Processing limits and accuracy standards for sheet metal parts

Sheet metal parts are used in our everyday life and in industry in a wide range of applications. In order for these manufactured sheet metal parts to be used interoperably, they must comply with certain accuracy standards and machining limits. These refer to specific parameters and tolerances with respect to width, length, thickness and geometric tolerances. The specifications must be taken into account when manufacturing sheet metal parts. The following examines various standards and limits in detail.

What characterizes sheet metal parts?

In mechanical engineering, sheet metal parts contribute significantly to the stability, functionality and aesthetics of machines and equipment as structural components, but also as shrouds and housings. Sheet metals are usually rolled products and are defined as a flat metal piece whose width and length is significantly greater than its material thickness.

Sheet metal parts have the following special features:

• Processing method: Some processing methods, such as rolling or pressing, may alter the microstructure of sheet metal, which affects strength and ductility.
• Material thickness: Sheet metal parts with a thickness of less than 3 mm are called precision sheet metal. If sheets are 3 mm or thicker, they are called plates.
• Formability: Due to their low thickness, precision sheet metal parts in particular can be easily reshaped using various methods such as stamping, embossing and bending. Complex geometries can as a result be produced.

What are sheet metal tolerances in sheet metal processing and what are they used for?

Sheet metal can be processed in a variety of ways, such as punching, laser-cutting, and bending. In sheet metal part production, the permissible tolerances must always be taken into account, as is the case when designing other components. It is almost impossible to manufacture workpieces to 100% scale. The standard-compliant dimensioning of sheet metal parts therefore plays an important role. Without further accuracy information, they are usually manufactured according to general-purpose tolerances. If details or a complete component are required at higher accuracy, the permissible deviation for these must be defined by specifying the tolerance in advance.

See this blog for an overview of ISO 1101 symbols used in engineering drawings.

The higher the degree of accuracy, the more expensive the production will be. Tolerances are used to ensure that such deviations are taken into account to a defined extent and that the finished products can be installed with an accurate fit. Tolerances must always be determined while keeping an eye on accuracy and feasibility, along with production costs. A upper and lower limit are defined. The tolerance field lies between these. The tighter the tolerance, the more accurate the workpiece is. Sheet metal processing relies on general-purpose tolerances for the following sheet metal features:

• Angle dimensions: Tolerances for angles, e.g. for bends.
• Shape: Shape tolerances such as flatness or roundness to ensure that the intended shape is maintained.
• Position: Ensure the exact position, e.g. symmetry when punching a hole into the sheet metal. This ensures that the sheet metal can be accurately assembled with other components.
• Bores: Tolerances exist, for example, for the dimensions of the bore, the distance from bore to bore, or the edge distance of the bore in steel. They are important to avoid cracking and deformation. In addition, a correctly maintained edge distance contributes to the strength and stability of the sheet metal.
• Thickness: Material thickness tolerances

Tolerance Analysis

How can you find the right tolerance field for the individual parameters? There are several ways to analyze tolerances:

• Worst Case Analysis: This method tests extreme tolerance combinations and then derives the maximum and minimum limits from these. All components with extreme limits are installed and must still work in combination.
• Statistical analysis: Statistical techniques are used to determine the probability that all components are within tolerance limits (assuming a Gaussian normal distribution). Although a sufficiently large amount of data is initially required for the assessment, it is then quite meaningful. Unnecessarily tight tolerances are also avoided.
• Tolerance Stack Analysis (RSS): Tolerance stack analysis calculates the total deviation of a system based on the assumption that tolerances are distributed independently and randomly. Although it is easy to use, it can at times be less accurate.

Determining sheet metal tolerances

Which tolerance is selected depends primarily on the intended use of the sheet metal part. The intended application decides factors such as material selection, size, position and shape as well as required dimensional accuracy. In many cases, general-purpose tolerances can be relied upon as sufficient for the required dimensional accuracy. General-purpose tolerances apply whenever engineering drawings do not call out specific tolerances.

A supplementary blog on the topic of test equipment and positioning elements for sheet metal parts can be found here.

General-purpose tolerances for position and shape

There are various DIN tolerances for metals. General-purpose tolerances for position and shape, e.g. DIN ISO 2768-2:

In the context of the standard, symmetry means that a component feature or several features must be distributed evenly around a reference axis. The symmetry tolerance indicates the maximum deviation from this axis. Symmetry tolerances are especially important for sheet metal parts that must remain movable or where load distribution plays a role.

The tolerance classes are defined as follows: Class H is "fine", K "medium" and L "coarse". Components that meet Class H requirements are therefore used for applications that require high precision. Class K is suited for medium precision and L for applications where major deviations are unproblematic.

For more information on tolerances and tolerance classes, visit this blog.

Flatness tolerance for sheet metal

Another important tolerance for sheet metal is the flatness tolerance. The flatness tolerance ensures that the sheet metal surface remains evenly flat within certain limits. Adherence to the flatness tolerance ensures that a sheet metal part has the correct fit and that various assemblies can be installed, e.g., tightly together. Failure to maintain the tolerance may result in incorrect load distribution and stresses. DIN ISO 2768-2 can also be used to determine the flatness tolerance:

Tolerances for bending sheet metal

Additional tolerances for sheet metal can be, for example, angular tolerance and perpendicularity tolerance. Because sheet metals are easily reshaped due to their often low material thickness and specific material properties, this is one of the most typical forms of processing. But this easy processing also makes the application of tolerances particularly important. MISUMI offers a wide range of fitting options for mounting brackets with different tolerances.

What are processing limits for sheet metal processing?

For sheet metal products, processing limits are set for each sheet thickness, material, shape and hole type.

If the value is outside the processing limits, the sheet metal part cannot be processed.

The following tables provide an overview of the possible processing limits of sheet metal parts at MISUMI:

Find more information about processing limits in MISUMI’s meviy.