Sheet Metal and Metalworking

The terms metalworking and sheet metal working cover a wide range of work with metals or sheet metals. In various processing steps, the materials are brought into the desired shape in order to produce components, parts or products. Often, the two terms cannot be clearly distinguished from each other because the finished assemblies or end products usually contain elements from both disciplines.

As a sub-area of manufacturing technology, metalworking deals with the production and processing of shaped workpieces made of metal in accordance with pre-defined geometric parameters and their assembly into functional units. A distinction is made between the following methods:

• chip removal processes (drilling, turning, grinding, milling, sawing, engraving, threading, etc.),
• chipless processes (pouring, bending, hammering, embossing, forging, stamping, rolling, drawing etc.),
• joining metalworking processes (welding, glueing, soldering, plating etc.).

This sub-discipline of metalworking encompasses a variety of manufacturing processes, all of which have sheet metal as their starting point. This is brought into the desired shape or design by severing, joining, forming and/or finishing. The most common sheet metal working processes include:

• Welding,
• Cutting,
• Bending,
• Stamping,
• Rolling, and
• Joining.

Classic products made of sheep metal are:

• Edged and bent parts (e.g., angles),
• Stamped parts (e.g., brackets, washers, body parts),
• Thermally cut or laser cut parts (e.g., blanks, flanges),
• Welded parts (e.g., brackets), and
• Welded assemblies (e.g., housings, containers).

The history of sheet metal and metal processing began more than 80,000 years ago. According to archaeological findings, the first man-made sheets were made of gold, which occurs naturally in pure form. They were still very small and were used to make simple pieces of jewelry. In the course of time, other metals such as copper and silver were added. In addition, the possibility of changing material properties with the help of alloying elements was discovered.

Iron has been used for more than 6,000 years. At 32 percent, this element makes up about one third of the earth's total mass. However, iron hardly ever occurs in its pure form. Therefore, our ancestors first processed iron meteorites, which consist mainly of iron and nickel. Due to the rarity of these solids of cosmic origin, the price of iron was sometimes up to 100 times higher than that of silver. This only changed when it became possible to extract iron from iron ore by smelting.

Initially, the ore was smelted directly at the site in bloomeries. These consisted of clay and iron and reached temperatures of up to 1,300 °C. However, this was not sufficient to obtain pure iron. Unwanted elements were therefore removed from the material by heating and hammering (forging). Wrought iron was very pure and contained hardly any carbon (<0.08 %). Therefore, it could not be hardened by heat treatments. The first wrought iron was created around 1,500 BC with the help of so-called low furnaces. The blast furnace was invented in China about 1,000 years later.

In the Roman Empire, metalworking played a major role in the military as well as in crafts, agriculture and construction. Precious metals and copper were used in Roman coinage. Precise medical instruments were also made of metal at that time, for example, forceps for pulling teeth or star needles for the treatment of cataracts. Iron and bronze were the most important raw materials for the military. The former was used to make swords, for example, while the latter was used to make helmets, breastplates, and greaves.

At a time when the first farmers were just settling in Central Europe and were firing clay pots, craftsmen in ancient Egypt were already joining jewelry made of gold and copper by spot heating. Soldering is considered the first method for thermally joining two metal parts. As early as around 2700 B.C., the Egyptians joined copper pipes for the urban water supply with the aid of fire welding. This technology can also be found on agricultural equipment, art objects, and weapons over a period of several thousand years.

Sheet metal processing came into use in Europe from the High and Late Middle Ages. Iron sheets were used mainly for making knights armor. Rolling metal was still unknown at that time. Instead, the sheets were worked with hammers until they had the desired thickness.

Since the work of iron extraction up to the present product was very laborious and time-consuming, sheet iron was a sought-after and expensive commodity. In addition, sheet metal working required a high degree of practice and experience, since the starting materials were processed exclusively by hand with the help of the simplest tools. It was not until large hammers driven by water power made their way into metal processing that significant improvements were noticeable. Nevertheless, the manufacturing process could still take several days, depending on the size of the sheet.

During the Industrial Revolution, which began in England in about 1780 and spread to other European countries in the 19th century, the first metalworking machines were developed. These machines made it possible to roll metals to produce sheet metal. This significantly shortened the production time, while at the same time lowering the price. As the quality of the sheets also improved in terms of their flatness and thickness, new fields of application were quickly found in a wide variety of sectors.

Today, there are a large number of methods available for processing metals or sheet metal that are suitable for a wide variety of metals, material thicknesses and formats. In many fields of application, the former manual work has now been replaced by automation solutions.

Metals and metal alloys such as steel, stainless steel, aluminum, copper, brass, and in some industries gold and silver can be used for sheet metal and metal forming. If these are in the form of sheet metal, they are a case for sheet metal working.

Sheet is a rolling mill product whose length and width are much greater than its thickness. A basic distinction is made between heavy plates and thin metal sheets. Thin metal sheets are defined as sheets with a thickness of less than 3 mm, while heavy plates are defined by EN 10079 as flat products with a thickness greater than 3 mm.

To be able to produce and form sheet metal, the material must have a certain strength and formability. These material properties can be achieved by adding various alloying elements. Some alloys have established themselves as standards in the metalworking industry.

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).

During the processing of sheet metal and other metal elements various tools and machines are used.

In the craft sector, a distinction is made between hand tools such as iron saws, pliers, hammers, and files as well as power tools such as percussion drills, soldering irons, welding guns, grinders, and cutting equipment.

In the industrial sector, processing is done with the help of machine tools such as:

• Metal band saws,
• Milling machines,
• Lathes,
• Bending machines,
• Grinding machines,
• Drill presses, and
• Pipe cutting machines.

These machines enable significantly higher piece counts than hand-held tools. Many now have computer control, which ensures consistently high quality over a large number of parts.

It is often possible to digitize old machines as part of a so-called retrofit process. This also enables smaller metal and sheet metal processing companies to tap into the benefits of Industry 4.0 for themselves.

Numerous professions have formed in the metal and sheet metal working industry. The 1660 Crafts Code lists blacksmiths, cutlers, foremen, long and short-time workers, hollow grinders, locksmiths, spur makers, watchmakers, gunsmiths, winch makers, needler makers, needle smiths, coppersmiths, plowsmiths, pipe smiths, and file makers.

Today, numerous industries rely on metal and sheet metal working, such as:

• Tool and mold making,
• Vehicle, ship and aircraft construction,
• Mechanical engineering,
• Bridge construction, and
• Jewelry making.

The spectrum of sheet metal and metal goods ranges from the smallest stamped parts (screws) to meter-high silos and pressure vessels. Matching products such as fittings, clamps, rails and welding elements (welding studs) are used in a wide variety of assembly work in trade and industry. In addition, many mechanical parts of machinery and equipment fall under the category of metal products.

Tools, fittings for doors and windows, heating boilers, and radiators are all made of metal, to name but a few.

Sheet metal is processed, for example, into cladding, vehicle bodies, control cabinets, welded tubes, signs and tin cans. Particularly thick sheets are used, for example, to make wind turbine masts, while very thin ones are used, for example, in model making and jewelry.