It comprises a supporting structure in which at least one arm is slidably attached. Each arm comprises first and second members hinged to each other. The first member is rotatably hinged to the supporting structure and it can be rotated about a longitudinal axis and the second member may receive a joint having at least two degrees of freedom for attaching a tool. The longitudinal axis of the first member is substantially perpendicular to an axis joining the first member and the second member to each other. A simplified architecture is obtained allowing for accurate and efficient spatial movement of the tool holding arm.