An adjustable depth router is particularly suited for routing openings in ducts beneath register openings in a floor, without use of router bit extensions. A small diameter pneumatic motor, whose casing is mounted at the lower end of an elongated air supply casing of the same diameter, holds the router bit at its lower end; this assembly is vertically adjustable in a router frame. When routing through a floor opening, the base is rested on the floor, the cylindrical assembly projects through the floor opening so the router bit will operate at the desired depth therebeneath; and the bearing of the cylindrical assembly against the edge of its floor opening guides the router. An opening is thus routed parallel to the floor opening and spaced inwardly by the difference between the radius of the casing and the radius of the router bit.