Abstract: An expandable mechanical stop is provided for positioning a workpiece during woodworking or other operations. The mechanical stop includes a positioning body which is adjustably attached to a track, the track having a vertical face that abuts an edge of the workpiece during operation. A stop arm is connected to the positioning body and includes a rear face that is substantially adjacent to the vertical face of the track. Anti-biasing means are provided to prevent the workpiece from dislodging the stop arm by sliding between the stop arm and the track. In one configuration, tabs are provided on the rear face and are received by grooves on the vertical face to serve as the anti-biasing means. In another configuration, grooves are provided on the rear face and receive tabs disposed on the vertical face.