25-08-25
Chapter 1 Introduction
Deep hole machining in the mechanical manufacturing industry has been applied in many fields, such as aerospace, shipbuilding, oil exploration, medical devices, and mold manufacturing. With the advancement of industry technology and the emergence of new materials, deep hole machining in various difficult-to-machine materials has become particularly challenging. Deep hole machining has become an essential and critical process in mechanical processing. Deep hole machining is performed in a closed or enclosed environment, preventing direct observation of the tool’s cutting progress. Furthermore, problems such as poor heat dissipation, chip removal, and poor process system rigidity directly impact part quality. Deep hole machining has always been a challenge in mechanical processing, and research on deep hole machining technology for difficult-to-machine materials has yet to achieve breakthroughs. This chapter focuses on the basics of deep hole machining.
1.1
Introduction to Deep Hole Machining Technology
1.1.1
Definition of Deep Hole
Hole machining can be divided into shallow hole machining, deep hole machining, and intermediate-depth hole machining. Generally, a hole is considered deep when the ratio of hole depth L to hole diameter d is greater than 5 (i.e., L/d > 5), while a hole is considered shallow when L/d < 5. Why is this regulation? Generally, standard twist drills are used to drill holes on solid materials. The helix angle of the twist drill is shown in Figure 1.1. In twist drill construction, the relationship between diameter d, helix angle β, and spiral groove lead P is:
In production practice, to ensure smooth chip removal, the drill depth L when drilling to the bottom (without exiting midway) is typically no more than 3/4 of the spiral groove lead P, i.e., L < 3P/4. Substituting this into equation (1.1) yields:
Under normal conditions, the life of a twist drill increases with increasing helix angle. A larger helix angle increases the rake angle, resulting in smoother cutting, lower torque and axial force, and improved chip removal. However, an excessively large helix angle weakens the cutting edge, worsens heat dissipation, and increases the chip removal distance and resistance. The appropriate twist drill helix angle is generally selected based on the properties of the material being processed. H-type helix angles (10°-15°) are suitable for machining hard materials such as hard plastics and brass, while N-type helix angles (15°-36°) are suitable for machining ordinary materials. Materials such as medium carbon steel and low carbon steel: W-type helix angles (38°-45°) are suitable for machining soft materials such as stainless steel, soft aluminum, and copper. The commonly used twist drill helix angle β is recommended to be between 25° and 30°. From formula (1.2), L/d < 4.08-5.05. The ratio of a twist drill’s drilling depth to hole diameter typically does not exceed 5. Therefore, holes with L/d > 5 are referred to as deep holes in engineering. The ratio of hole depth to diameter determines the rigidity of the hole machining process system and the characteristics of the tool structure. As L/d increases, the rigidity of the process system decreases, and chip removal and cooling and lubrication become more difficult.
With the development of mechanical technology, a large number of deep holes with aspect ratios of L/d > 50 have appeared in mechanical products. In production, deep holes with aspect ratios of L/d > 50 are often referred to as ultra-slender deep holes. As a key process in mechanical machining, ultra-slender deep holes are difficult to machine, and conventional machining processes are difficult to meet. Meeting production needs has become one of the bottlenecks of modern machining. Therefore, ultra-slender deep hole processing technology is an important topic in machining research.
1.1.2 Development history of deep hole processing technology
Deep hole processing technology has undergone a long period of development and has mainly gone through the following stages [1,2];
(1) In the early 20th century, the military departments of Western industrialized countries invented the single-edged drill, which was named gun drill because it was used to process gun holes. It is also called crescent drill or external chip removal deep hole drill.
(2) In 1943, Beisher of Germany developed the Bisnell system (i.e. internal chip removal deep hole drilling system), which was later improved by the International Association for Deep Hole Processing and named BTA drilling method.
(3) In 1963, Sandvik of Sweden invented the ejector drill, which cleverly applied the ejector effect to remove the chips. Using a lower cutting fluid pressure, the chips are pushed and sucked under the effect of pushing and sucking. Discharge, reducing the sealing of the system and improving
(4) In the mid-1970s, the single-tube double oil feed (double feeder, DF) drill developed by Japan Yakin Co., Ltd. combined the advantages of the two. It added an oil pressure head with a jet suction effect and successfully integrated the BTA drilling method with the jet suction drill
(5) In the 1980s, Chinese researchers invented the single-tube internal chip removal jet suction drilling technology (i.e. SIED technology) based on the summary of traditional deep hole processing technology. This technology improves the chip extraction design of DF drill. It improves the chip extraction capacity and optimizes the internal chip removal drill bit. The above five different stages of deep hole processing technology have their own characteristics in terms of drill rod structure, oil supply method, tool manufacturing cost, etc., and the applicable hole diameter range, depth range, processing accuracy and surface quality are different.
1.1.3 Research Directions in Deep-Hole Machining Technology
Deep-hole machining technology is a multidisciplinary, integrated application technology. Currently, both its theory and application are still immature, and further research into its theory and technology is urgently needed. With the rapid development and continuous integration of digital technology, sensing technology, and information technology, deep-hole machining technology will continue to develop towards intelligence, diversification, and environmental friendliness.
1. Research on Deep-Hole Tool Wear and Breakage
In CNC machine tools, machining centers, and automated production lines, deep-hole machining tools must not only be efficient and durable, but also extremely stable and reliable. Advances in science and technology and the increasing number of new, difficult-to-machine materials have placed higher demands on the precision and production efficiency of deep-hole machining. Furthermore, the emergence of deep holes with aspect ratios exceeding 500 or even 1000 requires considerable lengths in the cutting path, placing even higher demands on tool durability. Research on deep-hole tool wear and breakage is a major research topic for improving deep-hole tool durability. Emerging disciplines such as “metal cutting tribology” and “friction physics,” which study friction and wear during metal cutting, have taken shape (31).
2. Research on Cutting Methods
To adapt to deep-hole machining of new engineering materials with an increasing variety and increasing difficulty, traditional cutting methods have evolved into non-traditional cutting methods. Traditional cutting methods are based on mechanics and use single-edged or multi-edged tools. Non-traditional cutting methods utilize chemical, physical (electrical, acoustic, optical, thermal, magnetic) and other methods to process the workpiece material. Non-traditional cutting methods such as thermal cutting, cryogenic cutting, magnetized cutting, and vibration cutting can improve chip morphology, cutting forces, tool durability, and machined surface quality. For example, during cryogenic cutting, thermal wear of the tool has the greatest impact on its durability and machined surface quality. When the workpiece temperature drops to -20°C, the formation of built-up edge can be suppressed, while the friction state in the cutting zone is changed, avoiding the build-up of the machined surface. Surface scaling and furrowing appear, reducing surface microcracks and improving surface quality. Deep-hole electromachining and deep-hole ultrasonic vibration machining offer simple motion, zero deformation, minimal burrs, and low tool costs. They are primarily used for machining difficult-to-machine materials with high hardness and strength, and are particularly suitable for mass production.
3. Research on Deep-Hole Tool Materials
With the rapid development of aerospace, aviation, offshore oil drilling, and high-temperature and high-pressure technologies, special alloys with ultra-high strength, high hardness, high-temperature resistance, and creep resistance have emerged. Cutting these materials is particularly challenging. Despite their excellent physical and mechanical properties, some engineering materials are difficult or impossible to cut, hindering their industrial application. Therefore, the research and development of new tool materials is urgent. Research and development trends include: New ceramic tool materials, such as S-8 silicon hydride ceramic blades, have a hardness of 89-91 HRA and can achieve cutting speeds as high as 10000 rpm. 1828 m/min; new ceramic tool materials with excellent wear resistance and chemical stability; polycrystalline blocks and composite inserts, which offer high cutting speeds, low surface roughness, and excellent precision; and multi-layer wear-resistant coating technology for cutting tools. The multi-coating process involves sequentially applying a bonding layer, a transition layer, and a wear-resistant layer to a carbide substrate. Each layer is extremely thin and repeatedly applied. For example, the Widalon TK15 insert, coated with 13 layers, can be used in almost all cutting conditions. Deep-hole machining technology has undergone significant advancements through the refinement of generations of researchers since its inception. However, inherent defects in deep-hole machining persist, such as difficulty in chip removal, difficulty in delivering cutting fluid, and difficulty in observing. Further minimizing the impact of these defects on workpiece machining and quality is a key research priority.
1.1.4 Key technical issues in ultra-slender precision deep hole machining of difficult-to-machine materials
Ultra-slender deep holes are often characterized by: large aspect ratio, L/d>50; high dimensional accuracy, greater than IT7; low surface roughness, Ra<1.6μm. At the same time, the use of some new materials, such as high-strength and high-hardness difficult-to-machine materials (titanium alloy, pure titanium, oxygen-free copper, high-temperature nickel-based alloy, precipitation-hardened stainless steel, beryllium bronze, etc.), has brought a series of technical difficulties to deep hole machining, thus forming ultra-slender precision deep hole machining technology for difficult-to-machine materials.
Ultra-slender precision deep hole machining technology for difficult-to-machine materials is the main focus of deep hole machining at home and abroad, and its research mainly includes the following aspects.
1. Ultra-slender drilling and boring process for difficult-to-machine materials
The research on ultra-slender drilling and boring of difficult-to-machine materials mainly focuses on the following aspects:
(1) Ultra-slender deep hole drilling and boring methods. There are usually four ultra-slender deep hole drilling methods: gun drilling, BTA Drilling method, ejector drilling method and DF drilling method. Generally, BTA drilling method is selected for large aperture (aperture greater than 20mm), and gun drilling method is selected for small aperture (aperture less than or equal to 20mm). For example, for ultra-slender deep hole drilling of nickel-based high-temperature alloy GH4169 material, the drilling diameter is 36mm, the length is 5440mm, and the aspect ratio is 151, and BTA drilling method is selected6.
(2) Tool material and tool geometric parameters. According to the cutting characteristics of difficult-to-machine materials, the tool material and tool geometric parameters are designed through theoretical optimization. For example, high-temperature nickel-based alloy GH4169 material, titanium alloy material and 4145H When drilling ultra-slender deep holes with drill collar steel, the selected blade material determines whether the entire drilling process can proceed normally. From the perspective of the tool, the chip breaking performance of difficult-to-process materials is very poor. The angle of the tool directly affects the chip breaking performance and the strength of the blade. A reasonable tool geometry angle should be used.
(3) The problem of axial deviation of ultra-slender deep holes. The problem of axial deviation of deep holes is a technical problem that currently exists. When the aspect ratio of the drilling hole is greater than 50, the deviation of the axial line of the deep hole is generally unpredictable and uncontrollable. After the axial line of the hole deflects to a certain extent, it begins to change sharply. At this time, the straightness of the hole is greatly out of tolerance, and the drill bit may even drill out “horizontally” from the middle of the workpiece, causing the workpiece to be scrapped, the drill bit to be damaged, and the economic loss to be large. The control of the axial line deviation of the hole in the process of ultra-slender deep hole drilling mainly involves two key issues: the first is the amount of axial line deviation of the hole in the process of deep hole drilling. The first problem is the measurement problem; the second is the problem of corrective measures after the axis of the processed hole is found to be deviated. The first problem is to use an ultrasonic thickness gauge to continuously detect the wall thickness of the rotating deep hole part online, and to determine whether the deep hole tool is deviated and the amount of deflection by measuring the wall thickness of the part; the second problem can be corrected in two ways, using the additional external force correction method to control the deflection of the axis of the hole, that is, the additional external force correction on the workpiece and the additional external force correction on the tool.
(4) Control of chip morphology. When drilling and boring difficult-to-process materials in ultra-slender deep holes, the cutting path is relatively long and the material properties are excellent, so chip handling is particularly important. According to the chip morphology theory and chip breaking theory of metal cutting, the mechanical chip breaking mechanism and geometric chip breaking mechanism are used to achieve the purpose of chip morphology control and chip breaking, and solve the problems of chip removal, chip blockage and tool failure in the process of ultra-slender deep hole drilling.
2. Ultra-slim, long, and precise deep hole machining methods for difficult-to-machine materials
Ultra-slim, long, and precise deep hole machining for difficult-to-machine materials is mainly characterized by the control of dimensional accuracy and surface quality. By applying high-power honing technology to the ultra-slim, long, and precise deep hole machining process, and combining theoretical modeling and experimental research methods, a new honing head is designed, and the oilstone abrasive grains and honing dosage are rationally selected to achieve the control of dimensional accuracy and surface quality in the ultra-slim, long, and precise deep hole machining of difficult-to-machine materials. Deep hole honing technology has become a precise and efficient machining technology, also known as high-power honing technology. High-power honing technology is extremely effective in the field of precision deep hole machining. High-power honing has high pressure when grinding the workpiece, which is 5 to 7 times that of ordinary honing; large machining allowance, which is 10 to 20 times that of ordinary honing; high grinding efficiency, and is a precision machining process that integrates grinding, finishing, and polishing. The high-power honing process can eliminate the processes of rough boring, floating boring, and fine boring, and is very suitable for rough drilling or Honing is performed directly after rough boring, and pre-honing processes are less demanding. High-force honing technology generally achieves superior processing quality compared to conventional honing processes, with dimensional accuracy reaching IT5 and surface roughness Ra reaching 0.2µm. High-force honing technology effectively addresses the problem of machining ultra-slender, long, and precise deep holes in difficult-to-machine materials. Key issues lie in the design of the honing head, the selection of the honing oilstone, and the determination of the honing process parameters.
3. Deep-hole machining machine tools and structures
Although there are many types of deep-hole machining machine tools, none are universally applicable. To achieve the drilling and finishing of ultra-slender, long, and deep holes in difficult-to-machine materials, new deep-hole drilling and boring machine models and structures have emerged, including horizontal deep-hole drilling and boring machines with large aspect ratios, deep-hole gun drilling machines, deep-hole honing machines, and CNC three-axis deep-hole drilling and boring machines. For example, CNC three-axis deep-hole drilling and boring machines utilize modern CNC and servo technologies, integrating external and internal chip removal systems, as well as a DF system. This machine can drill ultra-slender deep holes on both rotating and non-rotating parts, achieving an L/D ratio of up to 400, a drilling diameter range of 6 to 32 mm, and a maximum drilling depth of 2500 mm. The machining system primarily features a deep-hole machining system with internal chip removal (BTA system), while also incorporating a deep-hole machining system with external chip removal (gun drilling system), creating a dual-system system in one machine. This expands the machine’s processing range, allowing both BTA and gun drilling bits to be selected based on production needs. The chip removal system utilizes a negative pressure chip extraction device (DF system), addressing the challenges of ultra-slender deep-hole drilling. This reduces chip removal pressure and expands the drilling diameter range, reaching a minimum of 6 mm. The structure and control utilize linear guides and three-axis servo control for continuously variable speeds in both main and feed motions. The spindle speed range is 200 to 3000 rpm, and the tool feed rate is 10 to 800 mm/min. Meet the process requirements for ultra-slender deep-hole drilling.
4. Precious Metal Trepanning Technology
Precious metal deep-hole trepanning technology uses a circular cutting method to drill holes in solid materials. This leaves a core after machining, saving material and reducing machine tool power consumption. Key issues to consider in deep-hole trepanning include chip control and tool head selection. Theoretical modeling and experimental methods are typically used to study the machining properties of typical precious metal materials (titanium alloy, oxygen-free copper, and aluminum alloy) to determine the structure, blade material, geometric parameters, and cutting parameters of deep-hole trepanning tools, enabling trepanning of precious metals. For example, for deep-hole trepanning of titanium alloys, the trepanning diameter ranges from 70 to 300 mm, the core shaft diameter ranges from 40 to 270 mm, and the length is 3000 mm. For deep-hole trepanning of oxygen-free copper (TU1), the trepanning diameter ranges from 70 to 200 mm, the core shaft diameter ranges from 40 to 60 mm, and the length is 3000 mm.
