Sheet metal and metal working processes

Sheet metal working encompasses a wealth of manufacturing processes that can be classified into forming, severing, joining, and finishing.

During forming, the workpiece is stressed beyond its elastic limit and thus plastically deformed. A distinction is made between the following process groups:

• Forming under compressive conditions according to DIN 8583 (rolling, free-forming, swaging, indenting, pushing through),
• Forming under a combination of tensile and compressive conditions according to DIN 8584 (drawing through constricted tool orifices, deep drawing, spinning, forming by raising, upset bulging, hydroforming),
• Forming under tensile conditions according to DIN 8585 (stretch reducing, bulge forming, stretch forming),
• Forming by bending according to DIN 8586 (with straight-line or with rotating tool movement),
• Forming under sheering conditions according to DIN 8587 (twisting, shifting).

In addition, there are other forming processes such as vault structuring, crumpling, high-pressure torsion and the Guerin process.

Cold and hot forming

In sheet metal working, a distinction is made between cold forming and hot forming.

In cold forming, the deformation takes place well below the recrystallization temperature of the material, usually at room temperature. This means that higher forming forces are required. The dislocations and residual stresses in the metal lattice caused by cold forming result not only in an increase in hardness and yield strength but also in a change in the magnetic and electrical properties. The initial permeability and electrical conductivity are reduced. In the case of steel, permanent magnetization is possible. Other advantages of cold forming are:

• Possibility to achieve tight dimensional tolerances,
• Increase in the strength of the material,
• Reduction of ductility, and
• No scaling of the surface.

The main cold forming processes include:

• Cold rolling,
• Deep drawing,
• Bending,
• Drifting,
• Peening,
• Hammering, and
• Pressing.

Hot forming comprises all forming steps which take place above the recrystallization temperature of the material. The hardening that takes place during forming is accompanied by recovery and softening processes in the material. As a result, high degrees of forming are possible despite low forming forces. The following points must be taken into account during hot forming:

• Hot forming has poorer dimensional tolerances and surface finishes than cold forming,
• Hot forming leads to slightly scaled surfaces.

Methods of hot forming include:

• Forging,
• Hot rolling,
• Mold hardening,
• Extrusion.

Severing refers to processes in which something is cut off from the workpiece. The final shape of the workpiece is already contained in its initial shape. In addition to a residual piece, this usually results in chips. Severing processes include:

• Severing according to DIN 8588 (shear cutting, tearing, splitting, breaking, wedge cutting, fracture cutting),
• Chip removal using a geometrically defined cutting edge according to DIN 8589-0 (turning, drilling, countersinking, milling, planing, sawing, filing, etc.),
• Chip removal using a geometrically indeterminate cutting edge according to DIN 8589-0 (grinding, honing, lapping, beam cutting, barrel polishing, etc.),
• Removal operations according to DIN 8590 (laser blasting, plasma etching, laser cutting, chemical etching, electro-erosion, water jet cutting, etc.),
• Disassembling according to DIN 8591 (disassembly, loosening of adhesive connections, desoldering, loosening of friction-locked connections, etc.),
• Cleaning according to DIN 8592 (separation of unwanted layers from the workpiece surface (e.g., blast cleaning, brushing, ultrasonic cleaning, lye removal, flaming, etc.).

Joining involves permanently joining two or more workpieces together. In some cases, a "formless material" (e.g., adhesive) is used. The cohesion of the joined parts is achieved by:

• Non-positive connections (screw connections, press fits),
• Material connections (welded, soldered, and bonded connections),
• Positive connections (e.g., tongue and groove connection, feather key, connecting fitting).

Connections can be detachable, conditionally detachable and non-detachable. Screw connections, for example, are detachable. Conditionally detachable connections include riveted connections. To separate these, the rivets must be destroyed, but not the joined components. Soldered connections can also usually be detached by desoldering. Undetachable connections are welded joints. These cannot be separated without destroying the components.

This term covers machining processes that serve to modify the functional and/or optical surface properties of the workpiece.

Examples include:

• The removal of the surface (e.g., electropolishing),
• Surface coating (e.g., painting, chrome plating, galvanizing, powder coating),
• Mechanical surface treatments (e.g., deburring, polishing, grinding, shot peening),
• Chemical surface finishing (e.g., degreasing, etching, cleaning).