In-Depth Analysis: A Complete Guide to Key Points and Precision Control in Cold Heading Process for Fasteners

Abstract: The cold heading process is a core technology in fastener manufacturing, widely adopted for its high efficiency, material savings, and superior product performance. This article provides a systematic analysis of how to enhance the manufacturing precision and quality of fasteners from five key aspects: process characteristics, factors affecting precision, equipment selection, material requirements, and lubricant selection.

Cold heading is a forging method that forms metal rods into shapes at room temperature using dies. Its main advantages include:

• Precision Forming: Based on the theory of metal plastic deformation, it achieves predetermined shapes and dimensions through pressure applied at room temperature.
• High Efficiency and Automation: Utilizing multi-station cold heading machines, it can continuously complete multiple processes such as cutting, forming, chamfering, and thread rolling.
• Performance Enhancement: Workpieces exhibit good surface quality and high dimensional accuracy. By rationally controlling the deformation amount, the effects of work hardening can be optimized to achieve excellent mechanical strength.
• Economic Benefits: Significantly reduces or replaces machining processes, making it the preferred method for mass production of bolts, nuts, rivets, etc.

Achieving high-precision cold heading requires strict control over the following variables:

• Chemical Composition: Pure metals generally have better plasticity than alloys; impurity elements can easily induce brittleness.
• Metallographic Structure: Grain size, segregation, and inclusions (such as pores, shrinkage cavities) directly affect material plasticity.
• Deformation Temperature: Although forming occurs at room temperature, material plasticity changes nonlinearly with temperature fluctuations.
• Strain Rate: Variations in rate have a dual and comprehensive impact on metal plasticity.
• Mechanical Conditions: A compressive stress state is conducive to exploiting plasticity; die design must optimize stress distribution.
• Deformation Continuity: Intermittent deformation can sometimes improve the forming performance of low-plasticity metals.

High-quality cold heading machines are fundamental to ensuring precision and should possess the following characteristics:

• Structural Strength: Machine body cast from alloy nodular iron with high tensile strength; crankshaft connections utilize high-wear-resistant alloy copper sleeves.
• Power System: Two-stage gear transmission combined with an air-operated clutch brake for efficient and energy-saving power transfer.
• Operational Stability: Guide plate-driven cutter bar ensures linear transmission of cutting force and excellent dynamic balance.
• Intelligent Monitoring: Equipped with fault detection and safety protection devices, capable of automatic shutdown to protect dies.
• Feeding Precision: The feeding box should include a thrust prevention device and support variable frequency stepless speed regulation.

Material quality directly determines the upper limit of finished product performance:

• Conditioning Treatment: Must undergo spheroidization annealing; the microstructure should be spheroidized pearlite, possessing a "hard surface, soft core" characteristic to optimize cut quality.
• Hardness and Plasticity: Cold-drawn stock should have lower hardness to reduce cracking tendency and protect dies.
• Surface Requirements: The lubricant film should appear as a uniform dull color, free from scratches, cracks, rust, or scale.
• Decarburization Control: The total thickness of the radial decarburization layer must not exceed 1% of the diameter.
• Hardening Sensitivity: Raw materials should be evaluated through cold heading tests; lower sensitivity is more favorable for forming.

Lubrication serves not only to reduce friction but also as a crucial guarantee for the forming process:

• Extreme Pressure Lubricity: Prevents die seizure, especially for difficult-to-machine materials like stainless steel.
• Heat Dissipation and Environmental Friendliness: Good heat dissipation prevents local overheating; low-smoke, low-odor formulations improve the working environment.
• Protective Properties: Reduces sludge generation, provides inter-process rust prevention, and significantly extends punch and die life.
• Broad Applicability: Meets the processing needs of products with large deformation amounts, such as hollow rivets and high-strength bolts.

Enhancing fastener precision is a systematic project involving multiple interrelated aspects such as equipment debugging, material screening, and lubrication environment optimization. Strict adherence to process specifications not only ensures product quality but also achieves cost reduction and efficiency improvement.