Mechanical Drilling Research Articles

Effect of the number of irradiation holes on rock breaking under constant laser energy

The use of mechanical drilling in accessing energy resources stored in deep and hard rock formations is becoming increasingly challenging. Thus, laser irradiation has emerged as a novel drilling method with considerable in this context. This study examines the variation of rock fracture length, fracture tortuosity, hole size, and rock breaking efficiency for a different number of holes and laser power, based on the constant total energy of laser irradiation. As indicated by the results, increasing the laser power increases the laser intensity, which helps increase the hole diameter and depth. Moreover, for the same laser power, increasing the number of irradiated holes reduces the laser energy absorbed by each hole, which is not conducive to increasing the hole depth. As the number of holes increases, the mass loss of the rock also increases, while both specific energy (SE) and modified specific energy (MSE) decrease. When the number of holes remains the same, the mass of the shale removed by low power is less than that removed by high power, while SE and MSE have an inverse relation with power. Therefore, high laser power and multiple-hole irradiation are more conducive to rock breaking. Besides, the fracture length and fracture tortuosity of the rock irradiated by the low laser power will increase first and then decrease with the increase in the number of holes, and reach the peak value when the irradiation takes place through three holes. When a high-power laser irradiates the rock, the fracture length and tortuosity will increase with the increase in the number of irradiation holes. This is because a rock irradiated by low power dissipates more energy, with the result that the energy absorbed by the sample with four irradiation holes is not enough to break the rock quickly. This study is expected to provide some guidance to break rock for drilling deep reservoirs and hard rock formations using laser irradiation.

Mechanical peck drilling of Inconel 625: Parametric analysis and grey fuzzy logic based multi-response optimization

Nickel-based superalloy Inconel 625 is considered a hard-to-cut material due to characteristics such as high hardness, strain hardening behavior, and affinity with tool materials. It is challenging to machine through micro-holes of less than Ø 500 µm with the desired quality on Inconel 625 using the mechanical micro-drilling (MMD) process under dry conditions. The machining parameters must be selected optimally to enhance hole quality and to avoid frequent drill breakages. In this study, through micro-holes are machined on Inconel 625 by the MMD process using solid carbide micro-drills under dry conditions using a peck drilling strategy. Experiments are carried out by varying machining parameters such as spindle speed (8000 rpm, 11000 rpm, and 14000 rpm), feed (5 µm/rev, 7.5 µm/rev, and 10 µm/rev), and drill diameter (Ø 300 µm, Ø 400 µm, and Ø 500 µm) at three levels based on full factorial design. Thrust force and hole quality features such as exit burr height, radial overcut, and taper angle are measured as output responses. The mean effect plots are used to study the influence of machining parameters on output responses. The drill diameter had a significant effect on thrust force and radial overcut. Whereas feed and spindle speed had a major influence on exit burr height and taper angle, respectively. Finally, multi-response optimization is carried out using the grey fuzzy logic method, and an optimal machining parameter setting for multiple responses is ascertained.

Experimental Investigation of Thrust Force in the Drilling of Titanium Alloy Using Different Machining Techniques

Titanium alloy is a kind of hard-to-cut material widely used in aerospace, military and medical fields, and mechanical drilling is the primary technique used for hole-making in titanium alloy materials. The drilling force is an inevitable concomitant phenomenon in the drilling process and thrust force is its most important component. During the drilling of titanium alloy, it is crucial to understand the fundamental characteristics and changing rules of thrust force for optimizing process parameters, improving machining quality and predicting tool failure. In this paper, four different techniques, such as direct drilling (DD), ultrasonic vibration drilling (UVD), peck drilling (PD) and ultrasonic vibration peck drilling (UVPD), were used to drill small holes into Ti-6Al-4V titanium alloy, the thrust force was measured and its mean, maximum and peak-to-valley value were acquired from the time-domain waveform. Then the time-domain and frequency-domain characteristics of thrust force under the four techniques were compared, and the changing rules of thrust force with vibration amplitudes during UVD and UVPD were investigated. The results showed that, when compared to DD, UVD decreased the mean thrust force Famean by about 18.6%, and the force reduction effect was more significant as the amplitude increased. The variable velocity cutting characteristics and the antifriction effect of UVD were the primary reasons for the reduction of Famean. The pecking motion and ultrasonic vibration had a synergistic effect on reducing thrust force; UVPD could simultaneously reduce the mean thrust force Famean and maximum thrust force Famax. When the amplitude A was chosen within the range of 2–3 μm, Famax and Famean were reduced by approximately 37% and 40% in comparison to DD.

Plasma drilling on Martian ice: Enabling efficient deep subsurface access to Mars' polar layered deposits

Mars' polar layered deposits (PLD) contain the best historical record of the Amazonian time period. Deep subsurface access at Mars' poles is challenging due to the restriction of Mars' atmospheric condition. Traditional terrestrial mechanical drilling methods using drilling fluids are not feasible in these conditions and the melting probe approach is very inefficient due to heat loss. For ice in Mars' polar condition, the increase in surface area due to cracks would greatly reduce its thermal conductivity. This will lead to a higher thermal efficiency and an easier access to polar layered deposits during melting or sublimation. The preliminary results with pulsed plasma ice drilling, using localized electric breakdown with electric energy input per pulse up to 80 ​J, were promising under both Mars and Earth conditions. To simulate Mars' polar condition, a vacuum vessel system was designed and implemented with a pulsed discharge circuit, ice plate, controlled liquid nitrogen cooling, and CO 2 gas feed-in. A simplified heat conduction model was established for estimating the heat conduction at various porosities for homogenous water ice and CO 2 gas mixture using COMSOL under two scenarios including constant thickness of mixture layer and constant thermal mass of the mixture layer under the heat source. The simulation results for both scenarios indicated more reduction of heat loss with an increasing porosity. Experimental studies of thermal conductivity measurement of granular ice under the Earth condition inside a freezer using a custom thermal needle probe with known porosity proved the concept that voids/cracks in ice can reduce the thermal conductivity. Based on simulation and experimental results, the classic melting probe model is extended with porous ice-CO 2 mixture to estimate the power consumption on Martian ice at varying descending speed. Plasma drilling with limited power is promising for accessing Mars' PLD and can also be expanded to planetary exploration in searching for life on other planets. • Plasma drilling method has no moving parts. • Pulsed plasma drilling improves the Rate of Penetration of the melting probe. • The cracks/voids in ice decreases its equivalent thermal conductivity. • Additional power requirement is acceptable within limited solar power access for Martian probe.

Delamination measurement in glass fibre reinforced polymer (GFRP) composite based on image differencing

Drilling-induced delamination is the most severe material defect in fibrous polymeric composites. Although many delamination measurement methods are available, the outputs of recent digital image processing (DIP) based delamination measurement methods are significantly affected by the non-drilling induced material defects and anomalies like cavities or dirty textured surfaces. Therefore, the main aim of the present research was to develop a method capable of drilling-induced delamination measurement in fibrous polymeric composites based on the image differentiation principle. In an attempt to implement this principle, images of the undrilled composites were referenced/compared with the images of the drilled composites. The proposed delamination measurement method is tested through mechanical drilling experiments in glass fibre reinforced polymer (GFRP) composites and compared with conventional DIP-supported manual measurements. The experimental results show a good agreement between the delamination detected by the naked eye, the conventional and the proposed method. Although the proposed method slightly increases the drilling operation time, the comparative differentiating makes it a good solution to detect drilling-induced material deformations in textured and semi-specular surfaces. The proposed method can separate drilling-induced delamination from other non-drilling-induced material discontinuities or anomalies. Considering the automated, reproducible and complex nature of the novel method, it may replace current qualification operations in high-end industries.

Experimental research on the thermal method of drilling by melting the well in ice mass with simultaneous controlled expansion of its diameter

During the seasonal work of the 64th Russian Antarctic Expedition in 2018-2019 at the “Vostok” drilling facility named after B.B.Kudryashov (“Vostok” station, Antarctic) specialists of Saint Petersburg Mining University conducted experimental investigations on the process of drilling by melting with simultaneous expansion of wells in the ice mass. A test bench and a full-scale model of a thermohydraulic reamer-drilling tool were developed, manufactured and tested for the research. The first bench tests of the full-scale model proved its efficiency and suitability for experimental drilling with simultaneous expansion of wells in ice mass; its operational capabilities were determined and the drawbacks that will be taken into account in future were found out. The article substantiates the choice of constructive elements for thermohydraulic reamer-drilling tool. It is determined that the technology of full diameter drilling with simultaneous expansion of the well in ice mass can be implemented by combining contact drilling by melting and convective expansion with creation of forced near-bottomhole annular circulation of the heated heat carrier. Dependencies of expansion rate on main technological parameters were determined: active heat power of heating elements in penetrator and circulation system, mechanical drilling rate, pump flow rate. According to the results of investigations, the experimental model of thermohydraulic reamer-drilling tool will be designed and manufactured for testing in conditions of well 5G.

Ultrasound-Guided Catheterization Using a Driller-Tipped Guidewire With Combined Magnetic Navigation and Drilling Motion

The endovascular intervention has been widely used to treat occlusive peripheral vascular disease (PVD). However, the current procedure has concerns such as low successful rate, undesired trauma, and radiation exposure. This article proposes a robotic solution to mitigate these limitations. A miniature magnetic driller-tipped guidewire (MDG) is designed, which performs 1) controllable bending under directional magnetic fields to navigate in complex vasculature and 2) mechanical drilling under rotating magnetic fields to pass through clogged regions. An integrated actuation system is adopted for magnetic control and ultrasound (US) imaging in a large workspace. A control framework composed of the preoperative and intraoperative stages is developed, which internally coordinates all system modules, addresses noisy US imaging, and provides convenient operation. Demonstrations under an optical camera verify the flexible steering ability and effective unclogging motion of the MDG. Furthermore, US image-monitored <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in vitro</i> experiments validate the overall hardware platform and control strategy. The designed MDG realizes dual objectives, including controllability at bifurcations and penetrability at clots. The introduced US imaging modality largely reduces radiation hazards in conventional catheterization. This article studies the intervention of the MDG under electromagnetic field navigation and US imaging guidance. The proposed workflow has the potential to improve operational safety and clinical outcomes of the occlusive PVD treatment.

Hydrogeological studies in the São Torcato Zone -Tabuaço with aim of optimizing the location of a special groundwater abstraction to plan a new Medical Spa in Portugal

The present work presents a case study, to evaluate the possibility that in an area of the territory of the interior of Portugal, there is potential for capturing special groundwater that allows its classification as natural mineral water for thermalism. In this sense, after an initial part, with the framing of the theme and the presentation of the work methodology, the geomorphological and geological elements are shown, giving more development to the hydrogeological aspects, highlighting the results of the monthly sequential hydrological balance, namely its annual surplus, crossing it with the groundwater sampling sites, and still, the existing aquifer systems and the characteristics of their waters are identified. As a main result, the conceptual geohydraulic model of the region is presented, to show that in the São Torcato zone, and particularly in the Qta do Plácido zone, there is the possibility of exploiting groundwater in an aquifer system of granitic rocks, from fissural type, which is supplied from relatively distant areas, in hydrographical basins other than São Torcato basin, provided relatively deep boreholes are to be drilled. Finally, the main conclusions are synthesized, pointing out the most favorable area to carry out groundwater abstraction with the potential to have stability and quality suitable for a medical spa. It should be noted that these elements are only previous studies, which should be considered as initial contributions, and that further work should be carried out in the future, including geophysical studies and mechanical drilling, flow tests, as well as, among others, isotopic studies in the waters of the region to assess the recharge areas and available reserves.

Numerical Study Regarding the Seismic Response of a Moment-Resisting (MR) Reinforced Concrete (RC) Frame Structure with Reduced Cross-Sections of the RC Slabs

In the first part of the current study, the effectiveness of the transversal cross-section reduction method for RC beams in marginal areas (by means of mechanical drilling) was validated. The said method “encourages” the formation of plastic hinges at the beam ends and, at the same time, allows for taking into account the bending stiffness of RC slabs, which is exerted upon the RC beams. In these conditions, the second part of the current research study (i.e., the current manuscript) highlights the real mode of reducing the lateral stiffness of the slabs upon the RC beams. These elements form a common body, together with the beam–column frame node. The same method as in the first part of the study—“weakening” the plates in the corner area through vertical drilling, without affecting the integrity of the reinforcing elements—was used. The analytical MR RC frame model, studied by means of the comparative method, highlights the efficiency of the transversal cross-section reduction method for RC slabs. Basically, the directing of the plastic deformations from the weakened slab areas towards the marginal areas of the reinforced concrete beams takes place. The beams rotate as far as the weakened slab areas allow its plastic deformation, thus being possible to observe the partial conservation effect of the beam–column frame joint. Furthermore, for the analytical model with the maximum number of vertical holes in the corner areas of the concrete plate, minimal plastic deformations are recorded for the marginal areas of the concrete columns. A partial conservation of the formation mechanism of the “beam-slab-frame node” common rigid block is also noted. Consequently, the dissipation of the seismic energy is made in a partially controlled and directed manner, in the “desired” areas, according to the “Strong Columns—Weak Beams” (SCWB) ductile mechanism of the lateral behavior to seismic actions for reinforced concrete frame structures. The mechanism is specified in current design norms for RC frame systems. The effectiveness of the method for reducing the transversal section of the RC plates in the corner areas by means of transversal drilling is highlighted and validated from the perspective of the local and global ductile seismic response of reinforced concrete frame structures. A significant reduction in the bending stiffness of the slabs upon the beams and a real development of the plastic hinges in the marginal areas of the beams (together with partial implications and plastic deformations) were observed.

A review on tool wear issues in drilling CFRP laminates

Carbon fiber reinforced polymer (CFRP) laminates are featured by superior mechanical properties and excellent structural functions, which have been a promising alternative to conventional alloys and steels in diverse industrial fields. Mechanical drilling is a compulsory operation to shape fibrous composites to desired workpiece quality and target dimensional accuracy. Progressive tool wear is an extremely crucial issue when drilling CFRPs as it may result in undesired machining consequences such as increased force generation, elevated temperatures, and deteriorated surface quality. Despite great endeavors already made to address the wear issues, a complete review of the wear phenomena for CFRP drilling is still significantly lacking. The present paper aims to report the key research advances in tool wear aspects when drilling CFRPs. The wear phenomena, wear modes, and tool failure mechanisms are carefully reviewed. The effects of wear progression on various drilling outputs of CFRP laminates, such as drilling forces, cutting temperatures, and surface quality attributes, are discussed. A particular focus is placed on the quantification and assessment of tool wear during drilling CFRPs. Some research prospects for both academia and industry are outlined. The paper intends to offer a comprehensive understanding of wear mechanisms controlling the drilling of CFRP laminates.

Study on the Electrochemical Deburring for the External Surface of the Microhole Caused by Mechanical Drilling Process

To improve the performance of electrochemical deburring for microhole drilling (MD-ECD), the distribution and dynamic change of the current density in the machining area during the electrolysis process were analyzed, and the synchronous change relationship between the current density and the burr profile was studied. The effects of process parameters, such as machining voltage U, initial machining gap d, electrolyte concentration C, and electrode radius r1 on the deburring process, were studied. The results show that the magnitude of the current density value in the burr area reflects the MD-ECD’s deburring performance. The current density near the burr tip is high, and the material is preferentially removed. The non-processed area has a low current density and slow material removal. As the machining progresses, the burr tip becomes blunt and the current in the burr area gradually transfers to the non-machining area, resulting in the transfer of the material removal area from the burr area to the adjacent non-machining area. Then, a chamfer is formed at the orifice; the chamfer width is larger than the chamfer height. When U = 4 V, d = 0.35 mm, C = 12%, and r1 = 0.4 mm, the burr removal efficiency and accuracy can be guaranteed. The chamfer width and chamfer height obtained from the test are 29 μm and 17 μm, and the burr removal effect is good.

A Combined Thermal Spallation and Melting Technology by Plasma Jet for Deep and Hard Rock Reservoirs

Summary With the increasing demand for oil, gas, and geothermal resources worldwide, the efficient and economical construction of wells in deep and hard rocks has become very important, but conventional mechanical drilling technology cannot achieve this. In this paper, a new noncontact combined thermal spallation and melting technology by plasma jet is proposed. This technology can excavate rock materials by disintegrating brittle rocks into small fragments and melting plastic rock. Especially for hard granite, this method exhibits high rock removal efficiency with low specific energy. Furthermore, the plasma bit is not in contact with the rock, thus avoiding bit wear. A thermal spallation and melting experiment system is established, and laboratory tests are conducted. The influence of plasma current, plasma gas flow rate, confining pressure, and the types of rock-on-rock removal efficiency and specific energy are also researched. Results show that this novel technology can efficiently remove hard rocks, and hard granite is more likely to be removed under confining pressure than under the absence of confining pressure, indicating the feasibility of accessing geothermal, oil, and gas resources from deep and hard rock formations.

Characteristic analysis and experimental study of rotary impact screw drilling tool

With the development of petroleum exploration and drilling, the current drilling tools are facing more difficulty situations, which lead to these problems such as low drilling speed, weak rock breaking ability or short service time. Based on this background, we propose a new drilling tool is proposed in this paper. By analyzing the force of the cam roller system, the dynamic differential equations of the horizontal drill string are established. The key parameters’ influences on the axial impact force of the new tool are studied first. Based on the new outputs, the vibration characteristics of drill string system are studied. For discovering the internal mechanism, the impact part dynamic analysis of this new tool is carried out, so as the contact force between roller and cam, the impact force between upper shaft body and pad. Furthermore, for verifying the rationality and accuracy of theoretical calculation, these parameters’ results are compared and analyzed under different drilling weights. Through laboratory and field tests, the impact frequency and impact force of this new drilling tool are analyzed, the effectiveness of theoretical models are also demonstrated. This research can provide an effective technical references to solve the current drilling difficulty in new situations. HIGHLIGHTS The rotary impact screw drilling tool has the function of impact while rotating, realizing the combination of shear and impact rock breaking, so as to improve the mechanical drilling speed. The impact load on the drill bit is produced by the combined action of hydraulic and mechanical forces. The impact force is generated in the form of mechanical structure movement, and it is a continuous and stable periodic impact force, which makes the bottomhole assembly produce benign vibration. Through experiments, the effectiveness of theoretical calculation and dynamic model is proved successfully, and the drilling tool can effectively improve the drilling rate.

A novel hydraulic balanced turbine for stability control and application in drilling tools

ABSTRACT Most turbine designs are based on the continuous rotating turbine structure. In this paper, a novel hydraulic balanced turbine (HBT) with a symmetrical blade structure that can be used in a hydro-environment is proposed. The turbine, which uses the hydraulic impact of the nozzle jet and balanced turbine blade design in stability control, could be used to control stability in certain applications, e.g. in drilling engineering. This is a completely new way of thinking about turbine design. Because of the special blade design of the HBT, when the nozzle deviates from the balance position, the HBT will be restored to the balance position under the action of external moment. The follow-up performance of the HBT is directly related to the rotational speed of the nozzle. The theoretical and numerical solutions in this paper have good consistency. Modern computational resources allow computational fluid dynamics to be an integral part of turbine design. The simulation results proved that this innovative design is feasible and has potential application prospects. The HBT realizes the ‘soft’ connection between the controlled stable platform and the actuator in the vertical drilling tool, which can significantly improve the control precision of mechanical vertical drilling.

Pitting corrosion prediction based on electromechanical impedance and convolutional neural networks

Corrosion induced thickness loss in metallic structures is a common and crucial problem in multiple industries. Therefore, it is important to accurately monitor the corrosion amount of the structure. Traditional corrosion monitoring methods are mainly based on electrochemical methods, and most of them are unable to quantify the corrosion amount. In our previous work, a new type of corrosion sensing mechanism based on the electromechanical impedance instrumented circular piezoelectric-metal transducer was proposed, in which the peak frequencies in the conductance signatures decrease linearly with the increase of the corrosion induced thickness loss. However, only the uniform corrosion with even metal thickness decrease was considered in the previous study. In this paper, the capability of the proposed sensing mechanism for the quantification and prediction of pitting corrosion was investigated using one-dimensional convolutional neural networks (1D CNN). Finite element modeling of the pitting corrosion was performed and the probability distribution of the corrosion pits was considered. In the experimental setup, corrosion pits were generated on the corrosion sensor using mechanical drilling. The 1D CNN was adopted to explore the regression relationship between the EMI signatures of the sensor and the mass loss induced by pitting corrosion. The results show that the proposed method has achieved high accuracy in the quantitative prediction of pitting corrosion. This paper lays the technical foundation for real-time and quantitative monitoring of pitting corrosion for metallic structures.

Application of W/O Drilling Fluid in Horizontal Well with Long Horizontal Sections in Oil Shale Formations

To address the challenges of oil shale formation well wall instability,micro and nano pore blocking, long horizontal section well friction and borehole purification.We have analysed the nature of formation lithology,matrix pore size and specific surface by using linear expansion method,rolling recovery method and nuclear magnetic resonance method,and developed a white W/O drilling fluid system with an oil-to-water ratio of 70:30 by developing emulsifiers,tackifying and shear strength-improving agents,plugging agents and preferably filter loss agents,and wetting agents.The system was evaluated by using the breakthrough pressure test triaxial compression test and relaxation measurement method in the laboratory. The evaluation results showed that the system could not only maintain the strength of the rock sample but also improve the plasticity of the rock sample after the drilling fluid soaked with the breakthrough pressure of 6.5MPa,and it had good suspension capacity after the relaxation value reached 60s.The field test results show that the system has stable rheological properties and solves the problems of well wall instability and borehole purification,with the well diameter expansion rate of only 3.8% and the mechanical drilling speed of up to 10.85m/h.The SY-1 well has set a record for the longest horizontal section of 1800m in the Liaohe River oil area.

Laser rapid drilling of bone tissue in minimizing thermal injury and debris towards orthopedic surgery

Traditional mechanical bone drilling is prone to heat generation and debris accumulation which will cause tissue damage in orthopedic surgeries. Therefore, research on contactless laser bone drilling has become a mainstream trend in the development of bone surgery. This study presents a laser rapid drilling strategy for bone surgery with minimizing thermal osteonecrosis and debris based on the design of dynamic focusing and spreading droplet cooling. The laser drilled hole suffered from low temperature (Tmax = 46.2 ○C) under clinically accepted threshold of 47 ○C, presenting osteocyte-filled lacunas, visible Haversian canals and increased pullout strength when compared with that following mechanically drilling. An intact bone column without debris was extracted from a 4-mm-deep hole in vitro on a sheep tibia which provides a clean surgical method with increased drilling rate of 0.94 mm3/s. The criteria dimensionless We constant related to spreading droplets leading to effective heat transfer is determined as2.8 × 103 × Re-1/2 ≤ We ≤ 1.9 × 10-2 × Re7, yielding a high convective coefficient of 0.11 W/mm2·K for cooling. The comprehensive results of laser bone drilling proved the effectiveness of the presented technology and setup, which is significant in realizing rapid, debris-free orthopedic surgery.

Effects of geometric array and size of internal voids on tensile ductility of AZ31 magnesium alloy sheets

This study aims to investigate the contributions of the geometric array and size distribution of perforated cylindrical voids to the tensile strain of a metallic sheet through the numerical correction of the plastic constraint factor with regard to the geometric array of internal voids. In AZ31 alloy sheets fabricated via strip casting, internal voids were formed via mechanical drilling in the form of perforated cylindrical holes. The tensile strain decreased significantly (from 0.18 to 0.01) as the void area fraction increased to 40%, and its nominal value decreased with an increase in the void size, even when the geometric array (ratio of void size to ligament distance between voids, a/l) was maintained. A plastic constraint factor describing the geometric array and size distribution of perforated cylindrical voids was proposed through the modification of an original form for isolated spherical voids, and the theoretical prediction using a modified constitutive model agreed well with the experimental results. The tensile ductility of a metallic sheet with perforated cylindrical voids depends more sensitively than isolated spherical voids on the variation in the void area fraction, because the effective void area fraction is overestimated by the exclusion of the strain-hardening exponent term in the modified plastic constraint factor. Additionally, increases in the void size and void area fraction at a fixed a/l ratio lead to a reduction in the tensile strain, by the increase of the plastic zone and the intensification of stress concentration around a void.

New Technology of Mechanical Cavitation in a Coal Seam to Promote Gas Extraction.

Realizing efficient gas drainage in low permeability coal seams has always been a difficult problem for coal miners. Based on this, this paper proposes a new technology of large-diameter mechanical cave-making to promote gas extraction in a coal seam. This technology mainly uses the pressure of a water injection pump to control the automatic opening and closing of a mechanical reaming device to realize mechanical cavitation, and the hole diameter can reach up to 500 mm. The gas drainage effect of mechanical cavitation drilling is analyzed by a numerical simulation, which shows that under the condition of the same drainage time, the larger the cavitation radius is, the larger the effective influence radius of gas drainage is. According to the field test results, the time of single cave-making is about 5 min, and the speed of cave-making is fast. The coal output of a single cave is 0.42 t/m, and the pressure relief effect is obvious. Compared with ordinary drilling, the gas drainage concentration of mechanical cavitation drilling is increased by 2 times and the net amount of drainage is increased by 1.8 times. Large-diameter mechanical cavitation technology can better improve the gas drainage effect of outburst coal seams with low permeability and has a good application prospect.

Simulation of Temperature Field in Micro-EDM Assisted Machining of Micro-Holes in Printed Circuit Boards.

High-speed mechanical drilling based on the micro-bit is the mainstream process technology for machining micro-holes in the printed circuit board (PCB). However, the above process to obtain PCB micro-holes is prone to defects, such as hole burrs and nail heads in the hole. In this paper, the micro electrical discharge machining (micro-EDM) was used as an auxiliary means for machining PCB micro-holes to effectively eliminate the defects such as hole burrs and nail heads. However, during the process of micro-EDM, the micro-bit will be gradually worn, thus negatively affecting the machining quality of PCB micro-holes. To solve the above problems, in this paper, the temperature field model of micro-EDM-assisted machining of PCB micro-holes was established to predict the micro-bit wear by analyzing the temperature field with COMSOL Multiphysics software. This paper made an extensive study of the influences of spindle speed, machining voltage, and pulse width on temperature field and micro-bit wear. The simulation results show that with the increase in machining voltage and pulse width, the temperature of PCB micro-hole machining increases, resulting in an increase in micro-bit wear. The spindle rotation is beneficial to the updating of the machining medium and the discharge of heat generated from EDM. Therefore, with the increase in spindle speed, the temperature of PCB micro-hole machining and the micro-bit wear is reduced.