A Detailed Explanation of Internal Thread Machining Technology: Methods, Applications, and Standards

 

As a core connecting element in mechanical engineering, the design, machining, and standardized application of internal threads are crucial for industrial manufacturing. This article will conduct an analysis from aspects such as the definition, machining methods, application scenarios, and standard dimensions of internal threads to help readers systematically master relevant knowledge.

 

I. Definition and Core Functions of Internal Threads

An internal thread refers to a helical structure formed on the inner surface of a cylindrical or conical hole. It is usually used in conjunction with external thread fasteners such as bolts and screws to achieve the functions of fixing, connecting, and supporting. Its design needs to meet the requirements of high precision and high strength to ensure the stability and reliability of mechanical components.

 

II. Five Major Machining Methods for Internal Threads

1. Tapping

Tapping is the most commonly used method for machining internal threads, which forms threads by cutting inside a pre - drilled hole with a tap.

- Operation Essentials:

  - Calculate the bottom hole diameter according to the material (such as steel, cast iron). For ductile materials, the bottom hole diameter ≈ nominal diameter - pitch; for brittle materials, the hole diameter needs to be appropriately reduced.

  - Keep the tap vertical during tapping and reverse it periodically to break chips to avoid tool jamming. For blind hole machining, additional depth (usually 0.7 times the nominal diameter) needs to be reserved.

  - Lubricant selection: Use machine oil for steel parts and kerosene for cast iron.

 

2. Extrusion Forming

Threads are formed by extruding the hole wall to cause plastic deformation. This method is suitable for machining larger diameters (such as above M6). Its advantages are no chips and high thread strength, but it has relatively high requirements for the ductility of the material.

 

3. Milling

A thread milling cutter is used for machining on a CNC machine tool, which is suitable for mass - producing complex threads (such as tapered threads). Its advantage lies in high flexibility, allowing for the machining of deep holes or threads at special angles.

 

4. Turning

Internal threads are cut with a forming tool on a lathe, which is suitable for single - piece or small - batch production. It can machine large - diameter threads, and the precision is controllable, but it has relatively high requirements for operating skills.

 

5. Grinding

Precision machining is carried out using a grinding machine, which is used for the final finishing of high - precision threads (such as aerospace components). The surface roughness can reach below Ra0.4μm.

 

III. Application Fields and Industry Requirements

Internal threads are widely used in:

- Mechanical Manufacturing: Connecting equipment supports and transmission components.

- Automobile Industry: Fastening engine blocks and chassis.

- Electronics and Electrical Appliances: Assembling the shells of precision instruments.

- Aerospace: Threaded connections of high - strength and lightweight components, which need to meet the reliability requirements under extreme environments.

 

IV. Standardization and Quality Control

The dimensions of internal threads follow the ISO standard, and the key parameters include:

- Nominal Diameter: The basic dimension that determines the thread specification.

- Pitch: The axial distance between adjacent thread crests, which affects the tightness of assembly.

- Pitch Diameter Tolerance: The core index for controlling the matching precision of threads.

During machining, a thread gauge should be used for inspection to ensure that the through - gauge can pass through and the no - go gauge does not exceed the limit. For difficult - to - machine materials such as stainless steel, it is recommended to use titanium nitride - coated taps and appropriately increase the bottom hole diameter (by 0.1 - 0.2mm) to reduce cutting resistance.

 

V. Common Problems and Solutions

- Tap Breakage: Check whether the bottom hole size is too small and whether chip removal is smooth.

- Thread Burrs: Optimize cutting parameters and increase the number of reverse tool retractions.

- Precision Deviation: Calibrate the coaxiality of the machine tool spindle and select high - precision taps.

 

Conclusion

When machining internal threads, it is necessary to comprehensively consider material characteristics, process selection, and standardization requirements. With the development of CNC technology, the efficiency and precision of methods such as tapping and milling continue to improve, providing better solutions for industrial manufacturing. Mastering the above technical points can significantly improve the quality and service life of threaded connectors.