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.     

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.     

麻花钻几何参数对不锈钢钻削性能影响的研究

采用ProE和Deform-3D软件分析了影响麻花钻钻411性能关健的几何参数,主要研究麻花钻横刃和顶角对不锈钢钻削过程中切削力、扭矩、刀具磨损的影响.介绍了缩短横刃长度和采用S形横刃螺旋面钻尖对不锈钢钻削力和扭矩的影响.重点分析了顶角影响主切削刃的长度、单位刃长的切削负荷、切削层中切削宽度与切削厚度的比例、切削中轴向力与扭矩、切屑形成与排屑情况.对于在钻削中,如何提高钻头的寿命,提高钻削加工的生产率和孔的加工质量具有重要的指导意义.     

Mechanistic modeling of bone-drilling process with experimental validation

In this paper, an improved mechanistic model is developed to predict the thrust force and torque for bone-drilling operation. The cutting action at the drill point is divided into three regions: the cutting lips, outer portion of the chisel edge (the secondary cutting edges), and inner portion of the chisel edge (the indentation zone). Models that account for the unique mechanics of the cutting process for each of the three regions are formulated. The models are calibrated to bovine cortical bone material using specific cutting pressure equations with modification to take advantage of the characteristics of the drill point geometry. The models are validated for the cutting lips, the chisel edge, and entire drill point for a wide range of spindle speed and feed rate. The predicted results agree well with experimental results. Only the predictions for the drilling torque on the chisel edge are lower than the experimental results under some drilling conditions. The model can assist in the selection of favorable drilling conditions and drill-bit geometries for bone-drilling operations.     

An analytical finite element technique for predicting thrust force and torque in drilling

An analytical finite element technique was developed for predicting the thrust force and torque in drilling with twist drills. The approach was based on representing the cutting forces along the cutting lips as a series of oblique sections. Similarly, cutting in the chisel region was treated as orthogonal cutting with different cutting speeds depending on the radial location. For each section, an Eulerian finite element model was used to simulate the cutting forces. The section forces were combined to determine the overall thrust force and drilling torque. Good agreement between the predicted and measured forces and torques was found in orthogonal and oblique cutting and in drilling tests. The drilling tests were performed on AISI 1020 for several drill diameters, spindle speeds, and feed rates. An extension of the technique for predicting drill temperatures has also been described.     

New method for determination of the pose of the grinding wheel for thinning drill points

Traditionally, twist drills are reconditioned by thinning the web so the correct chisel edge length is restored. Recently, thinning has been included in the original design of drills so as to reduce torque and tool force. Because the International Standards Organization (ISO) has a system which can comprehensively model conventional twist drills but cannot model thinning specifications, this paper presents a system for precise mathematical modeling and CNC control of a 6-axis grinding workstation for drill thinning. The presented method determines the position and orientation of the grinding wheel based on the evaluated rake and clearance angles of ISO standards for 2-flute twist drills. The mathematical model and background are discussed. For verification and demonstration, two experimental drills are produced to the identical ISO standard except that one is thinned. The modeling herein is of value to industry and research if incorporated into computer software for drill design and manufacture. It is suitable for linear notch-type cutting with controlled variable rake angle along the secondary cutting edge for purposes of thinning, notching, dubbing and advanced drill research.     

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.     

Experimental analysis of drilling fiber reinforced composites

In comparison with metals, long-fiber reinforced composites have a layered structure, with different properties throughout their thickness. When drilling such structures, internal defects like delamination occur, caused by the drilling loads and their uneven distribution among the plies. The current experimental analysis is focused towards determining the cutting loads distribution (axial and tangential) along the work-piece thickness and tool radius by analyzing the thrust and torque curves when drilling with 3 different drills carbon-fiber (CFRP) and glass-fiber (GFRP) reinforced composite plates. A wide range of cutting parameters is tested. The highest loads are found at the tool tip in the vicinity of the chisel edge for all cases. It is also found that the maximum load per ply varies mainly with the axial feed rate and tool geometry, while the spindle speed has little or no influence. The analysis is useful for selecting the cutting parameters for delamination free drilling and also for conducting drill geometry optimizations.     

The effect of chisel length and associated pilot hole on delamination when drilling composite materials

Drilling-induced delamination often occurs both at the entrance and the exit of the workpiece during drilling of composite material. Investigators have studied analytically and experimentally that delamination in drilling can be correlated to the thrust force of the drill. With a pre-drilled pilot hole, the delamination can be reduced significantly. Early reference reported models of drilling-induced delamination, however, the effect of chisel edge length and pilot hole diameter on delamination is rarely discussed. The optimal range of chisel edge length with respect to drill diameter is derived in this paper.     

Mechanical model for predicting thrust and torque in vibration drilling fibre-reinforced composite materials

Delamination is a dramatic problem associated with drilling fibre-reinforced composite materials (FRCMs), which, in addition to reducing the structural integrity of the material, also results in poor assembly tolerance and has the potential for long-term performance deterioration. The key to solving the problem lies in reducing the thrust force of drilling. In this paper, a theoretical analysis for predicting mean values of thrust and torque in vibration drilling FRCMs is presented. The model is based on mechanics of vibration cutting analysis and the continuous distributions of thrust and torque along the lip and the chisel edge of a twist drill. The result of a simulation study has shown a very good agreement between the theoretical predictions and the experimental evidence. On the same cutting conditions, the thrust and the torque by the vibration drilling method are reduced by 20–30 percent, compared with conventional drilling.     

Development and performance analysis of new spade bit designs

The purpose of this work is to present the development and experimental performance assessment of a new generation of spade drill bits. Rigorous point geometry and drilling force models that describe the topology of the drill and its cutting behaviour have guided the development of these new drills with unique topological features. It is shown, both analytically through simulations and through a systematic experimental study, that the performance of the newly developed topologies exceeds that of the commercially available designs. The new spade bits yield lower thrust and torque over the whole range of pragmatic operating conditions.     

Effect of pilot hole on thrust force by saw drill

The applications of composite materials are numerous, especially in the structural parts of aerospace, automotive and marine industries. Owing to the marked anisotropy and macroscopic heterogeneity of composite materials, the mechanics of machining used is different when compared to metals. Delamination is one of the most concerns of applying the fiber-reinforced composite materials in various industries. A hole is pre-drilled to eliminate the thrust caused by the chisel edge of twist drill; thus, the threat for delamination is significantly reduced. Saw drills eliminate the chisel and utilize the peripheral distribution of the thrust in drilling. An analytical approach to identifying the role of the pilot hole was proposed to reduce the thrust force-induced delamination during saw drilling. The predicted critical thrust force is in good agreement with the experimental values.     

Cutting performance of thick web drills with curved primary cutting edges

In order to improve the cutting performance of drills, a thick web drill with curved primary cutting edges was devised. The curved primary cutting edge was mathematically determined by changing the distribution of the tool orthogonal rake angle along the primary cutting edge. A three-dimensional finite element analysis based on the torsional rigidity of the drill was applied to obtain the “secondary” flute shape of the drill with curved primary cutting edges and to specify the web thickness. Experiments were conducted to evaluate the drill's cutting performance. Compared with conventional twist drills of different web thicknesses, the thick web drill with curved primary cutting edges shows greater effectiveness in reducing the thrust force, the torque, and the tool wear, thus providing a better cutting ability and a longer tool life.     

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 new mathematical model for multifacet drills derived by using angle-solid model

A new mathematical model for multifacet drills (MFD) is developed using angle-solid-block approach to find the relationship of design parameters. The function of chisel edge angle is defined as ψ=f(ρ′ ,τ ,α_r) in order that the uncertainty of chisel edge geometry determined by existed model can be avoided. The effects of the new mathematical model on the geometry of chisel edge and main cutting edge are shown in this paper. In addition, the new mathematical model has the potential of designing and researching for many tools.     

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.     

Mathematical model for helical drill point

Helical drill points provide a superior cutting performance in drilling operations, particularly in micro-hole drilling. This paper presents a comprehensive and straightforward method for the design of helical drill points. The proposed method has three particular features. Firstly, a mathematical model of the helicoid grinding surface is developed. This model allows the normal and tangential vectors of the abrasive wheel to be obtained explicitly. Secondly, the mathematical models of the flute and flank surfaces are integrated and therefore the cutting and chisel edges can be obtained by numerical calculation. Finally, the derivation of the model is straightforward and expresses the drill's characteristics (e.g. the semi-point angle, chisel edge, lip clearance angle, heel clearance, angle tool cutting edge inclination, normal rake angle and normal clearance angle) in accordance with all current international standards. The proposed model is capable of describing a wide range of helical drills. The methodology presented in this study facilitates the production of helical drills on a 6-axis CNC grinding machine.     

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.     

A new methodology for designing a curve-edged twist drill with an arbitrarily given distribution of the cutting angles along the tool cutting edge

The present study aims at the development of a new methodology for designing a curve-edged twist drill with an arbitrarily given distribution of the cutting angles along the tool cutting edge. The new methodology consists of 81 major mathematical equations and is developed using a method of mapping relevant planes and straight lines of a cutting tool (such as the cutting plane and the cutting edge) as corresponding image points and image lines on a projection plane. The developed methodology is used to intuitively and graphically analyze and determine the relationship between the orientation of the cutting edge and the cutting angles at each point on the cutting edge. A set of image points and image lines is established to calculate the cutting angles on the cutting edge of a twist drill, including the working tool rake angle, the working tool inclination angle, the working cutting edge angle, and the working normal rake angle. Three computer case studies are provided to show curved cutting edges that correspond, respectively, to a linear distribution of the working tool rake angle, a combined linear and uniform distribution of the working tool rake angle, and a linear distribution of the working tool inclination angle along the tool cutting edge. Finally, a set of metal drilling experiments is performed to compare the drilling torque and the thrust force between a conventional straight-edged twist drill and a new curve-edged twist drill that has a combined linear and uniform distribution of the working tool rake angle along the tool cutting edge. The experimental results show that the new curve-edged drill reduces the drilling torque by 28.5% and the thrust force by 24.6% on average.     

Mechanical modelling of high speed drilling. 2: predicted and experimental results

In this article experimental results on high-speed drilling are presented. The measurements are compared with the theoretical predictions of thrust force and torque to validate the continuous drilling model using the orthogonal cutting model and the oblique cutting model. This comparison is undertaken for the whole drill, because the lip and chisel edge are not studied separately in the experimental tests.