State of the Art

The ability to produce high-value products in small batches is critical in ensuring that European manufacturers are able to respond to rapid changes in the marketplace. The advent of re-programmable, multi-axis machining and assembly centres has opened up new opportunities for European manufacturers. By exploiting their flexibility, small batches of customised products can be rapidly and cost-effectively produced. The ability to do this has greatly enhanced the competitive advantage of European manufactures in the global marketplace.

Traditional fixturing and work holding methods are the key bottleneck in a truly flexible manufacturing environment. Current fixturing systems are, in most cases, designed for specific products and processes, making them highly inflexible to changes in product and process specifications. Modular fixtures have been developed as an attempt to overcome the inflexibility of traditional systems. Unfortunately, due to the piecewise nature of modular fixturing systems, they are more susceptible to tolerance stack-ups (Kusiak, 1992). This is expected, as modular systems consist of many joints and attachments, which compromise the rigidity of the overall structure (Hargrove & Kusiak 1994), (Rong & Zhu 1999).

As a result, modular systems have frequently failed to meet the repeatability requirements for high-precision manufacturing processes with tight tolerances. Also, as the components and processes become more complex, it will be increasingly difficult and time-consuming for even experienced engineering practitioners to plan, design and assemble the modular fixturing systems (Shirinzadeh 1995).

 

There is almost a complete absence of intelligence and control in current fixturing systems. Components are still commonly held using a series of structural hard-points on the fixturing system throughout the manufacturing process. Active control of the fixtures is generally limited to the configuration stage of the fixturing system (Benhabib et al. 1991) . Once the location and clamping points are configured, they are locked in place and the fixturing system acts as a traditional system with no active control. This presents a major problem, especially for the fixturing of flexible and difficult-to-handle parts in the presence of complex manufacturing forces.

The AFFIX system will overcome these issues by developing an active fixturing system that could be used to accurately identify and position the component, apply the correct amount of clamping forces and intelligently modify its fixturing schemes based on external influences on the part-fixture configuration.

"Plug & Produce”

"Plug & Produce" is a concept developed at the University of Tokyo using a framework of holonic production system (Arai et al. 2001). It utilises holons arranged in a “holarchy” to control the various devices that make up the system.

It is an attempt to develop a standard for manufacturing hardware and software systems, which is similar to the “Plug & Play” standard for PC systems. This will enable different machining, assembly as well as fixturing systems to be rapidly and cost-effectively reconfigured for the manufacture of different products.

The benefits of such a standard will only be realised with widespread acceptance and implementation. AFFIX will achieve this critical mass through the size and diversity of its partners in the consortium. The AFFIX platform could potentially become the first and foremost standard for an integrated and reconfigurable manufacturing-fixturing system in the world.

Using the AFFIX platform, service and maintenance depots will be set up to provide fixturing solutions on-demand to different manufacturers across Europe, opening up the possibility for even European SMEs to gain access to the benefits provided by the AFFIX fixturing system.

Case Studies

Automated assembly is generally confined to mass production environments such as the manufacture of cars and white goods. Even in this environment high-level automated assembly is restricted to the Original Equipment Manufacturers (OEMs), where production volumes are high and both flexibility and the ability to quickly reconfigure systems is not a major driver. At the higher levels of the supply chain, assembly is usually well planned, with good fixture design but little automation and little inherent flexibility.

The first challenge is to get the benefits of automated assembly into areas where basic production volumes cannot be used for justification. This can only be achieved through the introduction of highly reconfigurable systems.

When considering assembly processes, the aerospace industry is truly singular due to product size and the quality required. As an illustration, it is admitted that assembly process represents more than half of the total manufacturing costs for an aircraft, most of it being manual.

As a consequence, it seems vital to achieve low cost and high quality assembly operations in order to maintain a high industrial competitiveness in Europe and avoid major delocalization where workforce is economically more attractive.

The problem is further complicated by the move to large thin walled monolithic parts and the increasing use of composite structures. Monolithic structures have been introduced to reduce the cost of assembling large numbers of components as demonstrated in Table 1 below. Although the benefit of using monolithic parts is a large reduction in overall manufacturing cost the downside is a more difficult component to handle and assemble. If this problem can be overcome there are yet more savings to be achieved.

 

Table 1: Comparison of Monolithic and Conventional Aircraft Components

Original Part	Monolithic Part
Number of Pieces = 44	Number of Pieces = 6
Number of Tools = 53	Number of Tools = 5
Design & Machining time hr (Tools) = 965	Design & Machining time hr (Tools) = 30
Machining time (hrs) = 13	Machining time (hrs) = 8.6
Assembly Man-hours= 50	Assembly Man-hours= 5.3
Weight (Kg) = 3.77	Weight (Kg) = 3.37
Overall manufacturing cost =100 units	Overall manufacturing cost = 37 units

 

Figure 1: Combustion casing

A similar problem can be seen in the manufacture of aero engine combustion chamber outer casings (figure 1).

Here again the problem is a thin walled component but in this case manufactured from a difficult to cut material such as titanium, waspaloy or Inconel.

Machining the flexible structure and maintaining close tolerance is difficult, transferring to assembly is difficult and there are additional difficulties with drilling, tapping and fastening as the breakage of a drill or tap can cause the complete assembly to be scrapped.

While large automotive and white goods manufacturers can afford bespoke fixtures for each part due to the high volume of manufacture, these still need to be designed and built (and stored when new models come into production).  Economies of scale  mean that it would still be cheaper for them to have reconfigurable fixtures that could be used on any part (or even any of a family of parts) e.g. a fixture that could be used to hold any door panel regardless of the car model.

It might also be necessary to have several different fixtures to assemble one thing – for example one fixture to hold and assemble a car chassis, which is them moved to a separate station for the addition of wiring and yet another fixture for the addition of the body.  If you could use a single fixture for the whole assembly you would speed up assembly processes, use less fixtures and require less storage space.

 AFFIX will make basic science breakthroughs, but these are academic and of little use unless they can be transferred to industry.  Representative parts will be chosen by each of the large equipment manufacturers in the project.  These will be benchmarked, then the relevant science breakthroughs for each part will be applied through the plug and produce architecture.  The new AFFIX technologies can then be evaluated in an industrial setting.

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References