A method to decouple the geometric errors for rotary axis in a five-axis CNC machine

When using a ball-bar to measure rotary axis error, to keep the posture of the bar constant, a linkage motion of two linear axes with the rotary axis is required, which will cause the ball-bar length to contain both rotation axis error and linear axis error. In order to get the accurate rotary axis error, a decoupling strategy based on the actual position of the double balls on both ends of the ball-bar is proposed in a CFXYZA-type five-axis machine tool. Three measurement modes, axial, radial and tangential mode, are designed with a ball-bar instrument. Six groups of length variations of the ball-bar are measured by changing both the position and the height of the ball base mounted on the rotary table. Center offset error in measurement of the ball bar length is eliminated. The relationship between geometric error parameters of the rotary axis and ball-bar length variations is derived by the homogeneous coordinate matrix transformation from which the parameters can be identified. The displacement deviation aroused from the linkage motion of the linear axis with the rotary axis is also derived. Mathematical matching is operated between the feed rate of the rotary axis and acquisition speed of the ball-bar software. Center offset of the ball-bar length data acquired is eliminated to reduce the influence of installation. An obvious scaling mismatch between the axes diagnosed from the polar plot of ball-bar length variation shows that theC-axis has the tilt error around theX- orY-axes. Experiment results show that the proposed method can improve the identification accuracy. The decoupling method is both effective and convenient for identifying geometric error of the rotary axis in a five-axis machine tool.