Article Source: IMechE
Accurate prototype part and assembly manufacture is vital in almost every engineering sector.
Usually the prototype is the only component subjected to every testing and performance validation procedure, influencing key decisions on design modifications, published performance attributes, warranties and service intervals.
At the testing stage, prototypes must remain faithful to the original design. Any deviations can render test results unreliable or invalid.
If results are inaccurate, individual parts, or even the whole assembly, must be examined, remanufactured and reconfigured, entailing substantial delays and cost. Worse still, if inaccurate data are carried forward to the next production stage, ramifications can be severe: from shortened service life or failure to meet warranted standards to catastrophic in-service failure. A prototype must, therefore, be the best, most accurate part a company ever produces.
The problem with prototyping
Unfortunately, prototyping has not always received its deserved focus. Production prototypes are often expensive, particularly when compared to the final component’s cost per unit. There has also, arguably, been an inconsistent approach to prototype development.
Unlike the mass-production of finished parts – involving detailed operations sheets, photos, precise written instructions, and guidance on correct handling and fixing – prototype assembly is often done based on a CAD representation at best. Quality and repeatability can therefore suffer. With most projects having strict deadlines, re-engineering a component at a later stage incurs project delays and budget overruns.
Why, then, has prototype manufacture been so neglected? Traditionally, prototypes have been prioritised and progressed according to timeframe. Product development and project managers tend to focus on ensuring that all required components are available by a certain date. In the worst cases, only once the components have arrived do engineers start worrying about how, and indeed whether, they will fit together.
The number of tolerance and assembly issues identified at this stage is higher than many would care to admit. Fixturing can also cause problems; investment in adequate work-holding equipment when adding remaining components is often shelved to reduce project costs. This is a false economy: awkward working processes caused by incorrect fixturing can result in damage and sub-standard assembly.
A solution to the prototype problem
Each prototype manufacture and assembly step must be driven by final production objectives. Overall risk, cost and time can be cut, while increasing the chances of accurate production, reducing final assembly time and lowering rework costs.
Production Oriented Prototyping (POP) is a novel approach involving the individual component and full assembly designers from the application development phase. The aim is to build production standards into the prototype, while minimising cost and delivery time, and optimising safety.
Using virtual builds, simulation and validation via CAD, manufacturers can ensure that all components fit together for each design release and can be accessed and manipulated during the build. Any issues with compatibility, design or accessibility can be tackled much earlier and, crucially, before time and money have been invested in one-off component manufacture.
Using this methodology, the team can develop operation sheets as they go, making final assembly easier and more intuitive. Fixturing and work-holding are also considered earlier, allowing more time to specify and produce a bespoke system, if required. This is especially true of larger or more complex assemblies, which do not lend themselves to easy movement or manipulation.
In effect, assembly process design – including process, flow, tooling and fixturing – is completed before the physical components are ready to be assembled.
Through this approach, it’s easier to identify and fix issues faster than would be the case during final assembly, with programme time savings exceeding 25% often achievable.
Meanwhile, the extra efficiency afforded by POP allows assembly technicians to identify cost reduction opportunities for components and processes through ‘practice runs’ for future assembly. In one instance, the assembly time for the first prototype build of a complex automotive transmission was reduced from eight weeks to just eight days.
The approach can be applied in any sector where precision and replicability are vital to enable accurate testing and validation. We hope this will finally give prototypes the attention they deserve.