ISO Specifications of Complex Surfaces: Application on Aerodynamic Profiles

For many years, the use of complex surfaces is becoming widespread with the development of manufacturing process. CAD model describes nominal shape of the part. Specifications are then defined by annotations directly in the 3D model. This work is illustrated by an application on a turbine blade of an aircraft engine.To impose the widest possible tolerances on the whole surface, it is necessary to have a multi-scale approach. The main datum reference frame is built on the setting up surfaces of the part.In the first level, all surfaces are located with regard to the main datum reference frame with a wide tolerance in order to avoid interference with other parts of the mechanism. In a second level, specifications on restricted areas complete local requirements with lower tolerances. In the third level, orientation specification on small mobile zone with small tolerance detects micro defects as tool traces for example.This presentation shows several positions, orientation and form specifications on 3D surfaces and on 2D curves. The new modifier “For orientation constraint only” of the ISO 5459: 2011 standard lets us define only the orientation with regard to a reference surface. The overlapping of many orientation specifications on restricted areas limits parts defects inside a large global tolerance zone. Specifications with common zone of two surfaces face to face control the thickness of the part which can be useful for strength or mass reasons.These specifications can be detailed with a variable tolerance zone that defines the tolerance of all points of surfaces. The ISO standard only treats the 2D case; this paper presents a solution to specify 3D surfaces with variable tolerance by a hypothesis of proportional variation following the curvilinear distance on curves created on 3D surfaces.This set of specifications constitutes a tool-box for designers based on only one type of specification which allows them to verify all requirements with a partition of only one cloud of points.The 3D complex surfaces are the interface between the part and the outside environment and often require similar functions. This study in a context of aeronautic industry can easily be extended to other mechanical domains.