Precision Optimization Strategies for Diesel Engine Crankshaft Bore Machining

To ensure the geometric accuracy of the crankshaft hole in the diesel engine body, it is essential to combine contact stress analysis with actual usage data from the diesel engine. This involves detecting any coordinate deviations or straightness issues of the lower and side generatrix. By analyzing the factors that impact the machining accuracy of the crankshaft hole, conducting process tests, and organizing the analysis data, we can identify a process method to control the machining quality of the lower generatrix.

 

1. Introduction

The crankshaft hole is a crucial component of the large diesel engine body, and its machining accuracy significantly impacts the engine’s performance and longevity. Most manufacturers evaluate the geometric accuracy of the crankshaft hole by measuring the coaxiality or straightness of its centerline. However, many prominent foreign diesel engine manufacturers have established that their quality requirements for the crankshaft hole do not hinge solely on coaxiality or straightness. Instead, they focus on assessing the coordinate deviation and straightness of the lower and side generatrix of the crankshaft hole. This assessment is based on contact stress analysis between the crankshaft and bearings, as well as actual operational data from the diesel engine.

The crankshaft hole in the engine body demands extremely precise machining, which often necessitates the services of specialized manufacturers. When machining the diesel engine body, it is essential to consider variations in the centers of the crankshaft holes. Additionally, the impact of hole diameter on both the lower and side busbars must be taken into account, as this increases the risk of machining errors. This study analyzes the factors influencing the machining accuracy of the crankshaft hole, conducts process tests, and organizes the collected data to establish effective methods for controlling the machining quality of the lower busbar.

 

2. Diesel engine body structure and technical requirements

The dimensions of a large diesel engine body are as follows: 4760 mm in length, 1680 mm in width, and 1600 mm in height. The mass of the engine body is 13500 kg, and it has 20 cylinders made from GGG50 material. The structure of the diesel engine body is illustrated in Figure 1.

The size of the crankshaft hole is Φ296H7, with a surface roughness value of Ra = 1.6 μm. According to the drawing specifications, when the diesel engine is in operation with the oil pan facing downward, the coordinates of the lower busbars for the front and rear gears of the crankshaft hole are set to zero. The allowable range for coordinate changes in the lower busbars of the remaining gears is limited to ±0.05 mm. For three consecutive gears, the lower busbar coordinates of the front and rear gears remain at zero, while the coordinate change range for the middle gear’s lower busbar is restricted to ±0.02 mm.

Furthermore, the allowable range for coordinate changes in the side busbars of each gear is set to ±0.04 mm. For three consecutive gears, the change range for the middle gear’s side busbars is limited to ±0.02 mm. Since the gantry machine tool operates only along the X-axis and Z-axis when machining the crankshaft hole, it is relatively easy to control the side generatrix in the horizontal direction (i.e., the Y-axis). Therefore, the focus of this study is on the lower generatrix of the crankshaft hole.

The factors affecting the machining accuracy of the lower generatrix of the crankshaft hole will be analyzed, and improvements will be made to the process methods and control measures to ensure that the machining quality of the lower generatrix meets the specified drawing requirements and remains controllable.

The batch finishing equipment used for the crankshaft holes in small and medium-sized diesel engine bodies typically consists of customized boring machines. These specialized machines offer advantages such as excellent precision stability and consistency. However, they also have drawbacks, including limited adjustability for precision and challenges in repair if quality issues arise during processing. Given the manufacturing costs associated with these specialized machines and the production capacity needed for larger diesel engines, crankshaft hole processing usually employs a gantry pentahedron machining center equipped with an accessory head (see Figure 2).

3. The factors affecting the machining accuracy of the lower generatrix of the crankshaft hole are analyzed as follows.

(1) Machine tool accuracy

The repeatability of the Z-axis and the straightness of the X-axis are key factors that influence the accuracy of the crankshaft hole center position coordinates. Additionally, the perpendicularity of the X, Y, and Z axes of the machine tool also plays an indirect role in the three-coordinate evaluation of the crankshaft hole center for each gear.

Typically, machine tool detection focuses on static accuracy. However, various factors can affect dynamic accuracy, such as inconsistent temperatures across different parts of the machine during operation, transmission clearances, component matching, and changes in the stress state. The distinction between static and dynamic accuracy significantly impacts the coordinates of the crankshaft hole center.

 

(2) Ambient temperature

The ambient temperature significantly affects the accuracy of large gantry machine tools. Dynamic temperature variations lead to changes in both positioning and geometric accuracy of the machine tool.

 

(3) Tool and accessory head

The rigidity of the tool and accessory head will affect the surface roughness and cylindricity of the crankshaft hole, and will also affect the crankshaft hole center coordinates.

 

(4) Wear resistance and impact resistance of the blade
The wear resistance of the tool significantly affects the diameter, surface roughness, cylindricity, and processing efficiency of the crankshaft hole. Typically, there are oil grooves on the curved surface of the crankshaft hole. The hardness of the bearing cover and the body may not always match, leading to intermittent cutting and impacts during processing due to this inconsistency. If the blade’s impact resistance is inadequate, it can break, resulting in quality issues. Additionally, poor wear resistance of the blade can lead to excessive variation in the diameters of the crankshaft hole’s gears, which can negatively impact the coordinates of the lower generatrix of the crankshaft hole.

 

4. Processing Verification

4.1 Adjusting the Machine Tool Accuracy

The improved automatic fire water cannon comprises several key components, including the rotating seat, barrel seat, first hanging arm, and second hanging arm, all of which must be made from die casting aluminum. Of particular importance is the metal processing technology used for the rotating seat. After it has been cast, the inner circumferential surface at the upper end of the rotating seat must be turned on a lathe to create a second annular groove that meets specific process requirements.

To ensure the precision of the machine tool processing the machine body, the following checks are necessary: First, use a 2.5m marble bridge ruler to verify the straightness of the machine tool. The horizontal and vertical straightness in the X moving direction must be less than 0.01 mm. Next, employ a 1000mm x 1000mm x 1000mm marble block to assess the verticality between the X, Y, and Z axes, with each axis required to maintain verticality within less than 0.015 mm. An illustration of the machine tool accuracy test is shown in Figure 3.

During the machining of the machine body, only the X and Z axes are allowed to move. The center’s positioning is solely influenced by the repeat positioning accuracy of the Z axis, which must be less than 0.01 mm. If the machine tool does not meet these accuracy requirements, adjustments must be made to achieve compliance.

4.2 Tool selection

To enhance the stress distribution of the attachment head, the tool body must possess characteristics such as high strength, good balance, and lightweight construction. Taking into account the impact resistance of the blade and the requirements for surface roughness during machining, a tool tip radius of R=0.8mm has been selected. Based on production experience, a specific brand of conventional turning blades has been chosen, along with the following recommended processing parameters: cutting depth of 0.2 to 2mm, feed rate of 0.05 to 0.2mm/rev, and cutting speed ranging from 165 to 205m/min.

 

4.3 Trial processing and formal processing

Before CNC manufacturing processing, check the flatness of both the tooling positioning surface and the body positioning surface to ensure it is within 0.02 mm. After placing the body on the tooling, use a 0.02 mm feeler gauge to inspect the gap between the positioning surfaces to confirm a proper fit. Utilize a torque wrench to apply consistent torque across all clamping points.

The body crankshaft hole should initially be processed with a fine boring tool through two pre-boring stages. Adjust the processing data accordingly, and perform a final in-place size fine boring. The basic dimensions of the holes are φ294 mm, φ295 mm, and φ296 mm, with a precision tolerance of H7 set at (+0.052/0) mm. To enhance the accuracy of the lower busbar and minimize the impact of aperture accuracy on it, the hole processing tolerance evaluation standard has been revised to (+0.035/+0.010) mm.

Cutting parameters for the three processing stages and the surface roughness achieved post-processing are detailed in Table 1. Table 2 presents a comparison of the changes in aperture size (excluding the basic aperture) after the three processes, while Table 3 shows the comparison of the changes in the height coordinates of the lower busbar after the same processes.

From Table 1 to Table 3, it can be seen that in the first pre-boring, the cutting speed is 200m/min, the cutting depth is 0.15mm, and the feed rate is 0.10mm/r.
1) The surface roughness value, Ra, is between 1.6 and 2.2μm. The drawing specifies a requirement of 1.6μm, which means it is unqualified.

2) The aperture size was measured using a three-coordinate measurement system. The detected aperture varies between 0.010 and 0.035 mm, which is within acceptable limits. However, this size is at the edge of the tolerance setting, presenting a risk of exceeding the specified tolerance.

3) During three-coordinate measurement, the vertical coordinates of the front and rear gears of the crankshaft hole center were set to 0. The maximum difference in coordinate values for the remaining gears was found to be 0.025 mm. By subtracting the crankshaft hole radius from the coordinate values, we obtained the lower generatrix coordinates. Setting the lower generatrix coordinates for both front and rear gears to 0, we calculated that the maximum range of coordinate changes for the remaining gears is 0.043 mm. The drawing requirement is ≤0.05 mm, so this measurement is qualified. For three consecutive gears, with the lower generatrix coordinates of the front and rear gears set to 0, we calculated the maximum coordinate value for the middle gear to be 0.024 mm, while the drawing requirement is ≤0.02 mm, making this measurement unqualified.

 

For the second pre-boring, the cutting parameters were adjusted to a cutting speed of 180 m/min, a cutting depth of 0.12 mm, and a feed rate of 0.10 mm/r. The local coordinates of the machine tool program were fine-tuned based on three-coordinate detection data.

1) The surface roughness value (Ra) was measured at 0.7–1.5 μm, while the drawing requirement was 1.6 μm. This result is within acceptable limits; however, there is a risk of exceeding tolerances in the subsequent processing stages.
2) The aperture size was checked using three-coordinate measurement, revealing a change range of 0.013–0.032 mm, which is acceptable.
3) Calculations indicated that the range of the lower generatrix coordinate change was 0.033 mm, with the drawing requirement being ≤0.05 mm; therefore, this is compliant. For three consecutive gears, the maximum lower generatrix coordinate for the middle gear was found to be 0.028 mm, while the drawing requires it to be ≤0.02 mm, which does not meet the requirement.

 

For the third machining operation, the cutting parameters were modified to a cutting speed of 160 m/min, a cutting depth of 0.10 mm, and a feed rate of 0.12 mm/r. The local coordinates of the machine tool program were again fine-tuned based on the three-coordinate detection data.

1) The surface roughness value (Ra) measured between 0.8–1.3 μm, which conforms to the drawing requirements.
2) The aperture size was assessed through three-coordinate measurement, yielding a change range of 0.012–0.025 mm, which is acceptable.
3) The maximum change value of the center coordinate of the crankshaft hole for each gear was recorded at 0.01 mm. Upon calculation, the change in the lower generatrix coordinate was 0.023 mm, with the drawing requirement being ≤0.05 mm, indicating compliance. For the three consecutive gears, the maximum lower generatrix coordinate for the middle gear was 0.013 mm, also compliant with the drawing requirement of ≤0.02 mm.

In summary, by adjusting the machine tool accuracy and understanding its dynamic accuracy characteristics, we have progressively controlled surface roughness, hole diameter tolerances, and lower generatrix tolerances of the crankshaft hole through the selection of appropriate cutting speed, cutting depth, and feed rate.

 

5. Conclusion

This study analyzes the factors influencing the machining accuracy of the lower generatrix of the crankshaft hole in engine blocks. First, adjustments are made to the machine tool’s accuracy and the selection of cutting tools. Next, the tolerance of the lower generatrix is meticulously controlled, bringing it within the specifications outlined in the drawing by fine-tuning the cutting parameters. Finally, the methods for enhancing the machining quality of the lower generatrix are summarized. This information is valuable for improving the machining of crankshaft holes in large diesel engines.

 

 

 

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