Vibration Drilling Technology for High Aspect Ratio Deep Hole Applications
The machining of small, deep holes in increasingly complex hydraulic actuator components primarily encounters issues related to chip breaking, chip removal, and cooling. By utilizing mechanical vibration drilling technology on turning-milling machine tools and selecting appropriate tools for deep hole processing, along with optimizing cutting parameters, we can effectively address the challenges of machining deep holes with a high aspect ratio. This approach enables the efficient processing of large aspect ratio oil pipe holes on turning-milling machine tools.
1 Introduction
In the drilling process, “aspect ratio” refers to the ratio of the depth to the diameter of the hole being drilled (i.e., L/D). Generally, holes with aspect ratios greater than 10 are classified as “deep holes.” As the complexity and integrity of parts have increased, deep hole processing has become more common, and drilling technology has seen significant advancements.
Various drilling tools, such as deep hole twist drills, gun drills, BTA drills, ejector drills, and DF drills, have emerged. Coupled with new deep hole processing technologies like mechanical vibration drilling, ultrasonic vibration drilling, hydraulic vibration drilling, and electromagnetic vibration drilling, these innovations have greatly enhanced deep hole processing capabilities compared to the past.
However, deep hole processing presents unique challenges. The drill bit operates under semi-closed conditions when entering the workpiece, which imposes many restrictions during actual drilling. For example, the cutting conditions of the tool cannot be directly observed; heat generated during cutting is difficult to dissipate, necessitating an effective cooling method. Chip removal is also challenging, and the drill rod must possess adequate rigidity. These challenges can lead to varying results in practical applications, highlighting the need for further validation and analysis based on the actual processing conditions of the parts.
2 Typical deep hole processing problems
The local structure of a typical hydraulic actuator cylinder body part is illustrated in Figure 1. This component is made from 30CrMnSiA and has a hardness of 35-41 HRC after heat treatment. The manufacturing process focuses on complex shape milling, deep hole boring, and the precision processing of a hole system. It also involves the processing of three groups of oil pipe holes, each with a diameter of 9 mm and a maximum aspect ratio of 6:3. The effective depths of these holes are L1 = 559 mm, L2 = 347 mm, and L3 = 282 mm (hereafter referred to as the “559″ hole, “347″ hole, and “282″ hole, respectively). The straightness of each hole is required to have an offset of no more than 0.2 mm over a length of 100 mm, and the surface roughness should be Ra = 3.2 μm.
The traditional method for processing deep holes involves the use of a specialized machine tool. For every three groups of oil pipe holes that are processed on a part, three sets of special fixtures must be replaced. The specialized machine tool and fixtures used for this process are shown in Figure 2.
The traditional method for processing deep holes involves using a specialized machine tool. For each set of three oil pipe holes that need to be processed on a part, three sets of special fixtures must be replaced. The specialized machine tool and fixtures used in this process are illustrated in Figure 2.
The data presented in Table 1 highlights that the processing efficiency of specialized machine tools is low and the processing cycle is both complicated and time-consuming. For parts produced in large quantities, this deep hole machining process has become a bottleneck in production. Therefore, it is crucial to improve the processing methods and enhance efficiency to meet production demands.
3 Problem Analysis
3.1 Difficulties in Deep Hole Processing Deep hole processing mainly has the following difficulties.
1) Large Aspect Ratio: When processing deep holes with a length-to-diameter ratio (L/D) greater than 10, it becomes challenging to monitor and assess the tool’s status. Ensuring accuracy during this process is also difficult.
2) Difficult Chip Removal: The removal of chips is complicated due to their long path, which can lead to blockages. This blockage risks tool breakage and surface scratching, ultimately affecting the quality of the finished surface.
3) Poor Heat Dissipation: The cutting heat generated is hard to dissipate, making the cutting edge susceptible to overheating. This leads to increased tool wear. Additionally, materials that are difficult to process, such as titanium alloys and high-temperature alloys, further complicate the processing.
4) Poor Rigidity: Long drill rods tend to have poor rigidity, making them prone to deformation and vibration, which can cause hole deflection.
To address these challenges in deep hole processing, several effective measures can be implemented:
- Cooling and Lubrication: High-pressure internal cooling can cool, lubricate, and aid in chip removal, with oil-based cutting fluids being particularly effective.
- Guidance Improvement: Using guide blocks or enhancing the accuracy of guide holes can improve guidance precision.
- Chip Breaking: Employing methods such as self-excited vibration, forced vibration, or incorporating chip breaker grooves into the tool can facilitate rapid chip breakage and enhance chip removal.
3.2 Functions of milling machine tools
The use of advanced high-end CNC machine tools has led to a more focused processing approach. The structure of the milling machine tool is illustrated in Figure 3, where M1=3 and M1=4 indicate the forward and reverse rotation of the turning spindle S1, respectively, while M3=3 and M3=4 represent the forward and reverse rotation of the milling spindle S3.
This milling machine tool allows for various operations—including turning, milling, drilling, boring, reaming, gear processing, deep hole processing, and eccentric processing—to be completed in a single setup. This integration enhances processing flexibility and concentration, improving overall process efficiency. Moreover, it reduces the number of times parts need to be clamped during the entire processing workflow, which helps maintain processing accuracy.
Notably, the tool’s capability for deep hole processing is especially beneficial for manufacturing hydraulic actuator cylinders and piston components. The WFL milling machine discussed in this article features a high-pressure internal cooling function, capable of reaching up to 80 bar (with a maximum of 350 bar; 1 bar = 0.1 MPa), and its deep boring dovetail unit can efficiently handle deep hole processing tasks.
3.3 Gun drill tool
The tools used for deep hole processing include deep hole twist drills, gun drills, BTA drills, ejector drills, and DF drills. After considering the specific processing characteristics of the part, the hole size, tool costs, and other relevant factors, we have chosen a f 9.13 mm brazed gun drill for this part’s processing.
The gun drill is designed for external chip removal and is illustrated in Figure 4. It is primarily used to create deep holes with diameters ranging from 3 to 30 mm, achieving an aspect ratio of up to 100. The processing hole accuracy ranges from IT8 to IT10, and the surface roughness value can vary between Ra 0.8 to 3.2 μm.
The geometry of the gun drill is illustrated in Figure 5. The drill tip is not aligned with the center of rotation, and the cutting process occurs along the inner and outer cutting edges that intersect at the drill tip. The grinding geometry of the gun drill significantly affects its chip breaking ability and overall lifespan.
3.4 Mechanical vibration drilling technology
(1) Principle of Vibration Drilling
Vibration drilling is a technique that involves superimposing vibrations onto the drilling process. It can be categorized based on the type of vibration: self-excited vibration drilling and forced vibration drilling. Additionally, it can be classified according to the vibration device used, including ultrasonic vibration, electromagnetic vibration, hydraulic vibration, and mechanical vibration.
This paper focuses on the mechanical vibration drilling technology developed by MITIS in France, which aims to address issues related to short tool life and inconsistent processing quality in deep hole machining during turning and milling operations.
The mechanical vibration tool holder is a device that utilizes an electric motor to drive an eccentric cam, generating vibrations as illustrated in Figure 6. The mechanical vibration-assisted hole-making technology superimposes sinusoidal reciprocating motion on a constant feed motion, creating what is referred to as “micro-vibration feed.” This technique results in periodic cutting, enhancing conditions for heat dissipation and chip breaking.
(2) Factors affecting vibration drilling. The adjustment of amplitude and cutting parameters significantly affects chip breaking during drilling. The three primary parameters in vibration drilling are amplitude, frequency, and feed rate. These parameters greatly influence the actual drilling angle, the length of the chips produced, and the drilling torque required.
(3) Characteristics and advantages of vibration drilling. The main characteristics and advantages are as follows.
1) Improved Machining Accuracy and Surface Quality: Vibration drilling results in smooth and uniform surfaces on hole walls, without the formation of built-up edges. In contrast, traditional cutting processes often produce built-up edges that can increase the radial size of the cutting edge and lead to hole expansion. Vibration drilling eliminates the conditions that allow built-up edges to form, thereby preventing any diameter expansion caused by them.
2) Reduced Drilling Force and Torque: Vibration cutting involves a pulsed motion with constantly changing speeds and directions, which encourages the metal to transition to a brittle state. This reduces plastic deformation and lowers the friction coefficient, leading to decreased drilling force and torque, as well as reduced power consumption.
3) Lower Cutting Temperatures: Traditional drilling methods generate significant cutting heat due to their semi-closed cutting nature. This excess heat can cause thermal expansion and deformation of both the workpiece and drill bit, resulting in increased hole diameter. Vibration drilling, however, operates with reduced axial force and torque, and offers enhanced cooling and lubrication, which significantly lowers cutting temperatures and helps prevent hole diameter expansion.
4) Easier Chip Handling: The efficiency of chip removal during drilling directly impacts hole diameter stability. If chips are not effectively removed, they can interfere with the cutting process, leading to diameter expansion. Vibration drilling excels in chip breaking and removal, effectively addressing the challenges posed by chip blockage.
5) Reduced Tool Wear and Extended Tool Life: The combination of decreased drilling resistance and lower temperatures reduces tool wear and extends tool life. Additionally, the vibration effect in axial vibration drilling enhances the rigidity of the drill bit. This pulsed cutting force minimizes bending and deformation during drilling, significantly prolonging the tool’s service life.
4 Verification process
4.1 Turning and milling deep hole drilling process
Drilling deep holes on a turning and milling machine differs significantly from the methods used in traditional deep hole drilling machines. Professional deep hole drilling machines require special fixtures to ensure the accurate positioning of deep holes. In these systems, the drill sleeve on the fixture guarantees the precise location and accuracy of the hole.
In contrast, when drilling deep holes on a turning and milling machine, no fixtures or drill sleeves are needed. The gravity of the extended twist drill or gun drill, combined with the tool’s length-to-diameter ratio, makes it challenging to maintain the precise position of the hole during deep drilling. Therefore, the accuracy and location of the hole can only be secured by first drilling a guide hole. The processing steps involved are illustrated in Figure 7.
The main process is as follows:
1. Drill the guide hole: The diameter of the guide hole should be 0.01 to 0.02 mm larger than the diameter of the gun drill (IT7), with a minimum depth of 2.5 times the drill diameter (2.5D).
2. Initial drilling: Start at a low speed (≤30 r/min) with a low feed rate and reverse entry. Pause when you are 1-2 mm away from reaching the effective depth of the guide hole, then turn on the cutting fluid.
3. Increase speed: After the initial pause, increase to the normal cutting speed and begin the feed movement for drilling. Do not retract the tool, and pay close attention to the shape of the chips being produced.
4. Finish drilling: Once you reach the effective depth, quickly retract the gun drill to the guide drill position at the bottom of the hole. Stop the rotation and cooling, then swiftly move to a safe position.
4.2 Mechanical vibration drilling process
The process of mechanical vibration drilling offers several advantages. By utilizing mechanical vibration tool holders and gun drills, it is possible to superimpose sinusoidal reciprocating motion onto the tool’s linear feed motion. This combination creates a “micro-vibration feed” that aids in chip breaking, ultimately enhancing processing quality and extending tool life.
(1) Tool Clamping: This technology employs the mechanical vibration drilling method, with the French MITIS vibration tool holder chosen to secure the gun drill, as illustrated in Figure 8. Given that the tool handle features an ER32 retaining ring interface, a sealing ring is necessary to prevent the internal cooling pressure from being released.
(2) Processing procedure
In the turning-milling compound gun drilling CNC turning process, first drill and bore the guide hole, then use the gun drill to directly process to the effective depth. The program is compiled as follows.
N1; (DRLL D9.1 HOLE)
CDSTLCH1(“ZT9D1″, -90)
TLPREP1(“XTD9D15″)
MCMILLS1
MCUTLIM(15)
G54 G64
G95 S3=2500 M3=3 F0.06
G0 X1=59 Y1=-16 Z1=150 C1=0 M3=7
DRILL(0, 20, 3, 20)
G0 Z1=150
CDSM00
N2; (BORING D9.15 HOLE)
CDSTLCH1(“XTD9D15″, -90)
TLPREP1(“QZ9D13″)
MCMILLS1
MCUTLIM(10)
G54 G64
G95 S3=1200 M3=3 F0.05
G0 X1=59 Y1=-16 Z1=150 C1=0 M3=7
DRTURN(0, 20, 3, 19.8, 0, 0.05)
G0 Z1=150
CDSM00
N3; (DRLL D9.13 HOLE)
CDSDKM00
TLCH1H(“QZ9D13″, -90)
TLPREP1(“”)
MCMILLS1
MCUTLIM(10)
G54 G64
G95 S3=50 M3=4 F0.06
G0 Z1=20 C1=0
G0 X1=59 Y1=-16
Z1=5
G1 Z1=-1 F1
M00
G1 Z1=-15 F0.5
M3=5
M99
S3=1400 M3=3
G4 F2
Z1=-560.5 F0.015
G4 F2
S3=300
G0 Z1=-15
M3=9
G0 Z1=10
X1=200
M00
TLCH1H
M30
(3) Processing test parameters
The mechanical vibration drilling technology was employed, and the processing parameters were adjusted to increase both the amplitude and frequency. Taking into account the material characteristics of the parts, the heat treatment state, the clamping rigidity, and the tool material, we verified the chip breaking conditions under various cutting parameters and amplitudes. This allowed us to control the chip shape, improve processing quality, and enhance tool wear resistance. The amplitude, frequency, and feed rate significantly affect the wear degree of the tool and the chip breaking conditions. The specific test parameters are detailed in Table 2.
(4) Conclusion of the processing test
The main conclusions are as follows:
1) The use of mechanical vibration drilling technology results in effective chip breaking, with chip lengths typically measuring between 3 and 7 mm. In contrast, when mechanical vibration drilling technology is not employed, the chip lengths are generally between 18 and 35 mm. Figure 9 illustrates the comparison of chip lengths for the two processing methods.
2) The amplitude and accuracy of the matching of cutting parameters directly influence the formation of different chip shapes. In deep hole machining, the determination of chip breaking and removal is based on the ability to produce C-shaped chips during the process. The formation of these C-shaped chips provides several advantages, including improved surface roughness of the inner hole and enhanced tool life.
3) Mechanical vibration drilling significantly enhances tool life compared to conventional drilling methods. According to cutting statistics, when processing holes of the same depth, the tool life increases by over 30% when using mechanical vibration drilling technology, as opposed to using a side-fixed tool holder. Figure 10 illustrates the comparison of tool wear under identical processing conditions.
4) Processing quality and processing efficiency.
After using a mechanical vibration toolholder for drilling, we observed significant improvements in chip breaking and removal compared to a side-fixed toolholder. Data from multiple batches of parts confirmed that the processing quality is high, particularly regarding the straightness of the holes. The deviation remains within 0.12 mm over a length of 100 mm, which reduces the risk of the gun drill breaking inside the parts.
From the perspective of the comprehensive energy efficiency ratio during tool testing, the efficiency of processing oil pipe holes on a turning-milling machine with mechanical vibration drilling technology is over 7.5 times greater than that on a dedicated machine tool (when considering pure cutting time), as shown in Figure 11. Additionally, machining on a turning-milling machine eliminates the downtime associated with changing fixtures back and forth, significantly enhancing overall processing efficiency.
5 Conclusion
In the metal cutting process, the material removal rate during hole processing accounts for approximately one-third of the total material removal rate. While advanced tools, technologies, and methods for hole processing are enhancing efficiency and quality both domestically and internationally, challenges remain in the stable and efficient processing of micro holes and deep holes.
To address the hole processing issues encountered in on-site production, it is essential to both adopt new tools and integrate existing theoretical knowledge on hole processing. This integration should involve verification through processing technologies and methods. For instance, innovative techniques such as mechanical vibration drilling and ultrasonic vibration drilling represent new approaches to tackle the challenges of deep hole processing.
It is crucial to consider factors such as the specific machine tools in use, the tools themselves, the workpieces, workpiece materials, clamping rigidity, and cooling methods during the machining process. Gathering more data through verification and practical experience will help build a solid foundation for enhancing our manufacturing capabilities.
If you want to know more or inquiry, please feel free to contact info@anebon.com
At Anebon, we believe in the values of innovation, excellence, and reliability. These principles are the foundation of our success as a mid-sized business that provides customized machining parts, turning parts, and aluminum die casting parts for various industries such as non-standard devices, medical, electronics, auto accessories, and camera lenses.