Integrating Self-Centering Fixtures for Precision Drilling on CNC Lathes

Due to the limitations of traditional fixed drilling methods in turning, a self-centering drilling fixture was designed and manufactured. This fixture automatically adjusts the alignment of the drill bit with the lathe spindle during the machining process. It has demonstrated excellent results in drilling, reaming, and tapping applications.

 

PART.01 Introduction

Drilling is the fundamental process for creating holes. When machining holes that require a precision level of IT10 or below, a single drilling method or a two-stage process involving drilling and reaming is typically used. Key indicators for assessing the quality of a drilled hole include roundness, cylindricity, and straightness. From the perspective of the machining process, the main controllable factors that can lead to defects in a drilled hole consist of the quality of the drill bit, the precision of the machine tool, the drilling fixture, and the drilling process strategy.

Currently, traditional fixed rigid drilling fixtures are widely utilized in production lines. While these fixtures have been standardized and benefit from a well-established manufacturing process and excellent rigidity, they are highly dependent on the mechanical precision of the machine tool. Research has shown that CNC lathes used in production for a significant amount of time, particularly turret lathes, often display excessive coaxiality deviations between the lathe spindle and the tool holder centerlines. This discrepancy can easily result in quality issues during drilling.

To tackle this problem, this paper presents a simple and practical self-centering drill chuck designed to ensure coaxiality during the drilling process. This solution has demonstrated positive results in production practice. The following sections will detail the background, structural principles, and application methods of this self-centering drill chuck.

 

PART.02 Problem Statement and Solution

Traditionally, fixed drill chucks were utilized for drilling operations. However, during the trial production of the sliding sleeve depicted in Figure 1, we encountered several process difficulties that led us to explore the use of self-centering drilling and reaming techniques. The part’s structure is relatively straightforward, with the primary challenge being the machining of a Φ7mm deep hole. To ensure machining accuracy, we adopted a “drilling + reaming” process and employed a turret CNC lathe.

During the trial production, the use of a fixed drill chuck resulted in mostly tapered holes, which did not meet the specifications outlined in the drawings. After analyzing the process, we concluded that the problem stemmed from a misalignment between the machine tool turret center and the spindle, indicating a height difference. By upgrading to a more precise machine tool, we resolved this issue. Subsequent inspections revealed that all turret CNC lathes on the production line had varying degrees of center height difference problems. Since routine hole machining typically employs a “drilling + boring” process, this issue had been overlooked.

Given this analysis, we understand the importance of enhancing machine tool maintenance and regularly ensuring accuracy. Additionally, we plan to develop a process method that mitigates the impact of center height differences. As a result, we designed and manufactured a simple self-centering drill chuck.

 

PART.03 Design Principles and Structural Composition of Self-Centering Drill Fixtures

3.1 Design Principles

The mechanical structure of a CNC lathe indicates that when the boring bar of the turret is perfectly aligned with the machine tool spindle, the center point of the tool holder is positioned at the origin of a rectangular coordinate system, which has the spindle center as its reference point. In cases where they are not aligned, the actual center height of the tool holder will fall within a circular area of a certain radius around the origin of the coordinate system.

The principle of a self-centering drill fixture can be illustrated as shown in Figure 2. If we design a process device that allows for two degrees of freedom of movement, the center height of the installed tool (whether it is a drill bit or a reamer) can be adjusted back to the origin, aligning it precisely with the spindle center. This alignment ensures that the actual tool center height coincides with the spindle center. This concept forms the theoretical basis for designing a self-centering drill fixture.

 

3.2 Structural Composition and Function

Considering the nature of daily work, which primarily involves drilling, the goal is to design a lathe self-centering drilling fixture that offers low processing costs, simple operation, high torque, and improved drilling quality.

To achieve this objective, the mechanical structure consists of several key components: a clamping sleeve, connecting bolts, a “cross” floating slide, a drill chuck, a spring clip, a locking nut, and an elastic retaining sleeve. The overall structure is illustrated in Figure 3.

The clamping sleeve features a locking flat head machined on its outer circumference, with a round hole and a square hole machined in its center to accommodate the connecting bolt. The left end face of the clamping sleeve has a precision open keyway that allows for the sliding of the “cross” floating carriage. Similarly, the right end face of the drill chuck also has a precision open keyway for the movement of the “cross” floating carriage, along with an internal thread that matches the connecting bolt. The left end of the drill chuck is designed with an external thread to fit the locking nut and an internal conical surface for the spring clip.

Each end face of the “cross” floating carriage includes a raised key that fits into the keyways on the end faces of the drill chuck and the clamping sleeve, respectively. These two raised keys are positioned perpendicularly to each other, and a square hole, matching the specifications of the clamping sleeve, is machined in the center to hold the connecting bolt. This configuration is a critical component for achieving self-centering and transmitting torque. The difference in diameter between the square hole and the connecting bolt determines the floating capacity of the clamp. This structure is depicted in Figure 4.

As shown in Figure 5, the connecting bolts are standard hexagonal internal bolts. They connect the drill chuck through the clamping sleeve and the square hole of the “cross” floating slide. The spring clip and locking nut are standard components that work with the drill chuck to secure the drill bit. The elastic retaining sleeve, made of polyurethane, helps to constrain the drill chuck and the cross floating slide while counteracting the downward tilt caused by gravity. The actual floating drilling fixture is illustrated in Figure 5.

 

The self-centering process for this fixture is as follows: First, a centering hole is created on the end face of the workpiece using a center drill. Next, the connecting bolts of the self-centering drill chuck are adjusted to ensure proper clearance between the drill chuck and the clamping sleeve.

The drill bit is then fed at a low speed to engage the centering hole. As the feed force is applied, the drill bit vibrates, allowing the drill chuck to slide freely in two perpendicular directions via a cross-shaped floating slide. This process achieves automatic centering. Once the drill bit is fully inserted, the normal cutting speed can be used for drilling with the self-centering reference. The drill bit feeds along the axis of the hole to maintain coaxiality with the spindle. It is important to ensure that the diameter of the centering hole is maximized to facilitate effective centering. Additionally, high-quality grinding of the drill bit is crucial. Alternatively, the connecting bolts can be tightened to use the fixture as a standard drill chuck.

This non-stop fixture features a simple structure, is easy to manufacture, and significantly improves machining quality, particularly during deep hole machining. When machining 1:15 deep holes, the cylindricity of the hole can be controlled within 0.01 mm.

 

PART.04 Conclusion

The design and application of this self-centering drill chuck have effectively addressed technological challenges in production while also fostering innovative thinking. Although its structure is simple, it has pioneered the “self-centering drilling” process, demonstrating significant benefits in its application. This innovation has already been granted a utility model patent, with the patent number ZL 2023 2 0119427.8.

In conclusion, practical experience is the best teacher. As professionals, we can only advance our skills and become the knowledgeable, skilled, and innovative talents needed for the new era by continually achieving “the transformation from practice to theory.” In the future, further research can be conducted on the application of this self-centering drill chuck in various machining scenarios to optimize its structure and performance. This will expand its scope of application and enhance machining accuracy.

 

 

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