NEW DESIGN FOR MANUFACTURING AND ASSEMBLY APPLICATION ON THE DESIGN OF ASH64D HELICOPTER
25 September 2006 - Boothroyd Dewhurst

Design for Manufacturing and Assembly is a design philosophy used by designers when a reduction in part count, a reduction in assembly time, or a simplification of subassemblies is desired. It can be used in any environment regardless of how complex the part is or how technologically advanced this environment may be. It is gaining popularity where manufacturing costs are a concern. DFMA encourages concurrent engineering during product design so that the product qualities reside with both designers and the other members of the developing team.

DFMA is utilized by hundreds of domestic and international companies in an effort to cut down concurrent manufacturing and assembly time. Domestic companies like Allied-Signal, Motorola, Hughes Aircraft, and McDonnell Douglas Corporation have already implemented the DFMA philosophy throughout their product lines. The DFMA implementation process may be done at two different stages: when a new design requirement is established or when an existing design requires product optimization, such as the case of the Longbow Apache Helicopter. At the initial design stage, the designer develops a simplistic conceptual design by envisioning an assembly that requires a minimum of parts to perform the requirements previously established, and is easy to install. In the second stage the designer redesigns existing assemblies or designs new assemblies in order to implement design optimizations to ease manufacturing, and installation. This also meets reliability and maintainability requirements, moving the design towards cost reduction and customer satisfaction. In order to maximize the benefits of DFMA the designer must have a good working knowledge of the manufacturing processes available, and process capabilities to produce the parts. The design and manufacturing elements must work closely to determine the best manufacturing approach. A review of the State-of-the art manufacturing processes which increase the effectiveness of DFMA provides a means to understand this synergism, as well as the availability of Statistical Process Capabilities.

High Speed Machining
HSM was the primary tool used by the DFMA process in the airframe structural design area. It can be defined as the act of machining at speeds higher than 10,000 RPM. A High Speed Machine is a manufacturing tool that, when used in a DFMA environment, allows part count reduction by providing the machining capability to rapidly create complex geometrical parts normally designed with many mechanically fastened sheetmetal parts.

HSM supports DFMA with the utilization of improved machine cutting spindle technology which has created spindle speeds of 24,000 to 40,000 RPM. It combines rough and finish machining of material into a single machine operation. It reduces heat buildup and thermal growth allowing stable machining operations and close tolerance features.

It has been so successful that it is currently used by McDonnell Douglas Aerospace on several on-going aircraft programs, including the F/A-18 E/F Avionics Shelf, and Aileron Closure Rib , the T-45 Nose Landing Gear Door, and C-17 Cargo Floor Ramp.

With its application, complex assemblies are converted into simple part assemblies reducing part count and simplifying the assembly process. Reducing parts count cascades into savings in other areas. For example, it reduces part cost, fabrication and assembly time, and reduces tool design and fabrication costs. The tooling manufacturing process can be eliminated since the designs are transferred directly from a computer aided design system model like Unigraphics II, to the high speed machine itself, representing a major saving. This is for the Numerical Control and the Coordinate Measuring Machine programming. Additionally, it improves quality, provides design flexibility and reduces weight due to the absence of fasteners.

Composite Design
Composite design helps in the assembly process since it can help in minimizing the number of parts by combining several parts into one. The assembly time is reduced benefiting manufacturing in a similar manner to HSM. However, this manufacturing process is labor intensive due to the time it takes to set all the plies that build up the thickness of a part. Each ply must be cut to shape, stacked up, and then bonded together with adhesive at a later time as it is cured in an autoclave. It also creates the need for expensive tooling, since tools are used to form and shape complex parts. Tool wear and tear is another problem encountered in the composite manufacturing process, caused by the frequency of tool usage.

Composites are more susceptible to damage and ply dilamination due to humidity, creating costly future repair problems, depending on where and how they are used.

Superplastic Forming
Superplastic forming is the process in which a specific type of aluminum alloy, aluminum 2004 for example, is formed by blowing hot air against a sheet of metal, and over a tool inside an oven in order to shape the metal into the part's configuration. An advantage is the low part rejection since there is virtually no tool wear. Complex parts are simplified by integrating auxiliary parts into one part. Compound curvature shapes are also made possible since metal is formed at its plastic point. However, SPF is limited to small parts due to oven size limitations.

DFMA has already proven to be a savings tool as used by McDonnell Douglas Corporation in the military and commercial divisions. In the military division, significant savings have been obtained in the F/A-18 C/D Bay 4R shelf; savings of 84% in the number of parts, 73% cost, 11% weight, and 89% assembly time. In the conversion of the F/A-18 C/D into F/A-18 E/F there was a part count reduction from 1,744 to 1,048. In the commercial division; the MD-11 cargo liner had cost savings per aircraft of $86,000, the MD-11 #2 Bulkhead had cost savings of $4,000. In general the part count reduction done by McDonnell Douglas has been in the range from 36% to 96% on several of its aircraft component assemblies.

In the development of the Avionics upgrade on the U.S. Army's A Model Apache Helicopter, McDonnell Douglas Helicopter Systems utilized all design and manufacturing tools available. DFMA was one of these tools. DFMA was used on the AH-64D helicopter (Longbow) Program along with three auxiliary tools: High Speed Machine, composite design, and Superplastic Forming. HSM and SPF were used in the airframe structure design and composites in the Environmental Control System.

http://www.dfma.com

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