Precision Manufacturing Solutions for New Energy Vehicle Drive Components
This paper addresses the challenges of CNC machining aluminum alloy motor casings and alloy steel motor shafts used in new energy vehicles. It systematically analyzes key machining difficulties, including the deformation of thin-walled motor casings, the complexity of machining intricate cavities, the need for high-precision rotary surface machining of motor shafts, and the control of dynamic balancing.
To tackle these challenges, the paper combines theoretical analysis, simulation optimization, and practical verification. A comprehensive solution is proposed, which includes optimizing the entire process chain, selecting appropriate equipment and tools, innovating clamping methods, and implementing intelligent control. This approach provides thorough technical support for the precision machining of essential motor components in new energy vehicles.
1. Introduction
The global new energy vehicle market is experiencing explosive growth, and electric motors serve as the power source for these vehicles, which include pure electric, hybrid, and fuel cell models. Electric motors convert electrical energy into mechanical energy to drive the wheels and are one of the three core components—along with the battery and electronic control system—that distinguish new energy vehicles from traditional fuel-powered vehicles.
As the “heart” of new energy vehicles, the performance of the motor is critical to the vehicle’s power output and efficiency. The motor housing and motor shaft are essential structural components that provide support, protection, and power transmission. The quality of machining for these components directly impacts the motor’s operating precision, vibration and noise levels, and overall service life. Notably, the manufacturing costs of the motor housing and motor shaft account for over 60% of the total cost of the motor system.
Machining accuracy is vital; factors such as the coaxiality of the motor housing bearing hole and the dynamic balancing accuracy of the motor shaft are key to enabling high-speed operation (exceeding 15,000 r/min). Currently, aluminum alloy motor housings dominate the market due to their lightweight properties, which reduce weight by 40% compared to cast iron housings. However, their thin-walled structures (typically 3-5 mm) are susceptible to deformation of 0.04-0.08 mm during CNC machining, resulting in precision errors in bearing holes.
Motor shafts are generally made from high-strength alloy steels like 40Cr, and their aspect ratios often exceed 10. This characteristic makes them vulnerable to vibrations during turning and grinding, with a required dynamic balancing accuracy of G2.5 (where residual unbalance must be ≤ 2.5 g·mm/kg), posing significant machining challenges.
Statistics indicate that traditional machining processes can lead to a combined scrap rate of up to 12% for the motor housing and motor shaft, severely limiting production capacity and cost management. Therefore, it is essential to explore the key challenges and potential solutions for CNC machining of these components to promote the localization and high-end development of core elements for new energy vehicles.
2. Motor Housing Structural Characteristics and Processing
The motor housing is a crucial component of the powertrain in new energy vehicles. The top (open side) of the housing is connected to the inverter, while the bottom connects to the reducer and the spindle bearings through embedded bearing bushings. The sidewalls are typically attached to the subframe using mounts.
The motor housing is primarily constructed from 6061-T6 aluminum alloy, which has a tensile strength of 310 MPa, moderate cutting forces, and moderate tool wear. Its elastic modulus is 69 GPa, approximately one-third that of steel, which makes thin-walled structures prone to springback. The thermal conductivity of this alloy is 180 W/(m·K), allowing for rapid heat transfer during cutting and contributing to a significant temperature rise in the workpiece. Its thermal expansion coefficient is 23.6 × 10⁻⁶ /°C.
In terms of design, the motor housing features a thin-walled cylindrical structure with a wall thickness of 3-5 mm and an aspect ratio of 1. However, this results in insufficient overall rigidity, making it susceptible to vibration and deformation during machining.
To address these challenges, specific processing techniques are employed. Axial positioning tooling, such as an elastic expansion sleeve fixture, is used to reduce radial clamping forces, minimizing the risk of compressive deformation in the thin walls. The cutting process is divided into roughing and fine milling stages. For roughing, a margin of 0.5-1 mm is reserved, while in fine machining, a back cutting depth of ap ≤ 0.3 mm and a feed rate of f = 0.15-0.2 mm/r are implemented to reduce cutting forces. Additionally, the temperature of the workpiece is monitored in real time during processing, and infrared temperature measurements are utilized to control the temperature rise to ≤ 50°C, thereby mitigating the effects of thermal deformation.
(2) φ60H7 bearing holes at both ends
The bearing hole serves as the core assembly reference, and its coaxiality has a direct impact on bearing life. To ensure an optimal interference fit, the coaxiality must be within 0.02 mm, and the surface roughness must have a value of Ra ≤ 1.6 μm.
Regarding the processing technology, rough machining is performed using CNC boring, with a rotational speed of 800 to 1000 r/min and a feed rate of 50 to 80 mm/min. After semi-finishing, a grinding allowance of 0.2 to 0.3 mm is maintained. For finishing, precision honing technology is employed, utilizing diamond honing sticks. A floating fixture compensates for any slight deviation between the machine tool spindle and the workpiece axis, ensuring that the coaxiality meets the required standards. Additionally, a coordinate measuring machine is used to detect the hole axis deviation both before and after machining.
(3) Heat dissipation groove
The workpiece has a depth of 15 mm and an aspect ratio of 3:1, with a surface roughness value of Ra ≤ 3.2 μm. The large aspect ratio results in weak side wall rigidity, making the material prone to vibrations during milling, which can negatively impact surface quality.
To address these issues in the processing technology, a carbide end mill with a helix angle of 35° and an aspect ratio of ≤ 5 is selected. A cycloidal milling path is employed to minimize radial cutting forces. The milling process is carried out using multi-layer cutting, where each layer has a depth of 3 to 5 mm. The cutting width is limited to no more than 1/3 of the tool diameter (approximately 5 mm).
Additionally, high-speed spindle operation is utilized, with speeds ranging from 1500 to 2000 r/min, to enhance cutting stability. High-pressure internal cooling, with a pressure of 3 to 5 MPa, is applied to the cutting fluid to effectively remove chips and cool the tool, thereby preventing chip blockage and vibrations.
(4) Installation boss and bolt hole
Flatness should be ≤ 0.03 mm. This measurement is crucial as it affects the sealing assembly between the motor and the base, and the position of the bolt holes indirectly impacts the installation accuracy of the entire machine.
For the processing technology, the boss plane undergoes precision milling followed by grinding. The bearing hole axis serves as the reference point for positioning during the milling process. An electromagnetic chuck is utilized for grinding, ensuring a flatness error of ≤ 0.01 mm. The linear speed of the grinding wheel must be ≥ 30 m/s to maintain surface flatness.
The bolt holes are created through a “drilling-expansion-reaming” process, using a CNC drilling and tapping center, which offers a positioning accuracy of ± 0.02 mm. The workpiece coordinate system is established with the bearing hole as the reference, and rigid tapping is employed to guarantee that the verticality of the threaded holes is ≤ 0.05 mm per 100 mm.
3. Motor Shaft Structural Characteristics and Processing
The motor shaft is a key rotating component in the electric drive systems of new energy vehicles. It connects the motor rotor to the transmission system, which includes elements such as the reducer and wheel axles. Its primary function is to efficiently transmit the torque generated by the motor to the drive wheels, while also enduring the alternating loads, torque stresses, and vibrations that occur during operation.
Compared to traditional fuel vehicle motor shafts, those used in new energy vehicles have more stringent requirements regarding material selection, structural design, and processing precision. This is due to the high speeds, high power densities, and lightweight characteristics of electric drive systems.
The materials commonly used for motor shafts in new energy vehicles include 40Cr and 20CrMnTi alloy steel. For example, 40Cr has a tensile strength of ≥ 785 MPa, which means it can withstand significant force, but it also has a large cutting force that leads to rapid tool wear. After tempering, its hardness ranges from 220 to 250 HBW, making it suitable for bearing alternating loads. However, this high hardness complicates cutting processes. The elastic modulus of 40Cr is 206 GPa, providing good rigidity; however, shafts with a large aspect ratio can be prone to vibration. The material has a thermal conductivity of 36 W/(m·K), which results in slow heat conduction during cutting, increasing the risk of tool overheating. Its thermal expansion coefficient is 11.5 × 10⁶/°C, indicating relatively low thermal deformation in comparison to aluminum alloys.
Here are its typical structural features and their corresponding processing characteristics.
(1) Stepped shaft structure
The total length should be between 500 mm and 800 mm, with a minimum diameter of 30 mm and a maximum diameter of 80 mm. The aspect ratio must be greater than 10, and the overall rigidity is weak.
During processing, it is necessary to use a center stand or a tool rest to support the workpiece and control vibrations during turning and grinding. The turning process should be divided into rough turning and fine turning, with gradual corrections made to ensure geometric accuracy.
For grinding, it is essential to optimize the parameters of the grinding wheel to prevent surface scratches.
(2) Bearing position (φ50mm)
The accuracy requirement is at the IT5 level, with a roundness of ≤0.01 mm and a surface roughness value of Ra ≤0.8 μm, indicating that it must be a high-precision mating surface. The final processing should be carried out using precision grinding methods, such as an external cylindrical grinding machine. Prior to grinding, it is essential to ensure the coaxiality of the workpiece reference. A controlled environment with constant temperature is utilized to manage thermal deformation. If necessary, super-fine grinding may be performed to achieve the desired surface accuracy.
(3) Center deep hole (φ10mm)
The aspect ratio can go up to 20, which is important for balancing weights or lubricating oil circuits. However, chip removal and cooling become challenging during deep hole processing. Typically, technologies such as gun drilling or BTA (Boring, Transfer, and Advancement) deep hole processing are employed. Throughout the process, high-pressure cutting fluid is essential to maintain the straightness and surface quality of the hole. Additionally, deburring and inspecting the diameter accuracy of the holes are necessary after processing.
(4) Keyway and threaded hole
Position accuracy requirements must be ≤ 0.03 mm; therefore, the assembly accuracy of the transmission components must be ensured. The keyway should be processed using CNC milling or slotting, with the workpiece axis serving as the reference for positioning. The threaded hole should be completed with CNC drilling and tapping. It is crucial to calibrate the machine tool coordinate system during processing to avoid cumulative errors.
The required dynamic balancing accuracy must achieve a G2.5 level, with a residual imbalance of ≤ 2.5 g·mm/kg. A dynamic balancing test is necessary after the shaft body has been fine-machined. To meet the balance accuracy requirements, weight can be removed (through milling or drilling) or balancing weight can be added (using welding balance blocks) at specified positions, such as the flange end or shaft shoulder, until the balance accuracy criteria are satisfied. This ensures stability during high-speed operation.
4. Requirements for machining accuracy of motor housing
As a critical component in supporting and protecting the motor, the machining accuracy of the motor housing hinges on geometric tolerance control. This directly influences both the precision of the bearing assembly and the sealing performance of the entire machine.
(1) Key geometric tolerances
The coaxiality requirement for the bearing holes is ≤0.02 mm. It is essential to ensure that the axes of the φ60H7 bearing holes at both ends are consistent to prevent abnormal wear due to eccentric loads on the bearings. Additionally, the flatness requirement is ≤0.03 mm over a length of 100 mm. For the mounting boss and flange surfaces, it is important to ensure even distribution of force during bolt connection to avoid any assembly deformation.
(2) Surface quality control
The surface roughness of the bearing hole must be Ra ≤ 1.6 μm, which is achieved through either precision boring or honing. This ensures a reduction in contact stress between the bearing and the hole wall. Additionally, the surface roughness of the heat sink is required to be Ra ≤ 3.2 μm. Given that the height-to-width ratio of the axially distributed heat sink is 3:1, it is important to control vibration during the machining of the side walls. This will help maintain heat dissipation efficiency while also preventing stress concentration.
(3) Dimension tolerance requirements
The dimensional tolerance for the bearing hole is classified as H7 grade, which satisfies the interference fit requirements for the bearing. The tolerance for the wall thickness of the thin-walled structure is maintained at ±0.1 mm. It is essential to balance rigidity with a lightweight design to prevent dimensional deviations caused by elastic deformation during the cutting process.
(4) Special performance indicators
The airtightness requirement is set at ≤5×10⁹ Pa·m³/s. To ensure the sealing of the enclosed motor housing and prevent dust and water vapor from entering and affecting the motor’s lifespan, a helium mass spectrometry leak detection process is necessary. This airtightness is particularly crucial under high-speed or harsh operating conditions, as it directly impacts the motor’s protection level (IP level).
The motor housing is a vital component in the power system of new energy vehicles, and its manufacturing process significantly influences both the performance and cost of these vehicles. The precision requirements mentioned above are closely linked to the thin-walled cylindrical structure of the motor housing, which has a wall thickness of 3 to 5 mm and an aspect ratio of 1. During production, it is essential to manage thermal deformation and elastic rebound through techniques such as constant temperature cutting and tooling rigidity optimization. These measures help ensure the co-alignment of coaxiality and flatness within the porous system.
5. Requirements for machining accuracy of motor shafts
The motor shaft is the key rotating component responsible for transmitting torque. Its precision system focuses on rotational accuracy and dynamic balance, which directly affects the stability and lifespan of the motor. Machining accuracy primarily pertains to controlling the basic dimensional tolerances and geometric tolerances of the various parts of the motor shaft, specifically regarding its diameter and length.
(1) Rotational accuracy index
The roundness of the bearing seat must be ≤0.01 mm, and the dimensional tolerance is k5. To ensure a precise fit with the inner ring of the bearing and to minimize radial circular runout during high-speed rotation, a mirror-grade surface must be achieved through precision grinding, with a surface roughness value of Ra ≤0.8 μm. Additionally, the dimensional tolerance for each section of the stepped shaft is ±0.02 mm. For slender shaft structures with an aspect ratio greater than 10, it is crucial to control vibration deformation during turning and grinding to prevent axis deviation due to insufficient rigidity.
(2) Dynamic balancing requirements
The dynamic balancing accuracy must achieve G2.5, meaning the residual unbalance should be less than or equal to 2.5 g·mm/kg. A dynamic balancing test must be conducted and passed after fine machining. To eliminate centrifugal force imbalance during high-speed operation, the flange end or shaft shoulder must be deweighted or counterweighted, which helps prevent bearing overload and reduces vibration noise throughout the entire machine.
(3) Surface quality and stress control
The surface roughness of the journal must be maintained at Ra ≤ 1.6 μm to meet the requirements for sliding bearings or seals. Additionally, the residual stress should not exceed 80 MPa. By employing processes such as tempering and stress relief annealing, we can prevent shaft bending or fatigue fractures that may result from machining stresses. Controlling residual stress is crucial for enhancing the reliability of shaft components, particularly when subjected to alternating loads.
(4) Structural accuracy
The straightness and consistent diameter of the center deep hole (with a length-to-diameter ratio of 20) play a crucial role in the installation of the balance weight and the smooth flow of the lubricating oil path. Additionally, the accuracy of the keyway and threaded hole positions must be within ≤0.03 mm to ensure the correct circumferential alignment of transmission components, such as gears and couplings, thereby preventing phase deviation during torque transmission.
Considering the stepped shaft structure and the slender characteristics of the motor shaft, the manufacturing process requires the use of auxiliary support from a center frame, as well as constant temperature grinding and residual stress detection techniques. These measures are essential to ensure that the rotation accuracy and dynamic balance meet the standards, ultimately providing a solid foundation for the motor’s high-speed and high-precision operation.
6 Difficulties in Processing
6.1 Difficulties in Processing Motor Housings
Processing aluminum alloy motor housings presents two significant challenges: controlling deformation in thin-walled structures and achieving precision in complex cavity processing. The thin-walled nature of these structures causes notable deformation issues during manufacturing. These challenges primarily manifest in the following ways:
(1) Springback deformation caused by cutting force
Aluminum alloy has a low elastic modulus. When a radial cutting force of 50 N is applied, the springback of a side wall with a thickness of 3 mm can reach 0.04 mm, which exceeds the tolerance requirements for the bearing hole. In traditional three-axis machining, the cumulative springback error after multiple passes can reach 0.08 mm.
(2) Thermal deformation caused by cutting heat
Aluminum alloy has a high thermal expansion coefficient. When the temperature in the cutting zone reaches 200°C, the radial expansion of a φ60mm bearing hole can reach 0.027mm. In traditional machining, multiple clamping and uneven cooling can lead to cumulative thermal deformation of up to 0.04mm.
(3) Local deformation caused by clamping force
When the traditional pressure plate is clamped, the single-point clamping force, approximately 500 N, is concentrated. This concentration of force causes the surface of the workpiece, located within 10 mm beneath the pressure plate, to deform by 0.04 mm. Additionally, a rebound error occurs once the pressure is released. Controlling the machining accuracy of complex cavities presents significant challenges, particularly due to the intricate structure of the motor housing. These challenges can be observed in several specific aspects of the CNC machining process.
(1) The difficulty of posture control in five-axis linkage machining
Changing the tool inclination angle affects the distribution of cutting forces. As the angle (β) increases from 0° to 30°, the radial force (Fy) decreases by 25%, while the axial force (Fz) increases by 15%. This increase in axial force may lead to unintended axial movement of the workpiece. Additionally, the accuracy of five-axis interpolation motion (within ±0.01 mm) can accumulate errors during complex contour machining, causing the actual cutting path to deviate from the intended design.
(2) Flutter problem of heat sink sidewalls.
The heat sink has a height-to-width ratio of 3:1, and the sidewalls are not rigid enough, making flutter prone to occur during milling. This causes the surface roughness Ra to increase from 1.6μm to 3.2μm, accompanied by a 0.05mm wave-shaped deformation.
(3) Difficulty in ensuring coaxiality of porous systems.
The coaxiality of the bearing holes at both ends must be ≤0.02mm. Traditional three-axis machining requires three fixtures, leading to a cumulative positioning error of 0.05mm, which far exceeds the accuracy requirements.
6.2 Difficulties in Motor Shaft Machining
Machining motor shafts presents two main challenges: technical bottlenecks in achieving high-precision rotary surface machining, and the difficulties in maintaining accurate dynamic balancing.
In high-precision rotary surface machining, the large aspect ratio (greater than 10) and the stringent precision requirements lead to several technical difficulties:
1) Bending vibrations can occur during turning. When the aspect ratio exceeds 10, the vibration amplitude can reach 0.03 mm, resulting in the journal’s roundness falling out of tolerance. Additionally, uneven clamping force in the self-centering chuck—potentially differing by as much as 200 N—exacerbates deformation, which can cause roundness errors of up to 0.06 mm.
2) The low thermal conductivity of 40Cr alloy steel significantly affects machining quality. During the grinding process, the cutting heat tends to concentrate due to this low thermal conductivity. When the grinding wheel’s linear speed exceeds 150 m/s, the surface temperature can rise to 800°C. This leads to temper softening, resulting in a 15% decrease in hardness, an increased risk of grinding cracks, and deterioration of surface quality, with surface roughness exceeding Ra > 1.6 μm.
3) Conventional twist drills tend to produce substantial machining errors. When machining a central deep hole with an aspect ratio of 20 using a conventional twist drill, the straightness error can reach 0.1 mm per 100 mm. Additionally, the hole diameter tolerance is ±0.05 mm, which does not meet the requirements for dynamic balancing weights.
In terms of coordinated dynamic balancing accuracy control, its accuracy is significantly affected by the coupling of multiple factors, mainly manifested in the following aspects.
1) The cumulative impact of machining errors is quite significant. A combination of a 0.01 mm roundness error in the bearing seat, a 0.03 mm eccentricity in the deep hole, and a 0.02 mm keyway position error can lead to a residual unbalance of 12 g·mm/kg. This figure exceeds the G2.5 grade requirement, which is ≤2.5 g·mm/kg.
2) After the carburizing and quenching process of 20CrMnTi, the straightness deviation of shaft parts can reach 0.1 mm over a length of 100 mm. While straightening is necessary, the residual stress (≥80 MPa) that remains after this process can negatively affect the stability of dynamic balancing.
3) When the surface roughness of the bearing seat is greater than 1.6 μm (Ra > 1.6 μm), the aerodynamic imbalance increases by 30% during high-speed rotation. This results in a decrease in dynamic balancing accuracy.
6.3 Common Difficulties and Challenges
There are three common difficulties and challenges in the processing of motor housings and motor shafts, as follows.
(1) The problem of tool compatibility for processing different materials is prominent.
Processing aluminum alloys requires sharp tools and effective chip removal methods, often utilizing tools coated with AlCrN. In contrast, processing alloy steel demands tools that exhibit high hardness and wear resistance, such as those coated with CBN. When manufacturing both types of parts on mixed production lines, frequent tool changes occur, leading to decreased processing efficiency and increased costs and time associated with tool replacements.
(2) The contradiction between efficiency and precision is difficult to reconcile.
Increasing the cutting speed can enhance efficiency; however, it also leads to greater thermal deformation of the motor housing. On the other hand, reducing the grinding feed rate of the motor shaft can improve accuracy, but it will also extend the processing time. Traditional processes often result in a high scrap rate, making it challenging to strike a balance between efficiency and precision.
(3) Insufficient application of intelligent processing technology.
The current production line does not have real-time monitoring or adaptive control for cutting force, temperature, and other processing parameters. As a result, it struggles to address accuracy fluctuations caused by variations in part blanks and tool wear. This limitation hinders efforts to improve the stability and consistency of processing accuracy.
7. Process Chain Optimization Solution
7.1 Motor Housing Die-Casing-Machining Composite Process
The die-casting process employs a die-casting machine that integrates precision control techniques, five-axis layered milling, and fine turning datum correction. It operates with precise mold temperature control at 220°C, enabling rapid mold filling in just 0.08 seconds. With a bridge structure design, the process ensures stable control of the blank allowance within ±0.3mm, which reduces shrinkage defects by 60% and delivers high-precision blanks for subsequent processing.
In five-axis milling, a layered machining strategy is utilized. The roughing phase employs a backcut depth of 1.5mm and a cutting speed of 250m/min with a 15° spiral cut to efficiently remove excess material. During the semi-finishing stage, constant residual height milling is applied to maintain surface consistency. For the finishing process, a backcut depth of 0.2mm and a 20° tool inclination angle are used in down milling, which reduces cutting forces by 25% and minimizes thermal deformation to just 0.015mm.
The finishing turning process utilizes a high-precision CNC lathe equipped with hydraulic soft jaw clamping. This process accurately turns the bearing bore to a dimension of φ60H7, with roundness maintained within 0.015mm, effectively correcting for cumulative errors that may have occurred during die-casting and milling.
7.2 Integrated Turning-Grinding-Dynamic Balancing Process for Motor Shafts
A “rigid support turning-precision grinding-dynamic balancing closed-loop” process system has been developed. The cutting parameters were optimized as follows: for rough turning, the speed was set at 250 m/min with a back-cut depth of 1.5 mm, while for fine turning, the speed was adjusted to 220 m/min with a feed rate of 0.08 mm/rev.
To offset radial deformation, a 50 N axial thrust support was introduced, which improved the journal roundness from 0.06 mm to 0.015 mm. Grinding was carried out using a grinder equipped with CBN sandblasting. The grinding wheel, combined with micro-lubrication technology and a cutting fluid dosage of 50 mL/h, achieved a mirror-level precision for bearing seat roundness of ≤0.01 mm and a surface roughness value of Ra ≤0.8 μm. This method completely eliminated the risk of grinding burns.
Additionally, in the dynamic balancing collaborative control, the deep hole processing utilized a BTA high-pressure system (3 MPa) to maintain straightness within 0.05 mm per 100 mm. This system worked in conjunction with the dynamic balancing equipment to ensure precise weight balance, achieving a weight block distribution error of ≤0.02 mm. As a result, the residual unbalance was stabilized to ≤5 g·mm/kg.
7.3 Collaborative optimization of processes
By integrating die casting, five-axis milling, turning, grinding, and dynamic balancing processes, both types of parts can be processed collaboratively and efficiently.
(1) Process integration
Shifting away from the traditional independent processing mode, the motor housing and motor shaft key processes—such as the precision turning of bearing holes in the motor housing and the grinding of bearing seats on the motor shaft—now share benchmark detection equipment to minimize repeated tool setting time.
(2) Precision coordination
The uniformity of the die-cast blank allowance and the consistency of the turning and grinding reference points are designed to ensure that the process capability index for the coaxiality of the motor housing bearing hole and the roundness of the motor shaft bearing seat meet the required standards. This guarantees that the high-precision assembly requirements are fulfilled.
8 Equipment and Tooling Solutions
8.1 CNC Machine Tool Selection and Matching
The selection of CNC machine tools was guided by the fundamental principles of high-precision positioning, thermal deformation compensation, and minimal clamping integration.
1) For the motor housing-specific equipment, we utilized a five-axis machining center that offers a positioning accuracy of ±0.005 mm through five-axis linkage. This center is equipped with a thermal compensation system that automatically corrects for errors caused by temperature fluctuations (up to ±1°C) in real time. This capability ensures that we meet the high-precision machining requirements for thin-walled structures with complex curved surfaces, such as heat sinks and bearing bores.
2) The motor shaft precision machining equipment features a high-precision CNC lathe, with a spindle radial runout of ≤0.003 mm, which is driven by a linear motor. This setup enables the rigid turning of slender shafts. When combined with a grinder, it creates an integrated turning and grinding process. The high-precision grinding occurs at a wheel speed of 120 m/s, ensuring that the bearing seat roundness remains within ≤0.01 mm.
3) The turning and milling compound machine tool is designed for motor housings with shoulder or stepped shaft components. It can perform 80% of turning, milling, and drilling processes in a single clamping, which reduces the cumulative positioning error to 0.03 mm or less, a significant improvement over traditional three-way clamping methods. For instance, the compound five-axis gantry machine tool can handle the roughing, finishing, and online inspection of the die-cast blank for the motor housing in one clamping. This leads to a 40% increase in processing efficiency compared to conventional equipment.
8.2 Tool innovation and adaptation technology
The following special tools and intelligent management systems are developed for different material characteristics.
(1) The φ10mm AlCrN coated end mill is designed for processing aluminum alloys and enhances cutting performance. When used at cutting speeds of 150-300 m/min, this tool’s lifespan is improved by 50% compared to standard carbide tools. Additionally, it effectively reduces vibration along the heat sink’s side wall and maintains a surface roughness value of Ra ≤ 3.2 µm.
(2) CBN-coated turning tools made of alloy steel are ideal for machining 40Cr quenched and tempered steel. At a cutting speed of 220 m/min, the tool wear rate is reduced by 40%. Additionally, CBN grinding wheels enhance grinding efficiency by three times compared to standard grinding wheels, achieving a mirror finish on bearing surfaces with a roughness value of Ra ≤ 0.8 μm, all without the risk of burns.
(3) Intelligent tool management
Tool recognition technology automatically matches the machining parameters for the motor housing, which uses AICrN-coated tools, and the motor shaft, which uses CBN-coated tools. This technology reduces the tool change time from five minutes to just two minutes, preventing parameter mismatches that can occur with manual tool changes and significantly improving machining efficiency.
A comprehensive solution has been developed for the motor housing and motor shaft, taking into account their material characteristics and precision requirements. This solution encompasses equipment selection, tool adaptation, and intelligent management. By precisely matching equipment and tools, we achieve a closed-loop optimization of the “material-equipment-process” cycle.
The thermal compensation system of the five-axis machine tool effectively addresses the issue of thermal deformation in the motor housing, maintaining an error margin of ≤ 0.01mm. Additionally, the use of CBN tools enhances the grinding efficiency of the motor shaft by threefold. Our management system significantly reduces human error during tool changes by 80%, providing a solid foundation for high-precision processing, with the motor housing coaxiality maintained at ≤ 0.02mm and the motor shaft roundness at ≤ 0.01mm. This results in both high precision and high efficiency in the manufacturing of these two components.
9 Clamping Optimization Scheme
In view of the structural characteristics and clamping deformation problems of the two types of parts, special clamping schemes with high precision and low deformation are designed as follows.
(1) Flexible clamping scheme for vacuum adsorption of motor housing
The core problem is to solve the clamping deformation of thin-walled structures and the lack of rigidity in the suspended area.
① Main support system. A 150mm diameter annular vacuum suction cup is utilized to apply a uniform adsorption pressure of 0.08 MPa to the bottom surface of the workpiece, replacing the traditional single-point pressure plate clamping method. This new approach significantly reduces clamping deformation from 0.04 mm in the traditional method to just 0.01 mm, completely eliminating local depression rebound errors.
② Auxiliary support structure. For the suspended area distributed circumferentially in the heat sink (depth: 15 mm, aspect ratio: 3:1), three sets of adjustable nylon support blocks have been added under the side wall. The support force of each set is maintained between 5 and 10 N. By combining “vacuum adsorption for primary positioning” with “flexible support as an auxiliary,” the vibration deformation during side wall milling is minimized. This ensures that the surface roughness of the heat sink remains stable at Ra ≤ 3.2 μm.
(2) Motor shaft hydraulic soft jaw + axial support rigid clamping solution
Focus on addressing the issues of bending deformation and uneven clamping force in slender shafts with a length-to-diameter ratio greater than 10.
1. Clamping Optimization: We replaced traditional self-centering chucks with hydraulic soft jaws. A pressure sensor balances the clamping force in real time, reducing the difference in clamping force from 200N to 120N, which increases uniformity by 40%. This approach completely eliminates clamping deformation and decreases the journal roundness error from 0.06mm to 0.015mm.
2. Rigidity Enhancement: An axial thrust support device exerting 50N is installed at the tailstock end, creating a composite support structure of “two-end tightening + axial uniform clamping” in conjunction with the hydraulic soft jaws. This effectively offsets the turning bending vibrations associated with a large length-to-diameter ratio, reducing the amplitude from 0.03mm to 0.01mm. As a result, we establish a high-precision benchmark for subsequent precision grinding, ensuring that the roundness of the bearing seat is maintained at ≤0.01mm.
10 Intelligent Solutions
In order to solve the problems of precision control and efficiency optimization in the processing of motor housing and motor shaft, a full-process intelligent solution is built through AI technology, digital twin and intelligent management system.
(1) Five-axis machining AI thermal compensation technology
In the five-axis machining of motor housings, the advanced CNC system utilizes AI functions to learn about thermal deformation in real time during the machining process. By analyzing the rise in temperature alongside machining time, the system can dynamically predict the amount of thermal deformation that will occur. The measured compensation accuracy achieves an impressive 0.002 mm, effectively reducing dimensional deviations in aluminum alloy caused by cutting heat. This ensures that the thermal deformation effects on critical components, such as bearing holes, are significantly minimized, and the coaxiality of the porous system remains stable within the required high-precision specifications.
(2) Digital twin prediction of motor shaft grinding
The digital twin model for motor shaft grinding is developed using simulation technology to predict machining risks and achieve precision control from three key aspects:
1. Temperature Field Simulation: The maximum temperature in the grinding zone is maintained below 400℃. This control prevents the tempering effects that can lead to hardness loss and grinding burns in 40Cr alloy steel due to excessive heat.
2. Stress Field Analysis: Residual stress is kept within 80 MPa. This measure ensures that the residual stress from the straightening process does not negatively affect the dynamic balance stability of the shaft.
3. Surface Quality Prediction: The target surface roughness for the bearing seat is set to Ra ≤ 0.8 μm. This specification provides precise guidance for optimizing the grinding process parameters.
(3) Intelligent Production Management System (MES)
Develop a Manufacturing Execution System (MES) for the collaborative processing of two types of parts, focusing on three core intelligent management functions:
1. Automatically select the optimal processing parameters based on the material of the part (aluminum alloy or alloy steel) to prevent parameter mismatch issues during mixed-line production.
2. Dynamically predict the tool wear status through cutting force monitoring, with an accuracy of ±5%. When the tool back face wear reaches 0.1mm, the system will automatically notify the operator to change the tool. This improves tool utilization and avoids deterioration of surface quality due to excessive tool wear.
3. Thoroughly record all processing parameters, inspection data, and other lifecycle information for each motor housing and motor shaft. This ensures rapid quality traceability, significantly reduces the time needed for abnormality investigations, and achieves transparency and traceability throughout the processing workflow.
This intelligent solution leverages a framework of “real-time deformation compensation, virtual simulation prediction, and production system management and control.” It effectively addresses challenges such as the difficulty in controlling thermal deformation in traditional processing, high costs associated with process trials, and low efficiency in quality traceability. Ultimately, it promotes the transformation of motor core component processing to a data-driven, intelligent, and high-precision approach.
11 Conclusion
This study addresses the challenges associated with CNC machining aluminum alloy (6061-T6) motor housings and alloy steel (40Cr) motor shafts for new energy vehicles. It systematically analyzes the significant difficulties arising from the interaction of material physical properties—such as the high thermal expansion coefficient of aluminum alloy and the low thermal conductivity of alloy steel—with structural features and machining processes. Key structural features include the thin-walled cylinder of the motor housing, the heat sink, and the slender stepped shaft of the motor.
The study identifies several mechanisms that lead to multi-source deformation, including deformation during the cutting of thin-walled structures, thermal deformation, clamping deformation, and issues related to the rotation accuracy and dynamic balancing of the motor shaft. Solutions are proposed from four perspectives: process chain optimization, equipment and tool matching, intelligent clamping detection, and intelligent machining.
For the motor housing, a composite process is utilized involving “die casting, five-axis layered milling, and precision turning,” combined with vacuum clamping. For the motor shaft, dynamic balancing accuracy of G2.5 is achieved through an integrated approach that incorporates turning, grinding, and dynamic balancing. The collaborative machining of these two parts enhances overall efficiency.
Additionally, the introduction of AI thermal compensation, digital twin models for grinding, and a Manufacturing Execution System (MES) are driving the transformation of manufacturing towards data-driven processes. This provides the industry with a replicable high-precision machining technology system. By coordinating the optimization of materials, structures, and processes with the integration of intelligent technologies, this approach effectively overcomes existing challenges related to the accuracy and efficiency of core motor components, significantly impacting the high-quality development of motor manufacturing for new energy vehicles.
As the new energy vehicle industry increasingly demands higher efficiency, lighter weights, and more intelligent motors, future research in machining core motor components will concentrate on four innovative and engineering-valuable areas. This research aims to break through current bottlenecks through interdisciplinary technological integration in line with the “dual carbon” goals and advanced manufacturing technologies.
1) Deep integration and full-process application of digital twin technology. To tackle the current issue of inadequate real-time error control during machining, future research will develop a digital twin model that encompasses the entire machining process for motor housings and motor shafts, including die-casting, machining, inspection, and assembly. This model will utilize simulation software to integrate machine tool kinematic models with material removal dynamics equations.
Leveraging the machine tool’s Internet of Things (IoT) system, the model will gather real-time data on critical dimensions such as spindle torque, feed rate, and tool wear. This will facilitate the dynamic prediction of machining errors and the pre-optimization of process parameters. For instance, during the five-axis milling of motor casings, the digital twin system can forecast the risk of chatter on the heat sink sidewalls 30 seconds in advance. It can then automatically adjust the tool inclination angle and cutting speed to maintain surface roughness fluctuations (Ra) within ±0.2μm.
To combat the issue of grinding burn in motor shafts, the system will simulate the real-time temperature field, dynamically adjusting the grinding wheel pressure and cutting fluid flow to ensure that the maximum temperature in the grinding zone remains below 400°C. This innovative approach moves beyond the traditional “trial-cut-test-correct” method of hysteresis control and advances the machining process into “real-time closed-loop optimization driven by digital twins.” With this technology, we expect to increase the first-time pass rate for critical precision indicators to over 99%.
2) Breakthroughs in processing technology for new lightweight, high-strength materials. To meet the stringent material performance requirements for lightweight motors, there is an urgent need for research in the processing technologies of magnesium alloys and carbon fiber composites. Magnesium alloys have a density that is only two-thirds that of aluminum alloys, but their thermal expansion coefficient is 25% higher. Additionally, they are prone to sparking during cutting, which significantly increases the difficulty of maintaining machining safety and dimensional stability.
Future research will aim to optimize tool coatings, reduce cutting heat, and manage residual stresses during high-speed cutting of magnesium alloy motor casings. The goal is to control deformation to within 0.02 mm for thin-walled components with a thickness of 2-3 mm.
To tackle the anisotropy of carbon fiber composite motor shafts, which can lead to fiber tearing during cutting, we need specialized diamond-coated tools, ultrasonic vibration-assisted cutting technology, and interlaminar stress relief processes. These advancements are essential for overcoming challenges such as drill delamination and surface burrs, thereby promoting the engineering application of composite materials in high-speed motor shafts.
3) Innovation and engineering application of green and low-carbon machining processes. In response to the global manufacturing industry’s dual carbon goals, this initiative will focus on exploring the large-scale application of green machining processes, such as dry cutting, cryogenic cutting, and minimal lubrication in motor manufacturing.
For the dry cutting of aluminum alloy motor casings, we are developing a self-lubricating gradient structure tool. When combined with high-speed spindle dynamic balancing optimization, this tool facilitates efficient machining of heat sinks without the need for cutting fluids. This approach completely eliminates the high costs of wastewater disposal and the environmental risks associated with traditional emulsions.
For the cryogenic cutting of alloy steel motor shafts, we utilize liquid nitrogen cooling technology at -196°C. This rapidly reduces the workpiece surface temperature to below 50°C during the grinding process, suppressing grinding burns while extending the lifespan of CBN grinding wheels and reducing cutting fluid usage by over 90%.
This innovation enables the implementation of intelligent centralized chip removal and closed-loop waste fluid recovery systems, creating a green manufacturing cycle of cutting, chip removal, purification, and regeneration.
4) Integrated Development and Adaptive Control of Intelligent Machining Cells. An intelligent machining cell has been developed for motor housings and motor shafts by integrating AI algorithms, digital twin models, and IoT technologies. This cell combines machining, inspection, and decision-making processes and features an industrial robot at its core. It is equipped with a high-precision force-controlled fixture, an online visual inspection system, and an edge computing server, allowing for full-process adaptive control during machining.
When the system detects uneven stock allowance (±0.5mm), the AI algorithm automatically adjusts the cutting depth and feed rate. This capability enables comprehensive adaptive control, including self-compensation for tool wear and thermal deformation, as well as quality traceability. As a result, it fosters the development of highly intelligent and flexible machining systems.
These research directions focus on four core goals: precision, efficiency, sustainability, and intelligence. By innovating material processing mechanisms, developing intelligent algorithms, and achieving breakthroughs in equipment technology, this approach not only provides crucial technical resources for processing core motor components but also promotes a significant transformation in the mechanical processing field toward digitalization, sustainability, and intelligence. This transformation supports the new energy vehicle industry in achieving its strategic goal of becoming a manufacturing powerhouse.
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