Stamping Process
Theory
1. Introduction to Metal Stamping
Metal stamping is a high-speed, cold-forming manufacturing process used to convert flat metal sheets into precise and complex shapes. It involves placing sheet metal, either in blank or coil form, into a stamping press where a punch and die set apply controlled force to cut or form the material into the desired geometry.
The process relies on plastic deformation of the material under compressive forces, allowing the sheet to permanently change shape without melting. Because it is performed at room temperature, it is classified as a cold-working process, which often improves the strength and surface finish of the final component due to strain hardening.
Metal stamping is widely used in industries that require high-volume production with tight dimensional tolerances. In the automotive industry, it is used to manufacture body panels, brackets, structural components, and electrical connectors. In aerospace applications, stamping is used for lightweight structural parts. The electronics industry uses stamping for terminals, shielding components, and precision connectors.
Modern stamping operations may include multiple processes such as blanking, piercing, bending, embossing, and deep drawing, often combined into a single progressive die system. This enables rapid production rates, cost efficiency, material optimization, and consistent product quality.
Due to its productivity, repeatability, and ability to produce complex geometries economically, metal stamping remains one of the most important sheet metal manufacturing processes in modern industrial production.
2. Classification of Operations
Stamping operations can be broadly classified into cutting and forming operations.
Cutting Operations
Blanking
A cutting operation where a flat piece of metal is separated from a larger sheet.
The piece cut out is the desired product (the blank), and the remaining frame is scrap.
Piercing (Punching)
Similar to blanking, but the piece cut out is scrap (the slug), and the remaining sheet is the desired product.
Forming Operations
Operations that plastically deform the material without cutting, such as:
- Bending
- Drawing
- Coining
3. Mechanics of Shearing
The cutting of metal involves stressing the material beyond its ultimate shear strength. The process occurs in three stages:
1. Plastic Deformation
As the punch contacts the sheet, the material deforms plastically, penetrating a small depth into the die.
2. Shear Pattern Formation
As penetration continues, the material reaches its shear strength limit, and cracks begin to form at the edges of the punch and die.
3. Fracture
The cracks from the top and bottom surfaces propagate and meet, completing the separation of the material.
4. Die Design Components
Punch
The male component that enters the die opening to cut or form the material. It typically determines the size of the hole in piercing.
Die
The female component with an opening. It determines the size of the blank in blanking.
Stripper Plate
Used to strip the material from the punch during the return stroke.
5. Force Calculations
Maximum Cutting Force
The maximum cutting force F_max is critical for selecting the press tonnage. It is calculated as:
F_max = L × t × τ_max
Where:
- L = Perimeter length of the cut (mm)
- t = Sheet thickness (mm)
- τ_max = Ultimate shear strength of the material (N/mm²)
Energy Calculation
The energy required for the operation depends on the force-displacement curve. It can be approximated as:
E = K × F_max × t
Where:
- K = Factor (typically 0.4 – 0.7) representing the percentage of penetration before fracture
6. Die Clearance
Clearance is the gap between the punch and the die. It is a critical parameter affecting edge quality and tool life.
Optimum Clearance
Typically 5% – 12% of material thickness. Results in a clean cut where fractures meet effectively.
Insufficient Clearance
Secondary shear cuts occur, increasing force and wear.
Excessive Clearance
Large burrs are formed, and the cut edge is tapered.