Threaded connections are one of the most common fastening methods in machining and manufacturing. Threads can be classified into internal threads (female threads) and external threads (male threads), which are used together in various industrial applications. This article provides an in-depth analysis of the structure, applications, and selection criteria for internal and external threads.
1.What Are Internal and External Threads?
Internal Thread refers to helical grooves machined inside a hole, such as the inner walls of nuts and pipe fittings.
External Thread refers to helical ridges machined on the outer surface of a component, such as bolts and screw shafts.
Both types achieve mechanical connection through thread engagement and are among the most widely used fastening methods in industrial applications.
2.Four Key Differences Between Internal and External Threads
Processing Location
- Internal Thread: Cut inside a hole (e.g., tapping, turning).
- External Thread: Machined on the outer surface of a cylindrical component (e.g., die threading, CNC lathe cutting).
Functional Role
- Internal Thread: Acts as a “receptor” for fasteners (e.g., threaded holes in machinery).
- External Thread: Serves as an “active connector” (e.g., bolts, pipe joints).
Measurement Methods
- Internal Thread Measurement: Uses thread plug gauges, coordinate measuring machines (CMM).
- External Thread Measurement: Uses thread ring gauges, micrometers.
Common Failure Modes
- Internal Thread: Prone to thread stripping and hole wall cracking.
- External Thread: More susceptible to wear, deformation, or corrosion.
3. Application Scenarios and Industry Cases
Typical Applications of Internal Threads
Mechanical Assembly: Threaded holes in equipment bases
Pipeline Systems: Internal threads in gas/water pipe connections
Automotive Manufacturing: Engine block threaded holes
Core Uses of External Threads
Fastening Connections: Bolts, screws
Power Transmission: Lead screws, guide screws
Sealing Structures: High-pressure pipe fittings with external threads
4. Processing Methods and Key Techniques
Internal Thread Machining Methods
- Hand Tapping: Suitable for small-scale repairs (requires vertical alignment).
- Machine Tapping: CNC machine for high-efficiency processing (recommended for blind hole chip removal).
- Thread Milling: High-precision machining for large-diameter holes.
External Thread Machining Techniques
- Die Threading: Manual tool for quick shaping.
- CNC Turning: Precision thread cutting (Pitch accuracy ±0.01mm).
- Thread Rolling: Enhances surface hardness (efficiency increase of 300%).
5. Selection Guide: How to Choose the Right Thread Type?
Load Direction
- Axial Load: Choose triangular threads (e.g., metric threads).
- Radial Load: Trapezoidal/saw-tooth threads are recommended.
Sealing Requirements
- Gas/liquid sealing: Use tapered pipe threads (e.g., NPT, G-threads).
Disassembly Frequency
- Frequent disassembly: Choose coarse threads (better resistance to stripping).
6. Common FAQs
Q1: How to quickly distinguish between internal and external threads?
A: Observe the thread position—internal threads appear as grooves inside a hole, while external threads appear as raised ridges on a shaft.
Q2: What does the “M” mean in M20 thread notation?
A: “M” represents metric threads, and “20” indicates a nominal diameter of 20mm, with a default coarse pitch.
Q3: How to prevent thread loosening?
A: Use spring washers, thread-locking adhesives (e.g., Loctite 243), or switch to fine threads for increased preload.
Conclusion
Understanding the differences between internal and external threads is crucial for mechanical design, equipment maintenance, and hardware procurement. It is recommended to select thread specifications that meet ISO, DIN, and other relevant standards based on actual working conditions, and consult professional technical personnel when necessary.
