17.2 Introduction

(i.e., “cradle to grave”). Shop-floor execution systems are fed directly from the PPRhub environment, providing a direct reuse of CAD-based data as a basis for work instructions. As-built data are captured and available for use within the PPR hub for follow-on planning and validation as the product evolves throughout its life cycle.

In some ways, automobile-manufacturing technology is ahead of the aerospace industry by successfully implementing digital-manufacturing technology and advancing to futuristic visions. A successful automobile design can sell a million per year and last for a decade with minor modifications; whereas, in peacetime, fewer than 500 per year of a successful high-subsonic commercial transport aircraft are produced and none has yet reached the 10,000 mark in terms of total aircraft sales. The automobile industry can invest large sums in modern production methods yet keep amortization costs per car low.

17.1.1 What Is to Be Learned?

This chapter covers the following topics:

17.1.2 Coursework Content

Readers may compute the index of the design for customer. However, it is neither essential nor important because the industry is not adopting this system at this stage; more study is needed. However, the DFM/A considerations can be addressed in a second term. Such studies need not alter the finalized and substantiated configuration obtained thus far through the worked-out examples (in the industry, DFM/A is carried out in parallel during the conceptual design stage). It is beneficial to have an idea of DFM/A implications in aircraft design and operation. However, if it is a second-term topic, it may not be practical without specialist instructors using realistic data. This chapter provides only a glimpse of the scope of DFM/A during the conceptual study phase.

17.2 Introduction

Today, it is not the operators who are the only customer. The future trends suggest the entire society as a customer of the high-tech aerospace engineering, which could “make or break” any society depending on how the technology is used. This also is true for other types of technology, including nuclear and bioengineering.

In the past, trade-off studies were limited to the interaction among aerodynamics, structures, and propulsion, as discussed through Chapter 13 of this book.

Chart 17.1. DFM/A steps

Subsequently, during the 1990s, the need for DFM/A considerations in an IPPD environment gained credence. The IPPD process continues to evolve for the customer-driven design trends in order to minimize ownership costs without sacrificing integrity, performance, quality, reliability, safety, and maintainability. The recent economic downturn demands general and significant cost-cutting measures, severely affecting the commercial aircraft industry. In this economic climate, the roles of reliability, maintainability, recyclability, and so forth are designand manufacturing-process-dependent. This chapter introduces an index of design for customer and incorporates value engineering.

The eventual affordability for operators as the “best buy” (i.e., product value), in turn, will allow manufacturers to thrive. Design considerations should not impose difficulties in their manufacturability. The associated aerodynamic-shape and structural-design concepts facilitate parts fabrication, their assembly, enhanced interchangibility, and so forth. Bought-out items should be selected for efficient and cost-effective system integration that leads to better reliability and maintainability during the aircraft lifespan. Recent events have resulted in the additional constraints of cost-effectiveness and environmental issues, requiring increased attention. The issues of global sustainable-development and anti-terrorism require additional design considerations. The choice of materials from a recycling (i.e., disposal) perspective is an additional issue when the use of composites gains ground over metals.

17.3 Design for Manufacture and Assembly

The public domain proliferates with acronyms, such as DFM and DFA. These do not comprise a standalone concept; there is a relationship between design for manufacturing (DFM) and design for assembly (DFA) to meet the objective of lower production costs. In this book, fabrication and assembly are two components of the manufacturing process and are combined as DFM/A. Chart 17.1 shows the typical steps in DFM/A application.

DFM/A is concerned with the design synthesis of parts fabrication and assembly as an integral part of manufacturability. DFM/A analyses involve competition and risk – that is, balancing the trade-off between cost and performance. This eventually

ensures affordability for operators as the best buy. This multidisciplinary study searches for aerodynamic mould lines with surface-smoothness requirements (i.e., tolerance specification) to minimize performance penalties without imposing difficulties in manufacturability. The associated structural-design concepts facilitate parts fabrication and assembly (i.e., low manhours and low parts count, as well as enhanced interchangeability). Bought-out items are selected for efficient and costeffective system integration leading to better reliability and maintainability during the aircraft’s operational lifespan. Based on an awareness of customer affordability and requirements, designing and manufacturing target costs are established, which measure the objectives of lower production costs, improved quality, and reduced manufacturing cycle times, while increasing the product value without sacrificing design integrity, safety, and established specifications.

As a complex product, an aircraft is constructed of myriad parts. Assembleability, as a measure of the relative ease of product assembly, plays a prominent role for produceability. Following are the main goals of DFM/A considerations, which reduce parts count and assembly time:

improvement of the efficiency of individual parts fabrication

improvement of the efficiency of assembly

improvement of product quality

improvement of the assembly-system profitability

improvement of the working environment within the assembly system

product’s usefulness in satisfying customer’s needs

relative importance of the needs being satisfied

availability of the product relative to when it is needed

best cost of ownership to the customer

17.4 Manufacturing Practices

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 productspecific 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 precisionpositioned 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.