Drilling Performance Research Articles

Thermal drilling for application in gas and oil wells: Efficiency enhancement by acoustic inspection

Thermal drilling has recently attracted the attention of researchers due to its high efficiency in breaking hard rocks. However, the advent of low-efficiency processes, such as melting and evaporation, is challenging thermal drilling performance. The implementation of realistic guidelines to monitor spallation may help enhance the performance of this technology. In this article, the results of the continuous and intermittent application of butane oxygen flame on dry and wet limestone were compared with the results of diamond drilling based on the resulting rate of penetration and specific energies. A high-quality camera was operated as a device for visual inspection of the processes to determine any obvious macroscopic physical and chemical changes. In addition, acoustic monitoring of the processes involved during the thermal heating experiments was carried out using a microphone with a frequency response region below 20 kHz. The background noise in the recorded acoustic signals was eliminated by using noise reduction methods in MATLAB. The software performed Fast Fourier Transform (FFT) on the obtained clean signals to reveal the related frequency spectrograms. According to our experimental observations, the frequency response for spallation occurrence in wet and dry samples appears to be around 7 kHz and 15 kHz, respectively. However, the frequency response for fracture formation did not reveal a sensible dependence on moisture content in the limestone samples. Under continuous heating, the rate of penetration and the specific energy for wet samples were respectively 131% higher and 68% lower than those for the dry samples. In the dry samples, wetting between the two heating processes led to a considerable increase in the penetration rate and a dramatic decrease in the specific energy due to keeping the local temperature gradient high in the rock texture.

The Development and Application of Novel Water-Based Drilling Fluid for Complex Pressure System Formation in the South China Sea

The Y oilfield reservoir is characterized by ultra-depleted reservoir and multiple pressure systems in each well section. If conventional PLUS/KCL drilling fluid is used in directional drilling, the following problems are found: the rheological stability of drilling fluid is not sufficient, resulting in increased viscosity and thickening after aging, and the single plugging material may cause loss-circulation when the drilling differential pressure is more than 26 MPa, resulting in poor reservoir protection effects. To solve the above problems, based on the conventional PLUS/KCL drilling fluid formula, a PLUS/KCL drilling fluid system suitable for directional drilling in multi-pressure systems was formed by optimizing the addition of a treating agent to improve the rheological stability of the drilling fluid, optimizing the plugging agent and compound combination to improve plugging ability, and optimizing the particle size distribution of a temporary plugging agent to improve the reservoir protection effect. The laboratory test evaluation showed that the optimized PLUS/KCL drilling fluid had fine plugging pressure capacity, and the intrusion depth was only 4.1 cm on the natural core at 120 °C/35 MPa/12 h, effectively reducing the risk of loss-circulation. The permeability recovery rate of the core after cutting off the polluted end was more than 90%, indicating that the reservoir protection effect was good. The drilling fluid performance was stable, and the cuttings rolling recovery rate was over 90%. Field application showed that the optimized PLUS/KCL drilling fluid was used without any loss-circulation or wellbore instability, and the production of all wells was over-matched, effectively solving the problem of ensuring drilling safety and reducing reservoir damage under the differential pressure of multiple pressure systems.

Detecting downhole vibrations through drilling horizontal sections: machine learning study

During the drilling operations and because of the harsh downhole drilling environment, the drill string suffered from downhole vibrations that affect the drilling operation and equipment. This problem is greatly affecting the downhole tools (wear and tear), hole problems (wash-out), mechanical energy loss, and ineffective drilling performance. Extra non-productive time to address these complications during the operation, and hence, extra cost. Detecting the drillstring vibrations during drilling through the downhole sensors is costly due to the extra service and downhole sensors. Currently, the new-technology-based solutions are providing huge capabilities to deal intelligently with the data, and machine learning applications provide high computational competencies to learn and correlate the parameters for technical complex problems. This research presented a successful case study for developing machine learning models through a comprehensive methodology process for vibration detection using surface rig data through data collection, preprocessing, analytics, training and optimizing the models’ parameters, and evaluating the performance to have the best prediction results. Evaluating the models’ performance showed that obtained predictions have a great match with actual measurements for the different stages of training, testing, and even during models’ validation with unseen well data. Real-field horizontal drilling data was utilized to feed and train the models through different tools named radial basis function (RBF), support vector machines (SVMs), adaptive neuro-fuzzy inference system (ANFIS), and functional networks (FN) to auto-detect the three types of downhole vibrations (axial, torsional, and lateral). The study results showed a high correlation coefficient (higher than 0.9) and technically accepted average absolute percentage error (below 7.5%) between actual readings and predictions of the developed ML models. The study outcomes will add to the automation process of drilling operations to avoid many tools failure by comparing predicted vibrations versus downhole tools limits such as red zone and continuing drilling without interruption to the well total depth especially while drilling horizontal sections.

A New Method for Determination of Optimal Borehole Drilling Location Considering Drilling Cost Minimization and Sustainable Groundwater Management.

Drilling boreholes for the exploration of groundwater incurs high cost with potential risk of failures. However, borehole drilling should only be done in regions with a high probability of faster and easier access to water-bearing strata, so that groundwater resources can be effectively managed. However, regional strati-graphic uncertainties drive the decision of the optimal drilling location search. Unfortunately, due to the unavailability of a robust solution, most contemporary solutions rely on physical testing methods that are resource intensive. In this regard, a pilot study is conducted to determine the optimal borehole drilling location using a predictive optimization technique that takes strati-graphic uncertainties into account. The study is conducted in a localized region of the Republic of Korea using a real borehole data set. In this study we proposed an enhanced Firefly optimization algorithm based on an inertia weight approach to find an optimal location. The results of the classification and prediction model serve as an input to the optimization model to implement a well-crafted objective function. For predictive modeling a deep learning based chained multioutput prediction model is developed to predict groundwater-level and drilling depth. For classification of soil color and land-layer a weighted voting ensemble classification model based on Support Vector Machines, Gaussian Naïve Bayes, Random Forest, and Gradient Boosted Machine is developed. For weighted voting, an optimal set of weights is determined using a novel hybrid optimization algorithm. Experimental results validate the effectiveness of the proposed strategy. The proposed classification model achieved an accuracy of 93.45% and 95.34% for soil-color and land-layer, respectively. While the mean absolute error achieved by proposed prediction model for groundwater level and drilling depth is 2.89% and 3.11%, respectively. It is found that the proposed predictive optimization framework can adaptively determine the optimal borehole drilling locations for high strati-graphic uncertainty regions. The findings of the proposed study provide an opportunity to the drilling industry and groundwater boards to achieve sustainable resource management and optimal drilling performance.

Adjustment of the axial load on the bit when hole deepening by using the screw mechanism

The object of the research is a screw mechanism for adjusting the axial load on the bit during a hole deepening. Mathematical modeling of dynamic processes that occur in the drill string during the hole deepening in deep wells is considered. It is shown that in the process of hole deepening, the longitudinal oscillations are proportional in their intensity to the torsional ones. Obtained boundary conditions for mathematical models of the column with the use of a shock absorber or a screw amplifier. In the course of research, it was found that the screw working mechanism of the axial load creates a relationship between the axial load on the bit and the torque on the screw. This makes it possible to install safety devices that limit the torque of the drive shaft (reciprocating engine, rotor). It is shown that various types of energy supplied to the punch, which are transformed by the punching motor and amplified due to the axial (screw) amplifier installed above the bit, ensure the rotary-progressive movement of the ball bit and increase the drilling performance. It was determined that the obtained dependencies take into account the change in time of the axial load and torque depending on the parameters of the axial amplifier and the geological and technical conditions of drilling and the arrangement of the bottom of the drill string. The research results will be useful to scientists and specialists of the oil and gas industry during the physical modeling of the processes of adjusting the axial load on the bit during a hole deepening by using a screw mechanism.

Estimation of rocks’ failure parameters from drilling data by using artificial neural network

Comprehensive and precise knowledge about rocks' mechanical properties facilitate the drilling performance optimization, and hydraulic fracturing design and reduces the risk of wellbore-related problems. This paper is concerned with the failure parameters, namely, cohesion and friction angle which are conventionally estimated using Mohr's cycles that are drawn using compressional tests on rock samples. The availability, continuity and representability, and cost of acquiring those samples are major concerns. The objective of this paper is to investigate an alternative technique to estimate these parameters from the drilling data. In this work, more than 2200 data points were used to develop and test the correlations built by the artificial neural network. Each data point comprises the failure parameters and five drilling records that are available instantaneously in drilling rigs such as rate of penetration, weight on bit, and torque. The data were grouped into three datasets, training, testing, and validation with a corresponding percentage of 60/20/20, the former two sets were utilized in the models' building while the last one was hidden as a final check afterward. The models were optimized and evaluated using the correlation coefficient (R) and average absolute percentage error (AAPE). In general, the two models yielded good fits with the actual values. The friction angle model yielded R values around 0.86 and AAPE values around 4% for the three datasets. While the model for cohesion resulted in R values around 0.89 and APPE values around 6%. The equation and the parameters of those models are reported in the paper. These results show the ability of in-situ and instantaneous rock mechanical properties estimation with good reliability and at no additional costs.

Dynamic Analysis of a Downhole Regulator for Drilling in Interbedded Formations

Summary In this paper, we investigate the effect of a downhole passive regulator [anti-stall tool (AST)] on the dynamics of rotary drilling systems in interbedded formations. Drilling in interbedded formations can significantly affect the vibrational signature of these systems and the associated drilling performance. Hence, models to assess the impact of drilling in such formations are needed. Here, we construct a dynamic model of the drillstring system which incorporates the bit/rock interface laws for the transitional phase of bit motion between rock layers with distinct mechanical properties. In the model, the axial and torsional dynamics of the drillstring systems are coupled by these interface laws and cast in the form of discontinuous (state-dependent) delay differential equations. Next, we include the AST in the dynamic model to enable the assessment of the influence of this downhole tool on drilling performance, in particular in terms of rate-of-penetration (ROP) and drilling efficiency. Furthermore, we also investigate the mechanical specific energy (MSE) and steady-state power losses at the bit (due to frictional torque) for different operational conditions and rock layer thicknesses. The analysis reveals that an increased drilling efficiency and lower MSE are obtained by incorporating the downhole tool in the drillstring, resulting in a higher ROP and a lower frictional contact between the bit and rock in interbedded formations.

Study on the Methods for Determining Tooth Orientation of Cone Bit

There is an important effect for improving bit drilling performance to adjust tooth orientation of cone bit pertinently. In this paper, to determine tooth orientation reasonably, Tooth coordinate system was established. And a computational method for projecting and decomposing tooth velocity to the axes of tooth coordinate system was presented. Considering the layer condition, the tooth orientation was adjusted and determined. Although the instantaneous cone/bit speed ratio is ambiguity, there is some significance of practical application to use average cone/bit speed ratio when we compute. The methods for determining tooth orientation is easy to apply. It provides a theoretical basis for computing and determining tooth orientation, which has great guiding significance for optimal design of cone bit.

Study on the Dynamic Characteristics of Bit Anchor Cable Drilling in the Gravel Sediments of a Soft Rock Bottom Hole

During the pore-forming stage of floor heave treatment, the phenomenon of gravel sediment accumulation in the bottom hole often occurs, which causes great difficulties in the normal installation of the anchor cable. Based on this observation, a new anchor cable installation method was proposed, which involves installing a drill bit at the front end of the anchor cable to assist its drilling. With the help of an anchor cable drilling test bench, the influence of the bit, motion parameters, and hole environment on the performance of anchor cable drilling was studied. Through the co-simulation of the discrete element method (i.e., multibody dynamics (DEM-MBD)), the drilling process of the bit anchor cable was simulated, and the dynamic characteristics of the bit anchor cable drilling in the bottom hole gravel sediment were analysed. The results show that installing a drill bit in front of the anchor cable can greatly reduce the drilling resistance and hamper torque during anchor cable drilling. When the rotational direction of the bit anchor cable is consistent with the spiral direction of the anchor cable, the bit anchor cable can exhibit better drilling performance. The drilling resistance and hindering torque of the bit anchor cable are the largest when drilling in soaked particles, followed by drilling in dry particles, and the smallest occurs when drilling in moist particles. The research results can provide reference for the parameter setting of anchor cable auxiliary installation equipment.

Application of Taguchi coupled with genetic algorithm (GA) for optimizing surface quality in drilling of duplex stainless steel 2205

This paper aims to explore the drilling performance of duplex stainless steel (DSS) 2205 under sustainable machining, that is dry, MQL, and LCO2. Furthermore, for the correct choice of process parameters, the Taguchi statistical method is used to create a relationship between input parameters, that is cooling environment, spindle speed ( N), feed rate ( Vf), and responses, that is Ra, and hole deviation. From the ANOVA analysis, it was discovered that for measured Ra and hole deviation, the most effective parameter was the cooling environment. The model for Ra and hole deviation was obtained with a precision of 99.32% and 99.70%, respectively. In addition, a genetic algorithm (GA) was implemented for an optimum combination of process parameters and an optimized value of responses. From GA, it was obtained that the optimum process parameters were an LCO2 cooling environment with a high spindle speed (1120 RPM) and a low feed rate (0.02, 0.04 mm/rev), respectively. Also, the optimized value for Ra and hole deviation is 0.375271 and 0.0207519, respectively.

Modification and Application of Materials for Strong Inhibitory Drilling Fluid

Due to the strong inhibition of drilling fluid, many polymer fluid loss additives can not play their role. Therefore, it is necessary to develop some water loss reducing materials that can withstand the strong inhibition environment. In this paper, the surface of asbestos fiber was modified and treated by indoor experiments. The physical and chemical properties of the obtained drilling fluid treatment samples were evaluated to optimize the appropriate amount of reagents used for surface treatment of asbestos. Subsequently, the drilling fluid performance of the obtained treatment agent samples were evaluated at different temperatures to analyze the effect of temperature on the drilling fluid performance. The surface of asbestos fiber was modified by adsorbed cationic surfactant CTAC which better solved the problem of entanglement of asbestos fiber in drilling fluid. The mechanism of the action of the surface of asbestos fiber was analyzed by scanning electron microscope observation experiments. Subsequently, different amounts of modified asbestos fiber was added to the drilling fluid and the performance of drilling fluid was evaluated at different temperatures. The experimental results showed that the modified asbestos fiber reduced the filtration loss of drilling fluid and still maintained good filtration loss reduction effect at 200°C.

Experimental Study on Flexible Fiber Assisted Shear Thickening Polishing for Cutting Edge Preparation of Core Drill

To improve the cutting performance of the core drill, the flexible fiber assisted shear-thickening polishing (FF-STP) for cutting edge preparation was proposed to eliminate the microscopic defect and obtain proper radius of the cutting edge of the core drill. The flexible fiber was introduced into the shear-thickening polishing process to break the thickened agglomerates and improve the efficiency of cutting edge preparation. The influence of the polishing speed, abrasive concentration and the flexible fiber contact length with the core drill on the cutting edge radius r and surface morphology of the core drill edge was analyzed, and the drilling experiments were carried out after preparation, the cutting heat and drilled holes’ roughness were employed as evaluation indexes to evaluate the performance of the core drill. The results show that the cutting edge radius increases with the increase of polishing speed, abrasive concentration and contact length. However, too high a polishing speed and contact length reduce the abrasive particles involved in the polishing process, and then lead to a decline in preparation efficiency. Under the selected processing conditions, the cutting edge radius increases from the initial 5 μm to 14 μm and 27 μm with 4 min of treatment and 6 min of treatment preparation, respectively. The sharp cutting edge becomes rounded, the burrs and chipping on the cutting edge are eliminated, and the average roughness (Ra) of the flank face decreases from 110.4 ± 10 nm to 8.0 ± 3 nm. Nine holes were drilled consecutively by core drills after cutting edge preparation, and the cutting temperature and drilled holes’ roughness were recorded. The maximum cutting temperature (122.4 °C) in the process with the prepared core drill (radius r = 14 μm) is about 20 °C lower than that with untreated one, and the roughness of the drilled hole (Ra 510.5 nm) about 189.9 nm lower. The results indicates that FF-STP is a promising method for high consistency preparation of the core drill cutting edge.

Study of the dynamics of material removal processes in combined pulse laser drilling of alumina ceramic

Combined pulse laser (CPL), one of the hybrid laser processing technologies, has proven to be a reliable tool for the high-quality and high-efficiency material processing. In CPL drilling, the nanosecond laser has been proven that it can improve the drilling performance of the millisecond laser by increasing the laser absorption or improving the sputtering of the melt. However, the dynamics of material removal processes regarding CPL drilling have not been systematically investigated. In this paper, a numerical model of the CPL drilling of alumina ceramic was built to reveal the dynamic processes of the energy absorption, phase transformation, and micro-hole deformation process. The millisecond laser absorption enhanced by the ablated keyhole was quantified and considered as saturation (100 %), thus improving the drilling efficiency. In addition, the shock wave generated by the nanosecond laser accelerated the removal of the melt with a sputtering velocity of up to 23.8 mm/ms, which was much higher than the recoil pressure generated by the saturation vapor pressure, ensuring the drilling quality. Also, the experimental results showed that well drilling quality (1.09° taper angle, 0.96 circularity) were obtained for 1 mm thick alumina ceramic with a material removal rate as high as 1.9 × 104 μm3/μJ. The analysis and summary of the CPL drilling mechanism helps to further improve the drilling performance and provide a new technological route for high-speed processing of ceramic substrates.

PRÁTICA SISTEMÁTICA DE HABILIDADES NO EXERCÍCIO DE SOLO FEMININO

ABSTRACT Introduction Ground exercise is a high-level event of Chinese women's gymnastics. Maintaining the dominant strength of women's floor exercise in China has become one of the issues that coaches, athletes and gymnastics researchers should actively discuss. A research analysis on athletes’ competition in the National Games can better reflect and illustrate the fundamental level of athletes and their overall developmental status. Objective To verify the relationship between women's floor exercise skills and competition performance. Methods The analysis and study of 65 female athletes participating in floor exercises at the National Games was carried out. In this paper, the on-the-spot statistical method is used to perform statistics on the skills and the athletes’ completion in the four games. Movement training and choreography experts were consulted. Results The performance of women's floor drills in the National Games was discussed from three aspects: the difficulty and arrangement of movements, the initial score and bonus points, and the completion of movements. Conclusion Most athletes can adapt to the changes in the new rules and master many complex movements. This also gets them a higher score. However, the choreography of the athletes’ complete movements is relatively simple and lacks features. Strength training in the lower limbs should be the key to improving the athletes’ ability in floor exercise. Level of evidence II; Therapeutic studies - investigation of treatment outcomes.

Recent advances in the applications of graphene materials for the oil and gas industry.

Graphene is a material formed with carbon atoms connected by sp2 hybridization. It is extremely strong and very ductile, and is superhydrophobic and superlipophilic. It has important application prospects in materials science, micro and nano processing, energy, aerospace and biomedicine. Graphene also has some applications in the petroleum industry. As nanoscale materials, graphene-based materials can plug nano-pores and prevent water intrusion into clay minerals during the drilling process, they are suitable for sliding between layers and can be used as lubricants due to the two-dimensional structure. The adsorption properties of graphene-based materials allow them to improve the treatment rate when treating oily wastewater. This paper compiles recent advances in the application of graphene and its derivatives in oilfield extraction, including improving drilling fluid performance, enhanced oil recovery and oily wastewater treatment. We compare the performance advantages of graphene-based materials over other additives, and summarize the mechanism of action of graphene-based materials. The shortcomings of current research are identified and future research and improvement directions are envisaged.

A 1-year longitudinal study of physical fitness in Tunisian junior tennis players

Background: Tennis, a sport demanding high levels of physical fitness, combines various aspects of athleticism such as agility, endurance, strength, and speed. The development of these attributes in junior players is crucial for their progress in the sport. Understanding the impact of structured tennis training on the physical fitness of junior players can provide valuable insights into the effectiveness of training programs and guide future coaching strategies. Objective: This study aimed to assess the effects of 1-year of tennis practice on the physical fitness of Tunisian junior players. Methods: Sixty-two healthy participants were enrolled, divided into two groups, tennis players (TP) (n = 36) and controls, where the participants sedentary, (n = 26), each group was divided into two subgroups U14 and U16 age-groups. Physical fitness tests were conducted at baseline (T0) and after one-year (T1), including standing long jump, countermovement jump, medicine ball throw (MBT), 5m, 10m, and 20m sprints, sideways shuffle, Zigzag, spider drill, 20m shuttle run, and sit and reach (SAR) tests. Results: Jump performances increased significantly for TP in U14 and U16 compared to controls at T0 and T1. In both age-groups, MBT performance increased significantly for TP, with U16 for both groups were higher at T0 and T1 (p<0.05). The 5m and 10m sprints increased significantly for TP in both age-groups, though controls decreased in the 5m sprint in U14 (p<0.05). The 20m sprint increased significantly for TP in U14 and U16 (p<0.05), with U16 outperforming U14 at T0 and T1 (p<0.05). Sideway shuffle and spider drill performances significantly improved for TP (p<0.05), while decreased in controls (p<0.05). The Zigzag test and the maximum oxygen intake increased significantly for TP across both age-groups at T0 and T1, whereas controls' performance decreased in U14 (p<0.05). Conclusion: TP's SAR was significantly better than controls (p<0.05). In conclusion, tennis training enhances young athletes' physical fitness, improving agility, sprinting, jumping, and endurance compared to controls, reporting better results in U16 compared to U14 for most of tests.

Enhance the Properties of Lignosulfonate Mud by Adding Nanoparticles of Aluminum Oxide and Iron Oxide

Oil well drilling fluid rheology, lubricity, swelling, and fluid loss control are all critical factors to take into account before beginning the hole's construction. Drilling fluids can be made smoother, more cost-effective, and more efficient by investigating and evaluating the effects of various nanoparticles including aluminum oxide (Al2O3) and iron oxide (Fe2O3) on their performance. A drilling fluid's performance can be assessed by comparing its baseline characteristics to those of nanoparticle (NPs) enhanced fluids. It was found that the drilling mud contained NPs in concentrations of 0,0.25, 0. 5, 0.75 and 1 g. According to the results, when drilling fluid was used without NPs, the coefficient of fraction (CoF) was 44%, when added Al2O3 NP and Fe2O3 NP at 0.75g reduced CoF by 31% and 33% respectively. When Al2O3 and Fe2O3 NPs were used, particularly at a concentration of 1g, the amount of mud filtration decreased from 13.5ml to 9.3 ml and 8.5 ml respectively. Additional improvements rheological properties as well as swelling when Fe2O3NPs and Al2O3 NPs were added at 1g. Overall, it was found that adding NPs to the Lignosulfonate-WBM at a concentration of 1g can improve rheological, swelling, and filtration properties as well as lubrication at 0.75g.

Dynamic Characteristics of New Mechanical Rotary Steerable Tool in Directional Drilling

The characteristics of a drill string system have an essential influence on the performance of directional drilling. The appropriate control method of a drill string is important to drilling efficiency and downhole operation safety. Therefore, based on a new mechanical rotary steerable tool (RST), this paper aims to analyze the Dynamic Characteristics of RST in Directional Drilling. First, considering the thrust force generated by the RST and bottom hole assembly (BHA), the governing vibration equations of a drill string system are established. Furthermore, connected with fields drilling parameters, including the effects of a new RST on a drill bit and wellbore, as the boundary and initial conditions, the dynamic characteristics of a drill string system are compared and studied in directional and straight drilling. The results show that the axial vibration of the drill string increases extremely in directional drilling process, while the weight on bit (WOB) and inclination force change in small range. Moreover, the torque and azimuth force tend to be stable soon, even its initial fluctuations are significantly. Also, the new RST can effectively enhance the steering capacity, increase the drilling efficiency, and improve drill bit force performance. The research results can provide reference for developing directional drilling technologies, including related tool design and optimization.

Failure resistance research of the dual-stage hybrid bit

Vibration failure is mainly the abnormal fracture failure of cutters, including brittle fracture, impact spalling, impact fatigue fracture, and cutter loss. The weak core area of the existing drill bit is prone to develop into core failure. Vibration failure and core failure will seriously affect drilling performance of the drill bit. To limit vibration and improve stability, we proposed a dual-stage hybrid bit with a polycrystalline diamond compact bit as the primary cutting unit and a tri-cone bit as the secondary cutting unit. Polycrystalline diamond compact bit only breaks the outer rock to form a central rock pillar, while the tri-cone bit breaks the central rock pillar. The existence of the central rock pillar enables the hybrid bit to have a larger lateral contact area with the bottom hole, which can reduce lateral force and limit lateral vibration. The tri-cone bit strengthens the impact and wear resistance of the core area, which can effectively prevent core failure. The experimental results showed that the working parameters of the dual-stage hybrid bit fluctuated less, its drilling stability was better. The experiment verified that the lateral force of the dual-stage hybrid bit was smaller. The vibration test results showed that, with small weight-on-bit, the vibration accelerations of the two drill bits were relatively close. With higher weight-on-bit, the accelerations in all directions of the dual-stage hybrid bit were smaller than those of the polycrystalline diamond compact bit. Specifically, the lateral acceleration differed by more than 40%, which proved the lateral vibration resistance of the dual-stage hybrid bit. The axial and tangential accelerations were reduced accordingly. In the validation experiments, the lateral force of the dual-stage hybrid bit decreased by 11.7%, 21.3% and 26.7%, its vibration acceleration reduced by 12.2%, 26.4% and 14.9%. Therefore, the dual-stage hybrid bit could reduce vibration and enhance stability.

Electric discharge drilling of blind holes in titanium alloy Ti6Al4V

PurposeThis experimental study aims to deal with the improvement of process performance of electric discharge drilling (EDD) for fabricating true blind holes in titanium alloy Ti6Al4V. Micro EDD was performed on Ti6Al4V and blind holes were drilled into the workpiece.Design/methodology/approachThe effects of input parameters (i.e. voltage, capacitance and spindle speed) on responses (i.e. material removal rate, tool wear rate and surface roughness [SR]) were evaluated through response surface methodology. The data was analyzed using analysis of variance and multi-optimization was performed for the optimized set of parameters. The optimized process parameters were then used to drill deeper blind holes.FindingsBlind holes have few characteristics such as SR, taper angle and corner radius. The value of corner radius reflects the quality of the hole produced as well as the amount of tool roundness. The optimized process parameters suggested by the current experimental study lower down the response values (i.e. SR, taper angle and corner radius). The process is found very effective in producing finished blind holes.Originality/valueThis experimental study establishes EDD as a feasible process for the fabrication of truly blind holes in Ti6Al4V.