Fundamentals How does stamping and punching of sheet metal work?

Punching is a manufacturing process used primarily for piercing flat materials. The term comprises several forming and cutting processes.

Similar to cutting-out Christmas cookies from a dough, stamped parts are punched out of flat materials. Punching is used for cutting, joining and forming. Special punching and stamping tools, also known as cutting and forming tools, are used for this purpose. In addition to cutting, these tools can be used to shape materials using a cold forming process by stamping, drawing-out, bending and upsetting.

In practice, stamping is used, for example, in the production of household goods, packaging for the food, medical and pharmaceutical industries as well as in the electrical and automotive industries. In addition, punching is used to create profile and round perforations and to remove waste material. The most frequently used tool for punching in everyday life is probably the paper punch, which can be found in every office.

What is stamping?

Stamping involves the machining of flat parts made of various materials, in particular hard-to-machine metals. Stamping is primarily used to produce workpieces from metal strips and sheet metal strips, but also from paper, plastic, textiles and sealing materials. The tools for stamping are usually installed in presses with a lower and upper part. These interchangeable tools include forming tools, splitting tools and joining tools. The individual processes can also be combined in composite tools.

Splitting: This is a chipless cutting process which includes wedge cutting and shear cutting.

Forming: By means of forming, any change in the shape can be achieved. Different tools are used for forming, mostly bending tools.

Joining: Using this process, two or more workpieces can be joined by folding, riveting or press-fitting.

What is shear cutting?

Shear cutting is a cutting process involved in stamping. With this process, materials can be split by cutting. Terms related to shear cutting are cutting insert and cutting punch. Depending on the position of the cutting line, different manufacturing processes can be used for shear cutting.

In detail, the following options are available for shear cutting:

Blanking: Blanking involves the production of an exact shape of a work piece by means of a closed cut line.

Parting: In this case, cutting takes place along a cut line, which can also produce waste.

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Piercing:

Piercing by punching forms a closed cut line to create any desired internal shape.

Notching: In notching, surface parts are removed along the inner or outer edges.

Lancing: In this process, only a partial separation of the material is created in or on the workpiece. Lancing often serves as preparation for drawing and bending work.

Clipping: The clipping process is used to remove machining allowances or material from edges.

Trimming: Trimming removes excess material from forgings, castings and compression molded parts.

Shaving: To refine pre-cut surfaces, the shaving process is used to remove minimal edges.

Cutoff: In order to produce several workpieces out of one initial piece, the cutoff process is used.

Nibbling: In nibbling, pieces are cut out of a material.

Fine blanking: Fine blanking is used to obtain a square, precise and smooth cutting surface on thick workpieces.

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The history of stamping

The history of stamping began in the late 19th century. Stamping was first used for the production of bicycles in the 1890s. Although at that time, forged parts were considered to be of higher quality and more stable than those produced by stamping, the possibility of saving costs and achieving results more quickly was soon recognized. The first bicycle parts produced in this way were imported from Germany to the United States, which led US companies to establish custom stamping offered by tool manufacturers. West Rad was instrumental in the development and research of stamping at the time, which is why most of the bicycle parts were stamped by this company. In the course of the growing automotive industry in the USA, Henry Ford first and foremost purchased the components manufactured by stamping for his Ford Motor Company.

Preparatory work prior to stamping

Before a punching tool can be put into operation, the cutting die and clearance must be verified. This is because there must be a cutting die between the stamp cutting edge and the cutting die opening that is measured at right angles to the cutting plane. The extent of the die clearance depends on the strength and thickness of the material.

Furthermore, the type of tool design and the quality of the sheet's surfaces play an important role in defining the die clearance. This is usually 2 to 5 % of the material thickness. The best way to check the die clearance is to use an angle tester or height gauge. The size of the die clearance affects dimensional accuracy, cutting force, surface quality of the cutting surfaces, burr height at the cut piece and tool wear.

Which machines are used in stamping?

A basic distinction is made between manual machines and CNC stamping machines. The manual stamping machine is based on simple mechanical or physical principles. The workpieces are inserted manually and brought into position. The stamping stroke is only initiated by an action of the operator, whereby the punching force is generated either hydraulically or by means of levers. Manual stamping machines consist of the machine body, a mechanical power transmission or another drive as well as the tools required for stamping, such as dies, punches and sometimes also return springs and punch holders.

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CNC stamping machines have proven to be more effective in the production of small and medium-sized series. They usually consist of a tool carrier and a tool that can be changed quickly. The material, such as sheet metal, moves between the punching tools during processing. In order to move the material, it is retained at the edges. Depending on the desired shape, the tools used for CNC punching are nibblers, punches for circular disks, squares in various sizes and special tools for individual shapes.

In this video, Dimeco shows that stamping can also be a practical solution for applications directly from the coil.

Configuration of stamping machines and punching tools

Basically, a machine for stamping consists of two tool parts. It consists of a die holder and punch holder. As soon as the punch engages in the die, the material is cut or processed. Stamping machines are distinguished between machines for stroke stamping and machines for rotary stamping.

The stroke stamping device consists of a press equipped with a punching tool, usually a flat upper and lower tool. The stamping punches are located in the upper tool. A matching die is attached to the bottom tool. Due to a precise alignment of the tools to each other, the punch hits the die exactly where the material is pierced. During stroke stamping, the full punching force is applied to the entire surface to be worked.

The stamping tools for rotary stamping are mounted directly into the high-precision bearings. The tools with punch are clamped above a shaft, the die tool is arranged at the bottom. During stamping, the material is continuously pushed through between the two tools. The name of this stamping process comes from the rotating tools used. The punching force is lower than that of stroke stamping due to the punctiform load associated with rotary stamping. Rotary stamping machines can be used to process continuous materials faster and more accurately.

In addition, rotary stamping machines are the ideal partner for roll forming lines.

Alternatives to manual or CNC punching: Laser cutting machines

For the punching of large quantities, usually laser cutting machines or combined laser-stamping machines are used. Using laser cutters, solids are cut by pulsed and continuous laser rays. This type of punching is suitable for many different materials, such as metals and organic materials. To this end it is necessary to be aware of laser beam parameters such as pulse energy, pulse duration, wavelength and average power.

In this video Amada presents the combination of punching and laser cutting.

The process of laser cutting or laser punching is always used when complex shapes need to be produced quickly and precisely. Three-dimensional openings or cuts in hard-to-reach areas can also be produced using the non-contact laser punching process. Combined laser punching machines offer the advantage of simultaneously using punching and nibbling. These types of machines, which are equipped with high-performance lasers, such as gas lasers, solid-state lasers or fiber lasers, can cut out any shapes and work with a punching head at the same time.

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These materials can be punched o stamped

The list of materials that can be punched is long. However, they mainly include thin materials such as metal foils, plastic foils, papers and films. In addition, thicker materials such as composites, cardboard, leather and sheet metal can be punched. Punching and stamping play an important role especially in technically demanding industries, such as the automotive industry, the electrical industry or the space industry. In metal construction and the building industry use punching to produce profiles in sheet metal.

The maximum dimensions of the punching process are of course related to the selected punching machine. A pure punching machine with an automatic conveyor system can process sheet sizes of 1250 x 2500 mm. The maximum sheet density is up to 4 mm. As an example for the capacity of a combined laser-punching machine, the same sheet dimensions apply. The sheet thickness for punching and laser cutting may be up to a 8 mm. If stainless steel is to be laser cut, the maximum material thickness may be up to 6 mm.

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This article was first published by belchnet.

Original by: Frauke Finus

Translation by: Alexander Stark

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