I. Why Are Dimensional Tolerances Necessary?

Limitations of Manufacturing Processes

• When machining shaft parts on a lathe, tool wear causes gradual deviations in diameter dimensions;
• Fluctuations in the cooling shrinkage rate of castings lead to dimensional differences.

Assembly Compatibility and Functional Requirements

Fitting Relationships

• Interference Fit (e.g., bearings and shafts): The shaft diameter needs to be slightly larger than the hole; tolerances ensure a tight connection after assembly.
• Clearance Fit (e.g., bolts and through-holes): The hole diameter needs to be slightly larger than the bolt; tolerances guarantee flexibility in disassembly.

Functional Adaptation

• The inner diameter of a washer must be slightly larger than the bolt diameter: excessive tolerance causes wobbling, while insufficient tolerance makes installation impossible.
• The thickness tolerance of sealing washers for pressure vessels must be strictly controlled; otherwise, sealing performance may be affected.

Cost Control

II. Common Standards for Washer Dimensional Tolerances

1. Inner Diameter (d₁)

• Standard Value: Corresponds to the nominal diameter of the bolt (e.g., the inner diameter of an M6 washer is approximately 6.4mm).
• Tolerance Grades:
  • Grade A: Used for precision assembly series, with small tolerances (e.g., the inner diameter tolerance of M6 is +0.15/0mm);
  • Grade C: Used for rough assembly series, with larger tolerances (e.g., the inner diameter tolerance of M6 is +0.30/0mm).

2. Outer Diameter (d₂)

• Standard Value: Determined by the bolt grade and application scenario (e.g., the common outer diameter of an M6 washer is 12mm or 10mm).
• Tolerance: Usually negative tolerance (e.g., the tolerance for an outer diameter of 12mm is -0.36/0mm) to avoid insufficient assembly space due to an excessively large outer diameter.

3. Thickness (h)

• Standard Value: Common thickness ranges from 1 to 4mm (e.g., the thickness of an M6 washer is 1.6mm).
• Tolerance: Thickness tolerance is usually ±0.1~±0.2mm; in high-precision scenarios (e.g., aerospace), it can be controlled within ±0.05mm.

Comparison of International Standards

III. Key Factors Affecting Tolerances

Material and Process

• Stamping Process: Ordinary carbon steel washers are formed by stamping, with larger tolerances (±0.2mm), suitable for Grade C standards;
• Machining Process: Stainless steel or high-strength washers may be processed by turning, with tolerances up to ±0.05mm (Grade A or precision grade).

Industry Application Scenarios

• Ordinary Machinery: Adopts Grade C tolerances (e.g., agricultural machinery, construction steel structures), allowing larger errors;
• Precision Equipment: Such as medical devices and aerospace components, requiring customized high-precision tolerances (e.g., ±0.02mm), which may need 100% inspection and screening.

Coating Thickness

IV. How to Select Appropriate Tolerances?

Clarify the Application Scenario

• For high-pressure sealing (e.g., chemical pipelines), select Grade A tolerances + thickness tolerance of ±0.1mm to ensure a flat sealing surface;
• For ordinary fastening (e.g., furniture assembly), Grade C tolerances can meet requirements and reduce procurement costs.

Verify Standards and Drawings

Consider the Tolerance of Mating Parts