Modularity for aircraft part fixtures gains altitude

  • 30-Dec-2010 01:52 EST

Little movement or drift was seen over the course of an eight-month study of a BoxJoint used by Airbus UK for a carbon-fiber wingbox section.

Affordable Reconfigurable Tooling (ART) is a concept for adjustable and reconfigurable work-holding during the assembly process. Instead of dedicated aircraft fixtures, which are very time-consuming and expensive to deploy, ART fixtures enable affordable construction from a standard steel component kit. They can be assembled quickly and set within tolerance by means of a metrology system. Calculation of strength and stiffness can be made by simple, standard methods.

The first drafts of the concept were made by Saab (Linkoping, Sweden) about a decade ago. Further developments were made by Linköping University, Airbus UK, BAE Systems, Leica Geosystems, and DELFOi. The first industrial application is now being realized at Saab and tests are under way at Airbus UK.

The basic principle in ART is that the pick-up devices of the product are set into CAD-defined positions by means of adjustable supports and through guidance of an external measuring system. This contrasts the traditional way of building work-holding fixtures in which the pick-up devices are related to a common datum by means of a stack of accurately machined construction elements or by help of shimming and fettling.

One of the main benefits of ART is that the accuracy of the pick-up positions can be maintained even though less precise machining is required for the construction elements. This provides a potential for cost savings.

Another benefit is that it enables an easy way for trimming the pick-up positions when disturbances occur. Such disturbances may come from variable temperature, floor subsidence, unexpected shock loads, and the release of internal tensions in welds.

Still another benefit of ART is that it enables shorter lead times and concurrent engineering in the tooling development. The main structure of the fixture can be designed and even erected before the product design is finally settled. Late changes of the product resulting in modified coordinates for the pick-up devices can be handled by setting the adjustable supports into amended positions. Contributing to the shortened lead time in ART is the fact that the construction parts, to a great extent, can be supplied off-the-shelf as modular, standard parts.

An additional benefit in some cases is the possibility of enabling reconfigurability within a group of different products. This is especially beneficial in low-volume production. The costs of the fixture then are shared between the different projects.

Prior drafts of ART fixtures used extensionable stics, so-called pogostics, which were attached into a firm framework of welded steel beams. The requisite degree of geometrical variability for the products was achieved by resetting the pogostics into new positions and directions. The framework, however, was welded, which limited both product variability and accessibility of the measuring device, a laser tracker.

Then a new modular system for joining steel beams, called “BoxJoint,” was conceived. The first version of this system enabled the joining of crossing beams in square, reconfigurable patterns. The benefit of the system was that the beams could be moved into new relative positions, avoiding many of the problems related to clashes and difficult-to-reach areas of welded structures.

During further development of ART, it was found beneficial to combine the existing types of adjustable supports, such as flexapods or hexapods, with newly developed so-called miniflexapods and other product supports that offer more work space for adjustments.

Now, a modified version of the system has been developed with a set of new joints (angular, T-, and corner). In addition, two new modular sizes were integrated within the prior system to form a total construction kit with a very large span of sizes and capacities.

Results to date of an ongoing field study at Airbus UK on an A380 trailing-edge subassembly are positive.

This article is based on SAE technical paper 2010-01-1873 by Peter Helgosson of DELFOi, Gilbert Ossbahr of Linköping University, and David Tomlinson of Airbus Operations Ltd.

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