Method of making a fiber preform of varying thickness

A method of making a fiber preform of varying thickness to manufacture a part made of composite material, the method comprising superposing layers of fiber fabric and needling the layers together as they are superposed, and within the, or each, portion of the preform to be made in which the thickness of the preform is to be reduced so as to form a recess the steps of:

• interposing a mask between two adjacent layers, forming the mask from a thin film of a material suitable for allowing the needles to pass therethrough during needling while preventing significant quantities of fibers caught by the needles from passing therethrough;
• and
• cutting out each layer after needling along a line corresponding to the outline of the recess in the preform at the level at which the layer is situated within the preform;
• situating the mask between two cutout layers occupying an area that is not less than the area extending between the cutout lines of the two layers, such that the portions of the layers delimited by the cutout lines are not securely connected to the remainder of the preform by the needling and so that the recess can be formed, after all of the layers have been needled, by removing the cutout portions of the layers and by peeling off the masks.