To Carry Out Tolerance Analysis of an Aeronautic Assembly Involving Free Form Surfaces in Composite Material

Abstract

The increasing competition in aerospace industries has brought to cutting programs in manufacturing, design and assembly of aircraft structural frame. An aircraft is made of several assemblies including 3D complex shapes and the different functional requirements from design and manufacturing that these assemblies should respect are various. Nowadays the most important policy is the “Concurrent Engineering” which rule is to lead in a parallel way, design and manufacturing, making them communicate with the aim of reducing reworking times and discard products; such requirements are strongly felt in the aerospace industries. Tolerance analysis has a considerable weight in the Concurrent Engineering and represents the best way to solve assembly problems in order to ensure higher quality and lower costs. It is a critical step to design and to build a product and its importance has grown in the past years. In fact, the need to assign dimensional and geometric tolerances to assembly components is due to the standardization of the production and to the correct working of the assembly. The appropriate allocation of tolerances among the different parts of an assembly can result in lower costs per assembly and higher probability of fit, reducing the number of rejects or the amount of rework required on components. A product is designed and manufactured to perform a task, and its issue depends on one or more parameters of the assembly that are commonly called “project functions”. A project function is a dimension or a geometric variable of the assembly whose value depends by the dimensions, the geometry and the tolerances assigned to the components constituting the assembly. The nominal value and the tolerance range of the project function allow us to guarantee the assembly functionality. Practically, the dimensions and the tolerances of the assembly components combine, according to the assembly sequences, and generate the tolerance stack-up functions. Solving a tolerance stack-up function means to determine the nominal value and the tolerance range of a project function by combining the nominal values and the tolerance ranges assigned to the assembly components. Tolerance analysis may consider alternative assembly cycles in order to identify that one allowing to obtain the assembly functionality with the maximum value of the tolerance range assigned to the components. Huge problems may present during the assembly