Process combination for the manufacturing of deep holes with small diameters Marko Kirschner 1,a, Markus Heilmann 1,b, Dirk Biermann 1,c

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1 Advanced Materials Research Online: ISSN: , Vol. 97, pp doi:1.428/ 214 Trans Tech Publications, Switzerland Process combination for the manufacturing of deep holes with small diameters Marko Kirschner 1,a, Markus Heilmann 1,b, Dirk Biermann 1,c 1 Institute of Machining Technology (ISF), Technische Universität Dortmund, Baroper Str. 31, Dortmund, Germany a kirschner@isf.de, b heilmann@isf.de, c biermann@isf.de Keywords: Hybrid manufacturing, single-lip deep hole drilling, laser drilling, combination of drilling processes Abstract. The industrial relevance of bore holes with small diameters and high length-to-diameter ratios rises with the growing requirements on parts and the tendency of components toward downsizing. Examples are components for medical and biomedical products or fuel injection in the automotive industry. An adapted process design is necessary for the production of deep holes with very small diameters, especially when the conditions at the beginning of the deep hole drilling process are unfavorable. In these applications, a hybrid process consisting of a laser pre-drilling and a single-lip deep hole drilling can shorten the process chain in machining components with nonplanar surfaces, or can reduce tool wear in machining case-hardened materials. In this research, the combination of laser and single-lip drilling processes was realized and investigated for the very first time. In addition, results for the machining of workpieces with non-planar surfaces are presented. Introduction Due to increasing requirements on components concerning their specific properties, e. g. increasing power density or miniaturization, the dimensions of parts diminish in size. An adaptation of the manufacturing processes is used for the production of these parts as well as the development of new technologies is necessary. This tendency is of particular relevance in the automotive industry, the information technology industry as well as for medical and biomedical products [1-2]. In various applications, the production of bore holes with a high length-to-diameter ratio and small diameters is required. For the production of deep holes with small diameters, different manufacturing processes are used. In this context single-lip deep hole drilling, laser drilling, electro discharge machining, electron beam machining and electro chemical machining were used. Each of the processes mentioned above provides specific properties, especially concerning diameter range, drilling depth, accuracy, repeatability and process time [3-6]. The combination of different processes for bore hole production can enhance the bore hole quality and can shorten the production time. One possibility for a hybrid process in micro machining is the combination of laser predrilling and counter boring by electro discharge drilling [7]. The motivation for the presented approach is based on applications, where the conditions at the beginning of the deep hole drilling process are unfavorable. Due to asymmetrical tool designs and the long length of deep hole drilling tools, in mechanical deep hole drilling a guiding support for the tools is needed. For the realization of this guiding support, usually boring bushings or pilot-holes produced by mechanical drilling are used. Many workpieces from industrial applications have nonplanar surfaces, where the conventional process chain consists of face milling, pilot-hole drilling and deep hole drilling [8]. In this research, the combination of the production processes laser drilling and single-lip deep hole drilling at non-planar surfaces was analyzed for the very first time. The aim of this investigation is the manufacturing of a pilot-hole by a laser for guiding the singlelip drill in the process phase of pre-drilling. In comparison to conventional processes for machining workpieces with non-planar surfaces, with the new approach, a reduction of this process chain is intended. Furthermore, a decrease in tool wear for the machining of case-hardened materials is aspired by laser drilling through a hardened layer. An innovative machine tool concept, that integrates a Nd:YAG laser for laser drilling in a conventional deep hole drilling machine tool was All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-19/9/16,2:31:42)

2 266 WGP Congress 212 developed to realize the objectives of this research. Using this machine tool, the utilization of laser processes and single-lip deep hole drilling in one setup is possible. Experimental set-up A prototypical machine tool was used for the experimental investigations. The basis of this machine tool concept is a conventional deep hole drilling machine tool for single-lip deep hole drilling with small tool diameters in the range of d =.5 6 mm. To realize the hybrid process, a Nd:YAG-laser was integrated in the working area of the machine tool. This was done under consideration of safety regulations to limit the danger of fire or explosions because the deep hole drilling process requires oil as cooling lubricant. Because of that, the working area was divided into two parts by a safety door, so that the laser drilling and the single-lip deep hole drilling process are separated from each other. To realize the laser and mechanical drilling in one fixture, the x-axis of the machine tool was extended into the laser working area. The machine tool and its technical data are presented in Figure 1. TBT ML-2: Prototype Single-Lip Deep Hole Drilling Machine Tool with an Integrated Laser Single-Lip Deep Hole Drilling Bore hole diameter: d =.5 6 mm Spindle power: P s = 4 kw No. of revolutions: n max = 36. 1/min Drilling depth: l tmax = 45 mm Coolant pressure: p KSS = 25 bar Laser Laser power: P L = 8 W Peak power: P P =.5 8 kw Peak energy: E p =.4 8 Joule Pulse duration: τ =.8 1 ms y x w c z Figure 1: Machine tool specifications In these experiments, for the mechanical drilling, single-lip tools with a diameter of d =.5 mm were chosen. The tools consisted of a homogenous, uncoated cemented carbide substrate (ISO HW- K15) and contained an internal cooling lubricant supply channel. The cooling lubrication concept was an internal oil supply using a high oil pressure of p = 2 bar. The process parameters such as cutting speed and feed were varied. The range of the cutting speed was kept between vc = 3,...,5 m/min, and the range of feed amounted between f =.5,...,.7 mm/rev. The low tool diameters necessitate very small values for the feed rate. Because of that, the ratio of cutting edge radius to feed becomes significantly higher than 1. This results in negative effective rake angles and can lead to higher compressive stresses, higher specific cutting forces and higher thermal tool loads. Furthermore, the chip formation is influenced by these contact conditions. The cutting edges of the tools used were sharp. The cutting edge radii of the tools are between r = and show a high reproducibility of the tool grinding process. In all deep hole drilling experiments, a drilling depth of l = 12,5 mm corresponding to a length-to-diameter ratio (l/d-ratio) of l/d = 25 was intended. As workpiece material, the stainless steel with an austenitic structure, so-called AISI 316L, was used. The laser beam process was carried out using different operating principles. On the one hand single-pulse laser drilling,while on the other hand, the laser beam process as percussion drilling were used in the experimental investigations. Additionally, the helical drilling was carried out in the experiments. To simulate different conditions for pre-drilling, the workpieces were fixed on devices with angles to the drilling axis of α =, 15 and 3.

3 d +.1 Advanced Materials Research Vol Results and Discussion Design of the laser process for laser pilot-hole drilling. In the first step of the experiments conducted, the laser drilling process was analyzed with the aim of identification of suitable laser parameters, which allow the manufacturing of bore holes meeting the requirements on a guiding assistance in the deep hole drilling process. The targeted requirements on the pilot-hole dimensions are listed in Figure 2. Requirements on a pilot-hole Target values: Pilot hole diameter: D = 5 51 Pilot hole depth: l p,min =.75 mm Conicity: c < 1% Roundness dev.: t k < 1 > 1.5 x d Figure 2: Definition of the targeted pilot-hole dimensions For the guiding assistance of a single-lip deep hole drilling tool with the diameter d =.5, the pilot-hole must have a diameter with a lower clearance of dmax = 1. This clearance is necessary to avoid the contact of the deep hole drilling tool with the bore hole wall while moving it into the pilot-hole. Furthermore, this pilot-hole must have a low conicity and a low roundness deviation to avoid contact of the cutting edge with the workpiece material while the positioning of the tool into the laser drilled hole. The depth of the pilot-hole must be at least 1.5 times of the tool diameter. This is caused by the axial guide pad recess respectively the distance of the tool tip to the guide pad. The guide pad must be guided in the pilot-hole before the deep hole drilling process starts. Especially in laser drilling, a hardening at the bore hole ground and at the bore hole wall can occur which can lead to high mechanical loads on the deep hole drilling tool. Because of that, the influence of the laser on the peripheral zone should also be low. To fulfill these requirements, a variation of laser parameters was conducted and the resulting bore holes were evaluated concerning the targeted values diameter, roundness deviation, drilling depth, surface roughness and peripheral zone for the three angles. For every laser parameter combination, repetitions were performed to analyze the repeatability of the laser processes. The first criterion used to evaluate the bore hole was the diameter. Therefore scanning electron microscopy (SEM) photographs were taken to measure the entrance diameters of the bore holes produced. At first the influence of the laser drilling strategy was carried out. Therefore the laser drilling by single-pulse, percussion and helical drilling were adopted. The influence of the laser drilling strategy on the bore hole diameter is presented in Figure 3. The results show that the use of percussion laser drilling is unsuitable for the production of laser pilot-holes under the given conditions. The diameter is significantly lower as the targeted value and the bore hole shows a high roundness deviation. This is based on the higher amount of material molten by the laser. A part of this material solidifies at the bore hole wall before it exits the bore hole and recast layers occur. These layers reduce the bore hole diameter and influence the roundness of the bore hole negatively. This effect also occurs in helical laser drilling. In contrast to percussion drilling, the bore hole diameter can be adjusted by an adaption of the diameter of the helix DH. Due to the creation of recast layers, the reproducibility of the diameter is lower than in single-pulse drilling. Another disadvantage is the longer process time. In single-pulse drilling the aimed diameter approximately reached with the laser parameters presented. Furthermore, the bore holes produced show qualitatively a high degree of accuracy. Considering the short production

4 Diameter D 268 WGP Congress 212 times the single-pulse laser drilling was identified as the most convenient laser drilling strategy for the production of pilot-holes. Because of that, this strategy is discussed in detail below. Figure 4 shows the variation of laser parameters for single-pulse laser drilling. Laser: Nd:YAG Material: AISI 316L Peak power: P = 7 W Pulse duration: τ = 1 ms Pulse frequency: f = 1 Hz Number of pulses: n = Varied Focus position: s f = mm Process gas: Compressed air 1 Diameter D Roundness deviation t k Percussion Percussion-/Helical drilling: Pulse frequency: f p = 1 Hz Pulse count: n = 25 Helical D H =.1 mm Helical D H =.2 mm Helical D H =.25 mm Single pulse Roundness deviation t k l t = 4.7 mm l t = 5.2 mm l t = 5. mm l t = 4.5 mm l t = 2.2 mm Figure 3: Influence of the laser drilling strategy on the pilot-hole dimensions In the single-pulse laser drilling experiments conducted, the variables pulse duration and peak power have shown the highest impact on the bore hole quality. Because of that, in the following the influence of these parameters will be discussed. As mentioned before, the main requirement on a pilot-hole for the guiding assistance of a deep hole drilling tool is the diameter and roundness accuracy (Figure 4). The increasing pulse duration shows the tendency of leading to growing bore hole diameters and to decreasing roundness deviations. Due to absorption, reflection and heat transportation effects, the laser influenced area increases and the diameter increases when using longer pulse durations. In the parameter range viewed, the impact of the pulse duration on the roundness deviation is inverse to its effect on the diameter. In the experiments with the lowest pulse duration the highest roundness deviations result. Furthermore, the scanning electron microscopy views of the bore holes produced using the lowest pulse duration show a significant deviation of the bore hole cylindricity. This deviation can lead to a contact of the single-lip deep hole drilling tool with the bore hole wall in the process phase of positioning the deep hole drilling tool in the pilot-hole. This contact can effect tool wear or tool breakages in the area of the cutting edge corner. The occurring deviation can be explained by the Gaussian-shaped distribution of intensity. At low pulse durations, the energy of the laser is only at the center of the laser beam sufficient for material ablation. Due to effects leading to a loss of power, such as reflection, this area is reduced by increasing drilling depth and thus a conical bore hole shape is created. The peak power shows also an impact on the bore hole diameter. With increasing peak power, the diameter also increases. Based on effects such as absorption, heat transportation and reflection with higher peak power the diameter also increases. Regarding the utilization of the process combination, the most adequate laser pilot-hole dimensions with respect to the diameter and roundness deviation were reached using a pulse duration of =.5 ms and a peak power of P = 7 W as well as using a peak duration of = 1 ms and a peak power of P = 8 W.

5 Diameter D Diameter D Advanced Materials Research Vol τ =.25 ms 6 W 7 W 8 W D = = D = = D = = τ = 1 ms τ = 1 ms 6 W 7 W 8 W D = 48 D = 489 D = Roundness deviation t k τ =.5 ms 6 W 7 W 8 W D = 493 D = 55 D = Laser: Nd:YAG Material: AISI 316L Peak power: P = Varied Pulse duration: τ = Varied Pulse frequency: - Number of pulses: n = 1 Focus position: s f = mm Process gas: Compressed air Diameter D Roundness deviation t k Roundness deviation t k Figure 4: Influence of the peak power and pulse duration on the pilot-hole diameter and roundness deviation Beside the dimensions of bore hole diameter and roundness deviation, the laser parameters show an influence on the drilling depth. The cross-sections as well as the measured drilling depth are presented in Figure 5. As shown in the realized pulse duration range, the drilling depth increases degressively. Using a low pulse duration leads to bore holes with a small drilling depths and a significant deviation of the targeted cylindrical shape. High bore hole conicity is inappropriate for the subsequent single-lip deep hole drilling process and could lead to tool damages during the run-in. Moreover it is obvious that the influence of the peak power on the drilling depth is negligible. The analyses on the influence of laser parameters on the diameter and roundness deviation in Figure 4 have shown, that the requirements on the guidance for the single- lip deep hole drilling process are fulfilled setting a pulse duration of =.5 1 ms and a peak power of P = 7 8 W. In the experiments conducted with a pulse duration of =.5 ms a sufficient drilling depth was realized, though the bore holes have a strong conicity. The cross section of the bore hole reduces over the drilling depth because of reverberation effects as well as losses in power due to beam absorption. A cone-shaped bore hole entrance could be avoided by increasing the pulse duration. Thus a higher proportion of energy is penetrating the material and converted into heat. As shown in the SEM-pictures a pulse duration of = 1 ms leads to a moderate bore hole tapering.

6 27 WGP Congress 212 Drilling depth l t 3 mm 2 1,5 1,5 τ =.25 ms Drilling depth l t 3 mm 2 1,5 1,5 τ =.5 ms 6 W 7 W 8 W 6 W 7 W 8 W Drilling depth l t 3 mm 2 1,5 1, W τ = 1 ms 7 W 8 W Laser: Nd:YAG Material: AISI 316L Peak power: P = Varied Pulse duration: τ = Varied Pulse frequency: - Number of pulses: n = 1 Focus position: s f = mm Process gas: Compressed air Figure 5: Influence of the peak power and pulse duration on the drilling depth Beside the requirements on the geometric tolerances of the bore hole, the pilot-hole produced by laser should also have a high surface quality as well as no significant impact on the peripheral zone. A hardening of the peripheral zone can lead to tool breakages when positioning the single-lip drill into the pilot-hole or in the deep hole drilling process. Because of that, a low impact of the laser process on the peripheral zone is needed to avoid tool breakages, especially when using tools with small diameters. Due to that, the influence of the laser process on the peripheral zone and the surface topography was analyzed by microhardness measurements and 3D-confocal whitelight microscopy. In the experiments conducted by single-pulse laser drilling the measurements show no significant influence of the laser drilling process on the peripheral zone. The microhardness values detected in a short distance to the bore hole face alternate with the hardness of the base material. With respect to the surface quality a low arithmetic mean roughness and average surface roughness using single-pulse laser drilling was reached. In summary, laser drilling using single-pulse drilling can be used for the production of bore holes meeting the requirements for the use as guiding assistance in single-lip deep hole drilling processes. Combination of laser pilot-hole drilling and single-lip deep hole drilling. After the identification of suitable process parameters for laser drilling, the combination of laser drilling and single-lip deep hole drilling was conducted. Therefore, pilot-holes were produced using the laser parameter combinations allowing the production of bore holes meeting the listed requirements. After the production of pilot-holes, the single-lip deep hole drilling process was performed using the same fixture. For the deep hole drilling process an adapted strategy is required. In Figure 6 is shown the experimental setup of the process combination of laser drilling and the following single-lip deep hole drilling for the machining of workpieces with beveled surfaces.

7 Advanced Materials Research Vol Creation of a pilot hole by Laser beams Single lip deep hole drilling δ Workpiece δ: Drilling Angle Laser Nozzle Workpiece Single lip deep drilling tool (d =.5 mm) Figure 6: Process combination of laser drilling and single-lip deep hole drilling for the production of deep holes To realize the combination of these thermal and mechanical processes, a customized strategy had to be developed to avoid the tool breakage and process disturbances. For the pre-drilling process an adapted strategy was necessary. To avoid deflection of the asymmetric tool when rotating it, the tool was driven into the pilot-hole at a low spindle speed of n = 3 min-1. The feed motion was stopped at a distance from the base of the pilot-hole of s =.3 mm. At this position the spindle speed was increased up to n = min-1 which corresponds to a cutting speed of vc = 4 m/min and the targeted feed velocity was initiated. In the single-lip deep hole drilling process a drilling depth of lt = 12.5 mm corresponding a length-to-diameter ratio of more than 2 was realized. Low deviations named in Figure 2 do not influence the deep hole drilling process. The experiments have shown that it is sufficient when the single-lip deep hole drilling tool run in the pilot-hole with low spindle speed to a depth, in which the guide pads supports the deep hole drilling tool and can avoid deflections. Furthermore, the tapering does not affect the deep hole drilling process. The single-lip deep hole drilling tool cuts the material with a low width of cut resulting from tapered shape of the pilot-hole. An influence of this effect on tool wear at the cutting edge corner was not seen in the experiments conducted. The bore hole quality reached in the process combining laser and single-lip deep hole drilling is also comparable to the conventional process. This is shown in the example of the surface quality after laser drilling and single-lip deep hole drilling (Figure 7).

8 272 WGP Congress 212 Laser: Nd:YAG Material: AISI 316L Pulse power: P = 8 W Tool: Tool A, d =.5 mm Pulse duration: τ =.5 ms Cutting speed: v c = 4 m/min Pulse count: n = 1 Feed rate: f =.5 mm Focus position: s f = mm Oil pressure: p oil = 2 bar Process gas: Compressed air Drilling depth: l t = 12.5 mm Drilling angle: δ = Drilling length: l f = varied Arithmetic mean roughness Ra Single pulse laser pilot hole drilling Ra Rz Area of pilot hole after single lip deep hole drilling Area of deep hole drilling Average surface roughness Rz Figure 7: Influence of the single-lip drilling process on the surface quality The surface roughness reached by laser pilot drilling amounts to R z = 1.72 for the laser parameters used. After the drilling experiments, a surface roughness of R z = 1.43 was measured in the area where the laser pilot-hole was. The single-lip drill smoothened the surface created by laser drilling. This can also be seen in the SEM-views presented. In the phase of positioning the single-fluted gun drill enters the laser drilled pilot-hole and the cutting edge corner of the tool removes surface irregularities coming from the laser drilling process. Furthermore, the guide pad of the single-lip deep hole drilling tool smoothened the bore hole surface of the pilot-hole at the beginning of the deep hole drilling process. Here, the occurring radial forces are supported by the guide pad to the bore hole wall and effects an improvement of the bore hole surface. The surface roughness reached in the area of the pilot-hole corresponds to the value reached in the deep hole drilling process at higher drilling depths. The combination of laser drilling and single-lip deep hole drilling has no influence on the surface quality of the bore holes produced. For the evaluation of the process combination the tool life of the single-lip drills was analyzed (Figure 8). The tool life reached using the adapted strategy with a cutting speed of v c = 4 m/min and a feed rate of f =.5 is comparable to the conventional single-lip deep drilling process with guiding bush.

9 Advanced Materials Research Vol Laser: Nd:YAG Material: AISI 316L Pulse power: P = 8 W Tool: Tool A, d =.5 mm Pulse duration: τ =.5 ms Cutting speed: v c = 4 m/min Pulse count: n = 1 Feed rate: f =.5 mm Focus position: s f = mm Oil pressure: p oil = 2 bar Process gas: Compressed air Drilling depth: l t = 12.5 mm Drilling angle: δ = Drilling length: l f = varied 2 mm Drilling length l f v c = 3 m/min f =.5 mm Single lip deep drilling with guiding bush v c = 5 m/min f =.3 mm v c = 5 m/min f =.5 mm v c = 5 m/min f =.7 mm Process combination v c = 4 m/min f =.5 mm Figure 8: Comparison of the reached drilling length in the conventional process and the process combination As a function of the cutting data, the obtained drilling length in conventional single-lip deep hole drilling of stainless steels is in a range of l f = mm. The reached drilling length of l f = mm generated by the process combination, presented in this work, is comparable to the conventional process. Hence, there is no negative effect on the mechanical deep hole drilling process results by the laser pilot-hole with respect to tool life. The material properties of the difficult-to-machine stainless steel, the low tool stiffness as well as the limited feed rates generally cause the moderate drilling lengths. Because of the tendency to work hardening and the small adjustable feed rates the material separation takes place in the altered regions. This leads to high tool loads at the cutting edge. After examining the basic feasibility of the process combination for planar surfaces, experiments had been conducted for workpieces with more complex surfaces. In these experiments the process combination was realized for machining workpieces with angles to the drilling axis between α = 3 (Figure 9). When laser drilling workpieces with non-planar surfaces, the laser beam pulse comes into contact with the surface of the workpiece at different times when several contact points are viewed. Because of that, a time shift and a difference in absorbed energy at different points on the surface occur. Nevertheless, a variation of different process parameters led to machining of pilot-holes for all angled surfaces tested, which meet the defined requirements.

10 274 WGP Congress 212 Laser: Nd:YAG Material: AISI 316L Pulse power: P = 8 W Tool: Tool A, d =.5 mm Pulse duration: τ =.5 ms Cutting speed: v c = 4 m/min Pulse count: n = 1 Feed rate: f =.5 mm Focus position: s f = mm Oil pressure: p oil = 2 bar Process gas: Compressed air Drilling depth: l t = 12.5 mm Drilling angle: δ = Drilling length: l f = varied Arithmetic mean roughness Ra δ = Focus position: s f = +.8 mm δ = 15 Focus position: s f = mm δ = 3 Focus position: s f = -.5 mm Ra Rz Avearge roughness Rz Figure 9: Reached surface qualities using the process combination for different workpiece geometries Identically to the machining of planar surfaces, the laser pilot-holes are counter bored during the run-in of the single-lip drill. This is confirmed by the high surface qualities after the utilization of the process combination for machining workpiece with angled surfaces of α = 15 and α = 3, as shown in in Figure 9. In conclusion the process combination of laser drilling and subsequent singlelip deep hole drilling for angled parts is applicable without any restrictions regarding the surface quality. Conclusion In the investigations carried out a combination of laser pre-drilling and single-lip deep hole drilling with a tool diameter of d =.5 mm was realized for the machining of workpieces with planar as well as non-planar surfaces. Main requirements on the laser pre-drilling are the diameter, the roundness deviation, the drilling depth, the conicity, the surface quality as well as a low impact on the peripheral zone of the bore hole. The investigations have shown, that the process combination of pilot-hole drilling by laser and single-lip deep hole drilling with the diameter d =.5 mm is feasible. The bore hole quality produced is comparable to conventional single-lip deep hole drilling processes [9]. Furthermore, the experiments have shown that the tool life is also comparable to conventional processes. Because of that, the strategy described is an economical alternative to the conventional process for machining workpieces with non-planar surfaces. Here, usually three process steps are necessary. These steps are face milling of a plane face at the workpiece, production of the pilot-hole by drilling or orbital milling and the deep hole drilling process. This process chain is reduced significantly by the combination of laser drilling and single-lip deep hole drilling.

11 Advanced Materials Research Vol Future work In further investigations, other materials will be investigated. Here, the combination of laser drilling and single-lip deep hole drilling in machining workpieces with non-planar surfaces will be analyzed using quenched and tempered steels and a titanium alloy. Furthermore, the transfer of the process combination described to the machining of materials with case-hardened surfaces is possible. Here, the laser is used to produce bore holes in the hardened surface layer and the mechanical single-lip drilling will be done in the softer material. With this strategy, the tool wear of the single-lip drill shall be reduced in comparison to conventional deep hole drilling processes. In addition, the possibility of the production of pilot-holes with greater diameters using trepanning and helical drilling will be analyzed and, if possible, the combination of laser drilling and single-lip deep hole drilling of greater diameters will be done. Acknowledgments This work was supported by the German Research Foundation (DFG) under the grant We 1723/78. References [1] Alting, L., Kimura, F., Hansen, H. N., Bissacco, G., Micro Engineering, 23, CIRP Annals Manufacturing Technology, Vol.52, No. 2, pp [2] Weule, H. et. al., 24, International state of the art of micro production engineering, Production Engineering, Research and Development, Vol.11, No. 1, pp [3] Yu, Z. Y., Zhang, Y., Li, J., Luan, F., Guo, D., 29, High aspect ratio micro-hole drilling aided with ultrasonic vibration and planetary movement of electrode by micro-edm, CIRP Annals Manufacturing Technology, Vol.59, No. 1, pp [4] Ali, S., Hinduja, S., Atkinson, J., Pandya, M., 29, Shaped tube electrochemical drilling of good quality holes, CIRP Annals Manufacturing Technology, Vol.59, No. 1, pp [5] Heisel, U., Stortchak, M., Eisseler, R., 23, Determination of cutting parameters in deep hole drilling with single-fluted gun drills of smallest diameters, Production Engineering, Research and Development, Vol.1, No. 1, pp [6] Hung, J. C., Lin, J. K., Yan, B. H., Liu, H. S., Ho, H. P, 26, Using a helical micro-tool in micro-edm combined with ultrasonic vibration for micro-hole machining, Journal of Micromechanics and Microengineering, Vol.16, No. 12, pp [7] Li, L., Diver, C., Atkinson, J., Giedl-Wagner, R., Helml, H.J., 26, Sequential Laser and EDM Micro-drilling for Next Generation Fuel Injection Nozzle Manufacture, CIRP Annals Manufacturing Technology, Vol.55, No. 1, pp [8] Weinert, K., Löbbe, H., 22, Crankshaft manufacturing on machining centres, AMST'2, 6th International Conference, Vol. 437, pp [9] Sakuma, K., Taguchi, K., Katsuki, A., Takeyama, H., 1981, Self-guiding action of deep hole drilling tools, CIRP Annals Manufacturing Technology, Vol.3, No. 1, pp