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Sheet metal processing knowledge summary

Sheet metal processing

   is a key technology that sheet metal technicians need to grasp, and it is also an important process for forming sheet metal products. Sheet metal processing includes traditional methods and process parameters such as cutting, punching, and bending forming, as well as various cold stamping die structures and process parameters, equipment working principles and operation methods, and new stamping technologies and processes. The processing of metal sheet parts is called sheet metal processing.

Sheet metal processing is called sheet metal processing. For example, using sheet metal to make chimneys, iron drums, oil tanks and pots, ventilation ducts, elbow heads, round corners, funnel-shaped structures, etc. The main processes include cutting, bending edges, bending forming, welding, riveting, etc., which require a certain amount of geometric knowledge. Sheet metal parts are thin metal parts that can be processed through stamping, bending, stretching, and other methods. A general definition is parts with constant thickness during the processing. Correspondingly, there are castings, forged parts, machined parts, etc. 

Material selection

The materials commonly used in sheet metal processing include cold-rolled sheet (SPCC), hot-rolled sheet (SHCC), galvanized sheet (SECC, SGCC), copper (CU), brass, purple copper, beryllium copper, aluminum plates (6061, 5052, 1010, 1060, 6063, hard aluminum, etc.), aluminum profiles, stainless steel (mirror, brushed surface, matte surface). Depending on the product's function, different materials are selected, generally considering the product's purpose and cost.

(1) Cold rolled plate SPCC is mainly used for electroplating and baking paint parts, with low cost, easy molding, and material thickness ≤ 3.2mm.

(2) Hot rolled plate SHCC, material T ≥ 3.0mm, also uses electroplating and baking paint parts, with low cost but difficult to form, mainly using flat parts.

(3) Galvanized sheet SECC SGCC。 SECC electrolytic plates are divided into N material and P material. N material is mainly not surface treated and has a high cost, while P material is used for spraying parts.

(4) Copper; Mainly using conductive materials, the surface treatment is nickel plating, chromium plating, or no treatment, which is costly.

(5) Aluminum plate; Generally, surface chromate (J11-A) is used for oxidation (conductive oxidation, chemical oxidation), which is costly. Silver plating and nickel plating are available.

(6) Aluminum profiles; Materials with complex cross-sectional structures are widely used in various plug-in boxes. Surface treatment is the same as aluminum plate.

(7) Stainless steel; Mainly used without any surface treatment, high cost.

Common materials

Galvanized steel plate SECC

The substrate of SECC is a general cold-rolled steel coil, which becomes an electroplated product after undergoing degreasing, acid washing, electroplating, and various post-treatment processes in a continuous electroplating production line. SECC not only has the mechanical properties and approximate processability of general cold-rolled steel sheets, but also has superior corrosion resistance and decorative appearance. There is great competition and substitutability in the markets of electronic products, home appliances, and furniture. For example, SECC is commonly used for computer cases.

Ordinary cold-rolled sheet SPCC

SPCC refers to the continuous rolling of steel ingots into steel coils or sheets of the required thickness by a cold rolling mill. The surface of SPCC has no protection and is highly susceptible to oxidation when exposed to air, especially in humid environments where the oxidation rate accelerates and dark red rust appears. When in use, the surface should be painted, electroplated, or otherwise protected.

Hot dip galvanized steel plate SGCC

Hot dip galvanized steel coil refers to a semi-finished product that has been hot-rolled, pickled, or cold-rolled, cleaned, annealed, and immersed in a molten zinc bath at a temperature of about 460 ° C to coat the steel sheet with a zinc layer, and then quenched, tempered, leveled, and chemically treated. SGCC material is harder, less ductile (avoiding deep drawing design), has a thicker zinc layer, and has poor weldability compared to SECC material.

Stainless steel SUS304

One of the most widely used stainless steels, it has better corrosion resistance and heat resistance than steel containing Cr (chromium) due to its Ni (nickel) content. It has excellent mechanical properties, no heat treatment hardening phenomenon, and no elasticity.

Stainless steel SUS301

The content of Cr (chromium) is lower than that of SUS304, and its corrosion resistance is poor. However, after cold processing, it can obtain good tensile strength and hardness in stamping processing, and has good elasticity. It is mostly used for spring springs and EMI prevention.

Drawing review

To write the process flow of a part, it is necessary to first understand the various technical requirements of the part drawing; The review of drawings is the most important step in writing the process flow of parts. 

(1) Check if the drawing is complete.

(2) Diagram view relationship, whether the annotations are clear, complete, and indicate the units of dimensions.

(3) Assembly relationship, key dimensions required for assembly.

(4) Difference between old and new layout surfaces.

(5) Translation of foreign language images.

(6) Table code conversion.

(7) Feedback and handling of graphic issues.

(8) Materials.

(9) Quality requirements and process requirements.

(10) The official release of drawings must be stamped with a quality control seal.

matters needing attention

The unfolded diagram is a plan view (2D) based on the part drawing (3D).

(1) The unfolding method should be appropriate, convenient, and save materials and processing efficiency.

(2) Reasonably choose the gap and edge wrapping method, with a gap of 0.2 for T=2.0 or less, and a gap of 0.5 for T=2-3. The edge wrapping method uses long edges wrapped with short edges (for door panels).

(3) Reasonably consider the tolerance of external dimensions: the negative deviation should be reduced to the bottom, and the positive deviation should be reduced by half; Hole size: The positive difference should be reduced to the bottom, and the negative difference should be reduced by half.

(4) Burr direction.

(5) Draw a sectional view in the direction of tooth extraction, riveting, tearing, punching convex points (bags), and other positions.

(6) Check the material, plate thickness, and tolerance of the plate thickness.

(7) For special angles, the inner radius of the bending angle (usually R=0.5) needs to be determined by trial bending before unfolding.

(8) Areas that are prone to errors (similar asymmetry) should be highlighted.

(9) Enlarge the areas with larger dimensions.

(10) The areas that require spray protection must be indicated.

Processing flow

According to the differences in the structure of sheet metal parts, the process flow may vary, but generally does not exceed the following points.

1. Cutting: There are various cutting methods, mainly including the following.

① Cutting machine: It is a simple material that uses a cutting machine to cut strips of material. It is mainly used to prepare for mold cutting and forming, with low cost and accuracy below 0.2. However, it can only process strip or block materials without holes or cutting angles.

② Punch press: It is the use of a punch press to punch and shape various shapes of flat parts on a sheet metal after unfolding the parts in one or more steps. Its advantages are short labor time, high efficiency, high precision, low cost, and suitable for large-scale production, but mold design is required.

③ NC numerical control cutting, when NC cutting, the first step is to write a CNC machining program. Using programming software, the drawn unfolded diagram is written into a program that can be recognized by the NC numerical drawing machine tool. According to these programs, it can punch various shapes of flat parts on the flat plate step by step, but its structure is affected by the tool structure, with low cost and accuracy of 0.15.

④ Laser cutting is the use of laser cutting to cut the structural shape of a large flat plate. Like NC cutting, a laser program needs to be written. It can cut various complex shaped flat plates with high cost and accuracy of 0.1

⑤ Sawing machine: mainly used for aluminum profiles, square tubes, drawing tubes, round bar materials, etc., with low cost and low precision.

2. Fitter: countersunking, tapping, reaming, drilling.

The countersunk angle is generally 120 ℃, used for pulling rivets, and 90 ℃ is used for countersunk screws and tapping British bottom holes.

3. Flanging: also known as hole drawing or flipping, it is to draw a slightly larger hole on a smaller base hole and then tap it. It is mainly used for sheet metal processing with relatively thin plate thickness to increase its strength and thread number, and avoid slipping. It is generally used for shallow flanging with normal hole circumference and thin plate thickness. When the thickness is allowed to decrease by 30-40%, a height 40-60% higher than the normal flanging height can be obtained. When the plate thickness is relatively thin, such as 2.0, 2.5, etc., the maximum flanging height can be obtained by squeezing 50%. When the plate thickness is large, such as 2.0, 2.5, etc., it can be directly tapped.

4. Punch press: It is a processing process that uses molds to form. Generally, punch press processing includes punching, corner cutting, material dropping, punching convex hull (convex point), punching tearing, hole drawing, forming and other processing methods. Its processing requires corresponding molds to complete the operation, such as punching material dropping mold, convex hull mold, tearing mold, hole drawing mold, forming mold, etc. The operation mainly pays attention to the position and directionality.

5. Riveting: As far as our company is concerned, riveting mainly includes riveting nuts, screws, and fasteners, which are operated by hydraulic riveting machines or punching machines to rivet them onto sheet metal parts. There are also riveting methods, and attention should be paid to directionality.

6. Bending: Bending is the process of folding 2D flat parts into 3D components. Its processing requires a folding bed and corresponding bending molds to complete, and it also has a certain bending sequence. The principle is to fold the next blade that does not interfere first, and fold the one that will interfere later.

The number of bends is 6 times the thickness of the plate below T=3.0mm to calculate the groove width, 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。

Classification of folding bed molds: straight knife, curved knife (80 ℃, 30 ℃).

When bending aluminum plates, if there are cracks, the width of the lower mold slot can be increased to increase the upper mold R (annealing can avoid cracks).

Precautions for bending: Firstly, the drawing requires the thickness and quantity of the board; II bending direction; III. Bending angle; IV. Bending size; The appearance and chrome plated parts must not have creases. The relationship between bending and riveting processes generally involves riveting first and then bending. However, if there are materials that interfere after riveting, they need to be folded first and then pressed. Some processes require bending, riveting, and then bending again. 

7. Welding: Welding definition: The distance between the atoms and molecules of the welded material and the Jingda lattice forms a whole.

① Category: A Fusion Welding: Argon Arc Welding, CO2 Welding, Gas Welding, Manual Welding. Pressure welding: spot welding, butt welding, impact welding. C brazing: electric chromium welding, copper wire.

② Welding method: CO2 gas shielded welding. B argon arc welding. C-point welding, etc. Robot welding.

The selection of welding methods depends on actual requirements and materials. Generally speaking, CO2 gas shielded welding is used for welding iron plates; Argon arc welding is used for welding stainless steel and aluminum plates. Robot welding can save labor hours, improve work efficiency and welding quality, and reduce work intensity.

③ Welding symbol: Δ angle welding, Type I welding, V-shaped welding, single-sided V-shaped welding (V) with blunt edge V-shaped welding (V), Spot welding (O), Plug welding or groove welding (∏), edge rolling welding (χ), Single sided V-shaped welding with blunt edge (V), Blunt U-shaped welding, blunt J-shaped welding, bottom sealing welding, and butt welding.

④ Arrow lines and connectors.

⑤ Welding deficiency and its preventive measures.

Spot welding: If the strength is insufficient, convex points can be made to impose welding area

CO2 welding: high productivity, low energy consumption, low cost, strong rust resistance

Argon arc welding: low depth of dissolution, slow welding speed, low efficiency, high production cost, with tungsten inclusion defects, but with the advantage of good welding quality, it can weld non-ferrous metals such as aluminum, copper, magnesium, etc.

⑥ Reason for welding deformation: Insufficient preparation before welding, requiring additional fixtures. Improvement process for welding fixture defects. The welding sequence is not good.

⑦ Welding deformation correction method: flame correction method. Vibration method. Hammer striking method. Artificial time efficiency method.

Other applications

The processing steps of parts in the sheet metal workshop are: product pre testing, product processing trial production, and mass production of products. During the product processing and trial production steps, timely communication and contact with customers should be made to obtain corresponding processing evaluations before proceeding with mass production of the product.

Laser drilling technology is the earliest practical laser technology in laser material processing technology. Laser drilling in sheet metal workshops generally uses pulsed laser, which has high energy density and short time. It can process small holes of 1 μ m, especially suitable for processing small holes with certain angles and thin materials. It is also suitable for processing deep small holes and micro holes on parts with high strength, hardness or brittle soft materials.

Laser can achieve drilling and processing of burner components for gas turbines, and the drilling effect can be three-dimensional, with a quantity of up to thousands. The materials that can be drilled include stainless steel, nickel chromium iron alloy, and Hastelloy based alloy. Laser drilling technology is not affected by the mechanical properties of materials and is relatively easy to achieve automation.

With the development of laser drilling technology, laser cutting machines have achieved automated operation, and their application in the sheet metal industry has changed the traditional processing methods of sheet metal technology, realizing unmanned operation, greatly improving production efficiency, realizing fully automatic operation throughout the process, driving the development of sheet metal economy, and improving the drilling effect to a higher level. The processing effect is remarkable.