Many problems in sheet metal fabrication do not arise during production, but rather during the design phase. Bending radii that are too small, incorrectly accounted-for rolling directions, or inappropriate tolerances can cause components to crack, fail to assemble, or require unnecessary rework. At the same time, manufacturing quality and cost-effectiveness depend heavily on how the material, cutting methods, forming, and joining techniques are coordinated with one another. Understanding these interrelationships reduces the need for coordination, rework, and costs.
Materials and Material Behavior in Sheet Metal Fabrication
Sheet metal fabrication refers to all manufacturing processes used to transform flat metal sheets into functional components: cutting, forming, and joining. The raw material is produced through rolling, a process in which metal ingots are formed into thin sheets and given a directional microstructure that plays an important role during subsequent bending.
In practice, sheet metal up to about 3 mm thick is referred to as thin sheet metal; in our case, it is 2.99 mm. Thicker sheet metal is referred to as heavy sheet metal, which has a coarser surface texture and is also known as hot-rolled sheet metal. The exact classification may vary depending on the standard and application. Most precision applications—such as electronic enclosures, medical devices, or control cabinets—use thin sheet metal.
Overview of Materials
Steel (S235 / S355): It is cost-effective and high-strength, but prone to corrosion without protection. Steel sheets are therefore almost always given a surface treatment such as galvanizing, powder coating, or temporary corrosion protection.
Stainless steel: The chromium content of at least 10.5% forms a self-healing passive layer. Stainless steel is the material of choice wherever aggressive cleaning agents or hygiene requirements are a factor. It is more difficult to machine than unalloyed steel and is less forgiving of welding errors.
Aluminum: Lightweight, easily formable, and naturally corrosion-resistant. When bent, aluminum springs back more than steel and is more prone to breaking when bent to too small a radius.
Copper / Brass: Less commonly used for electrical and thermal applications in sheet metal fabrication, but indispensable for specific components.
Manufacturing Processes in Sheet Metal Fabrication: Cutting, Bending, Joining
1. Cutting
The cutting process produces what is known as the blank—that is, the cut piece with the required contour and cutouts.
- Laser cutting is the method of choice for complex contours. A focused beam of light heats the material to its melting point; the cutting gas blows the molten material out of the cut. The result is clean edges with minimal material loss. When cutting stainless steel, experienced shops use nitrogen instead of oxygen as the cutting gas. As an inert gas, it prevents oxidation and produces an oxidation-free, silvery edge.
- Stamping is faster and more cost-effective when geometries are repeated. Hole patterns and formings can be created directly in the sheet metal in a single step—typically, for example, in IT enclosures with perforations for passive cooling. The limits are set by the die geometry. However, the cost-effectiveness of punching requires sufficient production volumes, as the tooling costs for punching and forming dies are comparatively high. For small production runs or frequently changing geometries, laser cutting is generally more flexible.
- Waterjet cutting is a cold process that does not involve heat input. It is recommended for heat-sensitive or pre-coated materials.
2. Bending
Bending transforms the flat sheet into a three-dimensional component. This is where most design errors occur.
- Rolling Direction: Due to the manufacturing process, sheet metal has a preferred direction in its microstructure. If a thicker sheet is bent parallel to the rolling direction and the radius is set too small, the tensile stresses on the outer side of the bend exceed the elongation at break. This results in cracks, some of which are immediately visible, while others do not appear until later. Bending lines should therefore be oriented perpendicular to the rolling direction whenever possible.
- Bending radius: The material-specific minimum value must be observed. As a rule of thumb for the minimum bend radius: soft, unalloyed steel requires at least 1x the material thickness; stainless steel, 1.5 to 2x the material thickness; and aluminum, depending on the alloy, 1 to 3x the material thickness. High-strength steels, cold-rolled grades, or small inside radii require significantly larger minimum values.
- Bend allowance and K-factor: The material stretches on the outside and compresses on the inside. The neutral fiber, described by the K-factor (typically 0.3 to 0.5), determines how much length is lost during bending. If you do not take this into account in the development process, you will end up with parts that do not fit together properly.
- Springback: After bending, the metal springs back to some extent. Experienced manufacturers compensate for this by incorporating an overbend into the machine program.
- Surface-treated sheet metal: The tooling and setup process remain the same as for the raw material. What changes is the handling. Bending films placed between the component and the die, recessed dies, or pre-laminated material protect the surface throughout the entire process chain, all the way through to packaging.
3. Insert
MIG/MAG welding is the standard process for steel structures: economical, versatile, and suitable for many applications. TIG welding produces higher-quality welds and is the preferred method for stainless steel as well as for visible welds. Laser welding introduces only a small amount of heat into the component, thereby reducing distortion and enabling reproducible results in robot-automated systems. This is particularly important for precision parts.
In addition to welding, alternative joining methods are also used in sheet metal fabrication. Mechanical methods such as clinching, screw fastening, or press-fit fasteners are particularly suitable for thin-walled components, coated sheet metal, or assemblies where heat input is undesirable. Structural bonding is used when joining different materials or when uniform load distribution without local heat effects is required.
Tempering discoloration in stainless steel: After welding or oxygen laser cutting, discoloration ranging from straw yellow to bluish-violet occurs. This indicates that the passive layer has been weakened by a loss of chromium. The result is reduced corrosion protection and, in hygiene-critical environments, an increased risk of bacterial colonization. The heat-affected zone discoloration must therefore be removed by pickling or blasting.
In a nutshell:
- The rolling direction affects crack formation
- Minimum bending radii depend on the material
- Springback must be taken into account in the design
Tolerance Management – What Determines Fit Accuracy
In the worst-case scenario, dimensional deviations add up linearly: Five parts, each with a deviation of ±0.1 mm, result in a total assembly deviation of up to ±0.5 mm. This is the cumulative tolerance and one of the most common reasons why assemblies do not fit together, even though all individual parts are correct.
The recommended countermeasure is to manufacture individual parts as close as possible to the target dimensions, fully measure prototypes before mass production, and compensate for any deviations. From a design perspective, mating parts must be given opposing tolerances—one with a positive tolerance and the other with a negative tolerance. Relevant standards include ISO 2768 for general tolerances, as well as EN 10029, EN 10048, and EN 10051 for limit dimensions for sheet metal.
The Most Common Design Flaws
Certain errors occur time and again in sheet metal fabrication. The cause usually lies not in the manufacturing process, but rather in the design phase. Typical examples include:
- Rolling direction not taken into account:
If a sheet is bent in a direction perpendicular to the rolling direction, cracks will form on the outer side of the bend that cannot be repaired later. - Bending radius set too small:
If material-specific minimum bending radii are not observed, the risk of material cracks or deformation defects increases significantly. - Misinterpreted Tolerances:
If both mating parts are given positive tolerances, the assembly will not fit together mathematically, even though each individual part is dimensionally accurate on its own. - Forgot to include bend reliefs:
If there are no reliefs near the edges, the material may tear during bending. - Annealing color not removed from stainless steel:
Annealing colors remaining after welding or laser cutting weaken the passive layer and can lead to corrosion in the affected areas over time. - The Handling of Finished Surfaces Is Underestimated:
Scratches and damage often occur not during the machining process itself, but during automatic unloading, transport, or packaging.
Many of these errors can be avoided early on if the design and manufacturing departments review the drawing together before it is approved.
We see many of these fields regularly in drawings. We review your design before production begins—in a practical manner and without lengthy delays.
What Really Matters in Sheet Metal Fabrication
Most problems in sheet metal fabrication do not arise at the machine, but rather during the design phase, in the selection of materials, or in the tolerance specifications. Understanding the fundamental relationships allows you to avoid many mistakes even before production begins.
The key to a functional sheet metal structure is not machinery, but an understanding of the material, the rolling direction, the bending radius, and a well-defined tolerance concept.
The rolling direction, minimum bending radius, and springback vary depending on the material and must be taken into account during the design phase. When it comes to cutting, the choice of process depends on the geometry and the number of parts: Laser cutting is suitable for complex contours and high precision, while punching is suitable for recurring geometries and production parts. Stainless steel should be cut with nitrogen whenever possible to ensure oxide-free cut edges.
In bending as well, many errors originate in the drawing itself. Bending lines should run perpendicular to the rolling direction whenever possible, while the K-factor and bending allowance must be correctly factored into the developed drawing. The material’s springback is later compensated for by overbending. In welding, laser welding reduces warpage in thin-walled components and enables reproducible results. For stainless steel, the annealing color must subsequently be removed by pickling or blasting to restore corrosion protection.
Tolerance management is equally important. Cumulative tolerances must be planned for assemblies, mating parts must have opposing tolerances, and prototype parts must be fully measured before production begins. For surface-treated sheet metal, protective measures are also required throughout the entire process chain, right up to packaging.
The best results are achieved when the design and manufacturing teams collaborate early on, while changes are still possible.
BVS Blechtechnik manufactures precision sheet metal parts, from prototypes to production runs.
FAQ
What is the difference between thin sheet metal and thick sheet metal?
The threshold is 3 mm in thickness. Thin sheet metal is easily formable and suitable for precision applications. Thick sheet metal has a coarser surface texture and is used for load-bearing structures in steel construction and mechanical engineering.
Why does sheet metal tear when it’s bent?
In most cases, there are two possible causes: The bending radius is too small for the material and thickness, or the sheet was bent parallel to the rolling direction. Three measures can remedy this: The bend lines should be positioned perpendicular to the rolling direction; material-specific minimum radii should be observed (steel: 1x, stainless steel: 1.5 to 2x, aluminum: up to 3x the material thickness); and the drawing should be coordinated with the manufacturing facility before approval.
What is the cause of the discoloration, and why does it need to be removed?
The yellowish to bluish-purple discoloration that occurs after welding or laser cutting stainless steel indicates localized chromium depletion in the passive layer. Corrosion protection is compromised in these areas; bacteria are more likely to colonize them in hygienic environments. Removal by pickling or blasting is mandatory.
What does "bending shortening" mean, and why is it important?
During bending, the material stretches on the outside and compresses on the inside. The neutral fiber (K-factor, typically 0.3 to 0.5) lies between these two states and remains constant in length. If you do not account for the K-factor in the development, you will end up with parts that do not fit together properly after bending. With multiple bends, the error accumulates.
Which cutting method is best suited for which application?
Laser cutting is suitable for complex contours, high-quality edges, and flexible batch sizes, using nitrogen as the cutting gas for stainless steel. Punching is the right choice for production parts with recurring geometries and hole patterns.
