Measurement-assisted assembly: when the part, not the jig, is the reference

For most of aviation history, the assembly jig has been the reference. Parts are clamped into a large steel fixture, and the fixture decides where everything ends up. Measurement-assisted assembly (MAA) turns that around: the measured geometry of the parts drives the assembly, and the tooling becomes simpler as a result.

The problem with the jig as the master

Researchers at the University of Bath, working with Airbus UK, pointed out that aerospace assembly remains slow and costly largely because of rework carried out inside the jig. Other industries solved this with interchangeable parts that fit first time. Aerostructures have not followed, because interface tolerances are tight and the components are large and flexible, so true interchangeability has been out of reach for most structures [1].

The jigs themselves are part of the cost. Monolithic steel fixtures are expensive, take a long time to build and cope poorly with design changes [2].

What measurement-assisted assembly changes

MAA uses large-volume metrology, typically laser trackers, photogrammetry or indoor GPS, to measure parts and features during assembly and use that data to position, machine or shim them. The Bath group argued that this route can deliver many of the benefits of part-to-part assembly without needing fully interchangeable parts, and set out a roadmap from interface management through determinate assembly towards part-to-part assembly [1]. A later paper in the International Journal of Advanced Manufacturing Technology defined the MAA concept formally and listed the research priorities for bringing it into industry [3].

A related idea, measurement-assisted determinate assembly (MADA), goes further: it only pays off if the structure is designed for it from the start [4]. That is a design decision, not something added on the shop floor.

A 2014 review of flexible, measurement-assisted assembly in aircraft manufacturing traced how digital, reconfigurable fixtures have grown out of this thinking as fixed tooling struggles to keep up with changing aircraft variants [5].

What this means in practice

From working with this approach, three points matter more than the choice of instrument:

• Datum strategy first. Decide which features define the product and make sure they can be measured at every stage, not only at final inspection.
• Uncertainty against tolerance. A measurement system is only useful if its uncertainty is a small fraction of the tolerance it is policing. Work this out before buying hardware.
• Start with one interface. Proving MAA on a single difficult interface, such as a shimmed joint, builds the data and confidence needed before wider rollout.

References

1. Muelaner, J.E., Kayani, A., Martin, O. and Maropoulos, P.G. (2011). Measurement Assisted Assembly and the Roadmap to Part-To-Part Assembly. 7th International Conference on Digital Enterprise Technology, Athens.
2. Muelaner, J.E. and Maropoulos, P.G. Large scale metrology in aerospace assembly and related work on metrology-enhanced tooling (Semantic Scholar record).
3. Maropoulos, P.G., Muelaner, J.E., Summers, M.D. and Martin, O.C. (2014). A new paradigm in large-scale assembly: research priorities in measurement assisted assembly. International Journal of Advanced Manufacturing Technology, 70, 621–633.
4. Muelaner, J.E. and Maropoulos, P.G. (2010). Design for measurement assisted determinate assembly (MADA) of large composite structures. Coordinate Metrology Systems Conference.
5. Mei, Z. and Maropoulos, P.G. (2014). Review of the application of flexible, measurement-assisted assembly technology in aircraft manufacturing. Proc. IMechE Part B, 228(10), 1185–1197.