Overview
ISO 273:1979 is an international standard published by the International Organization for Standardization (ISO) that specifies clearance hole diameters for bolts and screws used in general purpose applications. The standard covers hole sizes appropriate for a range of bolt thread diameters, from 1 mm up to 150 mm. It provides tabulated diameters for clearance holes corresponding to fine, medium, and coarse thread series to ensure proper fit and mechanical performance in fastening assemblies.
The standard was developed by ISO Technical Committee 2 (Fasteners) and serves as a technical revision of the earlier ISO Recommendation R 273-1962. It ensures consistency and interchangeability in the design and manufacturing of fastened joints worldwide. Special applications requiring non-standard hole sizes should consider bespoke design requirements.
Key Topics
- Clearance Hole Diameters: ISO 273 details precise diameters for clearance holes in three thread series-fine, medium, and coarse.
- Thread Diameter Range: The standard covers thread diameters from as small as 1 mm to as large as 150 mm.
- Dimensional Tolerances: Recommended tolerance fields are provided for holes to ensure optimal fit:
- Fine thread series: H12 tolerance
- Medium thread series: H13 tolerance
- Coarse thread series: H14 tolerance
- Chamfer Recommendations: To avoid interference between the hole edge and the bolt underhead fillet, chamfering the hole is advised.
- Application Scope: General purpose clearance holes based on standard bolt and nut dimensioning. Custom clearance holes for special applications should be designed as per specific requirements.
Applications
ISO 273:1979 is essential for engineers, designers, and manufacturers in the fastener and mechanical assembly industries. Its practical value includes:
- Mechanical Design: Assisting mechanical engineers in selecting correct hole sizes for bolts and screws to ensure smooth assembly and load transfer.
- Manufacturing Processes: Providing machining and quality control teams with standard diameters and tolerances, reducing trial and error and improving productivity.
- Product Assembly: Ensuring compatibility and interchangeability of parts in global supply chains by adhering to internationally accepted hole sizes.
- Maintenance and Repair: Facilitating easier replacement of fasteners by using standardized clearance hole sizes.
Industries benefiting from ISO 273 include automotive, aerospace, construction, machinery manufacturing, and electronics, where reliable fastening is critical.
Related Standards
To complement ISO 273, the following standards are relevant for a comprehensive understanding of fastener dimensions and performance:
- ISO 4032 - Hexagon nuts, product grades A and B
- ISO 4762 - Hexagon socket head cap screws
- ISO 261 / ISO 262 - Metric screw threads - General pitch and limits of sizes
- ISO 965 - Tolerances for metric screw threads
- ISO 898-1 - Mechanical properties of fasteners made of carbon steel and alloy steel
These standards work in conjunction to cover the entire range of fastener design parameters, from thread dimensions and mechanical properties to nut sizes and torque specifications.
By complying with ISO 273:1979, organizations can assure the design, manufacture, and maintenance of bolted joints meet global standards for safety, performance, and interchangeability, streamlining international trade and engineering collaboration. This makes the standard an invaluable tool in any mechanical engineering or fastener-related project.
