Guía de procesos de chapa metálica: corte, mordisqueo, plegado y embutición profunda

Sheet metal fabrication is a cornerstone of modern manufacturing, used extensively across the automotive, aerospace, construction, and appliance industries. Understanding the core processes—such as shearing, nibbling, bending, drawing, embossing, and coining—is essential for engineers and fabricators seeking to optimize production efficiency and product quality.  

1.0What Are Metal Forming Operations? 

Metal forming operations involve shaping the material without removing any of it, meaning there is no material wastage. The sheet metal is stressed beyond its elastic limit but remains below its ultimate strength, ensuring it takes on a new, permanent shape.

Common metal forming operations include:

• Bending
• Drawing
• Embossing
• Forming
• Coining (also known as squeezing)

2.0What Is Shearing? 

Shearing is a process used to cut straight lines across metal sheets, strips, or bars. It consists of three main stages:

1. Plastic deformation
2. Fracture (crack propagation)
3. Shearing (material separation)

When a metal workpiece is placed between the upper and lower blades of a shearing machine, and pressure is applied, the material first undergoes plastic deformation. As pressure increases, cracks begin to form at the cutting edges of the blades. These cracks then propagate and meet, causing the material to shear.

Shearing machines—ranging from manual to hydraulic and mechanical models—are used to perform this process efficiently and accurately, especially in high-volume or industrial applications.

3.0What Is Nibbling?

Nibbling is typically used as a substitute for blanking. It is designed to cut flat parts from sheet metal and is suitable for shapes ranging from simple to complex contours. This process is mostly used for producing small quantities of components.

4.0What Is Bending?

Bending is a process in which a straight sheet of metal is transformed into a curved shape. During bending, the material is subjected to both tensile and compressive stresses, resulting in plastic deformation beyond the elastic limit but below the ultimate strength.

Common types of bending include:

• U-bending
  U-bending (also called channel bending) uses a die cavity shaped like a “U”, resulting in a component with a U-shaped profile. This operation is commonly performed using a press brake machine equipped with U-shaped dies.
• V-bending
  V-bending uses a wedge-shaped punch and a V-die. The angle of the V can vary from acute to obtuse, including 90°. It is one of the most widely used bending techniques in press brake operations due to its versatility and precision.
• Angle bending
  Angle bending is a general term for bending sheet metal at a sharp angle. It can also be carried out using a press brake, depending on the geometry and required angle.
• Curling
  Curling involves curling the edge of sheet metal around a form. Both the punch and die contain partial cavities to shape the material. After the operation, the punch retracts, and the part is ejected using a plunger. This method is used in making drums, pans, pots, and similar items.
• Roll bending
  Roll bending uses a set of rolls to gradually bend large sheet metal parts into curved sections. It is commonly used for fabricating large storage tanks, pressure vessels, pipes, and structural components.
• Bending in a 4-slide machine
  This method is used for relatively short pieces. These machines vary in design and use both vertical and lateral die movements to form complex shapes.
• Edge bending
  Edge bending involves cantilever loading, where a pressure pad holds the workpiece against the die while a punch forces the metal to bend over the edge. Press brakes equipped with wiping dies can also perform this type of bending. Edge bending is typically limited to angles of 90° or less, although wiping dies can be designed for larger angles.

Due to the complexity of pressure pads and wiping dies, this method is more costly but well-suited for high-volume production.

5.0What Is Drawing?

Drawing is a process in which a punch forces a flat sheet metal blank into a die cavity, causing the material to plastically flow and take on a cup-like shape. It is used to create hollow parts from flat sheets.

6.0What Is Embossing? 

Embossing is used to create raised or recessed designs on sheet metal for decorative or functional purposes. It can be used to imprint logos, trademarks, part numbers, or other identifying marks.

7.0What Is Forming?

In forming, the metal is stressed beyond its yield point so that it permanently retains a new shape, directly reproducing the contour of the punch and die. Unlike drawing, there is no significant metal flow. This process is used for manufacturing items such as door panels, steel furniture, and aircraft bodies.

8.0What Is Coining (Squeezing)?

Coining is a precision forming process in which a metal blank is placed between a punch and die, and high pressure is applied. The metal flows plastically in a cold state, filling the die cavity completely. This process is used to produce coins, medals, and ornamental components with fine surface detail.

9.0Understanding Bending Mechanics and Common Methods

In sheet metal bending, the metal is strained around a straight axis. The material on the inside of the bend is compressed, while the material on the outside is stretched. The metal is plastically deformed so that the bend retains its shape after the stress is removed. Bending generally does not significantly change the thickness of the material.

Two common bending methods are:

• V-bending: Performed using a V-shaped punch and die, this method allows for a range of bend angles from acute to obtuse. It is generally used for low-production applications and is often performed on a press brake. V-dies are relatively simple and inexpensive.
• Edge bending: This method uses a pressure pad to hold the base of the workpiece while a punch bends the sheet over the edge of the die. Edge bending is limited to angles of 90° or less, unless more complex wiping dies are used. These dies are more costly than V-dies but are suitable for high-volume production.

10.0Important Factors in Bending: Bend Allowance, Springback, and Force

Bend Allowance
When the bend radius is small relative to stock thickness, the material tends to stretch during bending. Accurately estimating this stretching is essential to ensure the final part meets its design specifications. The bend allowance (BA) is the estimated length of the neutral axis before bending and accounts for material stretch. It can be calculated using the general formula:

BA = (π / 180) × A × (R + K × t)

Where:

• BA = Bend allowance (in mm)
• A = Bend angle (degrees)
• R = Inside bend radius (mm)
• t = Stock thickness (mm)
• K = Factor to estimate the location of the neutral axis (commonly between 0.33–0.5, depending on material and bend conditions)

Stretching typically occurs when the bend radius is small compared tothe  sheet thickness.

Springback

After the bending pressure is released, the elastic energy stored in the material causes partial recovery toward its original shape — this is called springback. It is defined as the increase in the included angle of the bent part compared to the tool angle after unloading.

SB = θ₁ – θ₂

Where:

• SB = Springback (degrees)
• θ₁ = Included angle of the sheet-metal part after bending
• θ₂ = Included angle of the forming tool

Bending Force
The required force for bending depends on factors like punch and die geometry, material strength, sheet thickness, and part width. The maximum bending force can be estimated with:

F = (Kbf × TS × ω × t²) / D

Where:

• F = Bending force (N)
• TS = Tensile strength of material (MPa)
• ω = Width of the part (mm)
• t = Sheet thickness (mm)
• D = Die opening dimension (mm)
• Kbf = Bending force coefficient
  • 33 for V-bending
  • 33 for edge bending

11.0Drawing Operations: Producing Hollow Shapes from Flat Sheet Metal

Drawing is a sheet metal forming process used to create cup-shaped, box-shaped, or other hollow parts. A flat sheet-metal blank is placed over a die cavity and pushed into it with a punch. A blankholder holds the material in place during the operation.

Typical applications include:

• Beverage cans
• Ammunition shells
• Sinks and cookware
• Automobile panels

Mechanics and Stages of Deep Drawing

In the basic cup-drawing process:

A blank of diameter Db is drawn using a punch of diameter Dp.

Punch and die have corner radii (Rp and Rd) to prevent tearing.

A clearance C is provided between the punch and the die:

C ≈ 1.1 × t

Two forces are applied:

Punch force (F) for deformation

Blankholder force (Fh) to control metal flow

Stages of deep drawing include:

Initial Contact – Metal is bent over the die and punch radius.

Straightening – Previously bent areas straighten as they are pulled into the die.

Drawing & Compression – Material flows from the flange into the die cavity.

Friction (static → dynamic) resists flow.

Compression at the flange causes thickening and potential wrinkling.

Continued Drawing – Punch continues downward, drawing metal into the die.

Thinning may occur in the cylinder wall.

A balance between blankholder force and friction is critical.

Lubrication is typically required.

Common Defects in Drawing

• Wrinkling in Flange: Radial ridges due to compressive buckling.
• Wrinkling in Wall: Flange wrinkles pulled into the vertical wall.
• Tearing: Open cracks near the base due to high tensile stress.
• Earing: Uneven edges (ears) from sheet anisotropy.
• Surface Scratches: Caused by rough die surfaces or poor lubrication.

Stretch Forming Operations: Large Contoured Sheets for Aerospace and Automotive

Stretch forming creates large, accurately contoured sheets by stretching the metal beyond its elastic limit over a form block.

Springback is a key issue, influenced by:

• Material type
• Thickness
• Hardness
• Bend radius (larger radius causes greater springback)

Methods to Reduce Springback

Overstretching using V-type form blocks

Corner setting: Coining the corners to release residual elastic stress

Stretch Forming Methods

Form-block method

The blank is stretched over a single form block (male die).

Mating-die method

Uses both male and female dies for higher accuracy and repeatability.

12.0Conclusion

In summary, sheet metal processes like shearing, nibbling, bending, drawing, embossing, and coining each play vital roles in shaping metal sheets into functional and precise components.

A thorough grasp of the mechanics and variables affecting these processes—including bend allowance, springback, and tool design—enables manufacturers to minimize defects and optimize production. Whether you are producing simple flat parts or complex

Contoured panels, mastering these techniques is key to successful sheet metal fabrication.

13.0Preguntas frecuentes

Q1: What is the main difference between bending and drawing in sheet metal fabrication?
A1: Bending changes the shape by plastically deforming the metal around an axis without significant material flow, while drawing involves pulling material into a die cavity, causing plastic flow to form hollow shapes.

Q2: How can springback be minimized during bending?
A2: Springback can be reduced by overbending, selecting appropriate tooling, using material with less elasticity, and applying proper bend allowance calculations during design.

Q3: When is nibbling preferred over blanking?
A3: Nibbling is preferred for cutting complex contours or small production runs where blanking dies would be costly or impractical.

Q4: What causes wrinkles during deep drawing, and how can they be prevented?
A4: Wrinkles result from compressive stresses in the flange area. They can be minimized by optimizing blankholder force, lubrication, and tool geometry.

Q5: Why is lubrication important in forming and drawing processes?
A5: Lubrication reduces friction between the sheet metal and tooling, preventing surface damage, excessive tool wear, and defects like tearing or wrinkling.