THE CRITERIA FOR USING SCREWS

The ISO (International Organization for Standardization) metric system for screws is an international standard that defines the dimensions, tolerances, and mechanical properties of screws. This system is widely used in the mechanical and engineering industry to ensure the 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 important characteristics.


Classification of ISO Metric Screws


ISO metric screws are classified primarily based on the nominal thread diameter and pitch. The nominal diameter is the external dimension of the screw, 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, economical Metal structures, machinery
Stainless Steel Corrosion resistance, aesthetics Corrosive environments, food applications
Brass Good corrosion resistance, electrical conductivity Electrical components, decorations
Plastic Lightweight, corrosion resistance Lightweight applications, non-structural components

Surface treatments, such as galvanizing, phosphating, and nickel plating, can further improve the properties of screws, offering greater corrosion resistance and improved aesthetics.


Tolerances and Precision


Tolerances are a crucial aspect of the ISO metric system, as they ensure the interchangeability of screws and compatibility with associated components. Tolerances are defined based on the tolerance class, which indicates the degree of precision required for a given application.


Table 3: Tolerance Classes for ISO Metric Screws

TOLERANCE CLASS DESCRIPTION TYPICAL APPLICATIONS
6g Standard tolerance for external threads 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 aspects:



  1. Working Load: The choice of screw must take into account the static and dynamic load that the threaded joint will have to withstand.
  2. Operating Environment: The environment in which the screw will be used (e.g., corrosive, high temperature) influences the choice of material and surface treatment.
  3. Compatibility: Screws must be compatible with existing components in terms of dimensions, threading, and tolerance class.


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