Abstract: A method and apparatus for forming a fillet to fill a void between two L-shaped structural members which reinforce a composite material skin. A leg of one of the structural members is placed against a leg of the other member, with the second legs of the two members extending outwardly from the adjacent legs. The void is formed between the adjacent rounded outer edges between the legs of each of the members. The apparatus measures the radii of curvature of each of the rounded edges along the length of the member, stores the radii data in an electronic memory, and calculates the area of the void as a function of length along the member. The apparatus then controls a knife to trim an edge from a composite tape so that the trimmed tape has a cross-sectional area equal to the area of the void. The apparatus then forms the fillet from the trimmed tape.