Process Enhancements for Precision Machining of RF Shielding Housings
To tackle the issue of deformation in the shielding box during clamping and machining, we analyzed the structural characteristics of the component. We developed effective process procedures and improvement measures, then implemented CNC milling using suitable clamping methods, optimal tool paths, and appropriate cutting parameters. These actions effectively minimized deformation of the part’s contour during the cutting process, ensured machining accuracy, and significantly enhanced both product quality and production efficiency.
1 Introduction
In recent years, companies involved in communications, artificial intelligence, and electronic information technology have undertaken various tasks related to the production of signal shielding boxes, leading to a sustained increase in demand. During the initial trial production phase, the complex cavity design of the shielding box created challenges; the blank material had a significant cutting allowance and lacked overall rigidity. This caused considerable tool vibration at the corners during the cutting process. Additionally, the finishing allowance was unevenly distributed during cutting, resulting in poor machining stability and a high risk of deformation. Consequently, this led to a high scrap rate that continued to challenge production efforts.
To meet the strict technical requirements for dimensional accuracy and surface roughness while minimizing local deformation, a comprehensive process flow was developed. The team carefully selected appropriate tools and cutting parameters, as well as optimized tool paths. These measures effectively reduced contour deformation during cutting, thereby enhancing processing quality and production efficiency.
Using a specific model of shielding box as an example, the team analyzed common issues encountered during trial production. They conducted an in-depth review and made adjustments to the machining process and methods, successfully completing the first trial cut. This achievement met the established processing requirements and significantly improved processing efficiency.
2 Part Structure Analysis
The shielding box depicted in Figure 1 is a cavity-type component made from an aluminum alloy plate, measuring 169.5 mm × 166 mm × 12 mm. Most of the cavity material has been removed, leaving behind primarily rib structures. The radius at the base of the cavity has a 2 mm fillet, and the cavity walls are thin. The corner steps and the bottom walls on the front and back sides are only 1 mm thick. The four side surfaces feature irregular notches and through-holes, with root diameters of 2 mm and 4.5 mm, respectively. During machining, it is essential to maintain dimensional tolerances, parallelism, flatness, surface roughness, and hole positioning simultaneously, which makes achieving precision quite challenging.
3 Process Analysis
The shielding box consists of a body and a lid that are secured together using an elongated groove and a convex circuit board. The body features long, outward-facing flanges on its side panels, while the lid has inward-facing circular grooves on its side panels. This design ensures a tight connection between the lid and the body.
For proper assembly, the soldering surfaces between the shielding box body and the PCB must be flat and free of burrs. Both solder pads should be coated with a solder mask and clearly marked with white silkscreen. Additionally, the locating holes must be accurately sized and positioned to guarantee precise installation and secure fixation of the shielding box.
Given the structural characteristics and processing requirements of the shielding box, a vertical machining center was chosen for its fabrication. To address the challenges of complex multi-step machining, it is important to minimize the number of clamping operations. This approach enhances repeatable positioning accuracy.
The processing flow for the shielding box is detailed in Table 1. Although the process is relatively complex, it ensures sufficient rigidity in the machined parts, effectively eliminates residual stress after material removal, significantly reduces rib deformation, and ultimately improves the product’s yield rate.
4 Clamping methods and processing difficulties
Positioning and clamping are critical for the processing of the entire part. Each step in the process is interconnected, and the clamping method used can significantly impact subsequent processing stages. If the geometric tolerance from a previous step exceeds the acceptable limits, it becomes challenging to maintain the flatness, verticality, and alignment of the part.
Taking into account the specific circumstances of the enterprise, including economic costs and the suitability of CNC machine tools, the use of suction cup clamping has been ruled out. Based on the structural characteristics of the part and the arrangement of the process flow, flat-nose pliers and combined clamps have been chosen as the primary clamping methods.
The processing difficulties of the part are as follows.
(1) Shape and plane processing
A φ20mm face milling cutter is used with a spindle speed of 4000 r/min and a feed rate of 1000 mm/min. The processing size tolerance is set at ±0.1 mm. During processing, it was observed that if the first step is machined to the maximum limit of the specified tolerance, achieving consistent size tolerance for the part contour after repeated positioning and clamping becomes challenging. To enhance the repeatability of positioning, it is necessary to ensure that the processing size tolerance for the part in the first step is maintained within 0.02 to 0.03 mm.
(2) Internal cavity processing
In the actual processing trial, after milling the inner cavity, the notches on both sides of the back are milled. During the milling of the notch on the back, the contact surface position of the pressure plate is limited to only 1 mm. If the pressure applied by the pressure plate is too high, it can cause stress deformation and damage to the part. Conversely, if the pressure is too low, the part may shift during the milling process.
To begin, both sides should be processed first. The bottom surface of the part is installed on angle iron and clamped using the pressure plate. Since the processing surfaces on both sides are inclined, the center drill tip cannot accurately determine the center. Therefore, the ends should be drilled to the required depth first, followed by milling the inner R1 mm notch.
To improve production efficiency, a φ6 mm end mill is used for rough processing, while a φ2 mm end mill is utilized for fine processing. Because the roughing process requires a significant corner margin, and to prevent continuous tool breaks during finishing, the milling operation should be divided into multiple layers. The spindle speed should be increased to control the tool feed rate accordingly. Actual cutting results indicate a spindle speed of 4000 rpm and a feed rate of 300 mm/min.
The clamping method for sidewall machining is illustrated in Figure 2, while the combined fixture for milling the inner cavity is shown in Figure 3.
In the original plan for CNC milling working the inner cavity, the clamping plate was positioned on the step of the part’s notch to secure it during the process. This setup was used to perform rough milling on various areas of the cavity, leaving a 0.2 mm margin on each side. The tool selected for this operation was a φ10 mm end mill, with a cutting depth of 2 mm per cut, a stepover of 75% of the tool diameter, a spindle speed of 4000 rpm, and a feed rate of 500 mm/min.
However, after implementing this clamping and cutting parameter setup, several problems arose during machining. The tool path of the inner cavity was disorganized, leading to an uneven bottom surface and corners. Additionally, the surface roughness exceeded tolerance, and there were noticeable chatter marks as well as irregular surfaces. Furthermore, when the clamping plate was released, the part experienced uneven stress deformation.
5. Milling Internal Cavity Process Optimization
To address a series of issues encountered in the original milling internal cavity machining plan, the following optimizations were implemented.
1) Adjustment of the clamping scheme. To effectively control the clamping force and ensure that the pressure plate distributes pressure evenly on the part, a plastic sheet was inserted between the pressure plate and the workpiece. This cushioning helps prevent damage and deformation to the part during machining.
2) In terms of tool selection, we replaced the original φ10 mm end mill with a φ8 mm end mill. Additionally, we increased the spindle speed to 5,000 RPM and adjusted the feed rate to 800 mm/min to enhance machining efficiency and accuracy.
3) In the cutting strategy, a progressively decreasing depth of cut was innovatively implemented to enhance the machining process. Specifically, the cutting speed was gradually increased as the number of machining layers rose. This approach not only maximized the tool’s cutting capacity during the initial stages but also ensured part accuracy in the final finishing process.
During the rough milling phase, with the exception of the final layer, the cutting depth for each intermediate layer was carefully allocated based on actual machining conditions. These conditions included workpiece material properties, tool wear, and machine tool performance, allowing for the appropriate cutting depth for each layer.
When machining the final layer, a 0.2 mm allowance is reserved and processed in a single pass to ensure a consistent and reliable allowance. A φ4 mm end mill is then used for the subsequent finishing process.
4) To improve machining in difficult areas, it is essential to optimize corner machining. During the roughing phase, the allowance at corners is reduced to address the increased stress concentration and high cutting resistance typically encountered in these areas. This reduction effectively lowers the cutting load on the roughing tool, which creates better conditions for the subsequent finishing process with the φ4mm end mill. Additionally, it is important to comprehensively tackle issues like unevenness and vibration marks that may arise on the bottom surface and corners, ensuring overall part quality.
5) Optimize the application of cutting fluid by strategically positioning the nozzles to create a multi-angle, all-around spray pattern that evenly covers the entire machining area. This approach eliminates any “blind spots” where heat may not dissipate effectively. Additionally, the flow rate and velocity of the cutting fluid are precisely controlled to swiftly dissipate the heat generated during cutting, without causing excessive impact forces that could disrupt the machining process. By optimizing these factors, we can maintain consistent machining conditions, improve overall part quality, minimize defects caused by thermal and stress deformation, and ensure the stability of the machining process.
By optimizing and adjusting key machining conditions such as the clamping scheme, tool path, cutting strategy, and cooling system, a practical approach was developed to address the challenges inherent in the original milling solution for internal cavity machining.
6 Cutting Parameters and Programming Strategies
To facilitate programming, first create a simplified process diagram that outlines each machining step. During the roughing phase, maintain a consistent stock allowance to minimize fluctuations in cutting load during the finishing process. Aim to reduce tool changes and minimize movement between machining areas to ensure a steady cutting load and material removal rate throughout the entire cutting operation. Table 2 provides the tool specifications and cutting parameter settings for various machining steps based on the dimensions of the CNC turning components.
Mastercam software was chosen for automated milling operations. To ensure stability during cavity milling and minimize tool vibration at corners, CAM simulation software was utilized to analyze and compare tool paths. After thorough evaluation, the 2D dynamic milling strategy offered by Mastercam was selected, resulting in substantial optimization of tool paths at corners. Figure 4 illustrates the tool path for the cavity corners using Mastercam’s 2D dynamic milling. The step size and minimum tool path radius can be adjusted as needed at the corners, significantly enhancing machining stability, reducing tool vibration, and providing a smooth transition of the tool path during the cutting process.
To minimize the tool path and reduce the return stroke time, the down milling method is employed to enhance processing efficiency. To facilitate chip removal and prevent sudden changes in cutting force, Z-axis contour layer cutting is used to divide the workpiece into several layers, which are then processed one layer at a time. After all areas of a part are completed in a given layer, the process moves on to the next layer. The tool path for layered cutting parameters is illustrated in Figure 5. Each layer utilizes a spiral feed, and a circular tool path without sharp corners is implemented. This uniform depth layering combined with spiral feeding improves tool stability during operation and enhances the smoothness of the processing, thereby reducing tool tip wear and extending tool life. The inner cavity feed method is depicted in Figure 6.
In the cutting parameters, you will find the “Micro Tool Lift” dialog box. The “Tool Lift Distance” indicates how far the tool retracts from the workpiece surface after machining. The default setting is usually adequate for most applications. The “Tool Lift Feed Rate” specifies the speed at which the tool retracts. Since the tool is not cutting during this retraction phase, increasing this speed can enhance machining efficiency. Therefore, it is generally advisable to choose a rate that exceeds the cutting speed. The settings for the Micro Tool Lift parameters are illustrated in Figure 7.
When machining a 2mm flat surface on the reverse side, a φ32mm face milling cutter was used at a rotational speed of 5000 rpm and a feed rate of 800 mm/min. However, vibrations on the part’s surface resulted in unevenness, making it impossible to achieve flatness in a single pass and leaving vibration marks.
Upon analysis, it was found that the absence of a mating surface in the center of the cavity, combined with excessive tool removal, contributed to the vibration issues. To address this, the cavity wall was secured, and an insert was fabricated to fit the contour of the cavity, making it 0.02mm shallower than the actual depth. This design change prevented air from entering, ensuring a complete fit and maintaining a surface roughness of Ra = 1.6μm.
This approach improved the part’s strength while reducing stress and deformation. The details of the machine tool milling process are illustrated in Figure 8.
7 Conclusion
The shielding box consists of several sections with wall thicknesses as thin as 1mm. During the cutting process, varying cutting forces can lead to plastic deformation, which may result in wall misalignment or even cracking. When designing the process setup, it’s important to first consider whether the current step aligns properly with the datum from the previous step. This alignment can significantly impact surface roughness, perpendicularity, and other requirements established by the previous operation.
Proper planning helps to create a logical processing sequence. When using a torque wrench for clamping, be mindful of the clamping force applied by the vise and pressure plate. Uneven force distribution can lead to deformation or cracking of the machined part. Effective clamping not only enhances workpiece stability but also increases part rigidity, helping to minimize deformation.
Additionally, if conditions permit, companies should consider using cutting tools with superior machining strength and toughness. Utilizing Mastercam’s 2D dynamic milling strategies can optimize toolpaths, maximizing rotational speed, minimizing cutting volume, and increasing feed rates to further reduce part deformation.
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