Views: 0 Author: ANEBON Publish Time: 2024-12-03 Origin: Site
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● 2. Product characteristics and processing difficulties
● 3. Processing technology and existing problems
Using the motor bearing seat parts as an example, we explored processing methods to address the challenges of controlling dimensional accuracy and geometric tolerance, particularly the tendency for taper deviation during the machining of the deep blind hole's inner diameter. By incorporating online measurement techniques, we proposed a new processing technology that utilizes a five-axis machining center to grind the inner hole of the deep blind hole bearing seat.
Following these improvements, we achieved the required dimensional accuracy, geometric tolerance, and surface roughness for the inner hole. The dimensional accuracy and geometric tolerance became more stable and reliable compared to the original process, while processing efficiency saw significant enhancements.
The machining of high-precision deep blind holes has long posed a technical challenge in the industry. This paper focuses on a specific deep blind hole component as its research subject. Through a series of experimental processes, it presents a new method for efficiently machining deep blind holes and applies this method in practical applications.
The precision deep blind hole bearing seat discussed in this article is a critical component of a specific type of long-life motor. Its primary function is to secure the internal parts of the motor. Given that the motor shaft operates at very high speeds, the inner hole size and geometric tolerance of the bearing seat must be precise. The structure is illustrated in Figure 1.
The bearing seat is constructed from 9Cr18Mo and has a heat treatment hardness ranging from 48 to 53 HRC. It features a cavity structure with a regular shape and a thin wall, and its end face serves as the reference surface. The component requires high machining accuracy and surface quality.
The machining of the inner hole presents challenges, as it must meet the following specifications:
- The verticality between the end face and the inner hole should be ≤ 0.002 mm.
- The roundness of the inner hole must also be ≤ 0.002 mm.
- The straightness of the inner hole is required to be ≤ 0.002 mm.
Additionally, the blind hole is 70 mm deep, and the back cut measures 2.6 mm in width.
The original inner hole grinding process uses the outer diameter as a positioning support and employs electromagnetic centerless clamping to secure the workpiece. The steps involved are as follows: rough grinding of the inner hole, fine grinding of the inner hole against the bottom surface, and final grinding of the inner hole against the bottom surface.
This process cannot achieve the required inner hole specifications in a single pass using conventional equipment; instead, it must rely on subsequent grinding to meet the accuracy requirements. The existing processing challenges are as follows.
1) Detecting the inner hole accurately during the grinding of deep blind holes is challenging and requires frequent assessments. Conventional analog gauges are unable to measure blind holes. While three-dimensional coordinate measuring machines can use an extended probe for offline detection, this method has low efficiency. Additionally, the high accuracy required for inner holes makes them susceptible to temperature fluctuations during measurement, rendering this method unsuitable for in-process inspections.
2) The significant depth of the hole increases the likelihood of taper and dimensional deviations during grinding. Deep blind holes are often processed using internal cylindrical grinders. When the grinding wheel moves axially, it tends to create more wear at the front end than at the rear. The bending deformation caused by the radial cutting force on the grinding wheel's connecting rod contributes to this issue, resulting in the inner hole exhibiting a taper of 0.01 or more. Consequently, this affects both dimensional accuracy and geometric tolerances, which may not meet required specifications.
3) Inner hole accuracy is typically ensured through manual grinding; however, this method can result in inconsistent product quality and the occurrence of scratches. A grinding rod is employed to grind the inner hole, leaving a grinding allowance of 0.02 mm. During this process, a bell mouth may develop, while the requirement for inner hole straightness is 0.002 mm. The grinding process is relatively complex and requires repeated grinding and testing. Each individual grinding session takes approximately two hours, and the finished product must undergo a constant temperature test lasting four hours, making the process inefficient and susceptible to grinding scratches.
In order to solve the problems existing when machining the inner hole according to the original process, the inner hole machining process of the bearing seat needs to be optimized and adjusted.
4.1 Equipment selection
To meet the precision requirements for the inner hole's surface roughness and geometric tolerance as specified in the bearing seat design, we propose using a high-speed five-axis machining center with a maximum speed of 20000 r/min for processing. After securing the workpiece, we will conduct online measurement and alignment to ensure the inner hole axis is perpendicular to the end face. A CBN grinding wheel, no thicker than 2.6 mm, will be utilized, with the grinding wheel connecting rod installed on the tool holder for cylindrical surface grinding.
Our online measurement and intelligent correction technology can automatically detect the machining reference surface, dynamically reconstruct the machining coordinate system, and eliminate errors caused by reference transfer. Additionally, it can automatically detect the machining allowance, reduce the number of fixtures required, shorten the production cycle, and ultimately lower manufacturing costs.
4.2 Processing principle
The processing flow is illustrated in Figure 2. To begin, the processing program is compiled and the online detection macro program is executed. An induction probe is mounted on the tool holder, and the macro program initiates the movement of the probe. When the probe makes contact with the industrial CNC machining workpiece, its coordinate values are automatically recorded. This data is used to reconstruct the detection contour of the workpiece through multiple contact points. Furthermore, the extracted data is integrated into the processing program for automatic compensation and correction. The online detection process is shown in Figure 3.
The method of automatically aligning the horizontal plane is based on the principle of three-point plane determination. This involves extracting the coordinate values of the contact points between the probe and the plane. By adjusting each axis to ensure that the Z-axis coordinates of the three points are the same, we can guarantee that the measurement plane is perpendicular to the tool spindle.
During the processing stage, we first calibrate the test to determine any actual size deviation. This allows us to correct the online detection data, providing accurate information for setting the cutting amounts in the next process. Throughout this procedure, the macro program automatically handles measurement compensation without the need for manual intervention, enabling the product to be clamped just once and still achieve the necessary quality standards.
4.3 Clamping method
When using conventional clamping methods, the clamping force significantly affects the processing of the inner holes. To meet the dimensional accuracy and geometric tolerance requirements of the workpiece, it is recommended to use a step-fixing clamping method based on the structure of the outer ring. This clamping method, illustrated in Figure 4, effectively prevents deformation of the parts during clamping, thereby avoiding processing tolerances.
4.4 Process
Optimized Processing Technology:
- Use the outer diameter as positioning support.
- Perform rough grinding on the inner hole.
- Proceed with fine grinding of the inner hole against the bottom surface.
- Fix the workpiece with steps against the bottom surface.
- Conduct rough repair of the inner hole.
- Complete fine repair of the inner hole.
The fine grinding of the inner hole utilizes an internal cylindrical grinder, maintaining a grinding allowance of 0.03 mm and controlling the taper to be less than or equal to 0.01. The inner hole is repaired using a five-axis machining center, which relies on online detection for automatic adjustment. This ensures that the reference surface is horizontal while the grinding wheel oscillates and reciprocates in a small range, thereby improving surface roughness.
Due to the significant taper generated during the fine grinding of the inner hole, the grinding process is split into rough repair and fine repair stages. For rough repair, a grinding wheel with a smaller grain size is employed to increase the feed rate, enhance cutting efficiency, and reduce the taper of the inner hole. Conversely, for fine repair, a grinding wheel with a larger grain size is utilized to ensure dimensional accuracy and surface roughness while further minimizing the taper.
The machining process is illustrated in Figure 5. A CBN grinding wheel with a thickness of 2 mm is employed. During operation, the grinding head rotates at high speed and moves up and down for a stroke, completely retracting from the machining surface. Simultaneously, the spindle is tilted at a specific angle relative to the axis of the workpiece to achieve a perfect circular trajectory.
During the machining process, the CNC machining allowance is monitored by online measurement technology, and the grinding process parameters are set. The rough and fine grinding parameters are shown in Table 1.
The theoretical grinding time for three rough grindings, along with one online inspection, and one fine grinding followed by another online inspection, is approximately 95 minutes. This duration is significantly shorter than the time required for grinding a 0.02mm allowance and conducting a constant temperature inspection.
The CNC machining processing effects of both the original and new processes are illustrated in Figures 6 and 7. Figure 6a displays the surface of the inner hole processed using the original method. This inner hole exhibits both axial and circumferential grinding marks. Under a 40x magnifying glass, it is evident that the processing lines are fine (refer to Figure 6b), but their direction is irregular. In contrast, Figure 7a shows the surface of the inner hole processed with the new method. Here, the processing marks appear as clear axial stripes, and the lines are distinct (see Figure 7b), indicating good surface consistency.
The diameter test data at various depths of the inner hole are presented in Table 2. As shown in Table 2, both the online test and the three-coordinate test data satisfy the design requirements. Additionally, the cylindricality deviation measured between the online test and the three-coordinate test is 0.0002 mm. When processing and testing are conducted in a temperature-controlled room, the data regarding part size and cylindricality obtained from the machine can be utilized as a basis for evaluation.
The roughness test data are shown in Table 3. As shown in Table 3, the original process uses the grinding method, and the surface roughness of the inner hole is better than that of the new process. The surface quality processed by the two processes meets the design requirements.
In summary, the new processing technology meets design requirements for dimensional accuracy, geometric tolerance, and surface quality. It offers high processing efficiency and allows for strict control over these factors, thereby reducing the impact of human error on the consistency of inner hole quality. Additionally, this technology provides high reliability and enables automated finishing of inner holes with a single clamping. It also includes automatic detection, ensuring that only qualified products are removed from the machine.
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