Table of Contents

In mechanical manufacturing, deep-hole and tiny deep-hole machining have been attracting a lot of attention. The diameter of small holes is generally 0.1 to 3.0 mm, and micro small holes are < 0.1 mm, while deep holes are those with a hole depth-to-hole diameter ratio > 10. Based on the diameter, deep holes can be categorized as extra large, large, normal, small, and tiny deep holes. Usually, tiny deep holes and extra large deep holes are more difficult to machine.

Tiny deep holes are widely used in aerospace, military, hydraulic valves, injector nozzles, and medical devices. However, during the machining process, tool cooling and chip removal problems often lead to tool breakage, and traditional machining methods are unable to help some difficult-to-machine materials. In this paper, we will discuss the current micro deep hole machining methods, analyze their advantages and disadvantages, and look forward to future development, to provide a basis for further research.

Micro deep hole machining methods

 

Micro deep hole machining methods are divided into mechanical machining methods and special machining methods, of which, mechanical machining methods are mainly drilling; special machining methods mainly include EDM, electrolytic machining, ultrasonic machining, laser machining, and electron beam machining.

1. Mechanical processing of tiny deep holes

 

The main machining method for tiny deep holes is drilling, which is known for its fast machining speed and wide range of applicable materials, making it a cost-effective choice. Ordinary drilling is usually applied to holes with large diameters, small depth-to-diameter ratios, low molding accuracy, and low surface quality.

With the development of material science and technology, drilling technology has also advanced. In terms of equipment, high-speed motors, and electric spindles are used, such as those from Fisher (Switzerland) and Forest-line (France) with spindles up to 180,000 rpm. At the same time, air-bearing and magnetic-bearing technologies have improved the rotational accuracy of spindles, such as the precision spindles of Japan’s NSK, which can reach 1 μm.

On the process side, to improve hole accuracy and depth, drill sleeves guide the drill and allow step-by-step drilling. The introduction of ultrasonic high-frequency vibration also improves tool stiffness and chip removal, enhancing machining quality and speed. New materials and coating technologies, such as the MDSS-type carbide drills from Japan’s Jayo Electric, have a minimum diameter of just 30 μm and a maximum machining depth of 60 μm.

Deep hole machining is becoming more and more complex, requiring a balance between chip removal and cooling to achieve optimal cutting conditions. Beijing Aviation Precision Machinery Research Institute optimized the cutting edge and cooling hole structure of the gun drill, and developed an efficient machining method with a hole diameter of 2 to 10 mm and a depth-to-diameter ratio of 20 to 60, which improved the machining efficiency by more than 30%.

In terms of general material machining, drilling technology has significantly improved tool life and machining quality. However, in terms of difficult-to-machine materials, the efficiency is low and costly, and some materials can not even be processed.

2. Special machining technology for tiny deep holes

 

Special machining technology refers to the use of acoustic energy, electrical energy, thermal energy, light energy, chemical energy, and electrochemical energy, such as one or more kinds of energy composite, to realize the processing method of material removal. Special processing micro deep hole methods mainly include electric discharge machining, electrolytic machining, ultrasonic machining, laser processing, and electron beam processing.

2.1 EDM micro deep hole processing

 

EDM principle is based on the tool and the workpiece between the pulse spark discharge phenomenon generates a lot of heat to remove excess metal, its working principle is shown in Fig 1.

Fig 1 EDM micro deep small hole machining diagram

Compared with other processing methods, its processing characteristics are mainly expressed in the following aspects:

(1) Any conductive material can be processed;
(2) Various forms of holes can be machined;
(3) low strength and stiffness requirements for the tool.

However, EDM tiny deep holes also have shortcomings:

First of all, EDM tiny deep holes use thermal energy to remove metal, so a thermo-cast layer will be formed on the machined surface, affecting the service life of the part;

Secondly, the hole surface quality and machining accuracy are poor, and have a certain taper; again, the electrode preparation is not easy, the electrode is prone to deformation and loss, the machining efficiency is low, and the chip removal and heat dissipation are difficult, which is also the main problem affecting its wide application.

The most typical method is EDM perforation molding processing. In the process of processing, due to the small cross-sectional area of the electrode, easy to deform; the processing area is not easy to heat dissipation, chip removal is difficult, often caused by pulling the arc to impede the normal processing; electrode loss is large, and the electrode is too thin and too long easy to cause a short circuit. Therefore, the depth of the processed hole is limited, and the depth-to-diameter ratio is about 20.

Given the shortcomings of EDM perforation, EDM tiny deep hole processing in the electrode has been improved, that is, the use of double-hole tubular electrodes, and chipping electrodes, improves the processing efficiency, reduces the processing taper, reduces the electrode loss problem; the use of the workpiece inverted machining mode, improve the processing depth and processing stability.

Global research has been conducted on specialized EDM machines. The equipment developed by Japan’s Matsushita Seiki can stably process micro-fine holes of 5μm. The super tiny deep hole EDM machine tool developed by Beijing Yitong Electric Processing Research Institute has solved the problem of machining 2000mm tiny deep holes and improved the machining efficiency and machining accuracy.

At present, China’s production of CNC high-speed EDM machine tools for small holes in the process indicators has reached the international advanced level, and the processing of small holes in the depth ratio has been > 1000:1.

2.2 Electrolytic machining of small deep holes

 

Electrolytic machining is a special machining method based on the principle of anode ion dissolution, which has the advantages of fast machining speed, high surface quality, and a wide range of applications. It is widely used in aerospace, automotive, medical devices, and mold industries. However, due to the complex state of the gap during electrolytic machining, factors such as electrochemical, electric, and flow fields interact with each other, resulting in poor machining stability and accuracy. In addition, high and expensive corrosion protection requirements for equipment and proper disposal of electrolysis products limit its application.

Micro deep hole electrolytic machining can be categorized into electrolytic machining and composite electrolytic machining. Specific methods include electrochemical drilling, cathodic electrolytic processing of molded tubes, capillary electrolytic perforation, etc., and most of the electrolytes used are acidic. In-molded tube cathodic electrolytic processing, the cathode surface will be insulated to prevent corrosion. Currently, the main problems faced by electrolytic machining are insulation and stray corrosion of the cathode, as well as the undesirable shape of the machined hole (e.g., large taper).

Fig. 2 Principle diagram of electrochemical machining holes

Global research on electrolytic machining technology is gradually increasing. Prof. Di Zhu of Nanjing University of Aeronautics and Astronautics studied the pressure distribution in the electrolytic machining of tube electrodes and found that the voiding phenomenon in the machined area was mainly due to the changes in the electrolyte channel. He proposed that the use of a translational electrode can remove the cavitation region, thus improving the machining stability. Wang Wei et al. analyzed the flow field of a group-hole tube electrode and found that reducing the number of cathodes or the inner diameter can make the flow field more uniform, while wedge-shaped cathodes can effectively improve the flow field distribution. Fang et al, on the other hand, improved the electric field distribution in the machining gap by adjusting the potential difference of the anode tube electrode to enhance the precision of electrolytic machining and used a pulsating flow field to improve the stability of tiny deep hole machining.

Indian Institute of Technology (IIT) utilized 1% HCl and NaCl mixed electrolyte for the machining of nickel-based high-temperature alloys, studied the variation of hole diameter and depth, and realized the machining of bamboo cooling holes by adjusting the machining voltage and feed rate. In the pulse electrolytic machining test at Warsaw University of Technology, Poland, the pulse interval time helped to discharge the electrolysis products and improved the machining stability, but the efficiency was lower than that of the DC power source. S. Hinduja et al. of the University of Manchester, UK, on the other hand, established a mathematical model to analyze the effects of processing voltage, feed rate, electrolyte pressure, and concentration on the hole diameter and taper, which guides the processing of tube electrodes for bamboo cooling holes.

2.3 Ultrasonic machining of tiny deep holes

 

Ultrasonic machining is the use of ultrasonic vibration tools in a liquid medium with abrasive or dry abrasive material to produce abrasive impact, abrasive polishing, hydraulic impact, and the resulting cavitation to remove the material, or the workpiece along a certain direction of the application of ultrasonic frequency vibration for machining, the machining schematic shown in Fig 3.

Fig 3 Ultrasonic machining of small deep holes schematic diagram

Extensive research on ultrasonic machining technology has been carried out worldwide. For example, the Beijing University of Aeronautics and Astronautics found that rotary ultrasonic machining is significantly better than conventional drilling in terms of material removal rate and surface quality when processing carbon fiber-reinforced plastics, while tool loss is reduced. The Dalian University of Technology used an end chamfering tool to process potassium dihydrogen phosphate crystals with good results. Tianjin University has improved the efficiency and quality of processing small holes in engineering ceramics by driving tools through electromagnetic conversion technology.

Harbin Institute of Technology has processed 13μm micro-holes on silicon wafers and combined ultrasonic and EDM machining to realize efficient machining of small-diameter deep holes in titanium alloys. Nanjing University of Aeronautics and Astronautics research ultrasonic EDM and ultrasonic electrolytic composite machining, significantly improving the machining efficiency and quality. Kansas State University established a rotary ultrasonic machining mathematical model, Indian scholars of ultrasonic machining of titanium alloy test have also achieved good results, Swiss scholars have found that the twist drill tool in the tiny deep hole machining is more efficient.

2.4 Laser processing of small deep holes

 

Laser processing of small deep holes is the use of light energy by focusing the system at the focal point to reach a very high energy density, so that the instantaneous material melting, vaporization, and melting and vaporization of the explosive jet in the workpiece after the formation of small deep holes, processing schematic shown in Fig 4.

Fig 4 Laser processing of small deep holes schematic diagram

Compared with traditional mechanical processing methods, laser processing of small deep holes with fast processing speed, high efficiency, and small heat-affected zone, etc., is suitable for a variety of materials processing, the minimum hole diameter of the processed holes is up to 4 ~ 5 μm, the depth ratio of up to 10 or more. Such as the Demagi DML series laser processing center, the output power of up to 10 ~ 20kW, the surface roughness of the processed parts for Ra1μm, the material removal rate of up to 25m/min, the minimum diameter of the processed hole for 5μm, 20mm deep;

However, in general, the surface roughness of the processed hole is large, the roundness is relatively poor, and it is easy to form a flare mouth, it is necessary to reduce the flare mouth by an additional optical control axis, and the accuracy of the hole is generally lower than the IT8 level, and at the same time the high price of the laser equipment has also limited its application. At present, the global research on laser processing is mainly focused on the following 2 aspects.

(1) shorten the laser pulse width and increase the peak power. Femtosecond laser processing so that the thickness of its recast layer is reduced, and its thickness can generally be controlled at 0.02 ~ 0.05mm, the high energy density deposits produced within an instant will make the absorption of electrons and the way the movement is altered with ultra-high precision, spatial resolution and wide range of non-thermal fusion treatment process. Germany Hanover laser center using 150 fs, high energy density of femtosecond laser in 1mm(see Fig 5).

Fig 5 Femtosecond laser processing of small deep holes Fig

(2) Eliminate micro-cracks, recast layer. Laser composite processing mainly includes jet gas-assisted laser processing, underwater laser processing, water-guided laser processing, chemical-assisted laser processing ultrasound-assisted laser processing, etc., which can significantly improve the processing quality and reduce the micro-cracks, recast layer, and heat-affected zone. Xi’an Jiaotong University Mei Xuesong et al. precisely controlled and removed the laser-processed microcooling hole recasts, and reduced the thickness of recasts to less than 5 μm by optimizing the laser process parameters. The processed small holes without the recast layer are shown in Fig. 6

Fig. 6 Optical microscope view of the hole as a whole and local section

2.5 Electron beam machining of tiny deep holes

 

Electron beam machining is a method of converting the kinetic energy of electrons into thermal energy by using high-speed electrons to impact the workpiece. When a metal is heated in a vacuum, electrons escape from the atoms and form a high-speed beam of electrons. This beam of electrons, focused by a magnetic lens, has an extremely high energy density and instantly raises the surface temperature of the workpiece to several thousand degrees Celsius, causing localized melting and vaporization of the material, resulting in the formation of holes.

This method of drilling is highly efficient, with thousands to tens of thousands of holes per second, and good quality with virtually no flying edges or heat-affected layers. By controlling the speed of the electrons and the strength of the magnetic field, it is possible to achieve precise curved holes inside the workpiece. At present, the minimum diameter of electron beam hole punching can reach 0.003mm, the depth-to-diameter ratio of the hole can be up to 100:1, and the inner slope is about 1°~2°, which is suitable for jet engine cooling holes and wing adsorption screen holes and so on.

Electron beam processing in a vacuum environment, less pollution, and no oxidation of the surface, especially suitable for materials prone to oxidation, especially high-purity semiconductor materials. However, due to the high requirements of the equipment in the vacuum environment, the system cost is high, which limits its wide application. With the advancement of science and technology and the emergence of new materials, the processing of tiny deep holes in the future will rely more and more on composite processing methods, such as ultrasonic vibration drilling, electrolytic-mechanical composite processing, and so on.

Table 1 Analysis of micro deep hole machining methods

 

Micro deep hole machining technology development trend

 

With the development of science and technology, and the emergence of new materials, the future precision machining of tiny deep holes is difficult to realize through a single processing method, the composite processing method is bound to become the development trend. Composite processing mainly has ultrasonic vibration drilling, electrolytic mechanical composite processing, electrolytic EDM composite processing, electrolytic laser composite processing, and ultrasonic electrolytic composite processing.

1. Ultrasonic vibration drilling tiny deep holes

 

Ultrasonic vibration drilling is mainly drilling, ultrasonic vibration as a complementary composite processing. By applying ultrasonic high-frequency vibration on the drill bit, the drill bit and the workpiece produce periodic separation between the cutting force changes periodically, at the same time, the rigidity of the tool has also been improved.

The main modes of drill vibration are axial vibration, torsional vibration, and axial-torsional compound vibration (see Fig 7). In the process, ultrasonic high-frequency vibration drilling from continuous cutting to intermittent cutting improves the chip breakage and chip removal, cooling, and heat dissipation conditions, the formation of the pulse torque greatly reduces the friction factor between the drill bit and the workpiece, chips, reducing the drilling force, improving the service life of the drill bit and the quality and efficiency of the processed surface.

Fig 7 Ultrasonic vibration drilling schematic

Prof. Zhang Deyuan from Beijing University of Aeronautics and Astronautics and Zhao Bo from Henan University of Technology developed an ultrasonic vibration drilling test bench, which can adapt to different shapes and thicknesses of workpieces to realize ultrasonic-assisted drilling processing. The study shows that ultrasonic vibration can improve the crack toughness of SiC particle-reinforced aluminum matrix composites, enhance the surface roughness, and reduce the chipping phenomenon.

A comparative study by VARUN showed that ultrasonic vibratory cutting has lower cutting forces and tool wear compared to conventional cutting, with surface quality down to the nanometer scale.GHLANI et al. successfully drilled deep holes in nickel-based alloys and found that lower spindle speeds can reduce axial forces and improve surface quality. For new materials such as carbon fiber composites and titanium alloys, ultrasonic vibratory drilling can effectively reduce the axial force of the drill bit and lateral cutting force, thus reducing tool wear and improving the accuracy of the hole.

2. Electrolytic mechanical composite processing of tiny deep holes

 

Electrolytic mechanical composite machining mainly uses mechanical action to remove the passivation film produced in the electrolysis process. Therefore, electrolytic machining continues to be a composite machining technology mainly based on electrical machining, with mechanical machining as a supplement. A typical electrolytic grinding machining schematic is shown in Figure 8.

Fig 8 Schematic diagram of electrolytic grinding processing

Nanjing University of Aeronautics and Astronautics and Changzhou Institute of Technology conducted a series of studies on electrolytic grinding, the overall impeller blade profile five-axis linkage CNC spreading electrolytic grinding machining mechanism research, the overall impeller polishing efficiency increased by 12 times, solved the overall impeller blade profile finishing technical problems.

Professor Zhu Yu and others applied electrolytic grinding to finishing, and optimized the process parameters of electrolytic grinding on the processed small holes (see Fig. 9), which can meet the requirements of fuel nozzles. The introduction of mechanical action in the process of electrolytic machining, greatly improving the efficiency of electrolytic machining, has been widely used in large rolls, and large chemical container-shaped cavity polishing.

Fig 9 Optimal process parameters under the processing of the hole

3. Electrolytic EDM composite processing of small deep holes

 

Electrolytic EDM composite machining takes full advantage of the higher molding accuracy of EDM and electrolytic machining of better surface quality characteristics, in the same processing station, using different tools electrodes, first EDM molding process, and then electrolytic machining to remove the EDM generated recast layer, processing schematic shown in Fig 10.

Fig 10 Electrolysis – EDM composite machining diagram

Electrolytic EDM composite machining technology integrates the advantages of EDM and electrolytic machining while making up for their shortcomings. EDM has a higher machining accuracy, suitable for the processing of difficult-to-machine metal materials, but there is a recast layer on the surface of the processing; electrolytic machining has a good surface quality, high processing efficiency, no loss of tools, there is no cutting stress and the advantages of the recast layer, etc., but it is difficult to molding accuracy to a high degree of precision. By combining these two machining processes, high machining accuracy and surface quality can be obtained by utilizing their strengths and avoiding their weaknesses.

South Korea Yousei University ultrasonic electrolytic EDM composite processing deep hole test, single-ended insulated electrodes can increase the depth-to-diameter ratio of the hole Baza et al. first deionized water solution for EDM, and then the liquid will be replaced by an electrolytic solution containing phosphoric acid, electrolytic polishing of the micro-hole: to remove the recast layer, to get the micro-hole without a recast layer, and the processing effect shown in Fig. 11; its processing results.

Fig 11 Electrolysis – EDM polishing processing effect diagram

Japanese scholars use the same electrode in deionized water processing solution for EDM processing, and then replace the electrolytic power supply, electrolytic polishing processing in the same station, the electrode high-speed rotation to drive the micro-fine abrasive particles to remove the surface of the workpiece produced by the passivation layer, to achieve the mirror polishing of the surface of the workpiece (see Fig 12), the surface roughness of up to Ra0.07 μm.

Fig 12 Electrolysis – EDM grinding effect diagram

Harbin Institute of Technology has carried out non-conductive super-hard materials processing and other aspects of the exploration, the use of physical inflation methods and electrode ultrasonic vibration can improve the efficiency of electrolytic EDM processing;

Taiwan scholars use pulse power supply and rotating cathode to improve the precision of electrolytic EDM machining, and applied to glass processing; Zhu Yuejin composite electrolyte in stainless steel, and titanium alloy specimens in the processing of small deep hole test, the results show that the pulse current can effectively remove the recast layer, improve the machining efficiency and machining accuracy.

However, due to discharge and cathode loss and processing mechanism and process research is not deep enough, the technology is still in the experimental research stage.

4. Electrolytic laser composite processing of tiny deep holes

 

In electrolytic jet processing, laser east in the jet electrolyte beam under the guidance of the form of total reflection through the jet to remove the material, while the region of the electrolyte local temperature increases, improving the efficiency of electrolytic jet processing and electrolytic jet processing of the domain.

A laser beam of efficient processing reduces the taper of the processed hole, increasing the depth-to-diameter ratio, and electrolytic jet processing will also be produced by laser processing of recast layer, residual stress and micro-cracks, and other defects, “online” removal, the two composite, not only to maintain the surface quality of the electrolytic jet machining is good, but also take full advantage of the high efficiency of the laser machining features, to make up for the shortcomings of the respective processing. The respective processing of the deficiencies that exist. Processing schematic diagram as shown in Fig 13.

Fig. 13 Schematic diagram of electrolytic jet-laser composite processing

The technical research on laser-assisted jet electrolytic processing conducted by Prof. P.T. Paiak of the University of Scotland, Glasgow, shows that the introduction of laser assistance improves the precision and efficiency of electrolytic processing, while the surface is free of recast layer, residual stress, and microcracks.

Nanjing University of Aeronautics and Astronautics on the electrolytic jet laser-assisted processing of the hole surface quality and processing efficiency of the basic process research shows that the electrolytic jet laser composite processing can effectively remove the laser processing of the recast layer and spots produced by the processing of the surface of the processing of the holes without heat-affected zone as shown in Fig 14.

Fig 14 Electrolytic jet-laser composite processing of holes in the effect of the map

5. Ultrasonic electrolytic composite processing of small deep holes

 

In electrolytic machining, the generation of passivation film often hinders the process. To remove this film, it is usually necessary to use high current density or high electrolyte pressure, but this will increase the cost and reduce the quality and accuracy of machining. The use of linear electrolytes can also lead to stray corrosion, which can affect part quality.

Ultrasonic machining can improve machining accuracy and surface quality but is less efficient. Combining these two methods can effectively remove passivation films. The high-frequency vibration and cavitation of ultrasound not only remove the film but also periodically change the pressure and flow of the electrolyte, helping to renew the electrolyte and discharge the products, thus ensuring the continuity of processing.

In addition, the addition of fine abrasives to the electrolyte can enhance processing speed and surface quality. During composite processing, the cathode is subjected to ultrasonic vibration while being fed to the anode at a certain speed, the passivation layer is ruptured by the ultrasonic impact and abrasives, and the anode surface is reactivated. This alternating state of passivation and activation increases the speed of electrolytic processing. The processing schematic is shown in Fig 15

Fig 15 Schematic of ultrasonic electrolytic composite processing

A. Ruszaj et al. from the Institute of Advanced Manufacturing Technology in Poland studied pulsed-current electrolytic machining and ultrasonic electrolytic machining and found that ultrasonic electrolytic machining resulted in better surface quality, while the best results were obtained with the addition of abrasive powder.

S. Skoczypiec et al. analyzed the electrolyte flow in ultrasonic-assisted electrolytic machining by flow field simulation, and the results showed that ultrasonic vibration can change the pressure and the intensity of air pockets in the machining area, and the appropriate amplitude helps to reduce the polarization of the electrode.

B. Bhattacharyya et al. used low-frequency vibration of 150-200Hz for electrolytic machining and found that low-frequency vibration can effectively control the material removal rate and machining accuracy, and the effect is better than that of high-frequency vibration, which promotes the discharging of machining products.M. S. Hewidy et al. analyzed the effect of low-frequency vibration on electrolytic machining through mathematical models, and pointed out that the amplitude and the machining gap have a significant effect on the service life of the tool. M. S. Hewidy et al.

Research in China has also focused on ultrasonic electrolytic composite machining, which was first applied to polishing, using ultrasonic vibration to remove the passivation film on the surface of the workpiece and improve the quality of the metal surface. Hefei University of Technology, Zhu Yongwei designed a low-frequency vibration pulse electrolytic machining device, research shows that low-frequency vibration can improve the machining accuracy, surface quality, and material removal rate.

The Nanjing University of Aeronautics and Astronautics and Yangzhou University of ultrasonic electrolytic composite processing of the mechanism and technical advantages of the discussion confirmed that the technology can be used for microstructure fine processing. The microfabricated components processed by ultrasonic electrolytic composite microfabrication are shown in Fig 16

Fig 16 Ultrasonic electrolytic composite microfabrication micro components

Nanjing Agricultural University studied the rotary ultrasonic electrolytic composite machining of tiny deep holes and designed a new device that uses the internal spraying of electrolytes. In the study, they simulated and analyzed the flow field and electric field during the machining process, optimized the machining process, and achieved good results. In addition, they compared the effects of rotary ultrasonic electrolytic composite machining and rotary electrolytic machining (see Fig 17).

Fig 17 Comparison of 2 ways of machining tiny deep holes

The study shows that under the same process parameters, rotary ultrasonic electrolytic composite machining can significantly improve the machining quality of tiny deep holes and the process is more stable. The technological advantage is even more obvious after adding ultrasonic high-frequency vibration. In addition, the machining depth of the hole can be further increased by guiding the tool cathode through the guide sleeve.

Table 2 Development trend of micro deep hole machining technology

Conclusion

 

With the progress of science and technology, difficult-to-machine materials and new types of materials are emerging, and the requirements for machining methods are becoming higher and higher. Hole parts are widely used in the mechanical industry, the future precision machining of tiny deep holes will no longer rely on a single method, but the use of composite machining technology to improve machining accuracy and efficiency.

This paper briefly reviews the current situation of tiny deep hole machining, introduces the basic principles of traditional machspecial machining and its elopement dynamics, and proposes to combine vibration or electrolytic machining with other methods to form a composite machining technology to compensate for their respective shortcomings, thus promoting the development of future remanufacturing industry.