AbstractThis article describes the development of a one‐dimensional extensible ball bar designed to measure the positioning accuracy and repeatability of industrial robots. The ball bar has a linear travel of 5 cm (2 in) that is monitored by a built‐in electronic transducer having a resolution of 2.5 μm (0.0001 in). At each end of the bar is a precision steel sphere, one of which is attached to an universal joint mounted on a robot's wrist. The other steel ball is magnetically attached to a socket which is firmly located within the work zone of the robot. During the measuring process the robot wrist is programmed to move in an arc whose radius is the nominal length of the bar. The variation of the length of the bar is recorded from the linear transducer and used to obtain a one‐dimensional measurement of the positioning poerformance of the robot. Linear displacement and sag tests indicate that the ball bar has an accuracy of 32 μm (0.0013 in) and a repeatability of 20 μm (0.0008 in). Test procedures to evaluate a robot's performance are also presented.