Essential Factors Affecting Drilling Machining Efficiency

Drill bits are the most commonly used tools for hole processing and play a crucial role in mechanical manufacturing. They are especially important for creating holes in various components, such as cooling devices, tube sheets for power generation equipment, and steam generators. Their application is extensive and significant across many industries.

 

1. Characteristics of drilling

Drill bits typically have two main cutting edges. When in use, the drill bit rotates and cuts into the material. The rake angle of the drill bit increases from the center axis to the outer edge, resulting in higher cutting speeds closer to the outer circle. Conversely, the cutting speed decreases toward the center, where it reaches zero at the rotating center of the drill bit.

The chisel edge is positioned near the center axis of rotation. This edge has a large secondary rake angle and lacks a chip space, leading to lower cutting speeds and higher axial resistance. If the chisel edge is ground to type A or C, and if the cutting edge near the center axis has a positive rake angle, cutting resistance can be reduced, significantly enhancing the drill bit’s cutting performance.

Drill bits can be categorized into many types based on factors such as shape, material, structure, and function. Some common categories include high-speed steel drill bits (such as twist drill bits, group drill bits, and flat drill bits), solid carbide drill bits, indexable shallow hole drill bits, deep hole drill bits, sleeve drill bits, and interchangeable head drill bits.

 

2. Chip breaking and chip removal

The cutting process of a drill bit occurs in a narrow hole, requiring chips to be expelled through the grooves of the bit. As a result, the shape of the chips significantly impacts the cutting performance of the drill bit. Common chip shapes include flake chips, tubular chips, needle chips, conical spiral chips, ribbon chips, fan-shaped chips, and powder chips.

 

The key to drilling processing – chip control
When the chip shape is inappropriate, the following problems will occur: Fine chips can obstruct the groove, which affects drilling accuracy, shortens the lifespan of the drill bit, and may even result in drill bit breakage (this includes types like powder chips and fan-shaped chips).

On the other hand, long chips can wrap around the drill bit, impeding operations and potentially causing the drill bit to break. They can also prevent cutting fluid from properly entering the hole (examples include spiral chips and ribbon chips).

 

How to solve the problem of inappropriate chip shape:

You can improve chip breaking and removal by using methods such as increasing the feed rate, intermittent feeding, sharpening the chisel edge, and installing a chip breaker—either individually or in combination. These techniques can help resolve issues caused by chips.

For drilling holes, consider using a professional chip-breaking drill bit. For instance, incorporating a chip breaker into the groove of the drill bit helps to break chips into smaller, more manageable pieces. These smaller debris pieces can be smoothly discharged along the groove, preventing any clogging. As a result, the new chip breaker drill provides a much smoother cutting action compared to traditional drills. Additionally, the shorter, broken iron chips allow coolant to flow more easily to the drill tip, which enhances heat dissipation and cutting performance during operations.

Moreover, because the newly designed chip breaker extends throughout the drill bit’s groove, it retains its shape and function even after repeated sharpening. Beyond these functional benefits, this design also fortifies the rigidity of the drill body, significantly increasing the number of holes that can be drilled before any grinding is necessary.

 

3. Drilling accuracy

The accuracy of a drilled hole is influenced by several key factors, including hole diameter, positional accuracy, coaxiality, roundness, surface roughness, and the presence of burrs.

The factors affecting the accuracy of the processed hole during drilling include:

- Clamping Accuracy and Cutting Conditions: This encompasses the tool holder, cutting speed, feed rate, and the use of cutting fluid.
- Drill Bit Characteristics: This involves the size and shape of the drill bit, including its length, blade design, and drill core shape.
- Workpiece Geometry: This refers to the overall shape of the workpiece, the contour of the hole, its thickness, and the clamping status during drilling.

These elements collectively ensure the precision of the final drilled hole.

 

1 Hole expansion

Hole expansion occurs due to the swing of the drill bit during processing. The movement of the tool holder significantly affects both the diameter of the hole and the accuracy of its positioning. Therefore, it is important to replace a severely worn tool holder promptly.

When drilling small holes, measuring and adjusting the swing can be challenging. To achieve better results, it is advisable to use a drill with a coarse shank and a small blade diameter that ensures good coaxiality between the blade and the shank.

In addition, when using a re-ground drill bit, the loss of hole accuracy is often due to the asymmetry of the bit’s backside shape. Effectively controlling the height differences of the blade can help minimize hole cutting and expansion.

 

2 Hole roundness

The vibration of the drill bit can cause the drilled hole to take on a polygonal shape, often resulting in rifling-like lines on the hole wall. Common polygonal shapes include triangles and pentagons. The appearance of a triangular hole occurs because the drill operates with two rotation centers while drilling, which vibrate at a frequency of 600 rotations per minute. This vibration is primarily caused by unbalanced cutting resistance.

When the drill makes one complete turn, the resulting hole may not be perfectly round. This lack of roundness contributes to the unbalanced resistance experienced during the next cutting turn, leading to repeated vibrations. However, the phase of these vibrations is slightly offset, resulting in the rifling lines observed on the hole wall.

As drilling progresses to a certain depth, friction between the drill blade’s edge and the hole wall increases, which in turn reduces the vibration and eliminates rifling, leading to improved roundness. The longitudinal section of this hole type is generally funnel-shaped.

Pentagonal and heptagonal holes can also emerge during the cutting process for similar reasons. To mitigate these issues, it is important to control factors such as chuck vibration, differences in cutting edge height, and any asymmetry in the back and blade shape. Additionally, strategies like enhancing drill rigidity, increasing the feed rate per rotation, reducing the back angle, and sharpening chisel edges can be effective.

 

3 Drilling on inclined and curved surfaces

When the cutting or drilling surface of a drill bit has an inclined, curved, or stepped design, it can lead to reduced positioning accuracy. Since the drill bit performs single-sided cutting radially in these cases, its tool life is also decreased. To enhance positioning accuracy, the following measures can be taken: 1. Start by drilling a center hole; 2. Use an end mill to create a proper hole seat; 3. Choose a drill bit with excellent cutting performance and rigidity; 4. Reduce the feed speed.

 

4 Burr treatment

During the drilling process, burrs tend to form at both the entrance and exit of the hole, particularly when working with materials that have high toughness and when drilling thin sheets. This occurs because, as the drill bit approaches the end of the hole, the processed material experiences plastic deformation. At this point, the triangular section that is meant to be cut by the drill bit’s edge near the outer perimeter becomes deformed and bends outward due to the axial cutting force. Additionally, it curls further due to the chamfer of the drill bit’s outer edge and the edge band, resulting in a curled edge or burr.

 

4. Drilling processing conditions

General drill product catalogs typically include a “Basic Cutting Quantity Reference Table” organized by processing materials. Users can refer to this table to help select the appropriate cutting conditions for drilling. However, the suitability of these cutting conditions should be evaluated comprehensively, taking into account factors such as processing accuracy, efficiency, and drill life, ideally through trial cutting.

 

1 Drill life and processing efficiency

To effectively evaluate the proper use of a drill while meeting the technical requirements of the workpiece, we should consider both the drill’s lifespan and processing efficiency. The lifespan of the drill can be assessed using the cutting distance, while processing efficiency can be measured by the feed speed.

For high-speed steel drills, the lifespan is significantly influenced by the rotation speed, whereas the feed per revolution has a lesser impact. Thus, we can enhance processing efficiency by increasing the feed per revolution, provided that we also ensure a longer drill lifespan. However, it’s important to remember that if the feed per revolution is too high, chip thickness will increase, making chip breakage difficult. Therefore, it is essential to determine the optimal feed per revolution range that allows for effective chip breakage through trial cutting.

In contrast, carbide drills have cutting edges with a negative rake angle that features a larger chamfer, which limits the optimal feed per revolution compared to high-speed steel drills. If the feed per revolution exceeds this recommended range during processing, the drill’s lifespan may be compromised. However, carbide drills possess greater heat resistance than high-speed steel drills, meaning that rotation speed has less effect on their lifespan. Therefore, increasing the rotation speed is a viable method to enhance the CNC milling processing efficiency of carbide drills without adversely affecting their lifespan.

 

2 Reasonable use of cutting fluid

Drilling operations occur in narrow spaces, making the choice of cutting fluid and injection method crucial for the durability of the drill and the accuracy of the hole being processed. Cutting fluids can be classified into two main categories: water-soluble and non-water-soluble.

Non-water-soluble cutting fluids offer excellent lubrication, wettability, and anti-adhesion properties, as well as rust prevention. On the other hand, water-soluble cutting fluids provide effective cooling, do not produce smoke, and are non-flammable. In recent years, there has been an increased use of water-soluble cutting fluids due to environmental protection considerations.

However, if the dilution ratio of water-soluble cutting fluids is incorrect or if the fluid has deteriorated, it can significantly reduce the tool’s lifespan, so careful management is essential. Regardless of whether a water-soluble or non-water-soluble cutting fluid is used, it is important that the cutting fluid effectively reaches the cutting point during operation. Additionally, parameters such as flow rate, pressure, number of nozzles, and cooling method (either internal or external) must be strictly controlled to ensure optimal performance.

 

5. Drill bit resharpening

Drill bit resharpening judgment

The judgment criteria for drill bit resharpening are:
- Wear cutting edge, chisel edge, and edge face;
- Dimensional accuracy and surface roughness of processed hole;
- Color and shape of chips;
- Cutting resistance (indirect values ​​such as spindle current, noise, vibration, etc.);
- Processing quantity, etc.

In practical applications, it is essential to establish accurate and convenient judgment criteria from the indicators mentioned above, tailored to specific circumstances. When using wear as the judgment criterion, it is crucial to determine the optimal resharpening period that yields the best economic efficiency. The primary grinding areas of the drill are the back of the head and the transverse edge. If the drill experiences excessive wear, the grinding process will take longer, involve a larger grinding amount, and reduce the number of times the drill can be resharpened. This ultimately shortens the total service life of the tool, as the total service life equals the tool life after resharpening multiplied by the number of resharpening cycles.

When using the dimensional accuracy of the processed hole as the judgment standard, it is advisable to employ a column gauge or limit gauge to check for issues such as cutting expansion and non-straightness. If the measurements exceed the control values, the drill should be resharpened immediately.

When cutting resistance is used as a judgment standard, the machine can be programmed to shut down automatically if the set limit value (such as spindle current) is exceeded.

In instances where processing quantity limit management is implemented, it is important to combine the judgment criteria mentioned above to establish a comprehensive set of standards.

 

Drill bit sharpening method

When re-sharpening a drill bit, it is best to use a specialized drill sharpening machine or a universal tool grinder. This is crucial for ensuring the drill bit’s service life and processing accuracy. If the original drill shape is in good condition, it can be re-sharpened to match that shape. However, if there are defects in the original drill design, the rear shape can be improved, and the cutting edges can be sharpened based on their intended use.

Here are some important points to consider when sharpening a drill bit:

- Prevent overheating to avoid reducing the hardness of the drill bit.
- Remove all damage on the drill bit, especially any damage to the cutting edges.
- Ensure that the drill shape is symmetrical.
- Handle the cutting edge carefully during sharpening, and be sure to remove any burrs afterward.
- For carbide drill bits, the sharpening shape significantly impacts performance. The factory-designed drill shape is the result of scientific design and repeated testing, so it is advisable to maintain the original blade shape when re-sharpening.

 

 

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