What Are Sheet Metal Tolerances?

Natural variances in rolled sheet stock can occur during metal fabrication. The design needs to account for these differences for a fabrication shop to achieve the designing engineer’s goals. Sheet metal tolerances help operators have better control over their results for more consistent production. This guide will provide an overview of sheet metal tolerances and their uses.

Sheet Metal Tolerances: A Definition

The American Society of Mechanical Engineers defines tolerance as the total amount that a design’s dimension can vary. A part design can present tolerance as an upper limit and a lower limit or an allowable range. You may see a dimension’s tolerance written with the ± symbol. This symbol shows how far above and below a dimension can go outside of the specified design. For example, a 2-inch dimension with a tolerance of ± .05 inches has an acceptable range of 1.95 inches to 2.05 inches.

A sheet metal tolerance’s tolerance band refers to its range of allowable dimensions. Engineers and designers take different approaches to tolerance bands. A “looser” tolerance has a larger range between the upper and lower limits. Meanwhile, “tighter” tolerances have shorter tolerance bands. Tighter tolerances may indicate the importance of precision for a specific dimension.

Why Do Manufacturers Need Sheet Metal Tolerances?

Sheet metal rolls have small variations between one another that can influence the results they create during part production. Small variances in thickness can influence how machinery shapes the metal. Factors that change the structure of a sheet metal roll include:

• Impurities and contamination in the material
• The material’s grain
• Age of the material
• Heat-related changes from laser or plasma cutting
• Processing methods used

Defining tolerances in design provides the following benefits:

• Better fit: Allowing for tolerances promotes a better fit between parts.
• Reduced costs: Parts can become more expensive to produce when they have tight tolerances or no tolerances.
• Higher efficiency: Including tolerances in all of a design’s dimensions promotes clearer communication between the designer and shop for efficient manufacturing.

Tolerances make production more precise overall by accounting for the imprecisions in sheet metal.

What Factors Influence Sheet Metal Tolerances?

Engineers consider multiple aspects of the fabrication process when adding tolerances to a part. In addition to variations in material, these factors could influence a dimension’s tolerance band:

• Ease of assembly: Will holes and brackets line up properly during assembly?
• Cutting processes: How much variation can occur during the cutting process?
• Welding processes: What kind of heat and fit up will the welder use during the welding process?
• Functionality: How much or little space does the part need to function as intended?
• Repair requirements: Will the user need to change out parts frequently?

Looser tolerances tend to reduce costs compared to tighter ones, but factors such as those listed above also need consideration.