Depending on the manufacturing philosophy, jigs and fixtures need to be designed for the type of tooling envisaged for parts fabrication and assembly. Jigs and fixtures are special holding devices for making fast the workpiece for accurate fabrication and assembly of parts. Naturally, jig and fixture design starts early during Phase 2 of a project, along with planning for the facility and process layout. This can be expensive, requiring additional production-launch costs; however, there is a payback in saving labor costs when production starts. Investment in the aerospace industry is front-loaded.
Accurate dimensioning during fabrication and assembly is important for reducing manufacture and maintenance costs. The following are used to maintain dimensional accuracy (these are not precise definitions but make sense in context):
Tools: This equipment cuts and shapes material in the parts fabrication process. They can be handheld or fixed in place. Examples include drills, lathes, hammers, riveters, and welders. Tools, jigs, and fixtures work in conjunction with one another.
Gauges: These are measuring devices for accurately locating tools relative to the fixture in which a workpiece is held.
Fixtures: These are special working and clamping devices that facilitate processing, fabrication, and assembly. Fixtures are fixed frames designed to hold one or several workpieces in the correct position relative to one another. A gauge may be required initially to position a tool for cutting. Fixtures can be large, depending on the size of the workpiece. They should be solid and heavy structures to withstand any vibrations.
Jigs: These have a similar function as fixtures but they also incorporate guides for the tool. Jigs also are fixed items. Jigs typically are used for drilling, reaming, and welding.
Given herein are seven of many best-practices techniques that contribute to DFM/A practices. The basic idea of the seven techniques uses a modern manufacturing and tooling philosophy, moving away from the older, manual procedures to digital processing (see Section 17.10), where most tasks are performed. Modern methods make extensive use of CAD, CAM, and computer-aided process planning (CAPP) to ensure a high standard of accuracy and productivity. Numerically controlled (NC) machines are part of CAM.
1. Jigless Assembly: Designing for ease of assembly should not be restricted exclusively to the task of concept-design engineers. Tooling engineers contribute to the reduction of costs through a jigless assembly approach to manufacturing. Jigless assembly is an approach toward reducing the costs and increasing the flexibility of tooling systems for manufacture through minimization of product – specific jigs, fixtures, and tooling. During the development phase, tooling costs are high; consequently, savings in this aspect of aircraft manufacture are significant and they impact the time from concept to market as well. Jigless assembly does not mean toolless assembly; rather, it means the eradication or at least the reduction of jigs. Simple fixtures still may be needed to hold the parts during specific operations, but other methods are being found to correctly locate parts relative to one another. Assembly techniques are simplified by using precision- positioned holes in panels and other parts of the structure to “self-locate” the panels; here, parts serve as jigs. This process, known as determinant assembly, uses part-to-part indexing rather than the conventional part-to-tool systems used in the past.
2. Flyaway Tooling: Within the airframe-manufacturing industry, it is generally accepted that approximately 10% of overall manufacturing costs for each airframe can be attributed to the manufacture and maintenance of assembly jigs and fixtures. The traditional “hard-tooling” philosophy requires that the desired quality of the finished structure be built into the tooling. The tooling therefore must be regularly calibrated to ensure build quality. An alternative philosophy, “flyaway tooling,” was conceived to reduce tooling costs and improve build quality. This approach envisions future airframe components designed with integral location features with incorporated positional data that transfer to the assembly. This enables in-process measurement and aids in-service repairs. It also may be possible to design an aerospace structure with sufficient inherent stiffness, allowing the assembly tooling to be reduced to a simple, reusable, and reconfigurable support structure.
3. Gaugeless Tooling: This is achieved using a theodolite system linked through a central processor. Coordinated geometry, obtained directly from CAD, is used to establish the “hard points” to meet the build, interface, and interchangeability requirements. Gaugeless tooling is required for the manufacture and periodic inspection of the assembly process.
4. Inline Assembly: This provides a progressive and balanced assembly build sequence, utilizing the maximum number of subassemblies in a cellular-type environment, which improves interchangeability.
5. Automatic Riveting: The assembly is first slave-riveted on the fixture and then moved to the automatic machine. This improves productivity and accuracy; hence, the quality impacts from human error are minimized. The manpower engaged is also reduced.
6. Tolerance Relaxation at the Wetted Surface : Aircraft surface-smoothness requirements are aerodynamically driven with a stricter manufacturing tolerance to minimize drag – that is, the tighter the tolerance, the higher is the assembly cost. Trade-off studies between surface tolerance and aerodynamic drag rise can reduce manufacturing costs (see Section 17.6).
7. Six Sigma and Supporting Methodologies: An important framework in which DFM/A techniques should be conducted is that of concurrent engineering (i. e., IPPD) focusing on improvement of the product-development process by concentrating on the design stage for the entire life cycle of a product. Management strategies such as DFSS and LAM as well as effective personnel management also must be considered if improvements are to be made in assembly-system profitability. DFM/A should strengthen the team activity in all phases of the design process, thereby ensuring that the technical expertise of the participants is successfully utilized; this is a management tool.
Decisions made during product design have a major impact on cost, defects, and cycle time. In fact, about 70% of production cost is locked in during the design process. DFM/A helps reduce product complexity through minimization of parts and fastener counts, assembly and manufacturing time, and material costs. Additionally, DFM/A application reduces the potential for defects. Robust design, statistical tol – erancing, and geometric dimensioning and tolerancing actually help reduce defects. A better understanding of DFM/A in reducing the cost of production requires detailed studies in material selection and different process capabilities, which are beyond the scope of this book. The DFM/A concept assists the Six Sigma management strategy.