An investigation of the cutting fluid flow in self-piloting drills

The conditions of the coolant flow through the inlet annular channels, machining zone and outlet channels of self-piloting drills were investigated. Experiments were performed on B.T.A. and Ejector drills to determine the influence of the drill design parameters on the flow parameters. The influence of the inlet channel's clearance and eccentricity on the pressure distribution and energy loss was analytically examined. Experimental investigations of the static and dynamic pressure distribution of the cutting fluid in the machining zone were performed with different drill heads. Conditions for reliable chip removal from the machining zone and the boring bar were defined. The results obtained constitute a reference for designers of self-piloting drills and drilling processes.     

The performance of hydrogenated and non-hydrogenated diamond-like carbon tool coatings during the dry drilling of 319 Al

Aluminium alloys, though widely used in the automotive industry, are difficult to machine, particularly by drilling and tapping without the use of metal removal fluids, because of aluminium's strong tendency to adhere to the cutting tool. Tribological tests have revealed that carbon-based tool coatings, such as diamond-like carbon (DLC), promise an improved performance due to their low friction and adhesion. However, the tribological performance of DLC coatings depends on both their hydrogen content and the testing environments. Hence the experimental approach taken in this study was designed to understand the cutting performance of hydrogenated DLC (H-DLC) and non-hydrogenated DLC (NH-DLC) tool coatings during the dry drilling of a 319 Al (Al–6%Si) alloy. An experimental drilling station was built to measure torque and thrust force changes using a cutting speed of 2500 rpm and a feed rate of 0.25 mm/rev. The cutting performance was assessed by measuring the torques and thrust forces generated during the drilling of the first 150 holes or by drill failure—depending on which occurred first. The results indicated that superior cutting performance was achieved, in both torque and thrust force responses, using DLC-coated drills rather than uncoated high-speed steel (HSS) drills. The uncoated HSS drills failed after drilling only 49 holes as a result of excessive aluminium adhesion. At least 150 holes could be drilled using the DLC-coated drills, and both the torque and thrust forces generated during drilling were lower than those with uncoated HSS drills. In addition, a smaller proportion of holes exhibited abrupt increases in torque (at the end of the drilling cycle) during drilling with the DLC-coated drills. Scanning electron microscopy (SEM) investigations showed that the H-DLC drill flutes displayed minimal aluminium clogging—resulting in lower torque. H-DLC coating also diminished metal transfer and buildup edge formation on the drill's flank face and cutting edge. Thus, torque and thrust force measurements, supported by metallographic data, indicated that H-DLC-coated drills provided better dry drilling performance than NH-DLC.     

A study of high-performance plane rake faced twist drills.: Part I: Geometrical analysis and experimental investigation

A study of a modified drill point design with plane rake faces for drilling high-tensile steels is presented. A geometrical analysis has shown that the modified drill point design yields positive normal rake angle on the entire lips and point relieving in the vicinity of the chisel edge. This drill geometry can be expected to reduce the cutting forces and torque, and hence reduce the possible drill breakages when drilling high-tensile steels. An experimental study of drilling an ASSAB 4340 high-tensile steel with 7–13 mm titanium nitride (TiN) coated high-speed steel (HSS) drills has shown that the modified drills can reduce the thrust force by as much as 46.9%, as compared to the conventional twist drills under the corresponding cutting conditions, while the average reduction of torque is 13.2%. Drill-life tests have also been carried out and confirmed the superiority of the modified drills over the conventional twist drills. In some cases, the conventional drills were broken inside the workpiece, while the modified drills performed very well under the same cutting conditions. To mathematically predict the drilling performance and optimise the drilling process using the plane rake faced drills, predictive models for the cutting forces, torque and power will be developed in the second part of this investigation.     

Evaluation of drilling parameters on thrust force in drilling carbon fiber reinforced plastic (CFRP) composite laminates using compound core-special drills

Drilling is the mostly used secondary machining of the fiber reinforced composite laminates, while the delamination occurs frequently at the drill exit in the workpiece. In the industrial experiences, core drill shows better drilling quality than twist drill. However, chip removal is a troublesome problem when using the core drill. Conventional compound core-special drills (core-special drills and step-core-special drills) are designed to avoid the chip removal clog in drilling. But the cutting velocity ratio (relative motion) between outer drill and inner drill is null for conventional compound core-special drills. The current study develops a new device and to solve the problems of relative motion and chip removal between the outer and inner drills in drilling CFRP composite laminates. In addition, this study investigates the influence of drilling parameters (cutting velocity ratio, feed rate, stretch, inner drill type and inner drill diameter) on thrust force of compound core-special drills. An innovative device can be consulted in application of compound core-special drill in different industries in the future.     

Performance evaluation of endrills

This paper evaluates the performance of a relatively new type of drill called an endrill which is a cross between a drill and an endmill. Investigations into the effects of its cutting conditions on the drilling forces, surface finish, drill wear and hole oversize were carried out. It was found that endrills produced better quality holes than conventional twist drills, better surface finish and less oversize of the holes. Hence, with proper feed, speed and flow rate of the pressurized flushing coolant, a good finish of about R_a = 1 μm can be attained without reaming. Thus, the productivity of finished holes can be remarkably improved. Compared to twist drills, lower torque and thrust were observed which yielded improved tool life and reduced power consumption. No “walking phenomenon” was observed when this kind of drill was used and the amount of hole oversize was found to average about 0.7% of the drill diameter as compared to 1.6% when twist drills were used. Finally, general equations for the drill torque and thrust were derived from the experimental results.     

Tool length influence on wear behaviour of twisted carbide drills

The drilling of holes with an l/d-ratio greater than 12 is responsible for a significant amount of the overall production time and therefore has a high impact on the productivity. In the past gun drills were used for those higher l/d-ratios, providing good surface qualities and straight holes. However the productivity of gun drills compared to solid carbide twist drills is very low, due to the limited feed rate. With the use of solid carbide twist drills with l/d-ratios up to 40 and overall lengths up to 350 mm production time can be reduced extensively. Solid carbide twist drills can furthermore be used on standard machine tools. Former drilling tests show that the tool life of these drilling tools decreases abundantly clear with an increasing tool length. This paper presents the influence of the drill length and the dynamic behaviour of the drilling process on the wear behaviour of the drilling tool.     

Effect of MQL on the tool life of small twist drills in deep-hole drilling

Drilling of deep and small boreholes using twist drills has to be considered as one of the most difficult metal cutting operations. There are many reasons for this, one of them being that the cutting fluid has to be supplied externally. This research work investigates in how far the manner of supplying and the type of minimum quantity lubricant have an effect on the tool life of coated and uncoated high-speed steel twist drills of 1.5 mm diameter. Deep-hole drilling is performed as the holes, drilled into plain carbon steel, had a depth of 10 times the diameter. The feasibility of dry machining as an appropriate alternative to MQL in deep-hole drilling has also been investigated. This work shows that, compared to a continuous supply of the minimum quantity lubricant, a discontinuous supply brings about a significant reduction in tool life, especially in the case of heat-sensitive drills. With respect to the type of minimum quantity lubricant, a low-viscous type with a high cooling-capability gave rise to a notably prolonged tool life. It is also shown that dry drilling is associated with strongly accelerated tool wear for most of the twist drills tested, resulting in a significant reduction in tool life.     

Experimental studies of step drills and establishment of empirical equations for the drilling process

Various sizes of step drills were manufactured by a CNC grinder machine and used in the drilling process with different speeds and feed rates to produce single step holes in S1214 free machining steel. The performance of step drills was compared with that of conventional twist drills in the drilling of the free machining steel for the same task. The influences of drill size, feed rate and cutting speed on the performance of step drills were studied. Experimental results show that for better cutting performance, the small diameter should not be less than 60% of the large diameter. Also, most of the changes in the characteristics of the thrust force were influenced by the smaller drill of the step drill. On the other hand, the small diameter part of the step drill only contributed about 30% of the torque. From the experimental results, empirical equations for drilling thrust force and torque have been established for step drills.     

Modeling the mechanics and dynamics of arbitrary edge drills

This paper presents a new approach for modelling the cutting forces and chatter stability limits in drills with arbitrary lip geometry. The oblique cutting geometry at each point on the drill lip is modelled using parametric curve equations. The cutting force and process damping coefficients at different parts of the drill lip are identified empirically; the cutting force coefficients are identified from non-symmetric drilling tests, and the process damping coefficients are identified from chatter-free orthogonal turning tests. The presented approach provides a practical method for modelling the cutting forces and vibration stability without needing the detailed geometry of drill lips. The accuracy of presented model in predicting lateral and torsional-axial chatter stability limits is verified by conducting drilling tests using drills with various edge geometries.     

Machining stability in high speed drilling—Part 2: Time domain simulation of a bending–torsional model and experimental validations

In this paper, the torsional limits of stability in drilling are first obtained analytically based on Bayly's work [P.V. Bayly, S.A. Metzler, A.J. Schaut, K.A. Young, Theory of torsional chatter in twist drills: model, stability analysis and composition to test, Journal of Manufacturing Science and Engineering, 123 (2001) 552–561]. Subsequently, a time domain simulation model of chatter in drilling is presented. The novel simulation model, developed in this work, combines the effects of both bending and torsion. The major challenge in this model is the tracking of the instantaneous cutting parameters along the lips while vibrating in both modes. This challenge was met here successfully and the simulation results agreed closely with the analytical solutions. Cutting experiments were also conducted to verify the developed chatter models. Two drills, one “short” and one “long” were used in drilling a large number of holes with different pilot-hole diameters. The agreement between the cutting tests and theoretical predictions was not very close for the “short” drill due to inaccuracies in representing the boundary conditions in the mathematical model. On the other hand, the cuttings tests agreed very closely with the analytical and numerical predictions for the “long” drill.     

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.     

磁控溅射CrTiAlN涂层钻头的制备及其钻削性能研究

通过利用磁控溅射离子镀技术制备CrTiAlN梯度涂层M2高速钢麻花钻,对镀层进行了组织、结构分析,并在干式切削条件下对普通45#钢和30CrMnSiA高强度钢进行了钻削试验.由于CrTiAlN涂层本身具有良好的热氧化性能和良好的组织结构,M2高速钢刀具经CrTiAlN涂层后,在干式切削条件下钻削45#钢和30CrMnSiA高强度钢时,钻头寿命比未涂层提高约19倍和15.2倍.     

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.     

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.     

The torque and thrust force in twist drilling—II. Comparison of experimental observations with deductions from theory

Published data are shown to conform with previously derived expressions relating the torque and the thrust to feed and drill diameter in drilling operations conducted with a set of geometrically similar twist drills. The model from which the theoretical expressions are derived is shown to be capable of explaining the apparently deviant behaviour observed when drilling workpieces which exhibit high adhesion. The model is based on the assumption that the removal process at the drill lips is quasi-orthogonal which is valid provided the drill diameter is sufficiently large compared to the chisel edge length. The behaviour observed with small diameter drills is shown to be consistent with the removal process becoming noticeably oblique. The consequences of geometric dissimilarity of drill shape are discussed.     

Development of Drill Geometry for Burr Minimization In Drilling

The researchers carried out drilling tests using drills of various shapes to determine burr minimization. The ultimate objective of this study was to develop a compatible drill shape to minimize burr formation. For the experiment general carbide drills, round drills, chamfer drills and step drills are designed and manufactured. Burrs are generated under various cutting conditions using four different materials. A laser sensor was used to measure the burr dimensions. As a result of the experiments, step drill with specific step angle and step size Is suggested for burr minimization.     

Analytical and experimental investigation of coolant velocity in high speed grinding

Use of water-base coolant is a pre-requisite in an high speed grinding process to avoid thermal damage and to achieve better surface integrity as well as higher grinding ratio. However, the presence of hazardous chemical additives in the coolant causes environmental problems. As a result, stringent government legislation is being practiced for the coolant use and disposal, which consumes 7–17% of the total machining cost. This paper reports the coolant flux minimization through controlled jet impingement so as to prolong the coolant replenishment cycle. Control of coolant flux was achieved through development of a “metered quantity coolant” (MQC) nozzle which supplies the required amount of coolant to the grinding zone. Also, this investigation has shown that coolant velocity has a significant influence on the high speed grinding performance. When the coolant velocity is inadequate, coolant could not penetrate into the grinding zone. The increase in coolant velocity was realized with reduction in nozzle opening area and does not use a large quantity of coolant. This is of significance to reduce environmental pollution and machining costs through extended coolant replenishment period.     

钛合金深孔刀具耐用度及参数优化

深孔钻削钛合金难度大,刀具极易损坏。本文在原来自行设计和制造的内排屑钻孔（BTA）深孔钻头基础上．通过对钛合金材料的深孔加工试验,对改进后的深孔钻头的失效机理进行了分析研究,并探讨了钛合金深孔加工时进给量、刀具转速与耐用度的关系,优化了工艺参数和刀具结构、最终达到提高刀具耐用度的目的。     

Cu基钎料钎焊金刚石套料钻的试验研究

采用真空钎焊工艺,分别以不同成分的Cu基钎料(Cu80Sn20)90Ti10、(Cu90Sn10)82Ti18以及(Cu90Sn10)80Ti20制备了镀钛金刚石套料钻,并进行了花岗岩钻削试验。采用三维视频显微镜、扫描电镜观察了各种钎料钎焊金刚石套料钻的磨损情况。结果表明:3种配比的钎料对金刚石均已实现牢固结合;(Cu80Sn20)90Ti10合金较(Cu90Sn10)82Ti18合金和(Cu90Sn10)80Ti20合金钎焊金刚石套料钻有更长的使用寿命。