Precision Optimization Strategies for Cylinder Head Hole Diameter Control
The issue of random tolerance in the machining aperture of cylinder heads presents a significant challenge in the production line. The resulting scrap is difficult to identify, and there are substantial quality risks involved. Through long-term data analysis, production line operators have determined that the random tolerance in the machining aperture is primarily caused by spindle sticking.
To address this, a combination of table and internal system analysis methods has been employed. The table analysis focuses on evaluating the condition of the tool magazine, tool positioning, and spindle sealing. Meanwhile, the internal analysis examines the CNC program alongside factors such as cutting fluid pressure, flow, and the design of the spindle end cover.
By employing this comprehensive approach, a solution has been proposed to mitigate the spindle sticking issue, ultimately reducing quality risks in the production process.
1. Preface
The quality of machining for automobile cylinder heads is crucial as it directly impacts engine performance. To achieve high precision and efficiency during manufacturing, advanced machining equipment is essential. The horizontal machining center, with its unique design, allows for multi-faceted machining and can complete several processes in a single clamping. This significantly minimizes errors that may arise from multiple clamping of the workpiece, thus enhancing machining accuracy.
Equipped with a tool magazine and an automatic tool changer, the horizontal machining center can switch tools without stopping the machine. This feature is particularly important for cylinder heads, which often require complex processing. By using horizontal machining centers, manufacturers can not only increase production efficiency but also reduce uncertainties associated with manual operations, leading to greater consistency and stability in the machining process.
The implementation of horizontal machining centers in the processing of automobile cylinder heads demonstrates the modern manufacturing industry’s focus on precision and efficiency.
2. Existing problems
In recent years, the state has actively supported the integration of schools and businesses. It has encouraged educational institutions to collaborate with modern manufacturing companies, enhancing teachers’ practical skills and enabling them to use their expertise to assist businesses in resolving technical challenges. This collaboration allows students to engage directly with frontline products, immerse themselves in corporate culture, and develop a better awareness of product quality.
The cylinder head is a product that has been processed and fine-tuned using DMG MORI’s NHC4000 horizontal machining center. Since its mass production began, issues with random tolerance in its machining aperture have persisted, resulting in various challenges.
(1) High scrap rate
If the aperture size exceeds the specified tolerance range, the part will not meet the design requirements and cannot be used for assembly. This results in significant raw material waste and increased production costs. In 2022, the production line identified many quality defects stemming from random tolerances in machining the aperture, leading to the generation of 53 scrap parts annually.
(2) Long downtime
Quality problems require investigation and resolution, leading to increased machine downtime. This, in turn, adversely impacts overall production efficiency. Occasionally, it disrupts the company’s production plan, causing the output to fall short of completion.
(3) Decreased customer trust
Continuous product quality issues will damage the manufacturer’s brand reputation, affect customer relationships, and lead to a decrease in orders.
(4) Increased rework costs
If out-of-tolerance issues are found during later inspections, it may be necessary to rework or discard the parts that have already been produced. This not only increases costs but also consumes additional time and manpower.
(5) Safety hazards
The cylinder head is a crucial component of the engine, and the size of its aperture significantly impacts both engine performance and safety. An incorrect aperture size can lead to decreased engine performance, and in some cases, it may result in failures or accidents.
3. Problem Analysis
After analysis, the following reasons may be the causes of the random deviation of the machining center’s hole diameter.
(1) Spindle runout
Excessive spindle runout can cause the tool to deviate during machining, which affects the consistency of the hole diameter. To troubleshoot this issue, use a spindle, magnetic base, and dial indicator to measure the spindle runout and determine whether it exceeds the allowable range.
(2) Tool wear
Once the tool is used, its diameter decreases, causing the machined hole to be smaller than the intended size. To troubleshoot this issue, it’s important to regularly monitor tool wear and, if necessary, measure the tool diameter using an outside diameter micrometer.
(3) Unstable clamping
Unstable tool clamping occurs when the tool is not securely held in the tool holder, leading to movement during machining. To troubleshoot this issue, check the functionality of the tool clamping mechanism and use a dynamometer to verify if the clamping force is adequate.
(4) Insufficient cutting fluid supply
If the supply of cutting fluid is insufficient, it can result in inadequate cooling, leading to thermal deformation and affecting the accuracy of machining. To troubleshoot this issue, check the operational status of the cutting fluid pump to ensure that the flow and pressure of the cutting fluid meet the necessary requirements.
(5) Machine tool vibration
External or internal vibrations can cause the machine tool to move slightly, which affects processing accuracy. To troubleshoot this issue, a vibration analyzer should be used to detect the machine tool’s vibrations and identify any abnormal sources of vibration.
(6) Temperature change
Changes in ambient temperature or heating from the machine tool itself can cause its components to expand and contract, which impacts processing accuracy. To troubleshoot this issue, it’s important to monitor the ambient temperature surrounding the machine tool to maintain consistent temperature conditions. Additionally, check whether the machine tool’s internal cooling system is functioning properly.
(7) Poor guide rail lubrication
Insufficient lubrication of the guide rails increases movement resistance and affects feed accuracy. To troubleshoot this issue, check the lubrication of the guide rails and ensure that the lubrication system is functioning properly.
(8) Servo motor error accumulation
The feedback signal from the servo motor may contain errors, leading to processing deviations after long-term accumulation. To troubleshoot, use an oscilloscope or other electronic instruments to measure the servo motor’s position feedback signal.
(9) Mechanical clearance
The mechanical clearance in the transmission chain can lead to a misalignment between the actual position of the actuator and the commanded position. To troubleshoot this issue, check the clearances of the transmission components, such as gears and screws. If necessary, adjust or replace any faulty parts.
(10) Unstable control system
A failure in the control system, whether due to software or hardware, can result in unstable processing parameters. To troubleshoot this issue, it’s essential to check the operating status of the control system. This includes reviewing the software version and verifying hardware interface connections. If necessary, upgrade or repair the system to ensure proper functionality.
(11) Chips stick between the spindle end face and the tool holder end face.
Chips can get lodged between the spindle end face and the tool holder end face, leading to a loose tool installation. This can cause slight displacement of the tool during processing, which affects the consistency of the aperture. To troubleshoot this issue, disassemble the tool and carefully inspect both the spindle end face and the tool holder end face for any remaining chips. If necessary, use specialized tools to thoroughly clean these areas.
(12) Machine tool geometric accuracy inaccuracy
After prolonged use of the machine tool, key CNC component such as guide rails and slides can experience wear. This wear leads to a decrease in the geometric accuracy of the machine, which in turn affects processing precision. To troubleshoot this issue, it is essential to perform a comprehensive inspection of the machine’s geometric accuracy using precision level rulers, laser interferometers, and other tools. Special attention should be given to the straightness and parallelism of the guide rails.
(13) Improper cutting parameter settings
Ineffective settings for parameters like cutting speed and feed rate can lead to uneven tool load, which negatively impacts processing stability. To troubleshoot this issue, it’s important to re-evaluate the cutting parameters, adjust the cutting speed and feed rate, and ensure that the tool load is balanced and reasonable.
(14) Electrical interference of machine tools
The aging of electrical circuits and electromagnetic interference can cause unstable signal transmission in control systems, which negatively impacts the response accuracy of servo systems. To troubleshoot these issues, it is important to check if the electrical circuit connections of the machine tools are secure and if the shielding layer is intact. Additionally, using electromagnetic compatibility test equipment can help detect any electromagnetic interference present.
(15) Uneven clamping of workpieces
The clamping force on CNC machinery parts is unevenly distributed, leading to slight shifts during processing that affect accuracy. To address this issue, it is essential to evaluate the design of the fixture to ensure even distribution of pressure across all contact points. Additionally, using a pressure sensor can help test the distribution of the clamping force.
Following this method and conducting multiple tests with statistical data analysis, it was determined that the random tolerance in the machining hole diameter was due to spindle chip sticking. An optimization process was implemented to resolve these issues.
4. Implementation Plan
4.1 Optimize and replace the tool magazine door stopper
Replace the damaged tool magazine door stopper and widen it to ensure that the tool magazine door can be completely closed and truly isolate it to prevent chips from entering the tool magazine, as shown in Figure 1.
4.2 Installing a baffle on the cutter disc
Install an isolation baffle on the cutter disc (see Figure 2) to separate the tool chip area from the tool holder HSK area in order to prevent chips on the cutter head from falling into the tool holder.
4.3 Use a new spindle cover
After an extended observation period, it was discovered that the six low-pressure flushing holes in the spindle cover frequently became blocked. Due to the integrated design and stepped internal structure of the spindle cover, clearing these blockages proved impossible. To resolve this issue, the following three adjustments were made:
1) A new split spindle cover was designed with water spray holes that have a diameter of 1.5 mm. This modification greatly facilitates maintenance during future use, as illustrated in Figure 3.
2) Adjust the spindle low-pressure cooling pressure to 25-30 Bar (1 Bar = 0.1 MPa) to increase the flushing force on the HSK tool holder and end face.
3) Set the maintenance cycle for unblocking the spindle end cover cooling hole, generally once per quarter.
4.4 Add 2 nozzles in the tool magazine
1) Add 2 nozzles in the tool magazine to prevent the chips in the tool magazine from falling onto the HSK tool holder, as shown in Figure 4.
2) Ensure that the rotating nozzle in the tool magazine can work normally.
3) Adjust the position of the nozzle in the tool magazine.
4.5 Adjust the position of the tool on the tool disc
Change the position of the tool that tends to produce chips in the tool magazine to minimize chip generation.
4.6 Replace the inner seal of the spindle
If the seal fails, the cutting fluid could corrode the spring in the clamping mechanism, leading to insufficient clamping force and affecting the processing dimensions. The production line and maintenance teams regularly clean the inside and cross-section of the spindle and check the green seal. If the seal is intact, it will be cleaned and oiled. This inspection occurs once per quarter.
4.7 Optimize the program
1) Improve the placement of the M106 tool magazine door closing command in the CNC program. This will ensure that the workpiece is only processed once the tool magazine door is fully closed after changing the tool.
2) Enhance the M06 function in the CNC program by setting the tool flushing time before the spindle engages with the tool. For tools that are prone to chip sticking, set the flushing time to either 1 second or 1.6 seconds; for other tools, set the flushing time to 0.8 seconds.
3) Utilize the LAVE_ROUE program to automatically flush the tool magazine if the machining center has been idle for more than 1 hour. This ensures that the tool magazine remains clean.
4.8 Other measures
1) Use special tools to clean the processing area, as shown in Figure 5.
2) Add a new cutting fluid pipeline to increase the flushing flow.
5. Conclusion
When examining the overall improvement in processing with a focus on reducing scrap costs, it is notable that since the first-line ramp of the cylinder head in April 2023, there have been no instances of scrap caused by spindle sticking. This has resulted in a significant reduction in the scrap rate of workpieces due to aperture tolerance, minimizing raw material waste and effectively lowering production costs. Previously common defects are now well-controlled, leading to a marked enhancement in the company’s economic benefits.
Regarding the improvement of equipment startup rates, the resolution of the chip sticking issue has eliminated frequent machine stoppages for maintenance related to aperture tolerance. This has greatly increased production efficiency. The enhanced availability and reliability of the equipment allow for smoother production line operations, shortening production cycles and boosting overall production capacity.
From the standpoint of reducing the frequency of manual inspections, the improvements in aperture processing accuracy have enhanced the consistency of finished products. Consequently, the need for manual input in the quality inspection process has diminished. The requirement for frequent sampling inspections has been alleviated, saving human resources and improving work efficiency.
In terms of product reputation, the production of high-quality goods bolsters the brand image and strengthens customer trust. Stable processing quality and a reduced rework rate have contributed to a better market reputation for the company, facilitating an expansion of market share.
With regard to energy conservation and environmental protection, reducing scrap leads to less unnecessary resource consumption and waste emissions. Efficient raw material usage not only cuts costs but also aligns with green manufacturing principles, showcasing the company’s commitment to environmental responsibility.
From a production safety perspective, addressing the chip sticking problem has minimized potential safety hazards related to aperture tolerance, thereby enhancing the safety of the manufacturing process. A stable processing environment also helps maintain employee morale and contributes to a healthier workplace.
In summary, by resolving the issue of chip sticking on the spindle end face, the company has reaped significant economic benefits while also positively impacting production efficiency, quality control, environmental sustainability, and social responsibility. This achievement lays a solid foundation for the enterprise’s sustainable development.
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