Drill bits, as the most common cutting tools in hole machining, are widely used in mechanical manufacturing, especially for machining holes in parts such as cooling devices, tube sheets of power generation equipment, and steam generators. Their application is particularly extensive and important.
I. Characteristics of Drilling
Drill bits typically have two main cutting edges. During machining, the drill bit cuts while rotating. The rake angle of the drill bit increases from the central axis to the outer edge; the cutting speed is higher closer to the outer circle and decreases towards the center, reaching zero at the center of rotation. The chisel edge of the drill bit is located near the central axis of rotation. It has a large secondary rake angle, no chip space, and a low cutting speed, thus generating significant axial resistance. If the chisel edge is ground to type A or C according to DIN 1414, the cutting edge near the central axis will have a positive rake angle, which can reduce the cutting resistance and significantly improve cutting performance.
Drill bits can be classified into many types based on their workpiece shape, material, structure, and function, such as high-speed steel drill bits (twist drills, multi-hole drills, flat drills), solid carbide drill bits, indexable shallow hole drills, deep hole drills, nesting drills, and interchangeable head drills.
II. Chip Breaking and Removal
Drill cutting occurs in the confined space of a hole, and chips must be removed through the drill bit's cutting grooves. Therefore, the chip shape greatly affects the cutting performance of the drill bit. Common chip shapes include flake chips, tubular chips, needle-like chips, conical spiral chips, ribbon-like chips, fan-shaped chips, and powdery chips.
The Key to Drilling – Chip Control
When the chip shape is inappropriate, the following problems will occur:
① Fine chips clog the cutting grooves, affecting drilling accuracy, reducing drill life, and even causing drill breakage (e.g., powdery chips, fan-shaped chips, etc.);
② Long chips wrap around the drill bit, hindering operation, causing drill breakage, or preventing cutting fluid from entering the hole (e.g., spiral chips, ribbon chips, etc.).
How to solve the problem of inappropriate chip shape:
① Methods such as increasing feed rate, intermittent feed, re-sharpening the chisel edge, and installing chip breakers can be used individually or in combination to improve chip breaking and removal, eliminating problems caused by chips.
② Specialized chip-breaking drills can be used. For example, adding a designed chip-breaking edge to the drill grooves breaks the chips into more easily removed fragments. The fragments are smoothly discharged along the grooves, preventing clogging. Therefore, new chip-breaking drills achieve a much smoother cutting effect than traditional drills.
Simultaneously, the short, fragmented metal chips facilitate coolant flow to the drill tip, further improving heat dissipation and cutting performance during machining. Moreover, because the newly added chip-breaking edge penetrates the entire groove of the drill bit, it retains its shape and function even after multiple re-sharpening cycles. In addition to the aforementioned functional improvements, it is worth mentioning that this design strengthens the rigidity of the drill body, significantly increasing the number of holes that can be drilled in a single re-sharpening cycle.
III. Drilling Accuracy
The accuracy of a hole is mainly determined by factors such as hole diameter, positional accuracy, coaxiality, roundness, surface roughness, and burrs at the hole opening.
Factors affecting the accuracy of the machined hole during drilling:
① Drill bit clamping accuracy and cutting conditions, such as tool holder, cutting speed, feed rate, and cutting fluid;
② Drill bit size and shape, such as drill bit length, cutting edge shape, and core shape;
③ Workpiece shape, such as the shape of the hole sidewall, hole opening shape, thickness, and clamping status.
Enlarging
Enlarging is caused by the oscillation of the drill bit during machining. The oscillation of the tool holder significantly affects the hole diameter and positioning accuracy. Therefore, the tool holder should be replaced promptly when it is severely worn. Measuring and adjusting the oscillation is difficult when drilling small holes, so it is best to use a large-shank, small-diameter drill bit with good coaxiality between the cutting edge and shank. When using regrinded drill bits, the decrease in hole accuracy is often due to asymmetry in the rear shape. Controlling the cutting edge height difference can effectively suppress the hole's reaming.
Hole Roundness
Due to drill bit vibration, the drilled hole shape is easily polygonal, resulting in riveting patterns on the hole wall. Common polygonal holes are often triangular or pentagonal. The reason for triangular holes is that the drill bit has two centers of rotation during drilling, vibrating at a frequency of once every 600 degrees. The main cause of this vibration is an imbalance in cutting resistance. After one revolution of the drill bit, due to poor hole roundness, the cutting resistance becomes unbalanced during the second revolution, repeating the previous vibration, but with a slight phase shift, resulting in riveting patterns on the hole wall. When the drilling depth reaches a certain level, the friction between the drill bit's cutting edge and the hole wall increases, vibration decreases, the rifling disappears, and roundness improves. This type of hole appears funnel-shaped in longitudinal section. For the same reason, pentagonal and heptagonal holes may also appear during cutting. To eliminate this phenomenon, in addition to controlling factors such as chuck vibration, cutting edge height difference, and asymmetry in the shape of the flank and cutting edge, measures should be taken to improve drill bit rigidity, increase feed per revolution, reduce the clearance angle, and re-grind the chisel edge.
Drilling on Inclined and Curved Surfaces
When the drill bit's cutting face or penetration surface is an inclined surface, curved surface, or step, the positioning accuracy is poor because the drill bit performs radial single-sided cutting, reducing tool life.
To improve positioning accuracy, the following measures can be taken:
1. Drill a center hole first;
2. Mill the hole seat with an end mill;
3. Select a drill bit with good penetration and rigidity;
4. Reduce the feed rate.
Burr Removal
During drilling, burrs often appear at the hole's entrance and exit, especially when machining tough materials and thin plates. This is because as the drill bit nears penetration, the workpiece undergoes plastic deformation. The triangular portion that should be cut by the cutting edge near the outer edge is deformed outwards by the axial cutting force, and further curled under the action of the drill bit's chamfer and cutting edge, forming a burr or rolled edge.
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IV. Drilling Processing Conditions
Most drill bit catalogs contain a "Basic Cutting Parameter Reference Table" arranged by material being processed. Users can refer to the provided cutting parameters to select the appropriate cutting conditions for drilling. The appropriateness of the selected cutting conditions should be judged through trial cutting, considering factors such as machining accuracy, machining efficiency, and drill bit life.
1. Drill Bit Life and Machining Efficiency
Under the premise of meeting the technical requirements of the workpiece, the appropriateness of drill bit use should be comprehensively evaluated based on drill bit life and machining efficiency. The cutting path can be used as an evaluation metric for drill bit life; the feed rate can be used as an evaluation metric for machining efficiency. For high-speed steel drill bits, lifespan is significantly affected by rotational speed and less so by feed per revolution. Therefore, increasing the feed per revolution can improve machining efficiency while ensuring a longer drill bit life. However, it should be noted that excessive feed per revolution will thicken the chips, making chip breaking difficult. Therefore, it is essential to determine the optimal feed per revolution range for successful chip breaking through trial cuts. For carbide drill bits, the cutting edge has a larger chamfer along the negative rake angle direction, resulting in a smaller selectable range for feed per revolution compared to high-speed steel drill bits. Exceeding this range during machining will reduce drill bit life. Because carbide drill bits have higher heat resistance than high-speed steel drill bits, rotational speed has minimal impact on drill bit life. Therefore, increasing the rotational speed can improve machining efficiency while ensuring drill bit life.
2. Proper Use of Cutting Fluid
Drill cutting occurs in the confined space of a hole, therefore the type and application method of cutting fluid significantly impacts drill life and hole machining accuracy. Cutting fluids can be broadly classified into water-soluble and non-water-soluble types. Non-water-soluble cutting fluids offer better lubrication, wetting, and anti-adhesion properties, and also provide rust prevention. Water-soluble cutting fluids offer better cooling properties, are non-smoking, and non-flammable. Due to environmental protection considerations, the use of water-soluble cutting fluids has increased significantly in recent years. However, improper dilution ratios or deterioration of the cutting fluid can drastically shorten tool life, so careful attention must be paid during use. Regardless of whether the cutting fluid is water-soluble or non-water-soluble, it is essential to ensure that the fluid reaches the cutting point adequately. Furthermore, the flow rate, pressure, number of nozzles, and cooling method (internal or external cooling) must be strictly controlled.
V.
Drill Bit Resharpening
Drill Bit Resharpening Criteria
The criteria for determining whether a drill bit needs resharpening are:
1. Wear amount of the cutting edge, chisel edge, and cutting band facets;
2. Dimensional accuracy and surface roughness of the machined hole;
3. Color and shape of the chips;
4. Cutting resistance (indirect values such as spindle current, noise, and vibration);
5. Quantity of work completed, etc.
In actual use, an accurate and convenient criterion should be determined from the above indicators based on specific circumstances. When using wear amount as the criterion, the optimal retharpening period with the best economic efficiency should be identified. Since the main sharpening areas are the back of the head and the chisel edge, excessive drill wear leads to longer sharpening times, larger grinding volumes, and fewer regrinding cycles (total tool life = tool life after regrinding × number of regrinding cycles), ultimately shortening the drill's total lifespan. When using the dimensional accuracy of the machined hole as the criterion, gauges or limit gauges should be used to check the hole's reaming and straightness; if these values are exceeded, the drill should be re-sharpened immediately. When using cutting resistance as the criterion, methods such as automatically stopping the machine when a set limit value (e.g., spindle current) is exceeded can be used. When managing machining quantity limits, the above criteria should be considered comprehensively to set the appropriate standards.
Drill Bit Sharpening Methods
When regrinding drill bits, it is best to use a dedicated drill bit sharpening machine or a universal tool grinder. This is crucial for ensuring drill bit lifespan and machining accuracy. If the original drill bit is in good condition, it can be re-sharpened according to the original design. If the original drill bit has defects, the back shape can be appropriately modified and the chisel edge can be re-sharpened according to the intended use.
The following points should be noted when sharpening:
1. Prevent overheating to avoid reducing the drill bit's hardness;
2. Remove all damage to the drill bit (especially damage to the cutting edge);
3. The drill bit shape should be symmetrical;
4. Take care not to damage the cutting edge during sharpening, and remove any burrs after sharpening;
5. For carbide drill bits, the sharpening shape has a significant impact on drill bit performance. The drill bit shape at the factory is the optimal shape obtained through scientific design and repeated testing; therefore, the original cutting edge shape should generally be maintained when re-sharpening.
