Precision Machining Techniques for Hydraulic Slide Valve Sleeve Inner Bores

The phenomenon of motion jamming during the machining of longer spool valve components prompted an analysis of the structural, material, and design requirements for the valve sleeve components. By comparing the results of different aspect ratios for machining the internal bore of these valve sleeves, the team ultimately decided to use wire-cut electro-discharge machining (WEDM) instead of traditional boring methods. This machining technique effectively eliminated groove defects and localized dimensional deviations that can occur when machining elongated holes, while also ensuring the required cylindricity and dimensional consistency for the internal bore of the valve sleeve.

 

01 Introduction

Servovalves and control valves are commonly used in the aerospace industry. Key components of these servovalves and control valves include the valve core, valve sleeve, and spool valve. The proper functioning of these valves depends on the smooth movement between the valve core and the sleeve.

From a machining perspective, an analysis of the jamming phenomenon between the valve core and the sleeve revealed that factors such as dimensional accuracy, surface roughness, and geometric tolerances of the valve sleeve’s internal bore have a significant impact on the process. With advancements in technology, valve sleeve components are increasing in length, and the aspect ratio of the internal bore is also growing. However, deformation during machining complicates the machining process.

 

02 Valve Sleeve Classification and Processing Characteristics Analysis

2.1 Valve Sleeve Classification

In actual production, valve sleeves can be broadly categorized into two groups based on the aspect ratio of their inner bore. The first group consists of short valve sleeves, which have an aspect ratio of 10 or less (see Figure 1). The second group includes long valve sleeves, characterized by an aspect ratio greater than 10. (see Figure 2).

 

2.2 Material Analysis

Valve sleeve components are typically made from high-carbon chromium stainless bearing steel, specifically 9Cr18Mo bar. After the quenching process, these components demonstrate high hardness, excellent wear resistance, and stable tempering. The addition of molybdenum (Mo) improves the passivation effect of the stainless steel after machining. These components also maintain dimensional stability in both high and low-temperature environments, and the material is known for its outstanding machining performance.

 

2.3 Process Flow and Difficulties

In the finish machining of the valve sleeve inner bore, processes such as boring, reaming, and honing are typically utilized to achieve precise dimensions, optimal surface roughness, and specific geometric tolerances. The general process flow includes:

- Rough turning the outer shape of the part
- Gun drilling the inner bottom hole
- Machining the oil holes
- Vacuum quenching
- Reaming (including boring and honing) the inner bore
- Grinding the outer diameter
- Performing high and low-temperature cycling
- Aligning the part with a mandrel and cleaning

However, due to technological advancements, there has been an increase in the variety of hydraulic metal stamping products, leading to the development of long valve sleeves with aspect ratios greater than ten, as illustrated in Figure 2. When processing these long valve sleeves using the aforementioned process flow, it has been observed that their unique structure, combined with a heat treatment hardness of 58 to 62 HRC, can result in deformation during machining.

This deformation may lead to the formation of grooves and local dimensional deviations in the inner hole. Additionally, it can cause jamming during subsequent gap processing with the valve core. Such issues directly impact the performance, service life, and production cycle of the associated hydraulic products.

03 Problem Analysis

3.1 Internal Grooves

By closely examining the entire valve sleeve production process and evaluating the quality of each step, we identified that issues with internal grooves are most likely to arise during the gun drilling process in the rough machining phase. Gun drilling is known for producing consistent diameters and excellent straightness, making it efficient for machining short valve sleeves and resulting in fewer groove defects. However, when machining the internal bores of longer valve sleeves, the small diameter and extended length of the holes can hinder chip evacuation. This can cause waste chips to accumulate in the inner bore, leading to the formation of grooves.

 

3.2 Local Dimension Exclusions

Inspecting the internal bores of finished parts often reveals an incomplete surface finish and subtle steps, which can lead to localized dimensional deviations. Reaming and boring are commonly used processes for semi-finishing the inner bores of valve sleeves. Utilizing reamers and boring tools ensures that the internal bore dimensions, surface finish, and geometric tolerances of short valve sleeves comply with the specified drawing requirements.

For machining long valve sleeves, the length of the tools can limit machining efficiency. To address this, double-ended reaming or boring is often employed. However, this approach can result in misaligned bore axis lines, misaligned bore wall steps, and inconsistent bore dimensions. While boring generally produces slightly better results than reaming, it does not completely eliminate the steps during subsequent finishing processes. These steps and dimensional inconsistencies after semi-finishing complicate the finishing of the bore, a challenge that becomes more pronounced as the length of the valve sleeve increases.

 

3.3 Seizure Between the Valve Core and Sleeve

The long valve sleeve depicted in Figure 2 has external dimensions of 129.2 mm × 20 mm and an internal bore size of 8 to 8.022 mm. It requires an internal bore cylindricity of 0.004 mm, a coaxiality of 0.012 mm, and a surface roughness of Ra = 0.2 μm. According to the technical requirements outlined in the drawing, the inner hole of the valve sleeve must be compatible with the outer circle of the valve core to ensure a clearance of 0.005 to 0.008 mm, allowing for smooth movement without binding.

The inner hole is a through hole measuring a total length of 129.2 mm, which results in an aspect ratio greater than 15. Due to the challenges associated with machining this hole, the process flow was optimized and adjusted. A small amount of multiple machining was performed based on the finishing allowance to minimize machining stress. The specific process flow consists of the following steps: rough turning the part shape, gun drilling the inner hole bottom, machining each oil hole, vacuum quenching, reaming (boring and honing) the inner hole, subjecting the part to high and low temperature cycles, semi-finishing the inner hole and cleaning, grinding the outer circle, and finally matching with the mandrel and cleaning.

This process flow generally meets the drawing’s requirements for the inner hole. However, during precision metal machining of the valve core and valve sleeve, it was discovered that the valve core was stuck in the inner hole of the valve sleeve. Upon measurement and analysis of the stuck valve sleeve, it was determined that the combination of boring, honing, and grinding the inner hole could correct the roundness but not the straightness. The error in straightness led to the jamming of the valve core when trying to fit it into the valve sleeve gap during subsequent grinding and processing.

04 Solution

4.1 Boring, Reaming, and Honing

An analysis of the processes that lead to internal bore grooves and local dimensional deviations revealed that insufficient finishing allowance is the root cause, resulting in residual grooves and ongoing local dimensional inconsistencies. By increasing the finishing allowance, these issues can be effectively eliminated during the roughing and semi-finishing stages. However, raising the finishing allowance too much can significantly enhance the tool’s influence on the process, which may render finishing ineffective.

It is advisable to avoid the use of boring and reaming tools, as they can easily interfere with the finishing and correction of the internal bore. Instead, employing a vertical honing machine is recommended for correcting the cylindricity of the internal bore.

However, the relatively long length of this type of valve sleeve and the large machining allowance limit the effectiveness of vertical honing in actual machining and correction tasks. Additionally, the high workload and low production efficiency associated with this method make it unsuitable for mass production.

 

4.2 EDM and Wire Cutting

After measuring and analyzing the valve core and valve sleeve, we identified that the issue of sticking during machining was due to the inner bore of the valve sleeve. To resolve this problem, it was essential to improve the cylindricity of the inner bore. In other words, we needed to enhance the straightness of the valve sleeve’s inner bore. Considering the equipment commonly available on the production site, both Electrical Discharge Machining (EDM) and wire cutting techniques were evaluated as potential solutions to correct the straightness of the inner bore.

1) During trial machining, we discovered that the production efficiency of Electrical Discharge Machining (EDM) was low. Additionally, the micro-geometric features and surface properties of the machined part were altered. If these changes cannot be completely rectified in subsequent machining processes, the performance and service life of the valve sleeve could be negatively affected.

2) After the rough machining of the valve sleeve’s inner bore with an allowance, Wire Cutting was employed for semi-finishing. The inner bore was refined by removing the allowance three or four times. Subsequently, honing or grinding was utilized to eliminate any affected areas from the Wire Cutting process until the base material was exposed. This CNC machining process effectively addresses defects in the micro-geometric features and physical properties of the surface layer following EDM, achieving the required surface roughness for the inner bore pattern.

 

4.3 Comparison of Results

During the trial machining phase, data collected indicated that using wire-cut EDM for machining inner holes effectively removes grooves, enhances dimensional consistency, and minimizes the sticking of valve cores and sleeves during fitting. As a result, the fitting success rate exceeded 98%, leading to significant improvements in production efficiency and yield rates.

 

05 Conclusion

This paper examines the causes of motion jamming during the machining of valve cores and sleeves. It classifies the aspect ratios of the inner bores of valve sleeves and discusses the associated machining challenges and strategies for longer valve sleeve inner bores. By enhancing the machining process, a more effective solution was ultimately chosen. This innovative method employs wire-cut electro-discharge machining (WEDM) as a replacement for traditional boring and honing techniques. This approach eliminates groove defects, improves the cylindricity of the inner bore, and enhances dimensional consistency, resulting in a final product that meets design specifications. Additionally, this method has been successfully applied to similar products with favorable outcomes.

 

 

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