How should sheet metal tolerances be set? What happens if they are too tight?

In sheet metal design and manufacturing, tolerance is a key parameter ensuring that dimensions, hole positions, and assembly relationships meet design requirements. Reasonable tolerances not only guarantee product performance and consistency but also significantly reduce costs, minimize rework, and improve mass production efficiency.

However, many designers either set sheet metal tolerances too leniently, leading to assembly problems, or too strictly, causing costs to skyrocket.

This article will provide a comprehensive analysis of tolerance types, setting methods, common standards, and the actual impact of "overly strict tolerances."

What are Sheet Metal Tolerances?

Sheet metal tolerances refer to the range within which the actual dimensions of a part are allowed to deviate from the theoretical design dimensions.

For example:

• Design dimension: 50.0 mm

• Tolerance: ±0.2 mm

• Actual acceptable range: 49.8–50.2 mm

The smaller the tolerance, the stricter the requirements, and the higher the manufacturing cost.

Common Tolerance Types in Sheet Metal

• Linear Tolerance: Used for sheet thickness, edge length, and dimensions after bending.

• Hole Position Tolerance: Used for mounting holes and screw holes, affecting assembly accuracy.

• Bend Tolerance: The most common error in bending angle and external dimensions after bending.

• Flatness / Perpendicularity / Parallelism: Mainly used for large-area sheet metal or structural components.

• Sheet Thickness Tolerance: Determined by the raw material; for example, 1.0 mm cold-rolled steel typically has a tolerance of ±0.05 mm.

How are sheet metal tolerances set?

1. Follow standard tolerance systems

Common standards include:

• ISO 2768-m (medium tolerance)

• ISO 2768-k (wide tolerance)

For general mechanical parts, generally use: ISO 2768-m (medium)

Applicable to the vast majority of sheet metal parts, no additional specification required.

2. Distinguish between Functional and Non-critical Dimensions

• Functional Dimensions: Affecting assembly, sealing, and electrical interfaces, these critical tolerances can be set tighter, such as ±0.1–0.2 mm.

• Non-critical Dimensions: These are general dimensions, such as external dimensions and edge-to-edge distances. More lenient tolerances can be set, such as ±0.5–1 mm.

Common Engineer Tips:

Reducing the number of strict tolerances

Concentrating strict tolerances in critical functional areas

Using ISO 2768 automatic defaults for non-critical dimensions

3. Consider material thickness and bending compensation

The thicker the plate, the greater the bending error:

• <1mm: ±0.2–0.3 mm

• 1–2mm: ±0.3–0.5 mm

• 2mm: ±0.5 mm or more

4. Tolerance Allocation for Parts with Multiple Processes (Welding, Bending, Stamping)

Welded parts should not have their tolerances aligned with those of individual parts. Post-welding thermal deformation is significant, and dimensional errors may reach ±1 mm.

A common practice is to first ensure the mounting hole positions, then focus on the overall shape, and finally consider decorative dimensions.

What happens if the tolerances are set too strictly?

1. Significantly Increased Costs (More Complex Processes)

Tight tolerances typically lead to:

• The need for secondary processing (CNC recutting)

• The need for specialized fixtures

• The need for superior equipment (higher precision bending machine dies)

• Even requiring die stamping instead of laser + bending

Result:

Cost increases of 30%–300% are common

2. Decreased yield, surge in rework

Tighter tolerances increase the likelihood of defects exceeding acceptable limits:

• Slightly different bending angle → OUT

• Slight laser cutting vibration → OUT

• Slight welding deformation → OUT

Result:

Large-scale rework

Longer lead times

Decreased consistency

3. Impact on overall assembly efficiency

Tight tolerances are intended to improve assembly accuracy, but excessively tight tolerances can make parts too difficult to assemble.

For example:

• A bending deviation of 0.1mm can cause jamming.

• Two plates that are too tightly packed together cannot be inserted.

Tighter assembly is not necessarily better.

Conclusion

Tolerance setting is the most easily overlooked yet most impactful aspect of sheet metal design, significantly affecting both cost and quality.

A reasonable tolerance should strike a balance between the following three factors:

• Functional requirements (installable and usable)

• Manufacturing capability (mass production and stable operation)

• Cost control (avoiding excessive waste)