This paper proposes a bi-directional laser sequential step diagonal measuring method for three-axis vertical machining centers. Different measure paths are particularly designed for positive and negative directions, and corresponding error decoupling models are established. Based on the laser measuring data along these bi-directional paths, 18 geometric errors can be identified simultaneously, including 6 angular errors, 3 positioning errors, 6 straightness errors and 3 squareness errors. Compared with single directional step diagonal measurements, the proposed bi-directional mothed is more efficient and can identify more error items, namely 6 angular errors. Experimental tests were conducted on a three-axis vertical machining center. Decoupled error items obtained by the proposed bi-direction measurements coincide with those from direct measurements via a laser Doppler displacement meter, which verified the feasibility of the proposed method. Finally, the position error of the machine whole workspace was predicted by building volumetric error model and the machine accuracy was improved.

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