Introduction to Precautions and Processing Flow of Sheet Metal Processing
Concept: The processing of metal sheets is called sheet metal processing. For example, using sheets to make chimneys, iron barrels, oil tanks, oil cans, ventilation ducts, elbows, headers, round-to-square transitions, funnel shapes, etc., the main processes are cutting, bending, folding, forming, welding, riveting, etc., requiring a certain amount of geometric knowledge. Sheet metal parts are thin plate hardware pieces, which can be processed through stamping, bending, stretching, etc. A general definition is that the thickness of the part does not change during processing. Correspondingly, there are castings, forging parts, and machined parts.
Selection of Materials for Sheet Metal Processing
The commonly used materials in sheet metal processing include cold-rolled steel (SPCC), hot-rolled steel (SHCC), galvanized steel (SECC, SGCC), copper (CU) such as brass, purple copper, beryllium copper, aluminum plates (6061, 6063, hard aluminum, etc.), aluminum profiles, stainless steel (mirror finish, brushed finish, matte finish). Depending on the different functions of the product, different materials are selected, usually considering the use and cost of the product.
1. Cold-rolled steel SPCC: mainly used for electroplating and painting, low cost, easy to form, material thickness 3.2mm.
2. Hot-rolled steel SHCC: material T3.0mm, also used for electroplating and painting, low cost, but difficult to form, mainly used for flat parts.
3. Galvanized steel SECC, SGCC. SECC electrolytic plate is divided into N material and P material. N material is mainly not surface-treated, high cost, P material is used for spraying.
4. Copper: mainly used for conductive components, surface treatment is nickel plating, chrome plating, or no treatment, high cost.
5. Aluminum plate: generally used for surface chromate treatment (J11-A), oxidation (conductive oxidation, chemical oxidation), high cost, silver plating, nickel plating.
6. Aluminum profiles: materials with complex cross-sectional structures, widely used in various plug boxes. Surface treatment is the same as aluminum plates.
7. Stainless steel: mainly used without any surface treatment, high cost.
Precautions for Sheet Metal Unfolding
Unfolding diagrams are developed from part drawings (3D) into plane diagrams (2D).
1. The unfolding method should be appropriate, convenient, saving materials and facilitating processing.
2. Reasonably select gaps and edge wrapping methods, T=2.0 below gap 0.2, T=2-3 gap 0.5, edge wrapping method adopts long side wrapping short side (door panel type).
3. Reasonably consider tolerance dimensions: negative differences go all the way, positive differences go halfway; hole shape dimensions: positive differences go all the way, negative differences go halfway.
4. Burr direction.
5. Positions and directions of thread tapping, pressing rivets, tearing, punching convex points (packages), draw sectional views.
6. Verify material, plate thickness, by plate thickness tolerance.
7. Special angles, bending corner inner radius (generally R=0.5) need trial bending to determine the unfolding.
8. Places prone to errors (similar asymmetry) should be highlighted.
9. Add enlarged views where there are many dimensions.
10. Areas requiring spray protection must be indicated.
Sheet Metal Processing Technology Flow
According to the structural differences of sheet metal parts, the technology flow can vary, but generally includes the following points.
1. Cutting: There are various cutting methods, mainly including:
① Shearing machine: Using a shearing machine to cut simple material strips, it mainly prepares for mold cutting forming, low cost, precision less than 0.2, but can only process strip or block materials without holes or chamfers.
② Punch press: Using a punch press to punch out various shaped material pieces from the plate after the part has been unfolded, its advantage is short working hours, high efficiency, high precision, low cost, suitable for mass production, but requires mold design.
③ NC CNC cutting, when NC cutting, first write CNC machining programs, using programming software, convert the drawn unfolding diagram into a program recognizable by the NC machine tool, letting it step-by-step cut out each structural shape on the flat plate, but its structure is limited by the tool structure, low cost, precision 0.15.
④ Laser cutting, using laser cutting method, cut out the structural shape on the large flat plate, like NC cutting, requires writing laser programs, it can cut various complex shaped flat pieces, high cost, precision 0.1.
⑤ Saw bed: mainly used for cutting aluminum profiles, square tubes, round bars, etc., low cost, low precision.
2. Fitter: Counterboring, threading, enlarging holes, drilling holes. Counterbore angle is generally 120℃, used for pull rivets, 90℃ used for counter-sunk screws, imperial bottom hole threading.
3. Flanging: Also called drawing holes or flanging, which involves drawing a slightly larger hole on a smaller base hole, then threading, mainly used for thin sheet metal processing, increasing strength and number of threads, avoiding stripping, generally used for thin plates, normal shallow flanging around the hole, thickness basically unchanged, allowing 30-40% thinning of thickness, can get 40-60% higher flanging height, squeezing thin by 50%, can get maximum flanging height, when plate thickness is larger, such as 2.0, 2.5 and above, direct threading can be done.
4. Punch press: It's a processing procedure using molds to form, generally including punching, cutting corners, blanking, punching convex packages (convex points), tearing, drawing holes, forming, etc., the processing needs corresponding molds to complete operations, such as punching and blanking dies, convex package dies, tearing dies, drawing hole dies, forming dies, etc., mainly paying attention to position and directionality.
5. Press riveting: Regarding our company, press riveting mainly includes press riveting nuts, screws, non-loosening screws, etc., which is completed through hydraulic press riveting machines or punch presses, riveting them onto the sheet metal parts, there are also expanding riveting methods, need to pay attention to directionality.
6. Bending: Bending is folding 2D flat pieces into 3D parts. Its processing needs bending machines and corresponding bending molds to complete, it also has a certain bending order, the principle is to fold first where the next cut will not interfere, and fold later where interference will occur.
l Calculation of groove width for bending is 6 times the plate thickness for T=3.0mm and below, such as: T=1.0, V=6.0, F=1.8, T=1.2, V=8, F=2.2, T=1.5, V=10, F=2.7, T=2.0, V=12, F=4.0.
l Classification of bending machine molds: straight knife, curved knife (80℃, 30℃).
l When bending aluminum plates, cracks appear, groove width can be increased or upper mold R increased (annealing can avoid cracking).
l Precautions for bending: I Drawing requirements, plate thickness, quantity; II Bending direction; III Bending angle; IV Bending dimensions; VI Appearance, plated chromium parts should not have creases.
Relationship between bending and press riveting procedures, generally press riveting before bending, but if press riveting would cause interference, bend first then press rivet, some require bending, press riveting, then bending again, etc.
7. Welding: Definition of welding: the atomic and molecular distance of the welded material reaches lattice distance to form an integral whole.
①Classification: a Fusion welding: TIG welding, CO2 welding, gas welding, manual welding b Pressure welding: spot welding, butt welding, collision welding c Brazing: electric chromium welding, copper wire.
②Welding methods: a CO2 gas shielded welding b TIG welding c Spot welding d Robot welding.
Selection of welding methods depends on actual requirements and material types. Generally, CO2 gas shielded welding is used for iron plates, TIG welding is used for stainless steel and aluminum plates, robot welding can save time, improve work efficiency and welding quality, reduce workload intensity.
③Welding symbols: Δ Fillet weld, Д, I-type weld, V-type welding, single-sided V-type welding (V) V-type welding with blunt edge (V), spot welding (O), plug welding or slot welding (), edge rolling welding (), single-sided V-type welding with blunt edge (V), U-type welding with blunt edge, J-type welding with blunt edge, bottom welding, seam welding.
④Arrow line and joint.
⑤Welding defects and preventive measures.
Spot welding: Insufficient strength can add convex points, strengthen welding area.
CO2 welding: High productivity, less energy consumption, low cost, strong rust resistance.
TIG welding: Shallow penetration, slow welding speed, low efficiency, high production cost, has tungsten inclusion defect, but has the advantage of good welding quality, can weld non-ferrous metals such as aluminum, copper, magnesium, etc.
⑥Causes of welding deformation: insufficient preparation before welding, need to increase fixtures.
Improvement of welding fixtures.
Bad welding sequence.
⑦Correction methods for welding deformation: Flame correction method, vibration method, hammering method, artificial aging method.