Joining methods change to facilitate assembly automation

Citation

Messler Jr, R.W. (1998), "Joining methods change to facilitate assembly automation", Assembly Automation, Vol. 18 No. 4. https://doi.org/10.1108/aa.1998.03318daa.002

Joining methods change to facilitate assembly automation

Joining methods change to facilitate assembly automation

Robert W. Messler Jr

Robert W. Messler Jr is Associate Professor/Director of Materials Joining, Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, New York, USA.

Keywords Assembly, Joining, Snap-fit

Increasing pressures and incentives to automate assembly are placing new pressure on, and providing new incentives, for seeking and creating new methods and modifying and refining traditional methods of joining. While changes are more prevalent in the joining of mechanical assemblies, they will almost inevitably be more dramatic in the joining of electrical assemblies.

Joining is at once one of the most critical aspects of assembly in terms of mechanical integrity and achievement of function, and the most problematic in terms of accomplishment when assembly is to be automated. Installation of mechanical fasteners poses challenges to robots of both linear and angular positional accuracy (see the article in this issue by Setchi and Bratanov) to drill holes and align rivets, screws or bolts with those holes, or, even more challenging, nuts with bolts, and the ability to apply and react forces during hole drilling or rivet upsetting. Robotic fusion welding poses challenges with seam tracking, weld width, penetration control, and defect prevention or detection. Despite challenges with virtually every joining process (mechanical fastening, welding, brazing, soldering, and adhesive bonding), with the possible exception of adhesive bonding, changes are occurring.

Traditional methods of joining mechanical assemblies with fasteners, including predominantly nuts and bolts, screws, and rivets, is experiencing change with growing use of integral attachment epitomized by snap-fit features (see seven-part series by Genc et al., 1997-1998). Already prevalent in the assembly of plastic parts, snap-fits are finding their way into metal structures including snap-fit extruded aluminum truss members and cast aluminium fittings in spaceframes for advanced-concept aluminum-intensive automobiles, where they offer an alternative to fusion welding. They have also been employed in composite tower structures (Goldsworthy et al., 1994) and in the form of "composite integral fit" (CIF) joints (Lee and Hahn, 1996). Related approaches, referred to generically as "integral micro-mechanical interlocks" (IMMIs), have been proposed by the authors (Messler and Genc, 1998), that employ either elastic, plastic or rigid interlocking of microscopic features, closely akin to hook-and-loop systems and 3Ms dual-lock reclosable fastener systems ­ and these are just the beginning!

Another form of mechanical attachment is finding new niches; namely, mechanical interlock achieved by plastically deforming one piece-part into (or around) another. To find an alternative to either conventional fastening of aluminium structures (as in the aerospace industry) using match-drilled holes followed by placement of rivets or other mechanical fasteners, or to fusion welding, which requires careful process control and post-processing to relieve residual stresses, formed mechanical interlocks are beginning to emerge. Examples include:

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  "button fastening" (where portions of one sheet-gauge part are forced plastically into prepared recesses in a second part);

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  "tack-bonding" (which is quite similar, but is subtly different); and

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  "interface structure bonding" (which forces material plastically into a carefully prepared textured surface of a second part).

A recently described process is "hyper-pressure pulse bonding" using fluid jet pulses (Kolle, 1998).

As mentioned earlier, welding (especially using more common fusion processes like gas-tungsten arc and gas-metal arc) is undergoing slow but important changes with emphasis on use of process models as the basis for intelligent process control (including especially neural networks and fuzzy logic) and advanced sensor technology and feedback control to track seams, control penetration, control weld profile, control microstructure (by controlling heat input), and prevent defects.

For electrical assembly, automated die bonding on chips and mass soldering chip packages of printed circuit boards have predominated, enabling previously unparalleled levels of productivity and quality and reliability. But, as the dimensions of electronic devices both on chips or between chip packages on boards continue to shrink, inevitably approaching the atomic scale, diebonding and soldering will no longer be viable. Here, as-yet unimagined methods of joining will have to emerge, and when they do, they will be revolutionary, and so will the results in device and system performance (Messler, 1997).

It has been said: "Nothing about the future is certain, but that there will be change". Those of us involved in assembly must not only accept, but seek out and welcome, such change. After all, it is going to happen with or without us, and I for one do not want to be left behind.