The invention is directed to a rotary tool for attachment to a machine for working and smoothing an even surface of a base made of cement, concrete or similar hard material. The machine includes a multi-wheeled vehicle and at least one tool drive shaft arranged on the vehicle. The tool drive shaft defines a longitudinal axis extending perpendicularly to the base to be worked and has a tool holder for holding the rotary tool. The rotary tool includes a plate on which a plurality of cutter-wheel units are mounted about the periphery thereof. Each of the cutter-wheel units includes a frame attached to the plate and axles mounted so as to define respective rotational axes parallel to the plate and substantially parallel to a radius passing through the longitudinal axis of the tool drive shaft. A plurality of cutter wheels is mounted on the axles so as to rotate freely with respect thereto.