Analysis of the core technology of thread cutting: from process principles to efficient processing optimization

In the field of mechanical processing, thread cutting is a key technology for building precision connection and transmission systems, and its processing quality directly affects the reliability and performance of the equipment. This article focuses on the core of "thread cutting", integrates key contents such as process principles, processing methods, process skills, parameter optimization and application scenarios, and forms a systematic technical guide to help improve processing efficiency and precision.

1. Basic principles and core elements of thread cutting

The essence of thread cutting is to remove materials layer by layer and form a spiral tooth structure through the coordinated movement of the tool and the workpiece. Its core geometric elements include:

● Major diameter (D/d): the maximum diameter of the thread, which determines the connection strength;

● Middle diameter (D2/d2): the key parameter for measuring thread accuracy, affecting the tightness of the fit;

● Pitch (P): the distance between adjacent teeth in the axial direction, which determines the thread lead;

● Tooth angle (α): common 60° (metric thread), 55° (pipe thread), etc., which affects the force distribution.

During processing, the workpiece rotates at a constant speed (spindle speed n), and the tool is fed along the axis according to the pitch (feed speed vₓ = n×P), and the thread is formed by the trajectory envelope of the cutting edge. This process has extremely high requirements for machine tool transmission accuracy, tool positioning and parameter matching.

2. Comparison and application scenarios of mainstream thread cutting methods

Based on processing accuracy, efficiency and material properties, thread cutting is mainly divided into four major process systems:

(1) Turning thread: the "classic choice" for high-precision processing

● Process characteristics: Installing special thread turning tools on CNC lathes or ordinary lathes, suitable for small and medium diameter (M10-M100) and multi-material (steel, aluminum, alloy) processing, with an accuracy of IT6-IT7 and a surface roughness of Ra1.6-3.2μm.

● Advantages: High flexibility, can process single-start/multi-start threads and tapered threads; Disadvantages: Low processing efficiency for large pitches, requiring layered cutting (single cutting depth ≤0.5mm).

● Typical applications: automotive engine cylinder thread, reducer transmission shaft external thread.

(2) Milling thread: efficient processing of large diameter and non-standard threads

● Process features: Utilize the spiral interpolation function of the machining center or milling machine, and use a multi-edge thread milling cutter (integral/indexable), suitable for large diameter (＞M100), difficult-to-process materials (titanium alloy, high-temperature alloy) and special tooth shape (serrated, rectangular).

● Advantages: Single-edge cutting load is small, tool life is increased by 30%, and radius compensation is supported to achieve multi-specification processing; efficiency is 2-4 times higher than turning.

● Key parameters: cutting speed v=80-200m/min, feed per tooth fₜ=0.05-0.15mm/tooth

(3) Tapping and threading: a convenient solution for small diameter threads

● Tapping (internal thread): Use a tap to process in a prefabricated bottom hole (D = nominal diameter - 1.0825P), divided into machine use (mass production) and hand use (maintenance scenarios), suitable for M2-M20 threads, pay attention to bottom hole lubrication (internal coolant tap is recommended, pressure ≥0.5MPa).

● Threading (external thread): Cutting on round rods through dies, suitable for pipe threads (such as G1/2″), adjustable die head can be automatically centered, and efficiency is increased by 5 times.

(4) Grinding threads: the "ultimate solution" for precision machining

● Process characteristics: Grinding with a grinding wheel on a thread grinder, the accuracy can reach IT3-IT4 level, the surface roughness Ra≤0.1μm, specially designed for high-precision scenarios (such as ball screws, thread gauges).

● Core requirements: Constant temperature environment (temperature fluctuation ≤±0.5℃), diamond roller for precise grinding of grinding wheels, suitable for materials with hardness >30HRC.

3. Thread cutting process skills and precision control optimization

(1) Core skills for tool selection and installation

1) Material adaptation strategy

Choose tools according to workpiece material: High-speed steel tools are suitable for medium and low-speed processing of ordinary steel parts, carbide tools are used for high-speed cutting of stainless steel and quenched and tempered steel, and coated tools (such as TiN/TiAlN) improves the wear resistance of difficult-to-machine materials (titanium alloy, high-temperature alloy) and reduces chipping.

Optimization of geometric parameters: The tip angle of the turning tool strictly matches the thread tooth profile (such as 60° metric thread), the multi-edge design of the milling cutter reduces the single-edge load, and the spiral groove direction of the tap (right-hand chip removal, left-hand anti-blocking) improves chip removal.

2) Installation accuracy guarantee

The tool setting instrument calibrates the turning tool angle to ensure that the bisector of the tip angle is perpendicular to the workpiece axis to avoid tooth skew;

Use thermal expansion or hydraulic tool holders to clamp the tool to control the tool holder runout ≤0.005mm to reduce the accuracy deviation caused by vibration.

(2) Practical skills for adjusting process parameters

1) Speed ​​and feed matching

The cutting speed is graded according to the hardness of the material: medium speed for steel, high speed for aluminum alloy, and low speed for titanium alloy to avoid sticking or overheating;

The feed rate is strictly equal to the pitch. The CNC system synchronizes the spindle and feed axis through the electronic gearbox. Multi-start threads are based on the lead (number of heads ×

2) Layered cutting strategy

Rough machining uses decreasing cutting depth (slightly larger for the first cut, and decreasing for subsequent cuts) to quickly remove material and reduce single-edge load;

Fine machining leaves a 0.1-0.2mm margin, and a small cutting depth (≤0.1mm) with low feed to improve surface accuracy and mid-diameter dimensional stability.

3) Machine tool rigidity optimization

Large diameter threads reduce spindle speed and enable rigid tapping mode;

Regularly calibrate the machine tool lead screw pitch error and compensate in sections through the CNC system to ensure positioning accuracy.

(3) Key techniques for precision control

1) Tool and first piece inspection

After the first piece is machined, use a thread ring gauge/plug gauge to inspect the mid-diameter and use tool radius compensation (±0.01mm) to ensure positioning accuracy. =Level adjustment) to correct the size;

Set axial soft limit for blind hole thread to avoid tool collision with the bottom of the workpiece and cause scrapping.

2) Response to thermal deformation

When processing heat-sensitive materials such as aluminum alloy, use cold air cooling or reserve thermal expansion compensation;

Monitor the workpiece temperature during continuous processing, stop the machine for cooling when overheating, and prevent the center diameter from exceeding the tolerance due to thermal expansion.

3) Online detection and feedback

Equipped with a probe to measure the center diameter online and adjust the cutting parameters in real time;

During batch production, regular sampling is carried out, and the tooth profile half angle and pitch error are detected with a three-dimensional coordinate measuring machine to ensure consistency.

(4) Processing strategy optimization techniques

1) Path and tool planning

Milling threads use spiral cutting to reduce the empty cutting distance and reduce tool impact;

Turning multi-start threads uses spindle directional tooth division to accurately control the head division and avoid lead error.

2) Multi-edge and compound processing

Multi-edge milling cutters cut synchronously, which is more than 2 times more efficient than single-edge, and the cutting force is evenly distributed;

Complex threads (such as tapered threads) are processed by turning and milling, and multiple processes are completed in one clamping to reduce positioning errors.

3) Vibration suppression method

When cutting deep, use the machine damper or vibration-damping toolholder to control the amplitude within 5μm and improve the surface roughness;

For deep hole thread processing, use pecking feed, and retract the tool after each feeding to remove chips, so as to prevent chip blockage and tool breakage.

(5) Cooling, lubrication and tool maintenance

1) Medium selection and application

Emulsion (cooling) for steel parts, synthetic liquid (anti-sticking) for aluminum alloys, and extreme pressure oil (lubrication) for difficult-to-process materials to reduce cutting temperature by 20%-30%;

Internally cooled tools directly spray cutting fluid into the cutting area, which is especially suitable for chip removal and cooling of deep hole tapping.

2) Tool life management

Establish a tool replacement cycle (such as 2 hours for high-speed steel and 5 hours for cemented carbide), and force replacement when reaching the threshold;

Regularly check the edge wear (change the tool when it is >0.05mm or chipping >0.1mm) to avoid excessive wear affecting accuracy.

Summary:

Optimizing thread cutting requires starting from five dimensions: tools, parameters, precision, strategies, and lubrication: select the right tool to match the material, adjust the parameters to balance efficiency and precision, strictly control tool setting and thermal deformation to ensure size, plan the path to improve processing stability, and properly lubricate to extend tool life. Through the application of systematic techniques, the center diameter tolerance can be effectively controlled within ±0.02mm, and the surface roughness Ra≤1.6μm, meeting the high-precision requirements of most industrial scenarios.

4. Analysis of typical application areas of thread cutting

As an important connection and transmission element in mechanical structures, threads have extensive and critical applications in many industrial fields. The thread cutting process has become the basis for the realization of these applications due to its high requirements for precision and strength. The following briefly analyzes its main application scenarios from four core areas:

(1) Mechanical fastening connection: achieving efficient and detachable assembly

The most common use of threads is the fastening connection of mechanical parts, covering a variety of forms:

● Bolt connection is suitable for through-hole structures such as housings and brackets, which is easy to install and maintain;

● Stud connection is mostly used for heavy parts such as engines that need to be frequently disassembled and assembled;

● Screw connection is suitable for occasions where a nut cannot be installed on one end, such as internal fixation of small electromechanical equipment.

These connection methods have strict requirements on the dimensional consistency, processing accuracy and reliability of the threads.

(2) Transmission system: an actuator for precise motion

Threads are also widely used in power and displacement transmission. Common forms include:

● Trapezoidal threads are used in heavy-duty scenarios such as machine tool lead screws and have good guiding and anti-wear capabilities;

● Sawtooth threads are suitable for one-way force-bearing devices such as jacks and have strong bearing capacity;

● Rectangular threads are used in precision equipment with high transmission efficiency requirements but light loads.

These occasions place high demands on thread lead error and contact accuracy.

(3) Pipe system connection: ensuring sealing and reliability

In water, gas, oil and other fluid delivery systems, threads are often used to achieve detachable sealing connections.

● Pipe threads (such as G threads) are widely used in household and industrial pipes, and can achieve high-pressure sealing when combined with sealing materials;

● Special industries such as petrochemical and aerospace require higher strength and corrosion-resistant tapered pipe thread designs.

Thread accuracy directly affects the sealing effect and is the key to the safety of the pipeline system.

(4) Precision instruments and electronic equipment: miniaturized and highly integrated support

Threads are also important in fine-tuning, positioning and small assembly:

● Fine-pitch threads are often used in the fine-tuning structure of microscopes and optical platforms, and achieve precision control by self-locking;

● Electronic devices such as mobile phones and laptops widely use micro screws such as M1.2-M2 to fix internal parts, and have extremely high requirements for the coaxiality and small-size processing accuracy of the threads.

The thread cutting process runs through large-scale structural assembly to micro-precision equipment, requiring both strength and stability, as well as precision and controllability. It is an indispensable key link in the manufacturing field.

5. Summary

Thread cutting technology is deeply integrated into the four core areas of mechanical fastening, transmission, pipelines, and precision instruments. Its core competitiveness lies in the multi-dimensional adaptation of "precision - efficiency - materials": from ordinary carbon steel to high-temperature alloys, from millimeter-level large threads to micron-level micro threads, through the innovation of process methods (turning/milling/grinding) and tool technology (coating/multi-edge design), it continues to support the reliability and precision requirements of high-end equipment manufacturing.