Don't underestimate a threaded hole! 0.01mm precision behind "twisting" out high-end manufacturing

 

Different models of threaded holes show

Don't underestimate a threaded hole! 0.01mm precision behind "twisting" out high-end manufacturing

In the field of mechanical manufacturing, threaded holes, as a basic and key structure, carry the core functions of connection and fastening. From automobile engine blocks to aerospace precision parts, from smartphone shells to heavy machinery frames, threaded holes are everywhere. It is not only an important means for design engineers to realize functions, but also a core link in manufacturing quality control. This article will deeply analyze the principle, structure, processing technology and application of threaded holes, revealing its important position in modern industry.

1. Definition and structural characteristics of threaded holes

(1) Basic definition

Thread cutting is a means of processing internal and external threads. In fact, internal threads are threaded holes, which cooperate with external threads. The following is a detailed definition:

A threaded hole is a cylindrical hole with internal threads processed on a solid material (such as metal, plastic, composite material). Its function is to cooperate with external threaded fasteners (such as bolts and screws) to achieve mechanical connection. The essence of a threaded hole is to transmit torque and axial force through a spiral groove, thereby firmly combining two or more components.

(2) Key structural parameters

Major diameter (D): The nominal diameter of the threaded hole, which determines the matching size of the fastener.

Minor diameter (d): The diameter of the bottom of the thread groove, which affects the connection strength.

Pitch (P): The axial distance between two adjacent teeth, which determines the displacement per turn.

Thread angle (α): The angle of the thread tooth profile, commonly 60° (metric) or 55° (imperial), which affects the friction and self-locking performance.

Effective thread length: The part of the threaded hole with a complete tooth profile, which is directly related to the reliability of the connection.

(3) Classification and function

1) Classification by depth

Through hole: It runs through the entire workpiece, allowing the bolt to pass completely through, suitable for scenes that require double-sided fixing.

Through hole

Blind hole: It does not penetrate the workpiece, the bottom is flat or conical, and is often used for single-sided fixing or space-constrained structures.

Blind hole

2) Classification by tooth profile

Triangular thread: The most common type, used for fastening connections, with good self-locking properties.

Trapezoidal thread: The thread profile is an isosceles trapezoid, which is used to transmit power (such as a lead screw). It is highly efficient but complex to process.

Rectangular thread: The thread profile is rectangular, with the highest transmission efficiency, but low strength and is less commonly used.

3) Classification by direction of rotation

Right-hand thread: Tighten by clockwise rotation, which is the default type.

Left-hand thread: Tighten by counterclockwise rotation, which is used for special anti-loosening requirements or parts that cooperate with bidirectional motion.

2. Processing technology and technology of threaded holes

(1) CNC processing method

1) Tapping

Principle: Use a tap (cutting tool) to cut an internal thread in a pre-drilled bottom hole.

Steps:

Drilling: Use a drill to machine a bottom hole slightly smaller than the major diameter of the thread.

Chamfering: Machine a 45° chamfer at the hole mouth to facilitate tap positioning.

Tapping: Manually or mechanically drive the tap to rotate and cut the material to form a thread.

Difference between drilling and tapping

Advantages and disadvantages: Low cost and high efficiency, but limited by the hole diameter and material hardness, the tap is prone to breakage.

2) Milling

Principle: Use thread milling cutters to mill threads on CNC machine tools.

Advantages: Suitable for large diameters, deep holes or special tooth profiles (such as trapezoidal threads), with high processing accuracy and long tool life.

Application scenarios: Fields with high precision requirements such as aerospace and mold manufacturing.

A thread milling cutter is machining a metal threaded hole.

3) Turning

Principle: Use forming turning tools or thread combing tools on lathes to process internal threads.

Features: Suitable for single-piece or small-batch production, can process high-precision threads, but the efficiency is low.

(2) Special processing technology

1) Electrospark machining (EDM)

Principle: Use electric sparks to erode materials, suitable for difficult-to-process materials such as cemented carbide and hardened steel.

Advantages: No mechanical contact is required, complex shapes can be processed, but the cost is high.

2) 3D printing

Principle: Directly print threaded structures through additive manufacturing technology.

Limitations: High surface roughness, insufficient precision, and post-processing (such as tapping or grinding) is required.

Application: Rapid prototyping, customized small-batch production.

3) Processing technology optimization

Material adaptation: Carbide taps are used for hard materials (such as stainless steel), and forming taps are used for soft materials (such as aluminum).

Cutting parameters: Reducing cutting speed and increasing feed rate can reduce tool wear and improve efficiency.

Lubrication and cooling: Using special cutting fluid can reduce temperature and extend tool life.

3. Standards and quality control of threaded holes

(1) International standard system

ISO standard: Global standard for metric threads, such as ISO 261 (common threads).

ANSI standard: American national standard, such as ANSI/ASME B1.1 (unified threads).

JIS standard: Japanese industrial standard, such as JIS B0205 (metric threads).

(2) Tolerance and fit

Tolerance grade: 6H and 7H are commonly used for internal threads, and 6g and 8g are commonly used for external threads. The higher the grade, the higher the precision.

Fit type: Clearance fit (such as bolts and nuts), interference fit (such as shrink fit connection).

(3) Quality inspection methods

1) Go and no-go gauge inspection

Go gauge: Check whether the thread can be screwed in smoothly to ensure that the major diameter and pitch are qualified.

No-go gauge: Check whether the thread is too loose to ensure that the minor diameter and thread angle meet the requirements.

2) Three-dimensional coordinate measurement

Principle: The geometric parameters of the thread are obtained by a three-dimensional coordinate measuring machine with an accuracy of up to micron level.

Application: Quality inspection of high-precision parts (such as aircraft engine parts).

3) Optical inspection

Principle: Use laser scanning or microscope imaging to analyze the surface morphology and size of the thread.

Advantage: Non-contact measurement, suitable for complex shapes or tiny threads.

4. Typical applications and cases of threaded holes

(1) Automobile industry

Engine block: Threaded holes are used to install spark plugs, fuel injectors, etc., which are subjected to high temperature and high pressure and require high-precision processing.

Engine block threaded holes

Body structure: Threaded holes connect car doors, seats and other components to ensure driving safety.

(2) Aerospace

Aircraft landing gear: High-strength alloy steel threaded holes are subjected to huge impact loads and require strict inspection. Engine turbine blades: Threaded holes of high-temperature alloy materials need to be processed by electric spark machining.

(3) Electronic devices

Smartphones: Micro threaded holes (such as M1.4) are used to fix the motherboard and the housing, and the precision requirements are extremely high.

Communication base stations: Waterproof threaded holes prevent the signal module from getting wet and need to be coated with anti-corrosion coating.

(4) Industrial machinery

CNC machine tools: Trapezoidal threaded holes are used for screw transmission to achieve high-precision positioning.

Hydraulic systems: High-pressure threaded holes connect pipelines and need to be resistant to vibration and pressure shock.

5. Maintenance and solutions to common problems

(1) Common failure forms

Slippage: Thread wear causes loose connections, mostly due to overload or material fatigue.

Corrosion: Environmental humidity or chemicals cause threads to rust and reduce strength.

Breakage: Overload or processing defects cause cracks at the root of the thread.

(2) Repair and prevention

Slippage repair: Use a thread repair sleeve (such as Heli-Coil) or re-tap.

Anti-corrosion measures: Apply rust inhibitor, use stainless steel materials or galvanize.

Overload prevention: Use a torque wrench to control the tightening torque and avoid overload.

6. Conclusion

As the basic unit of mechanical connection, the importance of threaded holes far exceeds what is seen on the surface. From micron-level electronic components to heavy machinery several meters high, the design and manufacture of threaded holes directly affect the performance, reliability and safety of products.

As the manufacturing industry develops towards high precision and intelligence, the processing technology and detection technology of threaded holes are also constantly innovating. The following will popularize some common misunderstandings in the threaded hole processing process to help you quickly identify and correct them.

Common misunderstandings Q&A

Question 1: The larger the drilling size, the stronger the thread, right?

Answer: No. Too large a bottom hole will lead to insufficient thread forming material, incomplete thread profile, and significantly reduced strength and precision.

Too large a bottom hole will make the thread incomplete, with defects such as broken teeth and missing teeth, which will seriously reduce the tensile strength and service life of the thread.

Too large a bottom hole will also make the thread fit too loose, the pitch unstable, the connection torque and bearing capacity reduced, and the reliability impaired.

Question 2: As long as high-hardness taps are used, coatings and cutting fluids are irrelevant?

Answer: Misconception! Tap coatings and special tapping oils are essential for chip removal, wear reduction, and tool life extension.

Optimizing the tapping oil (cutting fluid) formula can significantly reduce cutting force, reduce chip adhesion, and prevent tap chipping, otherwise it will seriously affect the thread quality.

Coated taps are superior to ordinary taps in wear resistance and anti-sticking performance, and their life can be increased by more than 30%.

Question 3: If the thread plug gauge (go/no-go gauge) test passes, does it mean that the threaded hole is foolproof?

Answer: No. Go/no-go gauges can only verify the maximum and minimum mean diameters, and are powerless against pitch errors and half-angle errors.

When the mean diameter meets the requirements but the pitch or half-angle error is too large, the no-go gauge can still pass, but the actual contact height of the thread is insufficient, and such defects cannot be found by plug gauges alone.

It is necessary to combine the three-needle measurement method or three-coordinate measurement to conduct comprehensive quantitative verification of pitch, half-angle, coaxiality, etc.

Question 4: Can self-tapping screws be screwed in without pre-tapping?

Answer: Misuse of self-tapping screws will reduce connection reliability, and not all materials and hole diameters are suitable for self-tapping.

Self-tapping screws are suitable for soft materials, thin plates and other occasions, but for materials such as carbide and stainless steel, it is still necessary to pre-drill the bottom hole and tap to ensure the integrity of the thread.

Blind use of self-tapping will lead to incomplete thread forming, insufficient locking force, and easy loosening under vibration load.

Question 5: It doesn’t matter if the threaded hole is close to the edge of the part, as long as it does not penetrate?

Answer: If the threaded hole is too close to the edge, it will destroy the integrity of the hole wall, and it is easy to cause edge damage and fatigue cracks.

If the edge of the threaded hole is not left enough, the tool vibration will penetrate the edge during tapping, destroying the strength and appearance of the part.

Allow enough space at the edge of the threaded hole

The minimum margin requirement should be met during design (usually ≥1×thread diameter), and reinforcing ribs or bushings should be added to the edge of the hole to prevent delamination.

Question 6: Deep hole threads (＞3×hole diameter) are tapped as usual without any impact?

Answer: Deep hole tapping (depth exceeding 3D) is very challenging, and ordinary processes are prone to poor chip removal, vibration, and tap breakage.

Experts believe that deep hole threads require special long-residue taps and segmented chip removal technology, otherwise there will be risks of chip blockage and tap breakage.

It is recommended to use ultrasonic vibration tapping or segmented tapping, and cooperate with online chip removal devices to improve the processing reliability of deep hole threads.