional views.
6. Check material, plate thickness, and plate thickness tolerance.
7. For special angles and bending radius (usually R = 0.5), trial bending is required for accurate unfolding.
8. Emphasize areas prone to errors (such as asymmetry).
9. Include enlarged views for areas with multiple dimensions.
10. Indicate areas that require protective coating.
Sheet Metal Processing Workflow
The specific process flow for sheet metal fabrication may vary depending on the structure of the sheet metal parts. However, it generally includes the following steps:
1. Material Cutting:
There are various methods for material cutting, including:
a. Shearing: Simple materials are cut using a shearing machine. It is mainly used to prepare materials for forming with molds. It has low cost and precision lower than 0.2. However, it can only process materials without holes or cut angles.
b. Punching: Parts are cut and shaped into various shapes on the sheet metal using a punching machine. It has the advantages of short processing time, high efficiency, high precision, low cost, and suitability for large-scale production. However, it requires mold design.
c. CNC Cutting: CNC programming is used to generate machine-readable programs based on the developed flat pattern. The CNC machine then cuts various flat-shaped parts from the sheet metal according to these programs. The precision is around 0.15, and the cost is low.
d. Laser Cutting: Laser cutting is used to cut complex-shaped parts from large sheets using laser cutting technology. Similar to CNC cutting, it requires the development of laser programs. It has high cost and precision around 0.1.
e. Sawing: It is mainly used for cutting aluminum profiles, square tubes, rectangular tubes, round bars, etc. It has low cost and precision.
2. Sheet Metal Operations:
a. Metalworking: Operations such as countersinking, tapping, hole enlargement, and drilling are performed. Countersinking angles are generally 120° for rivet pulling and 90° for countersunk screws. Imperial bottom holes are used for tapping.
b. Flanging: Also known as punching or hole flanging, it involves enlarging a small base hole into a slightly larger hole and then tapping it. It is mainly used for thin sheet metal to increase strength and thread count, avoiding stripped threads. It is commonly used for thin sheet metal with normal shallow flanging where the thickness remains relatively unchanged. When the thickness is allowed to thin by 30-40%, the flanging height can be increased by 40-60% compared to normal flanging. The maximum flanging height can be achieved by thinning the material by 50%. For thicker materials such as 2.0mm or 2.5mm, tapping can be done directly.
c. Punching: Punching is a process that uses molds. It includes operations such as hole punching, chamfering, blanking, embossing, tearing, and forming. The process requires corresponding molds, such as punching and blanking molds, embossing molds, tearing molds, and forming molds. Attention should be paid to the position and direction during the operation.
d. Riveting: Riveting includes operations such as riveting nuts, screws, and loose fasteners. Hydraulic riveting machines or punching machines are used to join them to the sheet metal. Swelling riveting is also a method that requires attention to direction.
e. Bending: Bending involves transforming 2D flat sheets into 3D parts. It requires a press brake and corresponding bending molds. There is a specific bending sequence: the first bend should not interfere with the next one. If interference occurs, the subsequent bend should be done first. The number of bends is calculated as 6 times the plate thickness for T ≤ 3.0mm to determine the slot width. For example, for T = 1.0mm, V = 6.0, F = 1.8; for T = 1.2mm, V = 8, F = 2.2; for T = 1.5mm, V = 10, F = 2.7; for T = 2.0mm, V = 12, F = 4.0. Bending molds are classified as straight blades and bending blades (80°, 30°). When bending aluminum sheets, cracks may occur, so the width of the lower die groove can be increased or the upper die radius can be increased (annealing can prevent cracks). Considerations during bending include: I) drawing requirements for plate thickness and quantity; II) bending direction; III) bending angle; IV) bending dimensions; V) appearance, no creases allowed for chrome-plated parts. The relationship between bending and riveting processes is generally riveting before bending. However, if interference occurs after riveting, bending should be done first. Some parts require bending, riveting, and then bending again.
6. Welding:
Welding is the process of joining materials where the atomic and molecular distances between the welded materials form a solid bond.
a. Classification:
- Fusion Welding: Includes methods such as argon arc welding, CO2 welding, gas welding, and manual welding.
- Pressure Welding: Includes spot welding, seam welding, and projection welding.
- Brazing: Includes electric chrome welding and copper wire brazing.
b. Welding Methods:
- CO2 Gas Shielded Welding
- Argon Arc Welding
- Spot Welding
- Robot Welding
c. Welding Symbols: Delta weld, Phi weld, I weld, V weld, single-sided V weld (V), blunt edge V weld (V), spot weld (O), plug weld or slot weld (π), flange weld (χ), blunt edge single-sided V weld (V), blunt edge U weld, blunt J weld, backing weld, occasional weld.
d. Welding Defects and Prevention Measures:
- Spot Welding: If the strength is insufficient, protruding points can be added to increase the welding area.
- CO2 Welding: High productivity, low energy consumption, low cost, strong rust resistance.
- Argon Arc Welding: Shallow penetration, slow welding speed, low efficiency, high production cost, tungsten inclusion defects, but it has the advantage of better welding quality. It can weld non-ferrous metals such as aluminum, copper, magnesium, etc.
e. Welding Deformation Causes and Correction Methods:
- Insufficient preparation before welding, requiring additional fixtures.
- Poor welding fixture, improving the process.
- Improper welding sequence.
