Basic knowledge of fasteners

Definition of thread: A thread is a shape with a uniform spiral protrusion on the outer or inner surface of a solid.

Why use thread?

Tightening and connection: suitable for most screw products at this stage.

Transmission function (displacement function): such as a micrometer for QC to check the size.

Sealing effect: such as the connection sealing of the pipeline.

The development of thread

Machine Screw→Self Tapping Screw→Self Drilling Tapping Screw

Machine thread: When assembling, drill and tap on the assembly first. The internal teeth of the tapping are the same as the external teeth of the screw. Use a small torque for assembly.

Self-tapping thread: When assembling, first drill holes in the assembly without tapping the internal teeth, and use a larger torque for assembly.

Self-drilling thread: used directly on the assembly, the screw is drilled and tapped at once.

Screw processing method

Turning : By removing the material to make the material reach the desired shape.

Advantages: high processing accuracy, no mold restrictions.

Insufficient: high production cost and slow processing speed.

Forging: Forging materials by external force to deform them to achieve the desired shape.

Advantages: fast production speed, low cost, suitable for mass production.

Insufficient: The forming is limited by the mold, and the mold cost of complex products is high.

Cold heading process

Cold heading: It is a process of squeezing and deforming a metal wire rod by external force without heating the metal wire.

The cold heading process is only one type of forging process.

Commonly used raw materials for manufacturing screws

Metal and non-metal.

Metal: iron Fe, copper Cu, aluminum AI.

Iron:Carbon Steel, Alloy Steel, Stainless Steel.

Form of raw materials (wire and bar).

Raw material processing technology.

Current sources of metal materials: Japan/South Korea, Taiwan, Mainland China.

Introduction to common processes

1) Brief introduction of screw heat treatment process

Purpose: To make the screw get higher hardness and strength after cold heading.

Function: To realize self-tapping and locking of metal. Improve the mechanical properties of metal parts, such as torsion, tensile and wear resistance.

Classification: carburizing heat treatment (adding carbon elements to metal parts to improve their surface hardness, for metal materials with low carbon content); quenching and tempering heat treatment (not adding elements to metals, changing the internal structure of the metal with temperature changes To get better mechanical properties).

2) Introduction of electroplating process

Principle: Through the principle of electrochemistry, another metal is generated on the surface of the screw to sacrifice the new metal to slow the corrosion of the screw body.

Environmental protection plating (trivalent chromium)

Generally, it means that after plating another metal, the screw undergoes a passivation reaction to form a film on the surface, thereby improving the corrosion resistance.

In general, only the passivation film on the surface of the screw involves environmental management substances (four heavy metals).

3) Dispensing process

Purpose: Anti-loosening

When the internal and external threads are used together, the gap between the thread contacts is often dispensed due to their respective tolerances, in order to fill these gaps to reduce loosening of the fit.

Inspection method: Exit torque control.