The slope angle of the OMEGA helical groove is the angle formed by the drill axis and the tangent to the helical line of intersection of the front surface of the drill with the cylindrical surface, the axis of which coincides with the axis of the drill and whose diameter is equal to the diameter of the drill.
The surface of the tooth of the drill facing the cutting surface (the surface along which the chips are separated from the workpiece) is called the flank surface. The flank surfaces are reproduced when sharpening the drill and their shapes are determined by the accepted sharpening method. Flanks on twist drills are most commonly sharpened on tapered, helical and flat surfaces. The intersection lines of the leading and trailing surfaces of the drill form the cutting edge. With conventional twist drills, the straight cutting edges and the drill axis are crossing straight lines. The distance from the axis of the drill to the cutting edge is equal to half the diameter of the drill core. The 2FI angle between the cutting edges, which are located symmetrically relative to the drill axis, is called the tip angle.
The intersection of the flanks of both drill teeth forms a transverse cutting edge located in the center of the drill.
The angle of inclination of the transverse edge is between the projections of the transverse edge and the cutting edge on a plane perpendicular to the drill axis.
The clearance angle ALPHA between the flank and the cutting surface is measured in drills, usually in a cylindrical section concentric with the drill axis.
The angle at the tip of the drill 2FI plays the role of the main angle in the entering. With an increase in the angle at the tip of the drill, the active length of the cutting edge decreases and the thickness of the cut increases, which leads to an increase in the forces acting per unit length of the cutting edges, and contributes to an increase in the intensity of drill wear.
However, with an increase in the 2FI angle, the cut section remains unchanged and the degree of deformation of the cut layer decreases. At the same time, the value of the total component of the main cutting force, acting in the direction of the cutting speed, and determining the magnitude of the torque, decreases, which favorably affects the operation of such a non-rigid tool as twist drills. The total axial feed force of the drill increases with increasing angle 2FI. This is due to a change in the position relative to the drill axis of the plane normal to the cutting edge, as a result of which a smaller part of the forces acting on the cutting edges of the drill are mutually balanced. In addition, the rake angles on the transverse edge decrease with an increase in the nose angle, this worsens the penetration of this edge into the workpiece material and leads to an increase in axial forces during drilling. As a result, the risk of buckling and significant deformations of the drill increases. Experiments show that with a decrease in the angle 2PI from 140 to 90 °, the axial feed force decreases by 40-50%, and the torque increases by 25-30%.
With an increase in the angle at the tip, the angle between the cutting edge and the edge of the tape decreases, which leads to a deterioration in heat removal from the most intensively wearing peripheral zone of the drill.
With relatively low feeds used in the drilling process, a decrease in the 2FI nose angle can lead to extremely small values of the cut thickness, commensurate with the radius of rounding of the cutting edge. This leads to erratic results and more often than not to reduced tool life.
The angle at the tip of the 2FI of the twist drill affects the values of the rake angles and their change on the cutting part, as well as the direction and conditions of chip removal along helical grooves. It is known that the normal operation of the drill can take place when the chip is reliably provided along the grooves and there is no pinching and jamming. Studies show that an increase in the angle at the tip of the 2FI leads to a smoother change in the rake angles along the cutting edge, which has a beneficial effect on the cutting ability of drills.
Thus, the angle at the tip of the 2FI drill has a very contradictory effect on the drilling process and its optimal value depends on many factors that predetermine the nature of the drill. Therefore, in the literature, you can find various data and recommendations for the choice of the angle at the tip of the drill.
It should be borne in mind that, like cutters with different plan angles, it is possible to use drills with different angles at the tip of 2FI for given operating conditions and achieve satisfactory results at atom.
Based on experimental data and production experience, the angle 2FI at the tip of the drill can be roughly selected depending on the material being processed.
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