Can deep hole machining technology break through existing technical limitations?

According to statistics, at least half of the 29 manufacturing industries in China need deep-hole machining technology and equipment, and more than one-third of the industries have urgent needs for it. However, the lack of advanced deep-hole machining technology and cost-effective deep-hole machining equipment has become one of the bottlenecks restricting the rapid development of China’s equipment manufacturing industry.

In mechanical manufacturing, hole machining is usually divided into two categories: shallow and deep hole machining. Shallow hole machining refers to the hole depth and hole diameter ratio (length-to-diameter ratio L/D) being less than 5 holes, while deep hole machining refers to the L/D being greater than 50 holes. We usually call holes with L/D over 100 ultra-deep holes. As an important branch in machining, deep-hole machining has become the focus of much research. Scholars at home and abroad have also done a lot of research on the dynamic theory of deep-hole machining and explored how to improve machining efficiency and accuracy.

In this paper, the current development of deep hole machining technology will be outlined from many aspects, such as the mechanical behavior of drill pipe, deep hole machining process, and deep hole drilling monitoring. At the same time, the article will also introduce the progress of deep hole machining machine tools, analyze the technical difficulties that exist in them, and put forward an outlook on the future development trend.

Development status of deep hole machining technology

Deep hole machining technology originated in the military industry and was initially used for the processing of gun barrels and gun tubes. With the continuous progress of technology, deep hole machining technology continues to innovate, and a variety of new technologies have appeared, such as gun drilling, BTA drilling, spray suction drilling, and DF drilling systems.

1. Main systems sole machining technology

Gun drilling: It is mainly used for the processing of small diameter holes, suitable for Φ2~Φ30mm holes. It is the most common deep hole drilling method, widely used in small and medium-sized mass production.

BTA drilling: It is mainly used for deep hole machining with a diameter of more than 12mm, suitable for mass production, with the advantages of high load and continuous machining.

Jet suction drilling: It is a highly efficient method of deep hole machining, but is suitable for holes with a diameter of not less than 18mm.

DF drilling system: combining the advantages of BTA drilling and jet suction drilling, it is capable of machining larger hole diameters (as small as Φ6mm), which improves machining accuracy and efficiency.

Table 1 Comparison table of 4 main deep hole machining technologies

2. Research Direction of Deep Hole Machining

The development of deep-hole machining technology has been studied by a large number of scholars in the past decades. The main research directions include the mechanical behavior of drill pipe, deep hole machining process, and monitoring of the drilling process.

(1) Mechanical behavior of drill pipe

The force on the drill pipe in deep hole drilling is an important factor affecting the processing quality. Research shows that the mechanical behavior of drill pipe is closely related to the machining accuracy. Domestic scholars Bai Wanmin et al. proposed a mechanical model for deep hole drilling by studying the force on the drill lance and proposed a combination of measurement and calculation to analyze the cutting force.

In recent years, the dynamic behavior of the drill pipe has also gradually become a research hotspot, especially the vortex problem during the rotation of the drill pipe. Studies have shown that the vortexing of the drill pipe is often the result of self-excited vibration, which can affect the drilling accuracy and stability.

For example, Hu Zhanqi et al. analyzed the vortex phenomenon of rotating drill pipe in deep hole machining, studied the effect of cutting fluid on the vortex of the drill pipe, put forward the relevant mechanical mechanism, and pointed out that the main reason for instability is self-excited vibration.

(2) Deep hole machining process research

Process innovation in deep-hole machining is crucial for improving machining efficiency and quality. Wang Jun of North Central University invented the single-tube internal chip evacuation spray suction drill (SIED) technology in the 1980s, which solved the problem that the internal chip evacuation system could not process small-diameter holes and improved the cutting performance of the drill.

The multi-tooth folding edge deep hole drill developed by Xi’an Petroleum University has been structurally improved to improve the cutting force and centering to ensure the stability of the drilling process.

In addition, deep-hole machining technology has also successfully solved the problem of chip breakage in deep holes with the help of vibratory cutting technology. The vibratory deep hole machining equipment developed by Prof. Wanfu Xue has helped to improve the machining efficiency and reduce the risk of chip breakage.

(3) Deep hole drilling monitoring technology

Due to the large aspect ratio of the deep hole mac deep-hole machining process is usually in a closed or semi-closed state, which makes it difficult to directly observe the state of the tool to monitor the deep-hole drilling process in real-time, carried out a large number of studies.

For example, researchers from Xi’an University of Technology have successfully monitored tool wear by analyzing vibration signals through orthogonal wavelet transform; Beijing University of Technology has used fuzzy pattern recognition technology combined with acoustic emission signals to realize online identification of tool wear.

Some advanced equipment, equipment the automatic drilling machine of Japan’s Machida Iron Works, is equipped with a torque detection system, which protects the drill by automatically stopping the feeding system once the drill is subjected to excessive torque.

3. Development of international deep-hole machining technology

Deep-hole machining technology has made remarkable progress internationally. ATI Stellram of the United States focuses on the development of tools for difficult-to-machine materials, and its tool materials are in the leading position in the international market.

Sandvik of Sweden has serialized BTA deep hole drills, introduced drills suitable for different machining needs, and also has an advanced level in vibration-damping tools.

In Germany, BOTEK has standardized the specifications of gun drills to Φ0.9~Φ50mm, providing a wider range of choices for global deep-hole machining.

Fig. 1 Schematic diagram of deep hole machining system

4. Current Challenges and Future Trends

Although deep-hole machining technology has made significant progress, there are still challenges in microfine and ultra-long deep-hole machining, especially for drilling ultra-long microfine holes with diameters of less than 1mm and length-to-diameter (L/D) ratios of more than 200, for which a mature machining system has yet to be formed.

Future deep-hole technology will continue to develop in the direction of high efficiency and precision. With the progress of intelligent and automation technology, the monitoring and regulation of the deep hole machining process will become more accurate. New tool materials, optimized chip removal technology, and more advanced machining equipment will make deep-hole Mac technology play a more important role in various industries.

Development status of deep hole machining machine tools

Deep-hole machining is an integral part of precision manufacturing, however, the high price and maintenance costs of deep-hole machining machine tools have deterred many companies. Most companies do not have a dedicated deep-hole machining equipment line, often through the transformation of ordinary machine tools to meet basic needs.

For example, Zhang Yong and others from Soochow University have converted ordinary lathes into deep-hole machining machines with their electrical control system through PLC and AC frequency conversion technology, which effectively reduces costs and improves machining efficiency and precision.

However, it is still difficult for the modified ordinary machine tool to fully meet the high precision requirements of deep-hole machining. To obtain higher precision, specialized hole macdeep-holechine tools are gradually coming to the market. These machines include horizontal deep-hole drilling and boring machines, vertical deep-hole drilling and boring machines, deep-hole gun drilling, and deep-hole honing machines.

1. Common types of deep hole drilling machine

A deep hole drilling machine is the most widely used deep hole machining equipment. There are three main types:

Common single-coordinate deep hole drilling machine: this kind of machine is the most basic, usually used for simple deep hole drilling.

Specialized deep-hole drilling machines: These machines are designed for specific needs and offer higher machining accuracy.

Multi-coordinate CNC is deep-hole drilling machines: these machines are more advanced and capable of accomplishing complex deep-hole machining tasks with a high level of automation.

Table 2 Types of common deep hole drilling machines and their characteristics

For example, in 1989, Shandong Delong Machine Tool Company designed and manufactured Z2102 single-coordinate deep hole drilling machine, which can process holes with diameters ranging from Φ3 to Φ20 mm and a maximum depth of 1,000 mm, and it is China’s first single-coordinate deep hole drilling machine with gun drilling process, and in 2002, Shanghai Machine Tool Research Institute developed a special type of deep hole drilling machine with a small diameter, which can successfully satisfy the needs of the domestic industry for deep hole processing of a small diameter. The demand of the domestic industry for small-hole diameter deep-hole machining has been successfully met.

2. Development of international deep-hole machining machine tools

The development of foreign deep hole machining machine tools far exceeds that of the domestic market, especially in the advanced manufacturing countries, there are not only three-coordinate deep hole drilling machines, but also four-axis, five-axis, and even six-axis fully automatic CNC gun drilling machining centers. Such equipment is usually equipped with an automatic tool-changing system, which can complete more complex drilling, milling, tapping, and other process operations. Moreover, some deep hole drilling machines also adopt the internal chip drilling (BTA method) drilling function, which further extends their application scope.

Although there is similar equipment in China, there is still a certain gap with the international level in terms of precision, stability, and variety of specifications. For example, Germany’s five-axis CNC deep hole drilling and milling composite machining center, its design accuracy, and machining efficiency are in the world in a leading position.

3. Reconfigurable deep hole machining machine tool research

To improve the flexibility and adaptability of deep-hole machining machine tools, scholars have proposed the concept of reconfigurable machine tools. The University of Michigan in the United States began research on reconfigurable manufacturing systems in 1996, while the University of Stuttgart in Germany has made significant contributions to the modularity of machine tools, standardization of interfaces, and integration of functional modules.

In China, Prof. Wu Fujia and his team at North Central University are also conducting research on reconfigurable deep-hole machine tools. They have verified the feasibility of reconfigurable deep-hole machine tools through kinematic model analysis and the division of machine structure and functional modules. Such a machine can not only adjust its functions according to different needs but also greatly reduce the redundancy of machine functions and conversion costs.

Fig. 2 Reconfigurable deep hole machining machine tool

4. Design and Optimization of Deep Hole Machining Machines

The design and improvement of machine tools are critical to the accuracy and efficiency of deep-hole machining. In recent years, dynamic and static optimization of key components of machine tools, especially lightweight design, has become an important direction to improve machining performance. For example, Wu et al. conducted a modal analysis for the M2120A model internal grinder and proposed measures to improve the bed structure, which enhanced the dynamic performance of the machine tool. Similarly, Li Bin identified the weak points and optimized the sliding saddle of a horizontal machining center through dynamic analysis.

5. Development trend of deep hole machining technology

Although the BTA (internal chip drilling) method has not changed fundamentally since its introduction, progress has still been made in optimization. For example, the development of cutting tools, the optimized arrangement of guide blocks, the application of negative pressure chip removal devices, and other, azures have all played an important role in improving the precision and efficiency of deep holes Meanwhile, quasi-dry and dry deep hole machining technologies based on the green machining concept have also received more and more attention.

Modern deep-hole machining machine tools have been developing rapidly in the direction of high efficiency, precision, intelligence, and modularization. In particular, modular reconfigurable machine tools have made significant progress in research in recent years. However, there are still some technical challenges in the field of deep hole machining, especially in the machining of difficult hard-to-machine materials and special complex profiles, the traditional technology faces certain limitations. Therefore, special machining technologies such as laser machining, water jetting, EDM, and electrolytic deep-hole machining are opening up new development areas for deep-hole machining.

Fig. 3 Deep hole machining products

Summary

The continuous development of deep-hole machining machine tools and technology is pushing the manufacturing industry toward higher precision and higher efficiency. Despite the gap between domestic and international deep holholesy deep-hole hee flexibility and adaptability of deep-hole technology will be further enhanced with technological advances, especially the emergence of reconfigurable machine tools. In the future, deep hole Mac combined with green machining technology will become an important trend in the development of the industry.