Research on precision shaping technology of CVT for fairway cBN grinding wheel
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摘要: 无级变速器（CVT）具有结构简单紧凑、成本低、操纵方便等优点，是最理想的汽车变速器，而带轮、带轮轴球道的加工是CVT核心零部件加工的难点。电镀cBN砂轮具有磨削比高、磨削力小、发热少、环境友好、加工精度一致性好等优势，是球道加工理想的磨削工具。但球道的几何精度要求极高，特别是接触角允许公差极其严格，给砂轮的精密修整带来了极大难题。针对目前工程上光学引导法存在的技术难题，通过分析球道成形磨削原理、球道几何特征结构，梳理球道cBN砂轮几何精度控制原理，提出并采用比例修整法；并通过修整实验验证其可行性。经验证，该方法可使球道砂轮接触角误差稳定控制在 ± 1°以内，左右圆弧接触角偏差＜1°，具有广阔的应用前景。Abstract:
Objectives CVT technology has the advantages of a simple and compact structure, low cost, and convenient operation. It is the most ideal automobile transmission. The processing of its core component—the pulley block—is a difficult point in CVT manufacturing, especially the processing of the ball track on the pulley and pulley shaft. Electroplated cBN grinding wheels have the advantages of a high grinding ratio, low grinding force, less heat, environmental friendliness and good consistency in machining accuracy. They are ideal grinding tools for raceway machining. However, due to the high geometric accuracy required for the raceway—especially the allowable tolerance of the contact angle and its narrowness—it brings great difficulties to the precision dressing quality of the grinding wheel. Therefore, the precision forming and truing technology of cBN grinding wheels for CVT pulley raceway forming grinding is one of the key technologies that restrict the progress of CVT gearbox manufacturing technology in China. Methods Based on the technical problems existing in the current optical guided dressing method in engineering, this paper analyzes the principle of raceway forming grinding and the geometric feature structure of the raceway, and combs through the geometric accuracy control principle of the cBN grinding wheel in raceway grinding. The proportional dressing method is proposed and adopted. The feasibility of the method is verified by a dressing experiment. It is verified that the contact angle error of the raceway grinding wheel can be stably controlled within ± 1°, and the contact angle deviation of the left and right arcs is less than 1°. Results (1) The dresser "cross + ceramic grinding wheel up and down reciprocating" tool grinding wheel dressing method, can ensure that each point on the arc of the ceramic grinding wheel contacts the highest point on the cylindrical surface of the grinding wheel being dressed. This method is beneficial for ensuring the sharp angle dressing accuracy of the ceramic grinding wheel. The R value accuracy of arc dressing of the ceramic grinding wheel can reach 0.9980 and1.0020 mm, and the arc profile accuracy can reach 0.002 and 0.003 mm. (2) Within the tolerance range, the change of contact angle α with Xi and Rg is approximately linear, and the smaller the tolerance range, the better the straightness. (3) During the dressing process of the electroplated cBN grinding wheel in CVT raceway grinding, as the dressing process continues, the arc loss at the tip of the ceramic grinding wheel increases, and the arc values on both sides of the cBN grinding wheel gradually increase, resulting in a smaller contact angles on both sides. In the actual machining process, the dressing results can be detected in time, and the contact angle dressing accuracy can be controlled by adjusting the compensation value of the controller. The arc wear of the ceramic grinding wheel tip has little effect on the center distance of the double arcs on both sides of the tooth profile of the cBN grinding wheel. (4) When the ceramic diamond grinding wheel is used to dress the cBN grinding wheel, arc wear of at the ceramic grinding wheel tip is inevitable. In fact, a binder with better wear resistance can be selected, or the diameter of the grinding wheel can be increased to reduce the influence of ceramic grinding wheel wear on dressing quality. When adjusting the program, the contact angle is adjusted to a negative difference, and the allowable wear amount of the tool grinding wheel is increased to prolong the service time of the tool grinding wheel.Conclusions Due to factors such as drawing accuracy, minimum feed, and operation experience, the optical guidance method has problems such as poor trimming accuracy, time-consuming and laborious programming correction. At the same time, this method results in a low degree of smoothness in the grinding wheel contour, and sharp points often appear on the surface.This can easily cause quality problems such as scratches on the pulley ball track or unstable quality when the cBN grinding wheel grinds the ball track. Based on the optical-guided dressing method, this paper proposes a proportional dressing method. The dressing process of the grinding wheel is adjusted by the proportional adjustment principle, which can effectively control the dressing results. The dressing method of the tool grinding wheel using the dresser "cross + ceramic grinding wheel up and down" ensures that all points on the arc of the ceramic grinding wheel are in contact with the highest point on the outer cylindrical surface of the dressing grinding wheel being dressed. This method is conducive to ensuring the sharp angle dressing accuracy of the ceramic grinding wheel. -
图 4 砂轮理想齿形尺寸几何关系
Figure 4. Geometric relationship of ideal tooth shape size of grinding wheel tooth size
图 8 球道成型电镀cBN砂轮修整原理
Figure 8. Dressing principle of fairway forming electroplated cBN grinding wheel
图 15 双圆弧中心距2Xi偏差随修整数量的变化趋势
Figure 15. Change trend of arc center distance 2Xi deviation with number of trims
表 1 理论计算表（A ≈ −1.165 4）
Table 1. Theoretical calculation table (A ≈ −1.165 4)
项目 圆弧半径
Rg / mm模拟钢球半径
Rs / mm砂轮偏心距
Xi / mm接触角
α / (°)标准值 3.250 3 0.216 5 60.000 Rg 正差 3.253 3 0.216 5 58.844 Xi 正差 3.250 3 0.227 5 65.509 Rg 正差、Xi 正差 3.253 3 0.220 0 60.410 Rg 负差 3.247 3 0.216 5 61.229 Xi 负差 3.250 3 0.205 5 55.288 Rg 负差、Xi 负差 3.247 3 0.213 0 59.586 
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表 2 理论计算表（A = −0.866 0）
Table 2. Theoretical calculation table (A = −0.866 0)
项目 圆弧半径
Rg / mm模拟钢球半径
Rs / mm砂轮偏心距
Xi / mm接触角
α / (°)标准值 3.250 3 0.2165 60.000 Rg 正差、Xi 正差 3.253 3 0.2191 60.000 Rg 负差、Xi 负差 3.247 3 0.2139 60.000
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表 3 光学曲线磨床精度对比表
Table 3. Precision comparison table of optical curve grinding machine
μm 精度 P-ONE GLS-150D SPG-X 4轴 定位精度 ≤2 ≤4 ≤4 重复定位精度 ≤1 ≤2 ≤2 最小进给精度 0.1 0.1 0.1
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表 4 修整参数
Table 4. Trimming parameters
参数 数值 工具砂轮转速 n1 / (r·min−1) 6000 工件转速 n2 / (r·min−1) 70 走刀速度 v / (mm·min−1) 20 吃刀量 L / mm 0.002
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表 5 Rg补偿值计算（A = −0.886 0）
Table 5. Rg compensation value calculation (A = −0.886 0)
mm 项目 Rg实际值 程序修正值R修正 补偿量 左侧 3.252 9 3.256 2 0.003 3 右侧 3.253 4 3.249 6 −0.003 8
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表 6 补偿参数修正
Table 6. Compensation parameter correction
mm 项目 左侧 右侧 修整前 修正后 修整前 修正后 陶瓷砂轮圆弧半径补偿值 0.998 0 1.001 3 1.002 0 0.998 2
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表 7 实验条件
Table 7. Experiment condition
参数 类型或取值 机床型号 德国 EMAG HG310复合磨床 转速 v / (r·min−1) 12 730 进给速度 n3 / (mm·min−1) 220 加工余量 d / mm 0.4 磨削方式 切入式成型磨削
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