A large surface structural component, such as a rotor blade for a windmillr a blade root for a windmill rotor, includes a hollow core (6) of foam material supporting a shell (8, 11) of fiber reinforced synthetic material. Positive templates in the form of disks (5) are inserted into the core (6). These templates are preferably spaced from each other along the length of the structural component and have a shape which defines the cross-sectional contour of the finished component. Thus, initially foam sections having a shape convenient for their production and these templates are assembled on a rotatable arbor (2) which is then rotated in a machining operation such as cutting and/or grinding on a lathe type machine. The shape of these templates (5) provides a guide for the machining operation which is thus substantially facilitated while simultaneously permitting the use of foam cores having initially plane surfaces.