A sensitivity analysis method was proposed based on machining trajectories that might identify key geometric errors in CNC machine tool machining processes. Firstly, the spatial error model of the AC dual-turntable five-axis CNC machine tool was completed using multibody system theory and homogeneous coordinate transformation, and the geometric errors were measured using a laser interferometer and ball bar. A spatial error model with the machine tool position as the independent variable was further established, and a new sensitivity index was proposed based on the relationship between the error vector caused by 41 geometric errors in the machining trajectories and the spatial errors. Taking the machining trajectory of the S test piece as an example, 11 key geometric errors were identified. Finally, compensation tests were conducted on the 11 key geometric errors and the remaining 30 geometric errors, respectively. The results show that after compensation for the 11 key geometric errors, the machining errors reduce by 73.63%, while compensation for the remaining 30 geometric errors only reduce the machining errors by 11.28%, proving the correctness and effectiveness of the sensitivity analysis method.
建模时几何误差可定义为微小运动量,几何误差之间的彼此影响可忽略,此时空间误差矢量 E 可以看作41项几何误差单独作用产生的误差矢量之和。图3表示各项几何误差单独作用产生的误差矢量和空间误差矢量的传递关系,单项几何误差造成的误差矢量可由下式表示:
式中: Ei 为第i个几何误差单独作用时产生的加工误差;exi 、eyi 、ezi 为该几何误差在这个位置上x、y、z三个方向造成的误差分量。
由前文几何误差建模可知,位置无关误差为定值,位置相关误差是该轴位置为自变量的函数,可将空间误差模型简化成仅有机床五轴位置作为参量的函数。在实际加工前,通常都会使用工程软件生成工件的机床加工程序。而机床各个轴的运动数据可以通过加工程序进行后处理得到,各轴的位置与数据点一一对应,因此机床空间误差模型 E 和单个几何误差作用的误差传递模型Ei 可分别表示为
每项几何误差引起的误差矢量共同形成了最终的刀具位置误差矢量。图3中Pi 是 Ei 在 E 上的投影,可见 Ei 在 E 上的投影大小可以用来描述每项几何误差对刀具位置误差矢量的影响,可以将此作为依据定义灵敏度Si :
式中:Pi 为单个几何误差在数据点产生误差矢量 Ei 对总误差矢量 E 的投影;N为整个加工过程中所有数据点的个数。
基于前文通过拟合得到的位置相关误差多项式,将空间误差模型简化成以理想位置为参量的函数 E,在此基础上得到各项几何误差单独作用的传递模型 Ei。 Ei 在 E 上的投影大小可表示各项几何误差对空间误差的影响程度,因此可以将其定义为灵敏度指标。该灵敏度分析方法简化了空间误差模型参量,建立独立几何误差传递模型可直接基于工件加工轨迹进行分析。该方法考虑了空间误差为矢量的基础条件,同时体现了加工轨迹中各轴位置变化与几何误差大小变化的影响。
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