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When the part’s entire geometry is generated by machining operations, a large-sized machine tool is indispensable. On the other hand, Some large parts only require a set of machining operations within a small “local” region but many “local” features must be machined over the entire large part. For example, some large parts have many holes to be drilled, or many small pockets to be milled, and the requirement is in the accuracy of the position of holes or pockets. For such a machining application, as a more cost- effective alternative to large-sized machine tools, this paper proposes the concept of a “movable” machine tool; a small machine tool, which has the capability and the workspace to machine the given “local” feature, moves by long coarse linear axes to the location of each feature (see Fig.1). The core technology is in the three-dimensional measurement of the coarse axes' motion, and its real-time compensation to ensure the required positioning accuracy (see Fig.2). Traditionally, the accuracy and the stiffness of a machine tool is ensured by rigid mechanical design and mechanical adjustment of assembly. The objective of this study is to show that sufficient accuracy can be achieved by a significantly inexpensive mechanism based on real-time three-dimensional measurement and compensation. This study applies a laser tracker (see our past research) to measure the three-dimensional positioning accuracy of the coarse axes. To experimentally investigate the effectiveness of the present scheme, a preliminary test was conducted using a 6-DOF robot (see Figs.3 and 4). The robot is moved by “coarse” linear X- and Y-axes, driven by a ball screw and a servo motor. The coarse axes do not have sufficient positioning accuracy and precision. To ensure the accuracy in the position and the orientation of the robot hand tip, the real-time compensation of the position and the orientation of coarse axes is applied based on the three-dimensional position measurement using a laser tracker. This work was done with Mr. Kohei Yuasa (March, 2015). >> Publications: CE58 
Figure 1: Concept of "movable" machine tool.  
Figure 2: a) Concept of real-time control of volumetric errors of the coarse linear axis by using a laser tracker. b) Its application to robotic machining is also possible. 
Figure 3: Experimental setup. A 6-DOF robot for welding operations. Coarse positioning is by X- and Y- linear axes and fine positioning is by the robot. 
Figure 4: In our experiment, the laser tracker by Faro was used for position measurement of a retroreflector attached to the robot hand.
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