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

Wilma Polini

Dipartimento di Meccanica, Strutture, Ambiente e Territorio, Universita degli Studi di Cassino, Cassino

Italy

1. Introduction

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

process if the tolerance study on a sub-component was not carried out or was ineffectual (Whitney, 2004). It is even possible that the product design may have to be subsequently changed because of unforeseen tolerance problems not detected prior to actual assembly took place. In this case costs to the business will be high. It was estimated that 40%-60% of the production cost is due to the assembly process (Delchambre, 1996).

The study of the tolerance stack-up functions, during the design stage, is very critical for aeronautic field whereas the complexity of the structures, to which high performances are required; so advanced material, design techniques and assembly technologies are needed. In fact, the aeronautic structures involve free-form surfaces, which are often made in composite material. They may be considered as non-rigid parts that could be subjected to significant distortion after the removal of manufacturing forces. This condition, know as free-state variation, is principally due to weight and flexibility of the part and the release of internal stresses resulting from fabrication.

Many well-known approaches exist in the literature to tolerance analysis (Hong & Chang, 2002; Shen et al., 2004). However, these methods are not easy to apply, especially for complex aerospace assemblies, since they were born to deal with elementary features, such as plane, hole, pin and so on. So the aid of computer is called for. In the recent years, the development of efficient and robust design tools has allowed to foresee manufacturing or assembly problems during the first steps of product modeling by adopting a concurrent engineering approach. Today Computer Aided Tolerance (CAT) Software is readily available, but even if these tools provide good results they have not been widely used. Commercial CATs are not completely true to the GD&T standards and need improvement after a better mathematical understanding of the geometric variations. The user needs expertise and great experience combined with a through understanding of the packages’ theoretical base plus modeling principles to build a valid model and obtain relatively accurate results. Computer Aided Tolerance software efficiently deals with mechanical assemblies where the feature to align are planes, hole-pin, but it hardly treats of free-form surfaces to connect.

The present work deals with the tolerance analysis of freeform surfaces belonging to parts in composite material and that may be non-rigid. The aim of this chapter is to present the steps to carry out the tolerance analysis of an assembly involving free-form surfaces in composite material by using a commercial CAT software. The great effort of the present work is overcome the limits of the CAT software to deal with dimensional and geometric tolerances applied to free form surfaces in composite material. This paper tries to answer to some questions on tolerance analysis without clear answer: What are the functions of the product, how do we flow down these key product functions through into its detail parts? How to model a free-form surface for a tolerance analysis? How to deal with a composite material for a tolerance analysis? How to improve the assembly process in order to reduce the tolerance impacts on these functions?

An aeronautic component is considered as case study. This is an internal frame of a winch arm mounted on an helicopter. The part addressed in this paper is the after internal frame that allows to mount the fairing covers of the head of the winch arm directly on its structural beam. It is made in carbon fiber composite material. The frame is made with five layer of carbon fiber imbued in a matrix of epoxy resin for a total stack thickness of 1.65 mm. After the stratification process, a curing process is carried out by autoclave with a control of temperature and pressure. The weight of the part is about 350 g. Assembly operation among the parts are done by using special grub screws that are inserted in the holes of each component. No adhesives or resins are used to secure the surfaces, since a perfect adhesion is not required.

The paper is organized as follows: In Sec. 2, the mean of tolerance analysis is deeply discussed. In Sec. 3, the main models found in the literature for tolerance analysis are presented. In Sec. 4, the main functions of the commercial CAT softwares are shown. In Sec. 5, the steps of the proposed method for tolerance analysis of an assembly involving free­form surfaces in composite material are shown. In Sec. 6, the application of the proposed steps to an aeronautic case study is deeply discussed.