Precision Machining Solutions for Scroll-Type Components in EV Thermal Systems

This paper focuses on the manufacturing and processing of the scroll disk, a core component of the scroll compressor used in air conditioners for new energy vehicles. It discusses the factors that affect precision control and efficiency improvement in the production of these parts. Specifically, the analysis includes the impact of the scroll disk blank, machine tool precision, tool holders, tools, fixtures, and cutting parameters. By examining different precision data of parts processed under various working conditions, the paper summarizes methods for achieving the required processing precision and enhancing processing efficiency.

 

01 Preface

Scroll compressors are commonly used in automobile air-conditioning systems due to their benefits, including high efficiency, reliability, low noise, and cost-effectiveness. In new energy vehicles, scroll compressors are preferred as core components, with scroll disks being the most essential parts of these compressors. The quality of the manufacturing and processing of scroll disks directly impacts the performance of the air-conditioning system.

Given the rapid development of domestic new energy vehicles, the manufacturing and processing of scroll disks have become critically important technologies for automobile air-conditioning compressors. While advancements are being made quickly and technology is continually improving, several deficiencies still exist in current scroll disk manufacturing and processing. These deficiencies are primarily manifested in the following areas:

1. Insufficient technical support: Some technical specifications remain unfulfilled.
2. Low production efficiency: This is particularly evident in large-scale production.
3. Complex manufacturing processes: These complexities make it challenging to ensure consistent quality.
4. Underdeveloped processing technology: This results in high production costs.

As a result, the current objective for scroll disk manufacturing is to continuously optimize technology to improve both efficiency and quality.

 

02 Precision Requirements

The rotating and stator disks, together referred to as the scroll disk (see Figure 1), are essential components of scroll compressors used in new energy air conditioners. Due to the interlocking nature of these two disks, the precision standards for both are extremely high, with tolerances reaching the micron level. To ensure dimensional accuracy, high-precision CNC machining centers are widely utilized in the industry, and processing must occur in a climate-controlled workshop to prevent deformation.

The scroll profile is designed as a deep, thin-walled shape, and aluminum alloy materials are particularly susceptible to deformation. Factors such as cutting parameters, clamping methods, and clamping forces can all lead to distortions in certain features of the part. Inspection of characteristics such as diameter, roundness, flatness, parallelism, profile, and perpendicularity is critical and requires extremely high precision. This applies to features like bearing holes, locating holes, keyways, flat surfaces, and the scroll profiles themselves.

After undergoing multiple clamping operations across various stations, the part must meet tolerances for over 50 dimensions, achieving a sub-mirror finish without any color differences or visible imperfections. These requirements pose significant challenges in the machining process.

 

03 Process route

Scroll Disk Processing Route:

1. Blank Design:
Determine the material, design specifications, and allowances for the part. This stage should facilitate effective clamping and ensure processing efficiency and precision.

2. Fixture Design, Production, and Testing:
Create a fixture that is both efficient and precise, tailored to the shape and size of the workpiece.

3. Tool Holder Selection:
Analyze the pros and cons of spring tool holders versus heat shrink tool holders, and select the most suitable option.

4. Tool Design:
Develop a tool that offers high efficiency and precision, taking into account the specific processing characteristics of the scroll disk.

5. Machine Tool Precision Calibration:
Prior to processing, use a laser interferometer and other equipment to conduct precision measurements and calibration of the machine tool to ensure its accuracy.

6. PowerMILL Programming:
Examine the features of each part in conjunction with the designed tool. Determine the programming allowances, select various cutting parameters and strategies for testing and verification, and choose the optimal solution.

7. Part Cleaning:
Clean the parts thoroughly to achieve a bright surface finish.

 

04 Billet

4.1 Material Selection
Automotive air conditioner scroll components are typically made from aluminum alloy, stainless steel, or cast iron. In the context of new energy vehicles, which tend to have a heavier battery mass, forged aluminum alloy is often preferred for air conditioner compressor scrolls. This choice helps reduce the overall weight of the vehicle. Forged aluminum alloy not only meets the operational requirements of the compressor but also minimizes deformation during the finishing process, ensuring that precise tolerances are maintained.

Wrought aluminum alloys are advantageous due to their low density and high strength. They are highly resistant to corrosion, which helps maintain their structural integrity at elevated temperatures and protects against impact and tensile damage. The material properties of these alloys can approach or even exceed those of high-quality steel, while also offering excellent plasticity, making them suitable for various processing techniques. These alloys are primarily composed of aluminum, magnesium, zinc, and silicon (Al-Mg-Zn-Si). Although they contain a range of elements, their concentrations remain low, resulting in outstanding thermoplasticity and suitability for forging.

 

4.2 Clamping Aid Design
In designing the blank model, three clamping aids were created to align with the part’s maximum outer diameter (refer to Figure 2). Each clamping aid was positioned with a 120° rotational spacing to ensure effective clamping during multiple operations. Once all features of the part were finish-milled, the clamping aids were removed through the milling process.

 

4.3 Blank Allowance Design

The model of the scroll disc product is displayed in Figure 3, while the blank model is shown in Figure 4. During the production of the blank, we control the actual dimensions after forming—considering both radial and axial allowances—to be approximately 1 mm on each side. This allowance is crucial for facilitating clamping during processing and enhancing production efficiency, all while ensuring sufficient material for fine milling.

By setting these allowances, we can achieve a large axial depth of cut and allow for rapid radial feed of the milling cutter during machining. This enables rough milling to be completed in a single pass using the side cutting edge. As a result, the precision requirements for the rough milling fixture are reduced, simplifying the fabrication of the fixtures. Additionally, this reduces the positioning accuracy needed when removing and replacing parts during clamping, making operations easier and paving the way for automated production. Overall, these improvements contribute to enhanced production efficiency.

 

4.4 Blank Technical Requirements

After formation, the scroll disk blank must have a uniform surface profile, free from obvious bumps, sand holes, impurities, and burrs on the edges and corners to ensure accurate clamping and manufacturing.

 

05 Fixtures

5.1 Fixture Design and Fabrication

Three pneumatically driven clamps are strategically positioned on the three process-supporting clamping bodies of the scroll blank. Air pressure controls the clamp joint system, allowing for the clamping and releasing of the part. The structure of the pneumatic clamp is illustrated in Figure 6.

For production, four workstations are required. The pneumatic clamp designed for one-out-of-two production at these four stations is shown in Figure 7. During the design layout of the fixtures at each workstation, the model is replicated and moved longitudinally (along the Y-axis) to minimize precision errors in the machine’s longitudinal direction (X-axis). Each workstation fixture is equipped with a separate switch and pressure gauge.

At each workstation, the pneumatic cylinders are connected in parallel, ensuring that the clamps are pressed simultaneously and maintain consistent pressure while precisely aligning against the process-supporting clamping bodies. This setup prevents part deformation and helps maintain processing quality, clamping speed, and safety. Additionally, it simplifies future upgrades and the installation of a robot for automatic workpiece switching.

 

 

Due to the high precision requirements for the parts, the fixture manufacturing process emphasizes grinding first, followed by wire cutting, and finally milling with a CNC machining center. The most critical feature is the keyway boss on the fixture base of the third clamp, which aligns with the keyway of the scroll disc component, as illustrated in Figure 8. This keyway boss needs to be machined with a margin of 0.05 mm when processed individually

Once the fixture is assembled, it undergoes milling on a CNC machining center to ensure that the scroll disc component fits perfectly within the fixture. Since the base frequently accommodates different scroll disc components, it must have high wear resistance and stability. To achieve this, a quenching process is employed to increase the material hardness to a range of 48-52 HRC, ensuring quality and stability during long-term mass production.

 

5.2 Fixture Testing

A pressure gauge on each fixture station is capable of measuring the impact of different air pressures on part precision. After conducting multiple test cuts, we determined the optimal air pressure for varying cutting efficiencies. We concluded that for normal cutting (with a spindle speed of 10,000 rpm), an air pressure of 0.3 MPa provides good machining stability. When the spindle speed is increased to 20,000 rpm, the air pressure should be adjusted to 0.4 MPa to consistently maintain surface quality and precision while machining the involute surface of a scroll disk. This demonstrates that pneumatic fixtures can quantify clamping force using a pressure gauge and establish optimal pressure through testing.

Furthermore, the stability of the pneumatic fixtures allows for targeted improvement solutions to issues encountered during machining, enabling faster resolution. For instance, during our testing, we discovered that when a CNC machining center employed a milling cutter with a constant-height strategy to mill a circle, the diameter met the required tolerance; however, the cylindricity did not comply with technical specifications. Despite optimizing the tool path to fully implement circular interpolation, the cylindricity problem persisted when the CNC machine was operated. Only boring and reaming tools could achieve the desired results, exhibiting a high pass rate and efficiency.

To validate our findings, we produced ten products continuously. The 3D inspection data, as shown in Figure 9, indicates a 99% pass rate for the machined parts, with excellent vortex line profile accuracy, making them suitable for mass production.

 

06 Toolholders and Tools

6.1 Toolholders

The tool clamping system serves as the essential connection between the tool and the machine tool spindle, making it a vital factor that cannot be overlooked. The design of the tool’s clamping handle significantly influences machining accuracy, tool longevity, and efficiency, all of which ultimately affect the quality of the machining process. The commonly used types of tool holders in the industry include spring collet chucks and shrink-fit chucks, as illustrated in Figure 10.

 

Spring collet chucks are highly versatile and can help reduce the cost of toolholder purchases. However, they have relatively low precision and make chip removal during cutting inconvenient, which makes them unsuitable for scroll disk machining. In contrast, heat shrink chucks do not require any components between the tool and the cutter. They provide high clamping torque, excellent dynamic balance, low runout, high precision, strong clamping force, and robust rigidity. Additionally, cutting fluid can be easily sprayed onto the cutting area, effectively removing the heat and chips produced during cutting, making heat shrink chucks more suitable for scroll disk machining. Figure 11 illustrates machining with a spring collet chuck, while Figure 12 demonstrates machining with a heat shrink chuck.

 

6.2 Tools

Cemented carbide is known for its high compressive strength, excellent wear resistance, high hardness, elevated elastic modulus, and superior impact strength. It also offers good vibration resistance, strong corrosion resistance, and excellent dimensional stability. For these reasons, the base of cutting tools is often made from cemented carbide.

DLC-coated (diamond-like carbon) cemented carbide tools are particularly effective for cutting various aluminum alloys, graphite, and other non-ferrous metals. These tools ensure high processing quality and extended tool life.

To optimize production, it is preferable to design tools with a shorter effective length. A shorter tool length enhances rigidity, increases cutting efficiency, and stabilizes the cutting process, reducing the likelihood of tool bounce and improving overall processing accuracy. The difference in cutting efficiency between long and short tool blades is illustrated in Figure 13.

 

The cutting speed formula is vf = nznfz (1)

Where vf is the feed rate (mm/min); n is the rotational speed (r/min); zn is the number of blades; and fz is the feed rate (mm/z). From formula (1), we can see that the more blades a tool has, the higher the cutting efficiency. Therefore, under the condition of meeting the chip removal requirements, the more blades the better.

The roughing tool is designed with 4 blades to meet both efficiency and chip removal requirements; the finishing tool is designed with 6 blades to meet both precision and efficiency requirements. The 6-blade finishing milling cutter is shown in Figure 14.

 

07 Machine Tool Accuracy Inspection

Due to the high precision requirements for scroll disk components, particularly the machining accuracy of the scroll involute surface, CNC machining centers must adhere to even stricter precision standards. Precision inspection and calibration are essential prior to machining. Laser interferometers are utilized to measure displacement errors along the X, Y, and Z axes.

Inspection data from several Japanese Makino-V56i machine tools (with travel dimensions of 900mm × 550mm × 450mm) currently in operation in the workshop indicate that the displacement error on the Y axis, which has a smaller travel range, is generally maintained between 0.001mm and 0.002mm. Conversely, the larger travel of the X axis leads to increased error, resulting in a displacement error ranging from 0mm to 0.005mm. One specific machine tool demonstrated a displacement error of 0.003mm.

While achieving a correction to within 0.001mm would be ideal, if that is not possible, positioning the part as close as possible to both the machine tool setter and the tool magazine during clamping can help reduce precision errors. This positioning also enables the spindle to effectively reach the robot arm for tool changes, thereby shortening the tool change time. The machine tool setter and workbench are illustrated in Figure 15.

 

Use a marble level or micrometer to calibrate the machine table for horizontality, and employ a standard rod and micrometer to ensure the spindle’s verticality. The accuracy should be within 0.002 mm. According to technical specifications, a smaller correction error is preferable.

 

08 PowerMILL CNC Programming

8.1 Programming Key Points

Scroll disk programming and machining technology must prioritize high efficiency and precision. The programming process typically follows a “rough-semi-finish-finish” approach. Whenever possible, use the simplest 2D programming strategy to achieve faster calculation speeds, high-quality tool paths, and quicker machine operation. Additionally, the specifications for fine milling programming must strictly adhere to the intermediate tolerances outlined in the 2D technical drawings of the scroll disk part.

8.2 Rough and Semi-finish Milling

During rough milling, the stock allowance is controllable, and since the shortest tool solution is utilized, programming tolerances can be slightly larger (e.g., 0.02 mm) to enhance calculation speed. With a 0.33 mm allowance, a high-speed, deep-cut programming strategy is employed. The cutting parameters for the first pass of rough milling are outlined in Table 1.

For positioning during the second clamping, the top and side surfaces of the scroll involute profile are utilized. Field verification has indicated that a semi-finishing allowance of 0.3 mm is necessary after rough milling; otherwise, the part may not fit into the second clamping station.

Additionally, combining multiple programs into one and running them with the same tool can reduce tool change time. Please refer to Table 2 for the CNC machining process card.

8.3 Fine milling

(1) Bottom fine milling
The bottom features of the scroll disc are illustrated in Figure 16. At the center of the part is the bearing hole, which has very high accuracy and roundness requirements. A two-dimensional curve strategy is employed to define the circle, and the arc interpolation tool path trajectory is depicted in Figure 17. The resulting NC code is as follows:

G3X12.3461Y-16.9554I-.717J1.867F1337 X-12.3461Y-13.0446I-12.346J1.955F1671 X12.3461Y-16.9554I12.346J-1.955 X11.6294Y-15.0883I-1.975J.313F1337

Despite the inclusion of circular interpolation instructions in the code, the roundness of the resulting circular hole does not meet the required technical specifications. This issue can be effectively addressed through boring, which provides a precise solution. Additionally, reaming can be employed to improve the accuracy of positioning holes. Actual machining tests demonstrate that both boring and reaming methods yield the highest levels of accuracy, efficiency, and stability in hole machining.

 

The sliding grooves on both sides of the part require extremely high precision for two main reasons:

1. When assembling the compressor’s rotating and fixed scrolls, the rotating scroll moves back and forth along these grooves. High precision in the grooves ensures excellent meshing between the rotating and fixed scrolls, which guarantees a tight seal during air compression and enhances compressor performance.

2. During machining at the third station, locating holes and these grooves are essential for securing the part and preventing rotation. High precision is crucial for achieving a close fit with the fixture, which ensures the accuracy and concentricity of the top and side surfaces of the scroll involute profile at the third station.

Due to the narrow width of these grooves and their proximity to the outer diameter of the bearing hole, only a φ4mm × L16mm tool can be selected, leading to a large aspect ratio. When programming for this groove, a 2D curve strategy is employed for rough milling, requiring climb milling and rounding functions. A rounding value of 0.18mm is recommended to prevent tool bounce, which could negatively impact machining quality and tool life.

With a stock sidewall allowance of 1mm, multiple layers of cutting are necessary for the depth of cut to prevent tool breakage. The multi-layer tool path for rough milling the slider groove is illustrated in Figure 18. During fine milling, due to the small allowance, the side edge can be used to mill one layer to the finished product size. The tool path trajectory for fine milling the slider groove in one layer is shown in Figure 19, and it’s important to activate the 2D compensation function.

 

(2) Frontal milling

The third step involves milling the surface of the scroll. A key technical challenge in this CNC manufacturing process is to ensure that the contour of the scroll disk’s involute surface is uniform, while also meeting the parallelism and flatness requirements for the top and bottom planes. At a lower speed (n = 12,000 rpm) and feed rate (vf = 800 mm/min), it is easier to meet these technical specifications. The tool path trajectory operates in one axial layer, which helps to ensure there are no tool marks on the side surface and that the integrity of the material is maintained. The milling strategy used in PowerMILL is illustrated in Figure 20. This method requires high rigidity of both the tool and the tool holder to ensure that the verticality of the side surface meets the necessary standards. However, when the speed and feed rate are increased, the quality of processing can become unstable. The three-dimensional inspection data is displayed in Figure 21, where red indicates that the accuracy exceeds tolerance limits, yellow signifies a critical tolerance state, and green shows that the measurements are within the acceptable tolerance range.

 

 

 

 

Table 3 shows the test parameters for improving the efficiency of fine milling the scroll disk’s involute surface. After adjusting various parameter combinations, the optimal programming parameters for improving the efficiency of fine milling the scroll disk’s involute surface were obtained, as shown in Table 4. The clamping pressure was adjusted to 0.4 MPa. The high-speed cutting 3D inspection data for the product is shown in Figure 22, demonstrating that it met the technical requirements.

 

 

09 Part Cleaning

Residue from cutting fluid left on the surface of aluminum CNC parts after fine milling can cause oxidation of aluminum alloys, resulting in a loss of gloss on the scroll surface. To prevent this, it is essential to clean the surface immediately after removing the part from the machine. While using compressed air with an air gun or other cleaning fluids can be effective, it may also be costly or incomplete. We have discovered that cleaning with tap water effectively maintains a glossy surface finish and is both economical and convenient. A comparison of the surface before and after cleaning can be seen in Figure 23.

 

10 Conclusion

This paper examines the machining process for scroll disks used in air-conditioning compressors for new energy vehicles. The machining technology and methods for scroll disks were validated using on-site cutting data, enabling the mass production of these parts. The key conclusions are as follows:

1) Heat-shrinkable chucks are superior to spring collet chucks concerning machining accuracy and cooling ease when it comes to milling cutter holders.

2) For tool design, roughing cutters should ideally have four flutes, while finishing cutters can be designed with six flutes to strike a balance between efficiency and quality. Additionally, tool designs should follow the principle of minimum length.

3) If hole milling does not achieve the required roundness, boring and reaming should be utilized as alternatives.

4) In fixture design, each station should be configured to hold two parts simultaneously, with the workpieces positioned along the Y-axis to enhance both efficiency and accuracy.

5) For fine-milling the plane of the scroll disk involute surface, the spindle speed should be set at 22,000 RPM with a feed speed of 1,500 mm/min. When processing the side, the spindle speed remains at 22,000 RPM, but the feed speed should be increased to 1,600 mm/min. Additionally, the clamping air pressure needs to be adjusted to 0.4 MPa to improve efficiency and meet precision requirements.

 

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