CNC Drilling Process: Techniques And Applications

Views: 268     Author: ANEBON     Publish Time: 2024-12-03      Origin: Site

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CNC Drilling Process: Techniques And Applications

Content Menu

1. Characteristics of drilling

2. Chip breaking and chip removal

3. Drilling accuracy

4. Drilling Processing Conditions

5. Drill bit resharpening


Drill bits are essential tools for creating holes, playing a crucial role in mechanical manufacturing. They are particularly important for making holes in components like cooling devices, tube sheets in power generation equipment, and steam generators. Their application is both wide-ranging and significant.



1. Characteristics of drilling

Drill bits generally 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 you move closer to the outer circumference, the cutting speed of the drill bit also increases. In contrast, the cutting speed decreases toward the center, reaching zero at the very center of the drill bit. The chisel edge is situated near the center axis of rotation. This edge has a large secondary rake angle, lacks a chip space, and exhibits a low cutting speed, which leads to significant axial resistance.


When the chisel edge is ground to either type A or type C in DIN1414, and the cutting edge near the central axis has a positive rake angle, the cutting resistance can be reduced, significantly enhancing cutting performance.


Drill bits can be classified into various types based on factors such as shape, material, structure, and function. These 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, nesting drills, and replaceable head drill bits, among others.



2. Chip breaking and chip removal

The cutting process of a drill occurs within a narrow hole, requiring that chips be discharged through the drill groove. As a result, the shape of the chips significantly affects the drilling performance.


Common chip shapes include flake chips, tubular chips, needle chips, conical spiral chips, ribbon chips, fan-shaped chips, and powder chips.


A crucial aspect of the drilling process is effective chip control. When the chip shape is unsuitable, it can lead to various operational issues, the following problems will occur:


1. Fine chips can clog the groove, leading to reduced drilling accuracy, a shorter drill bit lifespan, and even the potential for the drill bit to break (examples include powder chips and fan-shaped chips).


2. Long chips can wrap around the drill bit, impeding operation. This may cause the drill bit to break or obstruct the flow of cutting fluid into the hole (examples include spiral chips and ribbon chips).


How to solve the problem of improper chip shape


Methods to improve chip breaking and removal include increasing the feed amount, using intermittent feeding, grinding the chisel edge, and installing a chip breaker. These techniques can be applied individually or in combination to effectively address issues caused by chips.


A professional chip-breaking drill bit is designed for drilling holes more efficiently. One key feature of this drill bit is the specially designed-chip-breaking edge located in the groove. This edge effectively breaks chips into smaller pieces, making them easier to remove. As a result, debris is smoothly discharged along the groove, preventing clogs that can occur with traditional drill bits. Consequently, this new chip-breaking drill provides a much smoother cutting effect.


Additionally, the shorter, crushed iron chips allow coolant to flow more freely to the drill tip, enhancing heat dissipation and improving cutting performance during machining. The chip-breaking edge extends throughout the entire groove of the drill, ensuring it retains its shape and functionality even after repeated sharpenings. Beyond these functional enhancements, this design also increases the rigidity of the drill body, significantly allowing for more holes to be drilled before requiring a new grind.

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3. Drilling accuracy

The accuracy of a drilled hole is influenced by several factors, including hole diameter, position accuracy, coaxiality, roundness, surface roughness, and the presence of burrs. Key factors that affect the accuracy of the processed hole during drilling include:


- Drill clamping accuracy and cutting conditions, such as the tool holder, cutting speed, feed rate, and use of cutting fluid.

- The size and shape of the drill, which includes drill length, blade shape, and drill core shape.

- The characteristics of the workpiece, such as the shape of the hole sides, the overall hole shape, thickness, and how the quoting machined parts is clamped.


Hole expansion

Hole expansion occurs due to the swing of the drill bit during operation. The stability of the tool holder significantly affects both the diameter of the hole and its positioning accuracy. Therefore, if the tool holder is substantially worn, it should be replaced promptly.


When drilling small holes, measuring and adjusting the swing can be challenging, so it is advisable to use a coarse shank small-diameter drill that maintains good coaxiality between the blade and the shank.


Additionally, when using a re-ground drill bit, a decline in hole accuracy is often attributed to the asymmetric shape of the back of the bit. By controlling the height difference of the blade, you can effectively minimize the amount of hole cutting and expansion.


Hole roundness

The vibration of the drill bit can cause the drilled hole to become polygonal, often resulting in rifling lines appearing on the hole wall. Common shapes of these polygonal holes are triangular or pentagonal. Triangular holes occur because the drill bit has two rotation centers during drilling, which vibrate at a frequency of 600 revolutions per minute. This vibration primarily results from unbalanced cutting resistance. As the drill bit makes one full rotation, the roundness of the hole is compromised, leading to unbalanced resistance during the subsequent rotation. This cycle repeats, but with a slight phase shift in the vibration, which creates rifling lines on the wall of the hole.


Once the drilling depth reaches a certain level, the friction between the drill bit's edge and the hole wall increases, causing the vibration to diminish, the rifling to disappear, and the roundness to improve. At this stage, the shape of the hole resembles a funnel when viewed in longitudinal section. Similar phenomena can cause pentagonal and heptagonal holes during the cutting process.


To eliminate these issues, it is essential to control various factors such as chuck vibration, differences in cutting edge height, asymmetry of the back face, and blade shape. Additionally, steps should be taken to enhance the drill bit's rigidity, increase the feed per revolution, reduce the back angle, and properly grind the chisel edge.


Drilling on inclined and curved surfaces


When the cutting or drilling surface of a drill bit is inclined, curved, or stepped, it tends to have poor positioning accuracy. This is because the drill bit primarily cuts from one side, which can lead to a reduced tool life.


To enhance positioning accuracy, the following measures can be implemented:

1. Start by drilling a center hole.

2. Use an end mill to create a flat surface for the hole.

3. Select a drill bit that offers good cutting performance and rigidity.

4. Reduce the feed speed.


Burr treatment

During the drilling process, burrs often form at both the entrance and exit of the hole, particularly when working with tough materials and thin plates. This occurs because, as the drill approaches the end of the hole, the material experiences plastic deformation. At this stage, the triangular section that should be cut by the drill’s outer cutting edge becomes deformed and bends outward due to the axial cutting force. Additionally, this section becomes further curled as a result of the chamfer on the drill's outer edge and the blade edge, ultimately leading to the formation of a curled edge or burr.



4. Drilling Processing Conditions

General drill product catalogs include a "Basic Cutting Quantity Reference Table" that is organized by processing materials. Users can refer to the cutting quantities listed in this table to help select appropriate cutting conditions for drilling. However, it is essential to evaluate whether these cutting conditions are suitable by considering factors such as processing accuracy, efficiency, and drill life through trial cuts.


Drill life and processing efficiency.

When processing a workpiece, it is essential to ensure that the technical requirements are met, but it is equally important to assess whether the drill is being used effectively. This assessment should consider both the drill's lifespan and processing efficiency. The drill life can be evaluated based on cutting distance, while processing efficiency can be measured by feed speed.


For high-speed steel drills, the drill's lifespan is significantly influenced by the rotation speed, whereas it is less affected by feed per revolution. Thus, to enhance processing efficiency, the feed per revolution can be increased while still maintaining a longer drill life.


However, care must be taken not to increase the feed per revolution excessively, as this can lead to thicker chips that are difficult to break. It is crucial to determine the appropriate feed per revolution range that allows for successful chip breakage through trial cutting.


In the case of carbide drills, the cutting edge is ground with a larger chamfer and has a smaller range of acceptable feed per revolution compared to high-speed steel drills. Exceeding this range during CNC machining processing can negatively impact the drill's lifespan. While carbide drills are more heat-resistant than high-speed steel drills, the rotation speed has less effect on their life. Therefore, increasing the rotation speed can be an effective method to improve the processing efficiency of carbide drills while still ensuring their durability.


Reasonable use of cutting fluid

Drill cutting occurs in tight spaces, making the choice of cutting fluid and the method of injection crucial for both the lifespan of the drill and the accuracy of the hole being processed. Cutting fluids are generally classified into two categories: water-soluble and non-water-soluble.


Non-water-soluble cutting fluids offer excellent lubricity, wettability, and anti-adhesion properties, as well as providing rust prevention. In contrast, water-soluble cutting fluids are known for their effective cooling performance, producing no smoke and being non-flammable.


Due to environmental protection concerns, the use of water-soluble cutting fluids has increased significantly in recent years. However, if the dilution ratio is incorrect or the cutting fluid has deteriorated, it can severely reduce the tool's lifespan. Therefore, careful management of these fluids is essential.


Regardless of whether the cutting fluid is water-soluble or non-water-soluble, it is imperative that the fluid effectively reaches the cutting point during operation. Strict control over the flow rate, pressure, number of nozzles, and the cooling method (whether internal or external) is crucial for optimal performance.

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5. Drill bit resharpening

The following criteria should be considered when determining whether a drill bit needs to be resharpened:


1. Condition of the cutting edge, chisel edge, and edge face.

2. Dimensional accuracy and surface roughness of the processed hole.

3. Color and shape of the chips produced during drilling.

4. Cutting resistance, as indicated by indirect values such as spindle current, noise, and vibration.

5. Quantity of material processed.


In practical applications, it is essential to establish accurate and convenient judgment criteria based on the indicators mentioned, tailored to specific circumstances. When using wear as the criterion, it is crucial to determine the optimal resharpening period that provides the best economic efficiency. The primary grinding areas of concern are the back of the head and the transverse edge. Excessive drill wear can lead to longer grinding times, larger grinding volumes, and a reduced number of re-grinding opportunities. This means that the total service life of the tool is calculated as the tool life after re-grinding multiplied by the number of times it can be re-ground. Consequently, excessive wear can shorten the overall service life of the drill.


When evaluating dimensional accuracy of the processed hole, it is important to check for hole expansion and non-straightness using a column gauge or a limit gauge. If any measurement exceeds the control value, the tool should be resharpened immediately.


If cutting resistance is used as the judgment standard, an automatic shutdown can be implemented to trigger when the set limit value (such as spindle current) is exceeded. Additionally, when managing processing quantity limits, the judgment criteria should be established based on the aforementioned evaluation standards.


Drill bit sharpening method

When re-sharpening a drill bit, it is advisable to use a drill bit sharpening machine or a universal tool grinder. This is crucial for maintaining the drill bit's service life and processing accuracy. If the original shape of the drill is in good condition, you can re-sharpen it to match that shape. However, if the original shape has defects, you can make appropriate modifications to the rear shape based on the intended use, and sharpen the cutting edges as needed.


The following points should be noted when sharpening:

1. Prevent overheating to maintain the hardness of the drill bit.

2. Remove all damage on the drill bit, particularly any damage to the cutting edges.

3. Ensure that the drill bit shape is symmetrical.

4. Be cautious not to damage the cutting edge while sharpening, and remember to remove any burrs afterward.

5. The sharpening shape of carbide drill bits significantly affects their performance. The original blade shape, designed through scientific testing, is usually the best. Therefore, it is advisable to maintain the original blade shape when re-sharpening.

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If you want to know more or inquiry, please feel free to contact info@anebon.com


Anebon adheres on the tenet "Honest, industrious, enterprising, innovative" to acquire new solutions continuously. Anebon regards prospects and success as personal success. Let Anebon build a prosperous future by hand with brass custom CNC machined parts and complex titanium CNC parts/die-cast toy. Anebon now has a comprehensive goods supply as well as selling price is our advantage. Welcome to inquire about Anebon's products.



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