Optimizing CNC Drilling: Problem Diagnosis and Resolution

Drill bits are the most commonly used tools for hole processing and play a crucial role in mechanical manufacturing. They are primarily utilized for creating holes in various components, such as cooling devices, tube sheets in power generation equipment, and steam generators. The application of drill bits is both extensive and significant in the industry.

 

1. Characteristics of drilling

Drill bits typically feature two main cutting edges. During operation, the drill bit cuts as it rotates. The rake angle of the drill bit increases from the center axis to the outer edge. As the distance from the center increases towards the outer circle, the cutting speed of the drill bit also increases. In contrast, the cutting speed decreases as you move toward the center, and at the center of the drill bit, the cutting speed is zero. The chisel edge, which is located near the center axis of rotation, has a large secondary rake angle. Due to the lack of chip space and the low cutting speed at this point, the chisel edge generates significant axial resistance.

Drill bits can be categorized into various types based on the shape, material, structure, and function of the workpiece. Some common types include high-speed steel drill bits (such as twist drills, group drills, and flat drills), solid carbide drill bits, indexable shallow hole drills, deep hole drills, trepanning drills, and replaceable head drill bits.

 

2. Chip breaking and chip removal

The cutting process of drilling involves creating a narrow hole, where it is essential for chips to be discharged through the drill’s groove. The shape of the chips produced has a significant impact on the cutting performance of the drill. Common chip shapes include flake chips, tubular chips, needle chips, conical spiral chips, ribbon chips, fan-shaped chips, and powder chips.

Key Considerations for Chip Control in Drilling

If the chip shape is not suitable, several issues may arise:

1. Fine chips can block the groove, which affects drilling accuracy, reduces the drill bit’s lifespan, and may even cause the drill bit to break (examples include powder chips and fan-shaped chips).
2. Long chips can wrap around the drill bit, hindering operation and potentially leading to drill bit breakage or impeding cutting fluid from entering the hole (this is a concern with spiral chips and ribbon chips).

 

Solutions for Inappropriate Chip Shapes:

1. To address issues with chip formation, you can implement techniques such as increasing the feed rate, using intermittent feeding, grinding the chisel edge, or installing a chip breaker. These methods can improve the chip breaking and removal process, thereby eliminating problems caused by unsuitable chips.

2. Another effective approach is to use a specialized chip-breaking drill. For instance, adding a chip breaker to the groove of the drill bit can break the chips into smaller, easier-to-remove pieces. This design allows for smooth chip discharge along the groove, preventing clogging. Consequently, chip-breaking drills provide a much smoother cutting effect compared to traditional drills. Additionally, the shorter and broken chips facilitate better coolant flow to the drill tip, enhancing heat dissipation and cutting performance during the drilling process. Notably, the chip breaker is designed to penetrate the entire groove of the drill bit, maintaining its shape and function even after multiple sharpenings. This design also strengthens the rigidity of the drill body and significantly increases the number of holes that can be drilled before requiring grinding.

 

3. Drilling accuracy

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

Key factors that affect the accuracy of the processed hole during drilling include:

1. Drill clamping accuracy and cutting conditions: This encompasses the tool holder, cutting speed, feed rate, and the use of cutting fluid.
2. Drill size and shape: Considerations here include the length of the drill, the shape of the blades, and the profile of the drill core.
3. Workpiece characteristics: This involves the shape of the hole, its dimensions, thickness, and how the workpiece is clamped.

By paying attention to these elements, one can significantly improve the precision of drilled holes.

 

1. Hole expansion

Hole expansion occurs due to the swinging motion of the drill during the machining process. The stability of the tool holder significantly affects both the size of the hole and the accuracy of its positioning. Therefore, it’s important to replace a worn tool holder promptly. When drilling small holes, measuring and adjusting the swing can be challenging, so it is advisable to use a coarse shank drill with a small blade diameter that ensures good coaxiality between the blade and the shank. If a regrind drill is used, any decrease in hole accuracy is often a result of asymmetry in the shape of the back. To effectively mitigate the issues with hole cutting and expansion, controlling the height difference of the blade is essential.

2. Hole roundness

Due to the vibration of the drill bit, the holes that are drilled can easily become polygonal, resulting in rifling lines on the walls of the holes. The most common polygonal shapes are triangular and pentagonal. A triangular hole occurs because the drill bit has two centers of rotation while drilling, which vibrate at a frequency of 600 rotations per minute. The primary cause of this vibration is unbalanced cutting resistance.

When the drill bit makes a full rotation, the roundness of the processed hole may be compromised, leading to unbalanced resistance during the subsequent rotation of the drill. This repetition can cause vibrations, with a certain phase misalignment contributing to the rifling lines on the hole walls. However, as the drilling depth increases, friction between the drill bit blade’s edge and the hole wall also increases, which reduces the vibration, eliminates the rifling, and improves the roundness of the hole. The shape of this type of hole is funnel-shaped in the longitudinal section. Additionally, pentagonal and heptagonal holes may also form during the CNC metal cutting process for similar reasons.

To address this issue, it is essential to control various factors, such as chuck vibration, height differences in the cutting edge, asymmetry of the back face, and the shape of the blade. Furthermore, steps should be taken to increase the drill bit’s rigidity, adjust the feed per revolution, reduce the back angle, and properly grind the chisel edge.

 

3. Drilling on inclined and curved surfaces

When the cutting or drilling surface of a drill bit is inclined, curved, or stepped, the positioning accuracy tends to be poor. In such cases, the drill bit engages in radial single-sided cutting, which leads to a reduced tool life.
To improve the positioning accuracy, the following measures can be taken:
① Drill the center hole first;
② Use an end mill to mill the hole seat;
③ Select a drill bit with good cutting performance and good rigidity;
④ Reduce the feed speed.

 

4. Burr treatment

During the drilling CNC manufacturing process, burrs can form at both the entrance and exit of the hole, particularly when working with materials that have high toughness or are made from thin plates. This occurs because, as the drill bit nearly penetrates the material, it experiences plastic deformation. At this stage, the triangular section that should be cut by the drill bit’s cutting edge near the outer edge becomes deformed and bends outward due to the axial cutting force. Additionally, the chamfer of the drill bit’s outer edge and the edge of the material cause further curling, resulting in the formation of a burr or curling edge.

 

4. Drilling processing conditions

Determining the appropriateness of cutting conditions requires a comprehensive evaluation of factors such as processing accuracy, processing efficiency, and drill life based on trial cutting.

 

1. Drill life and processing efficiency

The proper use of a drill should be evaluated comprehensively based on its lifespan and processing efficiency, while ensuring that the technical requirements of the workpiece are met. Drill life can be measured using cutting distance, and processing efficiency can be assessed by feed speed.

For high-speed steel drills, drill life is primarily influenced by rotation speed and less affected by the feed per revolution. This means that processing efficiency can be improved by increasing the feed per revolution, provided that drill life is maintained. However, it’s important to keep in mind that if the feed per revolution is too high, the chips may become too thick, making them difficult to break. Therefore, it is essential to determine the appropriate range of feed per revolution that allows for smooth chip breaking through trial cutting.

In contrast, carbide drills have a larger chamfer on the cutting edge in the negative rake direction, resulting in a smaller range of permissible feed per revolution compared to high-speed steel drills. Exceeding this range during processing can lead to a reduction in drill life. Although carbide drill bits have better heat resistance than high-speed steel bits, rotation speed has a minimal impact on their lifespan. Consequently, increasing the rotation speed can be an effective method for enhancing the processing efficiency of carbide drill bits while still ensuring their longevity.

 

2. Reasonable use of cutting fluid

Drill cutting is performed in narrow spaces, making the choice of cutting fluid and its application method significantly affect both the lifespan of the drill and the precision of the hole being drilled.

Cutting fluids are categorized into two types: water-soluble and water-insoluble. Water-insoluble cutting fluids offer excellent lubrication, wettability, and resistance to adhesion, as well as protect against rust. On the other hand, water-soluble cutting fluids provide superior cooling performance, produce no smoke, and are non-flammable. Due to environmental considerations, the use of water-soluble cutting fluids has become more prevalent in recent years.

However, improper dilution of water-soluble cutting fluids or the use of deteriorated fluids can greatly reduce the tool’s lifespan, so careful usage is essential.

Regardless of whether a water-soluble or water-insoluble cutting fluid is used, it is crucial that the fluid is effectively delivered to the cutting point. Additionally, factors such as the flow rate, pressure, number of nozzles, and cooling method (either internal or external) must be carefully controlled.

5. Drill bit resharpening

1. Drill bit resharpening judgment

The criteria for evaluating when to resharpen a drill bit include:

- Wear on the cutting edge, chisel edge, and edge face.
- Size accuracy and surface roughness of the drilled hole.
- Color and shape of the chips produced.
- Cutting resistance, indicated by spindle current, noise, and vibration levels.
- Quantity of material processed.

In practice, specific criteria should be established from these indicators based on the particular circumstances. When wear is used as the primary criterion, it is important to determine the optimal resharpening period that offers the best economic value.

The key grinding areas are the back of the head and the transverse edge. If a drill wears excessively, the grinding process will take longer, require more material removal, and reduce the number of times the drill can be resharpened. This means the total service life of the drill is equal to the tool life after resharpening multiplied by the number of times it can be resharpened.

When using the dimensional accuracy of the processed hole as the judgment criterion, it’s advisable to use a column gauge or limit gauge to check for issues such as hole enlargement and non-straightness. If any control value is exceeded, the drill should be resharpened immediately.

If cutting resistance is used as a standard, the machine can be configured to automatically shut down when a predetermined limit, such as spindle current, is surpassed. When managing processing quantities, it is beneficial to combine the above evaluation methods to establish a comprehensive judgment standard.

 

2. 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 longevity and precise performance of the drill bit. If the original shape of the drill is in good condition, it can be re-sharpened to match its original form. However, if there are defects in the original shape, you can make appropriate improvements to the rear shape and sharpen the cross edges based on its intended use.

Here are some important points to remember while sharpening:

- Prevent overheating, as excessive heat can reduce the hardness of the drill bit.
- Remove any damage on the drill bit, especially at the edge of the blade.
- Ensure that the drill shape is symmetrical.
- Be careful not to damage the cutting edge during sharpening, and remove any burrs afterward.
- For carbide drill bits, the sharpening shape significantly affects their performance. The original blade shape, established through scientific design and extensive testing, is typically the most effective. Therefore, it is advisable to maintain the original blade shape when re-sharpening.

 

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