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.     

A practical method to determine rake angles of twist drill by measuring the cutting edge

Rake angle and its distribution on the cutting edge of a twist drill are of vital importance to its performance. This paper presents a practical method to determine the cutting edge and rake angles. Firstly, 3D coordinates of the points on the cutting edge are inspected by using 2D measurement apparatus. Secondly, the flute profile is deduced from the data measured, and the rake angles are evaluated by numerical computation. Conventionally, rake angle calculation is based on a series of plane definitions and laborious work in plane projection is inevitable. To facilitate the process, a vector based procedure is presented. As an example, cutting edge of a standard drill is measured by using tool microscope and image-based instrument. In each case, the normal rake angle distribution is determined. Numerical results exhibit that average error and mean square error of the latter deviated from the former are 0.95^o and 0.33^o, respectively. It verifies the validity of the proposed procedure and the measurement device.     

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.     

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.     

Extracting cutting force coefficients from drilling experiments

Determining cutting force equations and the associated specific cutting pressures require a relatively large number of orthogonal cutting tests. These tests need to cover wide ranges of cutting speeds, feeds, and rake angles. Given the inherent variation of the rake angle and the tangential velocity over the drill's cutting lip, this work introduces a methodology for extracting these cutting force coefficients by performing a few drilling experiments on pre-drilled pilot holes.First, the contributions of the ploughing forces acting on the lip and margin are determined. Subtracting these edge forces from the measured total values, torque and thrust cutting forces and the corresponding cutting pressure distributions along the lip are derived. These distributions are then used to produce equations that estimate cutting force coefficients over a wide range of cutting parameters. The coefficients determined in this work from drilling experiments in Aluminium 6061-T6 compare favorably with others generated from orthogonal cutting experiments reported in the literature.     

Mechanical load distribution along the main cutting edges in drilling

Used in a very large variety of applications, drilling is one of the most complex manufacturing processes. Most of the research on drilling was done in the field of metal cutting since, in this case, high precision and quality are needed. The use of composite materials in engineering applications has increased in recent years, and in many of these applications drilling is one of the most critical stages in the manufacturing process. Delamination and extensive tool wear, affecting the quality and the costs, are among the problems which drilling of composite materials are currently facing. Understanding and predicting the cutting forces occurring during drilling of such materials would allow extending the currently used optimization methods and proposing new tool geometries and tool materials. The current paper introduces a new mechanistic model for predicting the cutting force distribution along the cutting edges of a drill. A simple, generic and effective method is proposed to relate drilling to oblique cutting using a direction cosine transformation matrix valid for any drill geometry. The oblique cutting model used considers forces on both rake and relief faces, and a simple system of empirical coefficients (their number is significantly less than other similar models). The empirical coefficients are calculated assuming the work-piece material is isotropic. The model is validated on experiments carried out on carbon-fiber and glass-fiber reinforced composites using two different drill types (tapered drill reamer and 2-facet twist drill), which are described in more detail in a previous published paper. The mathematical expression of the drill geometry is also reviewed; removing certain assumption, generalizing some definitions and introducing new drill geometry and features.     

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.     

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.     

Effects of micro-milling conditions on the cutting forces and process stability

Determining stable cutting conditions for corresponding cutting tools with specific geometries is essential for achieving precision micro-milling with high surface quality. Therefore, this paper investigates the influence of the tool rake angle, tool wear and workpiece preheating on the cutting forces and process stability. An advanced micro-milling cutting force model considering the tool wear is proposed. The micro-milling cutting forces are predicted and compared with experimentally obtained results for two cutting conditions and four edge radii measured at different stages of the tool wear. It is found that the cutting forces increase by increasing the edge radius. It is also observed that the cutting forces are higher at a rake angle of 0° compared with a rake angle of 8°. The increase of the cutting forces is mainly associated with the change of the friction conditions between the tool and workpiece contact. Stability lobes are obtained for different edge radii, rake angles of 0° and 8°, initial workpiece temperature and different measured static run-outs. The predicted stability lobes are compared with the micro-milling force signals transformed into the frequency domain. It is observed that the predicted stability limits result in good correlation with the experimentally obtained chatter free conditions. Also, the stability limits are higher at smaller edge radii, higher preheating workpiece temperature and positive rake angles.     

Intermittent metal cutting at small cutting depths—2. Cutting forces

Cutting forces in intermittent metal cutting at small cutting depths were investigated by single edge experiments. Single cutting strokes were performed in a modified Charpy pendulum tester which offers cutting and thrust force measurement and accurate selection of cutting speed and feed in ranges typical for many intermittent high speed steel (HSS) tool operations.

The cutting performance of a number of double rake HSS edges, with primary rake angles ranging from +20° (“parrot bill”) to −60°, all with a preground 0.1 mm flank length were tested in two steel grades (one plain carbon and one austenitic stainless). Some of the edge geometries were tested also in TiN coated condition. The relative performance of the different edges was investigated with respect to specific cutting and thrust forces. The influence of cutting length and depth, edge micro geometry, TiN coating and cutting speed is discussed specifically.

Among the most important observations were:

• The cutting and thrust forces at a fixed cutting depth may change significantly during the short (25–30 mm) cuts.

• The chamfer formed by a double rake geometry with negative primary angle increases the forces.

• For these chamfered tools the forces increase linearly with the projected flank length. TiN coating increases rather than reduces the forces during these short cuts.

The relationships between the varied parameters and chip formation phenomena like dead zone formation, chip curl and surface finish were presented in part 1 of this paper.     

The significance of normal rake in oblique machining

3D molecular dynamics (MD) simulations of oblique machining of an aluminum workmaterial with a single straight cutting edge were conducted over a wide range of normal rake angles (−45° to +45°) and inclination angles (0° to 45°). Three distinct rake angles, namely, the normal rake angle, α_n, the velocity rake angle, α_v, and the so-called effective rake angle, α_e, associated with oblique machining were considered. Variation of the three components of force (cutting, thrust, and oblique), force ratio (thrust force/cutting force), and specific energy (energy required for unit volume of material removed) with rake angle and inclination angle were determined. Based on the analysis of the simulation results, it is shown that normal rake angle is the angle of significance influencing the mechanics of oblique machining, especially from the point of view of cutting force and specific energy in machining, as reported at the macro scale by many in the literature.     

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.     

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.     

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.     

Effect of groove-type chip breakers on twist drill performance

This paper proposes the use of grooves placed on the rake face of a twist drill to facilitate chip breaking and reduce drilling forces and chip clogging. A procedure to design grooves including groove geometry and orientation is discussed. Two sets of experiments are presented that determine the effect of grooves on the forces, chip morphology, and critical depth, and compare the life of the grooved and ungrooved drills. The grooves are found to be very effective in breaking chips and preventing chip clogging. Experiments show the positive effects of the grooves including lower drilling torque and thrust, lower chip evacuation forces, lower cutting forces, increased critical depth and increased drill life.     

The location of the maximum temperature on the cutting edges of a drill

This study analyzes the temperature profile along the cutting edges of a drill and describes how the temperature on the chisel edge can exceed the temperature on the primary cutting edges. A finite element model predicts the temperature distribution in the drill, where the heat flux loads applied to the finite element model are determined from analytical equations. The model for the heat flux loads considers both the heat generated on the shear plane and the heat generated on the rake face of the tool to determine the amount of heat flowing into the tool on each segment of the cutting edges. Contrary to the conventional belief that the maximum temperature occurs near the outer corner of the drill, the model predicts that the maximum temperature occurs on the chisel edge, which is consistent with experimental measurements of the temperature profile.     

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.     

A study of high-performance plane rake faced twist drills. Part II: Predictive force models

Predictive cutting models for the thrust, torque and power in drilling operations using modified plane rake faced twist drills are presented and discussed. The models are based on the mechanics of cutting approach incorporating the many tool and cutting process variables and have been assessed by numerically analysing the predicted trends and by comparing with the experimental data. It is shown that the model predictions are in excellent agreement with the experimental results with an average deviation of about 5%. Simpler “empirical-type” thrust, torque and power equations are also established for use as an accurate alternative to the complex predictive models to facilitate practical applications.     

Compact core drilling in basalt rock using PCD tool inserts: Wear characteristics and cutting forces

The hardest component of the Martian surface is believed to be basalt rock, which is highly abrasive in nature. It will be important to operate a Martian drill under conditions that are conducive to minimal tool wear. In preparation for a Mars drilling project, this paper reports results of an experimental study of dry drilling in basalt and related tool wear. It also reports the effect of the tool wear on increasing the forces and torques required when drilling in basalt rock (on earth) using polycrystalline diamond (PCD) compact core drill inserts. Force and torque data measured for a variety of cutting conditions are shown along with experimental wear data obtained while drilling in basalt rock having different strengths. It is found that flank wear, VB, and cutting edge radius, CER, wear rates increase with rock strength, VB-wear rates and CER-wear rates exhibit opposite trends in their dependence on the remainder of the cutting parameters. For example, while VB-wear rates decrease with an increase in tool feed and spindle speed values, CER-wear rates increase with increases in tool feed and remains unchanged with increases in spindle speed. VB-wear rates decrease as the rake angle becomes more negative, while CER-wear rates increase as this occurs. It is found that basalt rock strength manifests itself via larger (smaller) generated forces/torques for rocks of harder (softer) composition. Strong correlations are found between both modes of tool wear (VB and CER) and the measured values of thrust force and torque. Equations for progressive tool wear as functions of the process variables are developed. A model for the changing drill forces and torques required by the progressive tool wear is developed for drilling in basalt.