Drill Point Geometry Research Articles

A Mathematical Model for Drill Point Design and Grinding

The performance of a drill is affected by the geometry of its point. The available mathematical representations of drill point geometry are inadequate to describe the variation of the drill point design. This paper develops a comprehensive mathematical model that describes the drill point geometry including the conical, the hyperboloidal, and the ellipsoidal drills. The model developed also enables one to precisely predict and control the drill point geometry and the process of grinding.

Computer analysis of drill point geometry

The difficulties in drill point geometry analysis are aggravated by the process of drill point design which now includes the hyperboloidal drill and ellipsoidal drill. This paper provides a tool based upon mathematical models for drill point and drill flute geometries which enables drill point geometry to be analyzed accurately and conveniently by computer. The explicit equations for the drill flank contour and the drill angles are derived and shown to be directly calculated from the grinding parameters. This methodology also enables one to calculate the shapes of cutting edges and chisel edges and their relationships with the grinding parameters. A flute contour is designed for the ellipsoidal drill to improve the excessive inclination angles of cutting edges near outside corners. The chisel edge shape is defined so that it can be measured quantitatively as well as qualitatively.

Measurement and control of the drill point grinding process

The accuracy of the drill point grinding is a problem of concern to the drill user, the drill manufacturer and the manufacturer of the drill point grinder. In this paper, an algorithm is developed to check and control the drill point grinding operation based upon the mathematical model and is illustrated using the conical, hyperboloidal, and ellipsoidal drills. The discrepancies of the drill point geometry estimated from the designed geometry, due to the errors of the estimation are investigated to evaluate the performance of the grinding process and to indicate where adjustments might be made. The coordinate system for the measurement of the drill flank configuration, the objective function, the initial guess values, the number of observations and the parameter for specifying the radial direction of the drill with the curvilinear cutting edges are discussed.

A study of the bone machining process—Drilling

Internal fixation of fractures with screws is a common procedure that has been an historical fact for a number of years. Unfortunately little research has been reported on the action of drilling when preparing for screw insertion. This paper presents some results of a study wherein bovine tibia mid-shaft sections were drilled using several commercially available and experimental drill point geometries under different conditions of feed rate and rotational speed while measuring torque and thrust. Results indicate that drill point geometry is critical when attempting to minimize drilling forces and that there appears to be a softening effect in bone when drilled at relatively high speeds.

An investigation of drill point sharpening by the straight lip conical grinding method—II. A criterion for selecting a solution

An attempt is made to obtain a further criterion to arrive at a solution to the conical grinding method for a given drill specification. From a study of the drill point geometry for a variety of grinding cones, a case for selecting the smallest permissible semi-cone angle is presented. This criterion leads to a plausible unique solution.

Analysis and Design of a Drill Grinder and Evaluation of Grinding Parameters

A prototype drill grinder was designed and built based on a computer aided drill point geometry analysis. The new grinder controls all essential drill point grinding parameters. The new grinder was evaluated by grinding drills, measuring their point geometry parameters, and comparing these measurements with their expected values. The effects of five parameters, consisting of three grinding and two cutting condition parameters, on the drill thrust and torque are determined by an experiment using a two-level factorial design.

Analysis of the Chisel Edge and the Effect of the d-Theta Relationship on Drill Point Geometry

Following the investigation of drill point geometry previously reported [1, 2], the chisel edge profile and the chisel edge angle are analyzed mathematically. The relationship between the cone semiangle, θ, and the x-coordinate of the cone vertex, −d, is derived for a given chisel edge angle and nominal relief angle at the outer corner. This relationship is used to study the effects of grinding cone parameters on drill point geometry. The analysis indicates the existence of an optimum θ value for best drill performance.

An Analysis of Drill Geometry for Optimum Drill Design by Computer. Part I—Drill Geometry Analysis

A comprehensive analysis of the twist drill point geometry is made in order that the high-speed digital computer can be used as an aid in the design of a drill. This subject is treated in two parts. In Part I, the drill geometry is analyzed with respect to the drill flute and flank contours by considering cross sections of the drill cut by planes perpendicular to its axis. Since several important drill angles are defined in planes inclined to the drill axis, the analysis is extended to cover the general case where the drill is cut by any plane inclined to its axis.

Effect of point geometry and dimensional symmetry on drill performance

Because the chisel edge (the edge across the web of the drill connecting the inner ends of the main cutting lips) is virtually a straight line perpendicular to the drill axis, the conventional chisel-point drill (Fig. 1) has no true point. Consequently the drill has no selfcentering action and has a tendency to walk to one side or the other. This causes the hole to start at other than its desired location, to have a bellmouth, to be out of round, to be crooked, to have poor finish, and to be more oversize. It is common practice then, to use a center-punched or center-drilled hole to provide a means for starting the drill on its desired location and to use guide bushings for holding the drill in line. Also, because of the chisel edge, the cutting action is extremely poor near the center of a conventionally ground drill, due to the very large negative rake in this region. An illustration of this is shown in Fig. 2 which is a photomicrograph of a section taken perpendicular to the chisel edge at a radius of 0.030 in. from the center of a 1⁄2 in, chisel point drill, while in the act of forming a chip. As shown the rake angle at this point is about minus 56 degrees. Because of the low cutting speed near the drill center the high negative rake angle is particularly bad since it has been previously shown [1] that high cutting speed is necessary for good chip formation when a cutting tool has negative rake. Furthermore, as the machined metal is virtually trapped between the tool face and the bottom of the hole, its escape sideways into the flute is very difficult. As a result, the action in this area is more an extruding action than a free cutting action. The thrust force in penetrating the work is therefore very high and a great deal of heat is generated. This also may result in excessive workpiece distortion. In view of these limitations of the conventional drill, research studies were undertaken, aimed at the elimination of the chisel edge. As a result of this research, a new drill point geometry and a practical method for applying the geometry to the common twist drill have been developed. Figure 3 shows the drill point configuration which has been developed as a result of this investigation. This point shape is produced, in a single grinding operation, by a mechanism which provides a controlled relative movement of the grinding wheel and drill, axially, radially, and circumferentially, all in properly timed relationship to the flutes of the drill. As indicated by the name spiral point, the entire flank surface extended rearwardly

The Spiral Point Drill—A New Concept in Drill Point Geometry

Abstract The drilling process is one of the oldest and most widely used manufacturing operations. Billions of holes are drilled each year, and over 100 million drills are manufactured annually in the United States alone. However, in spite of its economic importance, the geometrical shape of the drill point (the so-called “chisel edge”) has remained virtually unchanged for over a hundred years, even though its disadvantages of lack of centering, and poor cutting action at the center, have long been recognized. This paper describes an investigation into the cause of these disadvantages and a search for other geometrical forms which might result in improved performance. In the course of this investigation a “three-dimensional spiral” point shape was devised with interesting technical and performance characteristics, together with a unique method of point generation. Photomicrographs are presented showing sections through the drill point and partly formed chips at various points along the cutting edge, both for the conventional chisel point and the spiral point shapes, showing characteristic differences in chip formation. The results of a series of investigations of cutting forces and other performance criteria such as roundness, hole size, and drill life, are presented for both of these shapes, together with a few case histories of results obtained with the new geometry under repetitive manufacturing conditions. A description is given of a method for producing this shape on any type of drill. A modified form of the basic spiral point geometry is shown, which has additional unique characteristics for the drilling of sheet metal, and which therefore may have important applications in the aircraft and other sheet-metal fabricating industries.

Closure to “Discussion of ‘The Spiral Point Drill—A New Concept in Drill Point Geometry’” (1958, Trans. ASME, 80, pp. 1070–1071)

Closure to “Discussion of ‘The Spiral Point Drill—A New Concept in Drill Point Geometry’” (1958, Trans. ASME, 80, pp. 1070–1071)