CRITERIA FOR THE USE OF SCREWS
The ISO metric system (International Organization for Standardization) for Screws is an international standard that defines the dimensions, tolerances, and mechanical properties of Screws. This system is widely used in mechanical and engineering industries to ensure interchangeability and consistency of threaded components.
ISO metric Screws are identified by a combination of letters and numbers that describe the nominal thread diameter, pitch, and other essential characteristics.
Classification of ISO Metric Screws
ISO metric Screws are primarily classified based on nominal thread diameter and pitch. The nominal diameter represents the external size of the bolt, while the pitch is the distance between two consecutive threads.
Table 1: Classification of ISO Metric Screws
| Nominal Diameter (mm) | Standard Pitch (mm) | Fine Pitch (mm) |
| M1 | 0.25 | - |
| M2 | 0.4 | 0.25 |
| M3 | 0.5 | 0.35 |
| M4 | 0.7 | 0.5 |
| M5 | 0.8 | 0.5 |
| M6 | 1.0 | 0.75 |
| M8 | 1.25 | 1.0 |
| M10 | 1.5 | 1.25 |
| M12 | 1.75 | 1.5 |
| M16 | 2.0 | 1.5 |
Materials and Surface Treatments
ISO metric Screws can be manufactured from various materials, including steel, stainless steel, brass, and plastic. The choice of material depends on the specific application, considering factors such as corrosion resistance, mechanical strength, and cost.
Table 2: Common Materials for ISO Metric Screws
| Material | Main Properties | Typical Applications |
| Steel | High mechanical strength, cost-effective | Metal structures, machinery |
| Stainless Steel | Corrosion resistance, aesthetic appeal | Corrosive environments, food industry |
| Brass | Good corrosion resistance, electrical conductivity | Electrical components, decorative applications |
| Plastic | Lightweight, corrosion-resistant | Light applications, non-structural components |
Surface treatments such as zinc plating, phosphating, and nickel plating can further enhance bolt properties, providing increased corrosion resistance and improved aesthetics.
Tolerances and Precision
Tolerances are a crucial aspect of the ISO metric system, ensuring interchangeability and compatibility with associated components. Tolerances are defined based on the tolerance class, which indicates the precision level required for a given application.
Table 3: Tolerance Classes for ISO Metric Screws
| Tolerance Class | Description | Typical Applications |
| 6g | Standard tolerance for external thread | General applications |
| 4h | Fine tolerance for external threads | Precision applications |
| 6H | Standard tolerance for internal threads | General applications |
| 4H | Fine tolerance for internal threads | Precision applications |
Design Considerations
When designing components that use ISO metric Screws, it is essential to consider the following factors:
- Load Capacity: The selection of the bolt must account for the static and dynamic loads that the threaded joint will endure.
- Operating Environment: The environmental conditions (e.g., corrosive, high-temperature) influence the choice of material and surface treatment.
- Compatibility: Screws must be compatible with existing components in terms of dimensions, threading, and tolerance class.
By carefully considering these factors, engineers and manufacturers can ensure the reliability, durability, and efficiency of their mechanical assemblies.