Twist drill is a rotary cutting tool utilized for creating cylindrical holes in workpieces. Its name derives from the helical flutes that resemble the twisted shape of a hemp rope. The tool's main structure consists of the shank, neck, and working body, primarily manufactured from high-speed steel (HSS) or carbide. It is typically held and operated by equipment such as drill presses, milling machines, or lathes.

The geometric parameters of a standard twist drill's main cutting edges include the helix angle (typically 18°-38°), point angle (commonly 118°±2°), and chisel edge angle (usually 50°-55°). While the helical flute design facilitates efficient chip evacuation, the tool exhibits inherent limitations such as low core rigidity and insufficient cutting stability.

Twist drills find extensive application in metalworking, with diameters covering a broad range from 0.25 mm to 80 mm. Technological advancements have driven continuous upgrades in material science and processing. Examples include titanium coatings to enhance wear resistance and cobalt-alloyed grades specifically suited for machining stainless steels. High-end products incorporate solid Polycrystalline Diamond (PCD) tips sintered onto the body, breaking through traditional design constraints. This enables precision machining of composite laminates while significantly reducing the risk of delamination and edge chipping. Manufacturing processes are progressively transitioning towards intelligent production, where fully automated CNC machine tools ensure consistent quality, and non-standard customizations cater to diverse application requirements.

Helical flutes can have 2, 3, or more grooves, with the 2-flute design being the most prevalent. Twist drills can be mounted in various tools, ranging from manual or power-operated hand-held drills to stationary machinery like drill presses, milling machines, lathes, and machining centers. The drill bit material is generally high-speed steel or carbide.

Fundamental Angles

(1) Helix Angle (β): The helix angle is the angle between the unwound line of the outermost helix of the drill's flute and the drill's axis. Because the lead (axial distance for one complete revolution of the helix) is constant along the flute, the helix angle varies at different diameters. It is maximum at the outer diameter and progressively decreases towards the center. Increasing the helix angle augments the effective rake angle, which is beneficial for chip ejection, but concurrently reduces the drill's torsional rigidity and overall strength. Standard twist drills feature helix angles between 18° and 38°. For drills with smaller diameters, a smaller helix angle is typically adopted to ensure sufficient rigidity and prevent breakage.

(2) Rake Angle (γ_Om): The rake angle on a twist drill is not constant along the main cutting edge due to the complex geometry of the helical flute surface, which acts as the rake face. The rake angle is highest (approximately +30°) near the outer corner and gradually decreases towards the center, becoming significantly negative (around -30°) near the chisel edge. On the chisel edge itself, the rake angle is severely negative, typically ranging from -50° to -60°, creating a highly unfavorable cutting condition akin to extrusion.

(3) Clearance Angle (α_Om): The clearance angle at any selected point on the main cutting edge is defined in a cylindrical section. This cylindrical section is an imaginary surface formed by rotating a line parallel to the drill axis and passing through the selected point 'm' on the cutting edge, around the drill's axis. The clearance angle, denoted as α_Om in this reference system, is not uniform along the cutting edge. It increases towards the center of the drill. Typically, the nominal clearance angle at the drill's periphery (outer diameter) is ground between 8° and 10°. Near the chisel edge, the clearance angle is much larger, around 20° to 25°. This variation is deliberately designed to compensate for the decrease in the effective working clearance angle caused by the drill's axial feed motion during operation. It also helps to maintain reasonable cutting conditions that accommodate the significant variation in rake angle along the edge.

(4) Cutting Edge Angle / Approach Angle (κ_rm): This is the angle, measured in a reference plane (the tool's base plane), between the projection of the tangent to the cutting edge at a selected point 'm' and the direction of feed (drill axis direction). The base plane for any point on the cutting edge of a twist drill is defined as the plane containing that point and the drill's axis. Crucially, because the main cutting edges do not pass through the drill's axis, the orientation of this base plane changes for every point along the edge. Consequently, the cutting edge angle (κ_rm) is different at each point along the main cutting edge. Once the point angle is ground, the basic relationship and variation of the cutting edge angles along the edges are established. It is important to distinguish the cutting edge angle (κ_rm) from the point angle (2φ); they are related but distinct concepts describing different geometric aspects.

(5) Point Angle (2φ): The point angle is the included angle between the two main cutting edges as viewed in a plane parallel to them and the drill axis. A smaller point angle facilitates easier penetration into the workpiece and reduces the axial (thrust) force required. It also increases the active length of the cutting edge engaged in cutting, which results in a thinner undeformed chip thickness for a given feed rate. This promotes better heat dissipation and can enhance tool life. However, if the point angle is excessively small, the strength of the drill tip is compromised, tool deflection may increase, and the torque can rise, potentially leading to drill breakage. Therefore, selecting an appropriate point angle involves balancing these factors based on the strength and hardness of the workpiece material. The standard point angle for a general-purpose twist drill is 118°.

(6) Chisel Edge Angle (ψ): The chisel edge angle is the angle, measured in a projection onto a plane perpendicular to the drill axis, between the main cutting edge and the chisel edge (the edge connecting the ends of the two main cutting edges across the web). This angle is a consequential feature formed during the grinding of the drill's flank (clearance surfaces). As illustrated in technical diagrams (e.g., referenced Fig. 3-5), an increase in the chisel edge angle (ψ) results in a shorter chisel edge length. A shorter chisel edge generates lower axial thrust force, which is generally desirable as a large portion of the total thrust force originates from the inefficient cutting/ploughing action at the chisel edge. The standard chisel edge angle for a conventionally ground twist drill is approximately 50° to 55°.

Effects on Machining Performance

(1) Dimensional and Geometrical Accuracy: The diameter of a twist drill is inherently limited by the desired hole size. The helical flutes reduce the cross-sectional area of the drill's core, leading to relatively low bending and torsional rigidity. Guidance during drilling is primarily provided by only two narrow margin (land) lines running along the flutes. This limited guidance can cause the drill to deviate or "walk," especially upon initial engagement, resulting in misalignment and inaccuracies in the hole's axis and location. The presence of the chisel edge exacerbates this issue, as it does not positively center the drill but tends to "wander" on the workpiece surface before full engagement, requiring significant axial force and potentially causing the drill to wobble. Consequently, holes produced by standard twist drills often exhibit relatively large geometrical and positional errors.

(2) Tool Life and Dimensional Precision: The complex geometry of the twist drill, featuring curved rake (helical flute) and clearance surfaces, leads to continuous variation of both rake and clearance angles along the entire active length of the main cutting edges. Compounding this, the extreme negative rake angles at and near the chisel edge create very poor cutting conditions in that region, characterized by severe plastic deformation and ploughing rather than clean shear. Furthermore, the distribution of cutting speed along the cutting edge is highly non-uniform. The maximum cutting speed occurs at the outer corner, which, paradoxically, is often the geometrically weakest point (sharp corner, potential heat concentration). This combination of unfavorable mechanics and speed distribution leads to accelerated wear, particularly at the outer corners and margins. This wear directly impacts the final hole size, form, and finish, often resulting in reduced dimensional accuracy and potential hole enlargement.

(3) Chip Formation and Surface Finish: The entire main cutting edge is simultaneously engaged in cutting, and the cutting speed varies significantly from the outer diameter towards the center. This variation, coupled with the continuous engagement, promotes the formation of long, helical chips. Efficient evacuation of these chips from the deep, confined space of the hole can be challenging. Difficulties in chip removal lead to recutting of chips and intense friction between the chips, the drill flutes, and the newly machined hole wall. This interaction often results in scoring, galling, or scratching of the bore surface. As a direct consequence, the surface roughness achieved with standard twist drills is generally poor (i.e., high roughness value - Note: The original text stated "很低" implying very low roughness, which is contextually incorrect. The intended meaning is poor finish, hence high roughness). Achieving a good surface finish typically requires subsequent operations such as reaming or boring.