Optimized Machining: Integrating Vibration Drilling into Turning-Milling Operations

The processing of small deep holes in complex hydraulic actuator parts primarily encounters challenges such as chip breaking, chip removal, and cooling. By utilizing mechanical vibration drilling technology on turning-milling machine tools, selecting appropriate deep hole processing tools, and combining effective cutting parameters, we can address the difficulties associated with deep hole processing that has a large aspect ratio. This approach allows for efficient processing of large aspect ratio oil pipe holes on turning-milling machine tools.

 

01. Preface

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). Typically, holes with aspect ratios greater than 10 are categorized as “deep holes.” As the complexity and integrity of parts increase, deep hole processing has become increasingly common, leading to a wider adoption of advanced drilling technologies.

Several specialized drilling tools have been developed for deep hole processing, including deep hole twist drills, gun drills, BTA (Boring and Trepanning Association) drills, ejector drills, and DF drills. In addition, new deep hole processing techniques, such as mechanical vibration drilling, ultrasonic vibration drilling, hydraulic vibration drilling, and electromagnetic vibration drilling, have significantly enhanced deep hole processing capabilities compared to the past.

However, deep hole processing has unique challenges. When the drill bit enters the workpiece, it operates under semi-closed conditions, which imposes several limitations during the drilling process. Observing the tool’s cutting conditions directly is difficult, and dissipating cutting heat can be challenging, necessitating the use of effective cooling methods. Moreover, chip removal poses another challenge, and the drill rod must be rigid enough to withstand these conditions.

These difficulties can lead to varying results in actual applications, highlighting the importance of verifying and summarizing outcomes based on the specific processing conditions of the parts involved.

 

02. Typical deep hole processing problems

The local structure of a typical hydraulic actuator cylinder body is illustrated in Figure 1. This component is made from 30CrMnSiA, and after heat treatment, it achieves a hardness of 35- 41HRC. The manufacturing process focuses on complex shape milling, deep hole boring, and precision hole system processing. Additionally, it includes the machining of three groups of oil pipe holes, with a maximum aspect ratio of 63. The effective depths of these holes are L1 = 559 mm, L2 = 347 mm, and L3 = 282 mm (hereinafter referred to as the “559 hole,” “347 hole,” and “282 hole,” respectively). The straightness of each hole must be ≤ 0.2 mm over a length of 100 mm, and the surface roughness is specified to be Ra = 3.2 μm.

 

The conventional method for processing deep holes of this type involves the use of a specialized machine tool. For each set of three groups of oil pipe holes being processed on a component, three sets of special fixtures must be replaced. The specialized machine tool and the fixtures used are illustrated in Figure 2.

 

To complete the three deep hole processing of the part, the pure cutting time of a single piece is 6.58 hours. The cutting time statistics are shown in Table 1.

 

Table 1 shows that the processing efficiency of specialized machine tools is low, and the processing cycles are often complicated and lengthy. For parts that are mass-produced, the deep hole drilling process has become a bottleneck, making it urgent to enhance the processing methods and improve efficiency to meet production demands.

 

03. 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 observe and assess the condition of the tool. Additionally, measuring and ensuring accuracy is difficult.

 

2) Difficult Chip Removal: Chip removal can be problematic because chips tend to have a long removal path, leading to potential blockages. This blockage can cause the tool to break or scratch the machined surface, negatively affecting surface quality.

 

3) Poor Heat Dissipation: It is challenging to dissipate cutting heat effectively, which can result in overheating of the cutting edge and increased tool wear. The difficulty of machining materials such as titanium alloys and high-temperature alloys further exacerbates this issue.

 

4) Poor Rigidity: A drill rod that is excessively long suffers from poor rigidity, making it susceptible to deformation and vibration. This can cause the hole to become misaligned.

To address these challenges in deep hole processing, effective measures can be taken in several key areas, including cooling and lubrication, chip removal, guidance systems, and chip breaking. High-pressure internal cooling can provide cooling, lubrication, and assist in chip removal. Using oil-based cutting fluids is recommended. Improving the accuracy of guide holes or employing guide blocks can enhance guidance precision. Additionally, methods such as self-excited vibration, forced vibration, or incorporating tool chip breaker grooves can facilitate quicker chip breaking and enable better chip discharge.

 

3.2 Functions of Turning-Milling Machine Tools

The use of advanced high-end CNC machine tools has increasingly concentrated the processing capabilities. The structure of the turning-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 indicate the forward and reverse rotation of the milling spindle S3.

Turning-milling machine tools allow for a variety of operations—including turning, milling, drilling, boring, reaming, gear processing, deep hole processing, and eccentric processing—to be completed in a single process. This integration enhances the flexibility of the processing technology and streamlines the overall process, improving effectiveness while also reducing the number of times parts need to be clamped during manufacturing. This ultimately helps maintain processing accuracy.

Furthermore, the deep hole processing capability of these machines is particularly beneficial for manufacturing hydraulic actuator cylinders and piston components. The WFL turning-milling machine tool discussed in this article features a high-pressure internal cooling function capable of reaching 80 bar (with a maximum of 350 bar; 1 bar = 0.1 MPa), and its deep boring dovetail unit efficiently handles deep hole processing tasks.

3.3 Gun drill tool

Deep hole processing tools include deep hole twist drills, gun drills, BTA drills, ejector drills, and DF drills. After considering the processing characteristics of the part, the hole size, tool cost, and other comprehensive factors, a φ9.13mm brazed gun drill has been selected for this application. The gun drill is an external chip removal tool designed for deep hole drilling. It is primarily used to create deep holes with diameters ranging from 3 mm to 30 mm and can achieve an aspect ratio of up to 100. The processing hole accuracy is rated between IT8 and IT10, and the surface roughness can be as fine as 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 occurs along both the inner and outer cutting edges that meet at the drill tip. The grinding geometry of the gun drill significantly affects its chip breaking capabilities and overall lifespan.

 

3.4 Mechanical vibration drilling technology

(1) Principle of vibration drilling
Vibration drilling involves the process of drilling while simultaneously applying vibrations. This technique can be categorized based on the type of vibration: it can be classified as self-excited vibration drilling or forced vibration drilling. Additionally, various vibration devices can be employed, including ultrasonic, electromagnetic, hydraulic, and mechanical vibrations.

This paper focuses on the mechanical vibration drilling technology developed by MITIS in France, aiming to address the issues of low tool life and unstable processing quality encountered in deep hole turning and milling. The mechanical vibration tool holder utilizes an electric motor to drive an eccentric cam, which generates vibrations, as illustrated in Figure 6. This mechanical vibration-assisted hole-making technology superimposes sinusoidal reciprocating motion onto a constant feed motion, creating what is known as a “micro-vibration feed.” This approach 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 impacts chip breaking. The three main parameters in vibration drilling are amplitude, frequency, and feed rate. These factors greatly influence the actual drilling angle, the length of the chips produced, and the drilling torque.

 

(3) Characteristics and advantages of vibration drilling. The main characteristics and advantages are as follows.

1) Improve machining accuracy and surface quality.
The surface of the hole created by vibration drilling is smooth and uniform, with no built-up edge forming. In contrast, during conventional CNC cutting service, built-up edges often occur and can interfere with cutting. This interference increases the radial dimensions of the cutting edge and leads to cutting expansion. Vibration drilling effectively disrupts the conditions that allow built-up edges to form, completely eliminating the hole diameter expansion that these edges can cause.

2) It can reduce drilling force and drilling torque.
Vibration cutting involves a pulsing technique that constantly alters the movement speed and direction. This process causes the metal to become more brittle, which reduces plastic deformation and lowers the friction coefficient. As a result, vibration cutting not only decreases the drilling force and torque but also minimizes power consumption.

3) Cutting temperature is significantly reduced.
Traditional drilling is a semi-closed cutting method that generates significant heat during the drilling process. This heat can cause both the workpiece and the drill bit to experience thermal expansion and deformation, which can result in an increase in the hole diameter due to inadequate heat dissipation in the hole. In contrast, vibration drilling operates with reduced axial force and torque, allowing for better cooling and lubrication. As a result, the cutting temperature is significantly lower, which helps prevent the expansion of the hole diameter.

4) Easy chip handling.
The conditions for chip removal during the drilling process significantly influence the level of hole diameter expansion. If chip removal is not efficient, the chips can interfere with the cutting process, leading to increased friction and pressure that causes the hole diameter to expand. Vibration drilling effectively addresses this issue, as it enhances both chip breaking and chip removal capabilities, ultimately preventing hole diameter expansion caused by chip blockage.

5) Reduce tool wear and extend tool life.
In addition to the combined benefits of reducing drilling resistance and lowering drilling temperatures—which both help minimize tool wear and extend tool life—there is a deeper factor at play: the vibration effect of axial vibration drilling. This technique enhances the rigidity of the drill bit. During the drilling process, the application of a pulsed cutting force significantly improves the drill bit’s rigidity, making it less likely to bend or deform. As a result, the service life of the tool is effectively extended.

 

04. Verification process

4.1 Turning and milling composite deep hole drilling process

Drilling deep holes using a turning and milling machine differs significantly from the techniques employed by traditional deep hole drilling machines. Professional deep hole drilling machines necessitate the use of specialized fixtures to accurately process deep holes. In these machines, the drill sleeve on the fixture ensures the position and precision of the hole.

In contrast, when using a turning and milling machine to drill deep holes, special fixtures or drill sleeves are not required. The gravity of the extended twist drill or gun drill, combined with the tool’s length-to-diameter ratio, makes it challenging to precisely locate the position of deep holes. Therefore, the position and accuracy can only be ensured by first drilling a guide hole. The processing steps are illustrated in Figure 7.

 

The main process is as follows:

1. Drilling 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 diameter (2.5D).

2. Initial drilling phase: Set the drill to a low speed (≤30 r/min) and use a low feed rate, reversing entry as needed. Pause when you are 1-2 mm away from reaching the effective depth of the guide hole, then turn on the cutting fluid.

3. Drilling operation: Gradually increase the speed to the normal cutting speed and initiate the feed movement for drilling. Do not retract the tool during this phase, and closely monitor the shape of the chips produced.

4. Post-drilling procedure: Once you reach the effective depth, quickly retract the gun drill to the guide drill position at the bottom of the hole. Stop both rotation and cooling, then safely retract to a secure position.

 

4.2 Mechanical vibration drilling process

Based on the advantages of mechanical vibration drilling, this process utilizes mechanical vibration tool holders and gun drills. By superimposing a sinusoidal reciprocating motion onto the linear feed motion of the tool, a “micro-vibration feed” is generated. This technique aims to break chips effectively, thereby enhancing processing quality and prolonging the tool’s lifespan.

(1) Tool Clamping: The mechanical vibration drilling technology is implemented using the French MITIS vibration tool holder to secure the gun drill, as illustrated in Figure 8. Since the tool handle features an ER32 retaining ring interface, a sealing ring is essential to prevent the loss of internal cooling pressure.

 

(2) Processing procedure: According to the machining process of turning-milling compound gun drilling, first drill the guide hole, then bore the guide hole, and then use the gun drill to directly process to the effective depth. The program is compiled as follows.

N1; (DRLL D9.1 HOLE)
CDS
TLCH1(“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
CDS
M00
N2;(BORING D9.15 HOLE)
CDS
TLCH1(“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
CDS
M00
N3; (DRLL D9.13 HOLE)
CDS
DK
M00
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=-1F1
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: Mechanical vibration drilling technology was used, and the processing parameters increased amplitude and frequency. According to the material characteristics of the auto spare parts, heat treatment status, clamping rigidity and tool material, the chip breaking conditions during processing with different cutting parameters and different amplitudes were verified to control the chip shape, improve the processing quality and tool wear resistance. The amplitude, frequency and feed rate have a great influence on the wear degree of the tool and the chip breaking conditions. The specific test parameters are shown in Table 2.

 

(4) Conclusion of the processing test The main conclusions are as follows.

1) Using mechanical vibration drilling technology results in effective chip breaking, producing chips that are typically 3 to 7 mm in length. In contrast, when mechanical vibration drilling is not employed, the chips tend to be much longer, ranging from 18 to 35 mm. Figure 9 illustrates the comparison of chip lengths between these two processing methods.

 

2) The amplitude and accuracy of the matching of cutting parameters directly influence the formation of various chip shapes. In deep hole processing, the main criterion for assessing chip breakage and removal is whether C-shaped chips are produced during the machining process. The formation of C-shaped chips also offers certain advantages for the surface roughness of the inner hole and extends tool life.

 

3) Mechanical vibration drilling significantly improves tool life compared to conventional drilling methods. Cutting statistics indicate that when drilling holes of the same depth, tool life increases by more than 30% when using mechanical vibration drilling technology, compared to a traditional side-fixed tool holder. Figure 10 illustrates the differences in tool wear under the same processing conditions.

 

4) Processing quality and processing efficiency.
After using a mechanical vibration tool holder for drilling, the effects on chip breaking and chip removal are significantly better compared to a side-fixed tool holder. Data from multiple batches of parts confirm that the processing quality is high, particularly in terms of the straightness of the holes, which has an offset of only 0.12mm over a length of 100mm. This significantly reduces the risk of the gun drill breaking within the parts.

When considering the comprehensive energy efficiency ratio from the tool test cutting, the efficiency of machining oil pipe holes on a turning-milling machine that incorporates mechanical vibration drilling technology is over 7.5 times greater than that achieved on a dedicated machine tool (based on pure cutting time), as illustrated in Figure 11. Additionally, using a turning-milling machine eliminates downtime associated with changing fixtures back and forth, thereby greatly enhancing overall processing efficiency.

 

05. Conclusion

In metal cutting, the material removal rate during hole processing accounts for about one-third of the total removal rate. Although advanced tools, processing technologies, and methods—both domestically and internationally—are enhancing the efficiency and quality of hole processing, challenges remain, particularly in the stable and efficient processing of micro and deep holes.

To address the hole processing issues encountered in on-site production, it is essential to not only adopt new tools but also to integrate existing theoretical knowledge about hole processing. This involves validating processing technologies and methods. For instance, innovative approaches such as mechanical vibration drilling and ultrasonic vibration drilling are promising solutions for tackling deep hole processing challenges.

These new processing technologies must be considered alongside several factors, including the on-site machine tools, the tools themselves, workpieces, workpiece materials, clamping rigidity, and machine tool cooling methods. Gathering more processing data and accumulating experience through verification and practice will provide a solid foundation for enhancing our manufacturing capabilities.

 

 

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