Abstract: A manipulator includes a carriage, a base and a longitudinal guide for positioning the carriage. The longitudinal guide contains two guide parts attached to the base to provide a sliding mount for the carriage along an axis. A spindle is coupled to the carriage for moving it longitudinally and is provided with a rotating mount on the base. The manipulator also includes two wedge arrangements between the side parts of the base and the guide parts of the guide to center the carriage and prestress the guide parts in the direction of the axis. Each wedge arrangement comprises two cooperating slant faces one of which is an inner face of each side part of the base and the other of which belongs to a wedge which is moved with respect to the respective guide part for centering the same relative to the spindle. After completion of the centering operation the guide parts will be secured to the base, e.g. by screws.