A ballbar is used to assess the accuracy of relative motion between an arbor and a table 4--of a machine tool. Mounting components are secured respectively to the arbor and the table-4. Each mounting component comprises a block that has provision on three orthogonal surfaces for receiving support means for a ballbar. By appropriate selection of the mounting faces, it is possible to set up the ballbar to check motion in all three of the operating planes of the machine. The mounting component comprises a ring for fitting over the arbor of the grinding machine, and the mounting block is connected to the ring by a rod.
