Experimental characterization of micro-drilling process using nanofluid minimum quantity lubrication

This paper discusses the experimental characterization of the micro-drilling process with the nanofluid minimum quantity lubrication (MQL). The miniaturized drilling machine tool system is developed, and then a series of micro-drilling experiments are conducted in the cases of compressed air lubrication, pure MQL and nanofluid MQL. For the nanofluid MQL, nano-diamond particles having the diameter of 30 nm are used with the base fluids of paraffin and vegetable oils. For the micro-drilling process, an uncoated carbide twist drill having the diameter of 200 μm is used for making holes in the aluminum 6061 workpiece. The experimental results show that the nanofluid MQL significantly increases the number of drilled holes and reduces the drilling torques and thrust forces. In addition, the nanofluid MQL effectively eliminates remaining chips and burrs to enhance the quality of drilled holes.     

Sequential Laser and EDM Micro-drilling for Next Generation Fuel Injection Nozzle Manufacture

High quality holes of diameters less than 145 μm are required for the manufacture of next generation diesel fuel injection nozzles for improved combustion efficiency and reduction of emission to the environment. The current practice of using electro-discharge machining (EDM) drilling of fuel injection nozzles is limited in terms of the hole size it can produce effectively and the length of time needed to drill. In addition, the tooling cost is high. This paper reports on an investigation into a sequential laser and EDM micro-drilling technique for the manufacture of next generation fuel injection nozzles. A laser-drilled pilot hole is rimmed out by EDM drilling. It was found that this hybrid process has eliminated the problems of recast and heat affected zones typically associated with the laser drilling process. The new process has enabled a 70% reduction in total drilling time compared to standard EDM drilling as less material is removed by the EDM. The quality of the holes is as good as direct EDM drilling, thus eliminating the need for re-certification of the drilling process. Various combinations of laser/EDM drilling conditions have been examined. Optimum diameters for the pilot hole and the EDM electrode have been identified for a particular diameter of fuel injection nozzle, giving the minimum total drilling time and the best quality holes. A special system was designed to enable the alignment of nozzles to be controlled to within ± 20 μm. The technique has enabled valuable cost savings and increase in production capacity for next generation fuel injection nozzle manufacture.     

Tool life improvement by peck drilling and thrust force monitoring during deep-micro-hole drilling of steel

An unstable drilling process results from the insufficient supply of cutting fluid and bad chip removal as machining depth increases. These causes of unstable drilling lead to serious problems in micro-drilling. All drills were broken while drilling the first hole in the micro-deep-hole drilling of steel with an aspect ratio over 10, regardless of the cutting conditions. Peck drilling, which utilizes an intermittent feed, is widely used in drilling deep holes. Generally, the one-step feed-length (OSFL) of peck drilling is one and a half times longer than the drill diameter in conventional drilling. An OSFL between one half and twice the micro-drill diameter was used by some workers for micro-drilling. However, this range of OSFL was an arbitrary decision. This paper proposes the peck drilling method using thrust force signal monitoring. The monitoring parameters for peck drilling (MPPDs) are introduced through the analysis of thrust force in both the time and frequency domain. The monitoring system was embodied using LabVIEW. Through this monitoring system, the proper OSFL for stable machining in the deep-micro-hole drilling of steel was determined to be about a tenth of the tool diameter.     

钎焊金刚石磨粒钻钻削C/SiC陶瓷基复合材料孔时切屑对钻削过程的影响

钎焊金刚石磨粒钻适合钻削碳纤维增强碳化硅陶瓷基复合材料孔,但大量切屑会对孔的钻削过程产生不利影响。为此,针对切屑排出过程,分析切屑形貌,研究钻削时切屑对轴向钻削力、孔加工质量、钻头磨损的影响。结果表明：切屑对轴向钻削力有影响,尤其钻削深孔时影响显著。切屑对孔进口的加工质量几乎没有影响,只表现为孔进口处的轻微崩边;切屑对孔出口的加工质量影响显著,可引起严重的纤维断裂、撕裂缺陷以及基体的大区域脱落。同时,切屑加剧钻头磨损,使钻头不仅出现崩刃、微裂纹等轻微磨损,而且还产生基体剥落、金刚石剥落等严重磨损行为。     

Twin gas jet-assisted laser drilling through thermal barrier-coated nickel alloy substrates

Laser fabrication of cooling holes in certain parts of the aero-engine components involves percussion or trepan drilling at acute angles to the surface. These parts are often covered with plasma sprayed ceramic thermal barrier coatings (TBC) to protect them from reaching excessive temperatures in hot engine environments. Delamination of the TBC is the main problem of laser drilling acute angled holes in the coated components. The present study investigates a novel technique enabling to drill through thermal barrier-coated materials without delamination. The technique is based on the control of the melt flow trajectories and impact angle on the hole walls utilizing a secondary gas jet. Delamination-free laser-drilled holes inclined at 30° to the workpiece surface have been demonstrated. Finite volume-based numerical modelling of melt flow during the drilling process was developed to understand the effect of secondary jet in the prevention of delamination.     

Dry and minimum quantity lubrication drilling of cast magnesium alloy (AM60)

Dry and minimum quantity lubrication (MQL) drilling of cast magnesium alloy AM60 used in the manufacturing of lightweight automotive components have been studied. The maximum and average torque and thrust forces measured during drilling using distilled water (H₂O-MQL) and a fatty acid-based MQL fluid (FA-MQL), both supplied at the rate of 10 ml/h, were compared with those generated during flooded (mineral oil) drilling. Tool life during dry drilling was inadequately short, due to excessive magnesium transfer and adhesion to the (HSS steel) drill causing drill failure in less than 80 holes. The use of MQL reduced magnesium adhesion and built-up edge formation, resulting in an increase in tool life as well as reductions in both average torque and thrust forces—prompting a performance similar to that of flooded drilling. The maximum temperature generated in the workpiece during MQL drilling was lower than that observed in dry drilling, and comparable to flooded condition. The mechanical properties of the material adjacent to drilled holes, as evaluated through plastic strain and hardness measurements near the holes, revealed a notable softening in the case of dry drilling, but not for MQL drilling. MQL drilling provided a stable drilling performance, which was evident from the uniform torque and force patterns throughout the drilling cycles and also resulted in desirable machining characteristics, including a smooth hole surface and short chip segments.     

Thrust force, torque, and tool wear in drilling the bulk metallic glass

The thrust force, torque, and tool wear in drilling of Zr-based bulk metallic glass (BMG) material are investigated. Drilling the BMG at high speed generates the chip light emission, high tool temperature, and severe tool wear. At low spindle speed, the BMG work-material builds up at the major and margin cutting edges and may break the drill. A range of feasible spindle speed and feed rate for the efficient drilling of BMG without the detrimental chip light emission and cutting edge work-material build-up has been identified in this study. Under the same drilling condition, the WC-Co tool generally requires less thrust force and about the same torque than the high-speed steel tool. The progressive wear of the major and margin cutting edges for BMG drilling is examined. Severe drill wear is associated with the bright BMG chip light emission. Without chip light emission, the drill wear is visible but not severe. This study concluded that precision holes in BMG could be generated with proper selection of tooling and process parameters.     

Prevention of Exit Crack in Micro Drilling of Soda-Lime Glass

In micro drilling of glass plates, cracks occur at the exit surface. In this work, a back-up glass plate was used to prevent the exit crack. The mechanical lateral constraint was applied to the workpiece and the back-up glass plate to reduce tensile stress near the drill corner. In this way, crack-free holes could be produced. In order to avoid careful fixturing for the mechanical constraint, two glass plates were adhered with various liquids. It is shown that the liquid adhesion of the back-up glass plate without the mechanical constraint is a more effective method to prevent exit crack.     

双台肩钻杆接头刺漏原因分析

双台肩钻杆接头具有大幅度提高抗扭强度、增大水眼的优势,对此类钻杆接头的失效进行分析,具有规范使用操作或改善接头结构性能的意义。某井在双台肩钻杆接头使用中发生两起接头刺漏失效事故,为查明刺漏失效原因,对其中1支刺漏接头进行分析,包括刺漏冲蚀形貌宏微观观察、化学成分分析、力学性能测试及受力情况模拟。分析结果认为：钻杆接头发生刺漏冲蚀原因主要是接头上扣扭矩不足导致接头主台肩面接触压力不足,螺纹疲劳强度下降,螺纹牙底因应力集中形成多源疲劳裂纹,最终高压泥浆从副台肩及刺穿的裂纹冲蚀出。建议钻具使用时按照推荐的上扣扭矩进行上扣,避免类似的情况再次发生。     

Design characteristics of new types of drill and evaluation of their performance drilling cast iron—II. Drills with three major cutting edges

This paper describes the performance characteristics of a new type of carbide head twist drill with four flutes, four major cutting edges, and one chisel edge. This drill shows great potential for significantly improving drilling accuracy and productivity. The drill produces holes that are as good as reamed holes. The body and point geometries and the cutting characteristics of the four-flute drill are described, along with the accuracies of hole location, angularity, size and roundness. Cutting forces, drill wear and chip morphology during cast iron drilling are also discussed. The four-flute drill deflects and vibrates much less than two-flute drills, especially in interrupted cutting cases. A patent is pending for this drill.     

Shaped tube electrochemical drilling of good quality holes

This paper is concerned with the shaped tube electrochemical drilling (STED) of small diameter cooling holes in turbine blades. Following an initial identification of the common defects occurring in STED, a partial factorial design of experiments (DOE) was carried out to identify the more important process parameters that affect hole quality. For these process parameters, a full factorial design of experiments was conducted to study their effects not only on hole diameter but also on hole taper. Guidelines and equations for determining appropriate operating conditions to drill a good quality hole are presented for one drill diameter.     

Identification and control for micro-drilling productivity enhancement

Micro-hole drilling (holes less than 0.5 mm in diameter with aspect ratios larger than 10) is gaining increased attention in a wide spectrum of precision production industries. Alternative methods such as EDM, laser drilling, etc. can sometimes replace mechanical micro-hole drilling, but are not acceptable in PCB manufacture because they yield inferior hole quality and accuracy. The major difficulties in micro-hole drilling are related to the wandering motions during the inlet stage, high aspect ratios, high temperature, etc. However, of all the difficulties, the most undesirable ones are the increases in drilling force and torque as the drill penetrates deeper into the hole. This is mainly caused by chip-related effects. Peck-drilling is thus widely used for deep hole drilling despite the fact that it leads to low productivity. Therefore, in this paper, a method for cutting force regulation is proposed to achieve continuous drilling. A proportional plus derivative (PD) and a sliding mode control algorithm will be implemented and compared for controlling the spindle rotational frequency. Experimental results will show that sliding mode control reduces the nominal torque and cutting force and their variations better than PD control, resulting in a number of advantages, such as an increase in drill life, fast stabilization of the wandering motion, and precise positioning of the holes.     

Investigation of underwater laser drilling for brittle substrates

The aim of this paper is to present and discuss data on underwater laser drilling techniques for brittle substrates. Drilling under water reduced substrate defects that typically result from conventional laser drilling in air. In this study, the substrates were submerged 1 mm beneath the water surface. A CO₂ laser was used to drill holes into LCD glass and alumina substrates. It was found that the underwater laser drilling quality for these materials is much better than that from laser drilling in air. Underwater laser drilling reduced the phenomena of micro-cracking and minimized the size of the area affected by heat from the laser. Also in this study, single-hole drilling and array-holes drilling by percussion and trepanning were conducted and analyzed both in air and underwater. The minimum distance between the two neighbour holes that can be obtained was much shorter in water than in air. The SEM photographs of the holes illustrate the contrast in drilling quality. The relationship between laser power, pulse repetition rate, and the hole quality was also evaluated.     

Effects of exit back-up on delamination in drilling composite materials using a saw drill and a core drill

Machining of composites has caught greater attention in manufacturing of structural parts in aerospace, automobile and sporting goods. Composite materials have advantageous features in strength and stiffness coupled with lightweight compared to the conventional metallic materials. Amongst all machining operations, drilling is the most commonly applied method for generating holes for riveting and fastening the structural assembly. Delamination is one of the serious concerns in drilling holes in composite materials at the bottom surface of the workpiece (drill exit). Quite a few references of the drilling of fiber-reinforced plastics report that the quality of cut is strongly dependent on drilling parameter as well as the drill geometry. Saw drills and core drills produce less delamination than twist drills by distributing the drilling thrust toward the hole periphery. Delamination can be effectively reduced or eliminated by slowing down the feed rate when approaching the exit and by using back-up plates to support and counteract the deflection of the composite laminate leading to exit side delaminations. The use of the back-up does reduce the delamination in practice, which its effects have not been well explained in analytical fashion. This paper predicts the effects of backup plate on delamination in drilling composite materials using saw drill and core drill. The critical drilling thrust force at the onset of delamination is calculated and compared with that without backup. The well known advantage of industrial use of backup can be understood fundamentally by the fact that the threshold thrust force at the onset of delamination is increased making the delamination less induced.     

Effect of deviation on delamination by saw drill

Various cutting techniques are available to drill holes, but drilling is the most common way in secondary machining of composite materials owing to the need for structure joining. Twist drills are widely used in the industry to produce holes rapidly and economically. Since the twist drill has a chisel edge, increasing the length of a chisel edge will result in an increase in the thrust force generated. Whereas, a saw drill has no chisel edge; it utilizes the peripheral distribution of the thrust force for drilling. As a result, the saw drill can achieve better a machining quality in drilling composite laminates than twist drill. The deviation of cutting edge that occurs in saw drill would result in an increase of thrust force during drilling, causing delamination damage when drilling composite materials in particular. A comprehensive model concerning delamination induced by the thrust force of a deviation saw drill during drilling composite materials has been established in the present study. For a deviation saw drill, the critical thrust force that triggers delamination increases with increasing β. A lower feed rate has to be used with an increasing deviation saw drill in order to prevent delamination damage. The results agree with real industrial experience. A guide for avoiding the drill deviation during drill regrinding or drill wear has been proved analytically by the proposed model, especially when the deviation ratio (β) affects the critical thrust force. This approach can be extended to examine similar deviation effects of various drills.     

Examination of wear damage to rock-mining hardmetal drill bits

WC/Co mining bits from a drill head used for drilling holes for roof support bolts in a mine were examined using a focused ion beam scanning electron microscope (FIB-SEM). This was combined with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analyses to study the chemical interaction between the drill bit and the rock. It was found that at the surface of the buttons there was depletion of cobalt, change in chemistry of the remaining binder regions, and changes to the morphology of the WC grains. Tribochemistry calculations were done to understand the possible formation of silicides at the surface of the drill bits, and thus emphasise the importance of quartz content in rock on wear. The evidence of mechanical damage combined with chemical reactions is another step towards understanding the complete wear process in hardmetal mining tools.     

High-throughput drilling of titanium alloys

Experiments of high-throughput drilling of Ti–6Al–4V at 183 m/min cutting speed and 156 mm³/s material removal rate (MRR) using a 4 mm diameter WC–Co spiral point drill were conducted. The tool material and geometry and drilling process parameters, including cutting speed, feed, and fluid supply, were studied to evaluate the effect on drill life, thrust force, torque, energy, and burr formation. The tool wear mechanism, hole surface roughness, and chip light emission and morphology for high-throughput drilling were investigated. Supplying the cutting fluid via through-the-drill holes has proven to be a critical factor for drill life, which can be increased by 10 times compared to that of dry drilling at 183 m/min cutting speed and 0.051 mm/rev feed. Under the same MRR of 156 mm³/s with a doubled feed of 0.102 mm/rev (91 m/min cutting speed), over 200 holes can be drilled. The balance of cutting speed and feed is essential to achieve long drill life and good hole surface roughness. This study demonstrates that, using proper drilling process parameters, spiral point drill geometry, and fine-grained WC–Co tool material, the high-throughput drilling of Ti alloy is technically feasible.     

基于枪钻的薄壁深孔加工工艺设计

在分析热交换板的深孔特点和枪钻加工工艺的基础上,提出了通过减小刀具径向切削力、增加导向条长度和增大薄壁处强度来改善薄壁深孔加工中孔轴线偏斜问题的加工方法,依此设计了4种枪钻加工工艺方法,并通过试验进行了对比,获得了理想的加工效果。     

超声波磁流变钻削硬脆材料小孔研究

硬脆材料在航空航天等领域的应用日益广泛,在硬脆材料上加工出微观表面形貌损伤少的小孔,一直是制造领域的技术难题之一。采用超声波磁流变复合钻削方法加工硬脆材料小孔,通过建立材料去除理论模型,揭示了超声波磁流变复合钻削材料去除规律。实验表明:超声波磁流变钻削硬脆材料小孔方法可较好地提高零件的表面加工质量。     

Feasibility study of the rotary ultrasonic elliptical machining of carbon fiber reinforced plastics (CFRP)

Carbon fiber reinforced plastics (CFRP) are used for various aircraft structural components because of their superior mechanical and physical properties such as high specific strength, high specific stiffness, etc. However, when CFRP are machined, rapid tool wear and delamination are troublesome. Therefore, cost effective and excellent quality machining of CFRP remains a challenge. In this paper, the rotary ultrasonic elliptical machining (RUEM) using core drill is proposed for drilling of holes on CFRP panels. This method combines advantages of core-drill and elliptical tool vibration towards achieving better quality, delamination free holes. The cutting force model and chip-removal phenomenon in ultrasonic elliptical vibration cutting are introduced and analyzed. The feasibility to machine CFRP for RUEM is verified experimentally. The results demonstrate that compared to conventional drilling (CD), the chip-removal rate has been improved, tool wear is reduced, precision and surface quality around holes is enhanced, delamination at hole exits has been prevented and significant reduction in cutting force has been achieved.