Chisel edge and finishing of twist drill

Twist drills, as one of the most common and widely used hole-making tools, depend not only on the material and manufacturing precision but also, to a large extent, on the quality of sharpening. Among the entire sharpening system, understanding and sharpening the chisel edge is one of the key factors determining drilling performance.

The chisel edge refers to a short cutting edge at the center of the drill tip that has no cutting capability. Because the two main cutting edges of the drill bit cannot form an effective cutting geometry at the center, a structure resembling an "extrusion edge" is formed in the central region of the drill bit—this is the chisel edge. Geometrically, it does not have a positive rake angle like the main cutting edges but usually exhibits a large negative rake angle, its function being more like "extruding material" than true cutting. Therefore, during drilling, the chisel edge region not only has low cutting efficiency but also generates significant axial force, being one of the main sources of drilling resistance.

Because the chisel edge has the characteristic of "extrusion rather than cutting," its length and angle have a significant impact on drilling performance. If the chisel edge is too long, it means the "ineffective cutting zone" in the center of the drill bit expands, requiring a larger feed force during drilling and easily leading to problems such as drill slippage, difficulty in centering, and hole misalignment. Simultaneously, chips are difficult to form and remove, resulting in increased temperature rise and accelerated tool wear. Therefore, in practical applications, shortening the chisel edge length and improving its rake angle through proper chisel edge re-grinding is an important means of improving drilling performance.

The core objectives of chisel edge re-grinding can be summarized in three points: first, shortening the chisel edge length to reduce axial resistance; second, improving the chisel edge rake angle, transforming it from a negative rake angle to near zero or even a slightly positive rake angle, thereby enhancing the "cutting" rather than "extrusion" capability; and third, optimizing the cutting state at the center of the drill bit, improving centering ability and hole machining accuracy. Through these adjustments, the "bite" of drilling can be significantly improved, making it easier for the drill bit to enter the material, while also resulting in smoother cutting and chip removal.

In actual operation, chisel edge re-grinding is usually not performed in isolation but is completed simultaneously with the grinding of the main flank face. Common chisel edge grinding methods include cross-grinding, chip flute grinding, and core thinning. Among these, core thinning is the most basic and widely used method. Essentially, it involves grinding away a portion of the material from the center of the drill bit, shortening and thinning the chisel edge. This method is relatively simple to operate, but requires a high level of operator skill and experience. Improper grinding can easily disrupt the symmetry of the two main cutting edges, leading to drill misalignment or vibration.

Besides the chisel edge itself, the overall geometry of a standard twist drill is equally crucial. For example, the point angle is typically controlled at around 118° (with an allowable error of ±2°), a commonly used range balancing cutting efficiency and tool strength; the clearance angle is generally between 10° and 14°; too large an angle results in an excessively thin cutting edge and increased vibration, while too small an angle increases friction and causes severe heat generation; the chisel edge bevel angle is generally between 50° and 55°, directly affecting the contact state between the chisel edge and the workpiece. These parameters are interrelated; any deviation in any angle can affect overall cutting performance.

However, theoretical parameters are only the foundation; what truly determines the effect is the operational skill during the actual sharpening process. Many beginners, when learning to sharpen drill bits, are often eager to start, neglecting the importance of observation and understanding, leading to increasingly chaotic sharpening. In fact, the most crucial aspect of drill bit sharpening is not the action itself, but the understanding of positional relationships and geometry. Experienced sharpeners often emphasize "sharpen less, observe more," which essentially means establishing a correct spatial concept before starting.

In specific operations, several classic rules are extremely helpful for beginners. First, "level the cutting edge against the wheel surface," emphasizing that the main cutting edge must maintain horizontal contact with the grinding wheel surface; this is a prerequisite for ensuring even sharpening. Second, "angle the drill axis to create a sharp angle," meaning the drill axis forms approximately a 60° angle with the grinding wheel surface, thus forming the correct apex angle. Third, "grind from the cutting edge to the back face," requiring a transition from the cutting edge to the back face, ensuring the correct grinding sequence and facilitating heat dissipation. Finally, "avoid lifting the tail during up-and-down swinging," this is the most critical action; it must create a back angle without disrupting the cutting edge shape.

The "up-and-down oscillation" motion is often the most difficult part for beginners to master. If the oscillation turns into rotation, it will damage the other cutting edge; if the tail rises, it will dull the cutting edge. Therefore, the correct motion should be a small oscillation led by the front end, while the rear end remains stable. This motion directly determines the quality of the clearance angle and affects the drill bit's sharpness.

After sharpening the two main cutting edges, symmetry needs to be checked. The lengths of the two cutting edges must be equal and their angle with the axis must be consistent; otherwise, wobble will occur during drilling, leading to enlarged hole diameter or irregular hole shape. Experienced operators usually observe the drill tip against the light, judging the symmetry of the two cutting edges by the light and shadow. This method is simple but very effective and is a commonly used inspection method in practice.

For larger diameter drill bits, the drill tip also needs to be properly re-sharpened. Since a small plane often forms at the intersection of the two cutting edges, which affects the drill bit's centering ability, a slight chamfer needs to be applied at the root of the cutting edge to reduce this plane. It is particularly important to note that this step must not damage the main cutting edge, otherwise it will change the rake angle and affect cutting performance.

Returning to the chisel edge re-grinding itself, its improvement effect is very obvious in actual machining. Shortening the chisel edge significantly reduces the axial force required for drilling, which is especially important on equipment with limited feed capacity, such as hand drills. At the same time, due to the improved chisel edge rake angle, the material is more easily cut into rather than squeezed, resulting in smoother cutting and easier chip breakage and removal. Furthermore, the positional accuracy and surface quality of the hole are also improved.

In some specific cases, the drill bit can also be re-grinded in a "non-perfectly symmetrical" manner according to actual needs. For example, when the hole diameter accuracy requirement is not high, the two cutting edges can be slightly unequal in height, resulting in a slightly larger hole opening. This reduces friction between the cutting edge and the hole wall, thereby reducing cutting resistance. Although this method is not standard re-grinding, it has practical value under certain working conditions.

Furthermore, adjusting the point angle can also adapt to different machining conditions. For example, in handheld electric drill machining, due to the relatively small feed force, the point angle can be appropriately reduced to increase the cutting edge's pressure on the material, making it easier for the drill bit to "bite in". However, if the point angle is too small, it will affect the rake angle and reduce the tool strength, so a balance must be found in practice.