Advanced Drilling Programming Powered by CATIA

This paper uses the example of CNC turning for complex blind hole parts to introduce the mutual reference principle. It discusses how this principle can be applied to re-plan the processing route as well as the design and manufacturing stages. The goal is to enhance the efficiency of self-centering fixtures and improve overall processing efficiency, all while maintaining processing accuracy.

 

PART. 01 Introduction

In the aerospace industry, many parts have unique structural characteristics to meet specific functional requirements. These components often feature complex shapes and structures, along with precise specifications that can be challenging to achieve with conventional processing methods. As a result, innovative processing techniques must be employed, and specialized equipment and tooling are necessary for successful fabrication.

For example, consider a blind hole part with a relatively complex shape and structure, as shown in Figure 1. This part has a cylindrical tubular design, and its inner hole is classified as a blind hole. It is categorized as a blind hole structural component. The blind holes at both ends of the part require high coaxiality relative to a common reference. Throughout the processing, multiple clamping operations are necessary during turning to meet the accuracy requirements of the component. However, repeated clamping and changes in the reference positioning can lead to significant processing errors, which may result in the part failing to meet design specifications. Furthermore, the frequent clamping, positioning, and alignment during the manufacturing process can negatively impact overall processing efficiency.

Innovation of turning process for blind hole parts based on datum conversion1Innovation of turning process for blind hole parts based on datum conversion1

 

PART. 02 Analysis of processing status and difficulties

The inner holes on both sides of the part require a high degree of coaxiality (φ0.01mm) in relation to the same outer circle datum. However, due to the stepped design of these inner holes and the current processing conditions, it is not feasible to clamp and process both inner holes simultaneously using the outer circle as the datum. Therefore, the inner holes need to be clamped and processed separately.

This separate clamping leads to repeated positioning and potential misalignment of the datum, which can compromise the geometric accuracy of the parts. Additionally, each clamping requires alignment operations, significantly impacting processing efficiency—especially during batch processing, where these issues become more pronounced.

 

In the actual production and processing process of this part, there are the following shortcomings.

1) The current processing procedure is quite cumbersome and relies on conventional methods. Initially, the outer circle is processed with both rough and fine machining to achieve the correct dimensions. After that, the outer circle serves as the reference point for clamping, allowing for the machining of the inner holes at both ends of the part. Once the inner hole on one end is completed, the part must be flipped and re-clamped to process the inner hole on the other end.

This process requires multiple clampings, which increases the likelihood of positioning errors due to misalignment of the reference points. As a result, the precision of the workpiece, particularly regarding its geometric accuracy, may be compromised.

 

2) To ensure the accuracy of the part during the turning and clamping process, it is necessary to align the workpiece after re-clamping. This is especially important because the coaxial requirements for the inner holes at both ends of the part must be maintained within φ0.01 mm relative to the reference datum. Achieving this precision after repeated clamping can be time-consuming, which not only impacts the processing efficiency but also increases the labor intensity for the operator.

 

PART. 03 Processing measures

After analyzing the characteristics of the workpiece, according to the principle of mutual reference [1], it was decided to first process the inner hole at one end, use this inner hole as the positioning reference, process the inner hole at the other end, and then use the inner holes at both ends to jointly position to process the dimensions of the outer circle, and finally obtain a part that meets the requirements.

3.1 Replanning the processing route

Due to the challenges encountered in processing this component, a reverse thinking approach was adopted to address the limitations of traditional processing methods. By applying the principle of mutual reference, the processing route was altered: instead of positioning based on the outer circle, we shifted to positioning based on the inner hole. This means that the inner hole of the part was processed first, serving as the reference for subsequent operations. This approach ensures greater processing accuracy and enhances overall efficiency.

The improved processing route (refer to Figure 2) begins with rough processing of the outer circle of the part blank to establish a rough reference. Next, using the outer circle for positioning, we proceed to both rough and finish the inner blind hole at one end—specifically, the end with a longer length and a more complex structure. Finally, this inner blind hole serves as a positioning reference for the completion of the remaining processing of the part.

Innovation of turning process for blind hole parts based on datum conversion2

 

While this processing scheme effectively eliminates the issues associated with misalignment of multiple clamping datums and the need for alignment, it is still essential to design and manufacture a specialized positioning fixture. This fixture serves as the interface between the equipment and the part, ensuring proper positioning and clamping of the part on the equipment, which is crucial for facilitating smooth processing.

 

3.2 Blind hole positioning fixture design

A special fixture was designed based on the structural characteristics of the blind hole in the part, as shown in Figure 3. The fixture consists of two main components: a base and an extension shaft. The base connects to the processing equipment and can be clamped by the self-centering chuck of the lathe. The extension shaft serves the primary function of positioning and clamping the part.

The extension shaft is divided into two sections: an optical axis and a threaded section. The optical axis must be manufactured with high dimensional and geometric accuracy. Its diameter should correspond to the actual size of the inner hole in the part, ensuring that the fitting clearance between the two components remains within 0.01 mm. This precise fitting limits the degree of freedom of the part and enables accurate positioning of the inner hole.

Because the inner blind hole of the part features a threaded structure, an external thread is designed at the base of the extension shaft of the fixture to facilitate clamping. During actual production, the clamping method of the fixture can be tailored to the specific structure of the part being processed. Figure 4 illustrates how the part is positioned on the fixture. Once the blind hole of the part is accurately positioned and clamped by the fixture, rough and fine processing of the inner hole at the other end can proceed.

Innovation of turning process for blind hole parts based on datum conversion3

 

Innovation of turning process for blind hole parts based on datum conversion4

 

After finely precision CNC machining the inner hole at the opposite end, a clamping method utilizing one clamp and one top is employed, as illustrated in Figure 5. This clamping method positions the inner hole using the top, ensuring that the inner holes at both ends are aligned. Consequently, the outer circumference of the part can be processed uniformly, which maintains the required coaxiality between the inner holes and the outer circles at both ends. By eliminating the need for repeated clamping and alignment processes, we can significantly enhance processing efficiency.

Innovation of turning process for blind hole parts based on datum conversion5

 

3.3 Innovation of efficient self-centering fixture

While the previous fixture can effectively process blind hole parts and ensure the accuracy of processing, it still has some shortcomings, as illustrated in Figure 6. This is mainly evident in two key aspects.

Innovation of turning process for blind hole parts based on datum conversion6

 

1) The positioning component of the fixture aligns precisely with the inner hole of the part. However, if the inner hole is machined to its maximum limit size, the resulting clearance may be excessively large, surpassing the acceptable range of positioning errors. This can ultimately lead to the part’s geometric accuracy falling outside of the specified tolerance.

2) The part and the fixture are connected by screws and secured through the friction between the end face of the part and the fixture. Since the area of the end face is small, the locking force is correspondingly low. Although the right-hand thread is theoretically tightened during the normal turning process, it may still loosen under certain conditions, such as vibration.

 

Due to the identified shortcomings, the fixture has been innovatively redesigned. The updated fixture is illustrated in Figure 7. This new design can automatically achieve centering and clamping through the tightening force between the part and the fixture.

The effective length of the fixture is slightly greater than the depth of the workpiece’s blind hole. Additionally, the fixture base’s positioning area is divided into eight equal sections by eight evenly spaced slits around its circumference, as shown in Figure 8. When the workpiece is threaded into the fixture, the bottom surface of its blind hole presses against the fixture’s top rod. This action causes the tapered block to be pushed by an elastic element, which expands the fixture base for effective positioning and clamping.

The role of the elastic element is crucial: once the fixture is expanded and clamped, the workpiece can continue to be tightened so that its end face fits snugly against the fixture base’s positioning surface, as depicted in Figure 9. The actual installation of the self-centering fixture and parts is shown in Figure 10.

Innovation of turning process for blind hole parts based on datum conversion7

 

Innovation of turning process for blind hole parts based on datum conversion8

 

Innovation of turning process for blind hole parts based on datum conversion9

 

The base of the fixture needs to be machined on a CNC lathe turning. To ensure that the narrow slits are cut evenly on the base, the fixture must be removed from the lathe after this process. Once the narrow slits are completed, the fixture is clamped onto the CNC lathe again for further machining. This process requires repeated clamping, which can lead to significant runout errors in the positioning part of the fixture base.

To address this issue, the threaded part of the fixture base will not be machined initially; instead, the positioning section will have an appropriate finishing margin left. After the narrow slits on the fixture base are processed and it is re-clamped on the lathe, the threaded part will be machined. Since the positioning part contains a narrow slit, it cannot be turned on the lathe. Therefore, an external cylindrical grinding head will be used to grind it for finishing. The diameter of the positioning part should be determined based on the actual size of the inner hole of the part to ensure a matching clearance of 0.01 to 0.015 mm.

To maintain the necessary tightening force, the elastic component of the clamp will be made from a harder rubber sheet. The thickness of this sheet should be chosen based on the depth of the inner hole of the part, ensuring that the positioning end face of the clamp base and the head of the ejector extend 1.5 to 2 mm beyond the depth of the inner hole.

Since the positioning part of the clamp has an expandable design, it can compensate for any gap errors between it and the inner hole of the part within a specific range. As a result, the clamp can be reused multiple times after a slight grinding of the positioning part. The threaded section of the clamp base and the thread of the part will have medium matching accuracy to provide only axial tightening capability when screwed together; radial positioning is achieved through the clamp’s positioning part.

When the part is installed on the clamp and the thread is tightened, the positioning rod comes into close contact with the inner wall of the part. However, due to the presence of the elastic element, the part and the clamp are not locked in place. After the part is processed, it can be unscrewed using a flexible strap wrench or similar tools.

 

The self-centering fixture has the following characteristics.

1) The tightening force applied during the installation of the part is intended to eliminate any gap between the fixture and the inner hole, facilitating rapid clamping and ensuring self-centering positioning.

2) By increasing the contact area between the fixture and the inner hole of the part, the friction force is enhanced, which ensures a reliable locking force between the part and the fixture.

3) The elastic element of the fixture compensates for any depth errors in the inner hole of the part being processed. This design allows the part to maintain its screw connection while achieving self-centering positioning and proper alignment of the end face.

4) The effective self-centering characteristics of the device make it suitable for mass production of blind hole parts in intelligent manufacturing production lines. In practical applications, the fixture is primarily utilized for processing batch parts due to its simple structure and convenient assembly. Although its reuse rate may be low, it remains cost-effective when considering the benefits of maintaining the overall quality of batch parts and improving processing efficiency.

 

PART. 04 Conclusion

For blind hole parts with complex structures, the processing route is redesigned based on the principle of mutual reference. This approach ensures that the processing allowance remains uniform throughout the entire process, which helps maintain the accuracy of the parts. Additionally, the use of efficient self-centering fixtures reduces auxiliary processing time and boosts production efficiency. Moreover, these fixtures enable the integration of parts into intelligent manufacturing production lines, facilitating high-efficiency and high-quality mass production.

 

 

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