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Options in design with metals

Options in design with metals

Whether the choice is metals, ceramics, plastics or some other material, the decisions about part manufacture that product designers reach during Design for Manufacture and Assembly (DFMA)TM analyses are driven by cost and by functional requirements. Among competing materials, most metals still have distinct cost advantages; they are relatively inexpensive to purchase and to process. They also have a long product life, are widely available and are familiar to designers in many segments of industry.

The functional requirements of a design will also often dictate the use of metals. For example, polymers do not have temperature resistance over 500F. For gas turbine applications, where parts must meet temperatures approaching 2000F, designs are limited to high-temperature alloys or possibly ceramics. Once these limitations are recognized, a DFMA analysis will focus on achieving design efficiency and on optimizing the cost of manufacture.

Boothroyd-Dewhurst's DFM Concurrent Costing shows how various design features contribute to the cost of casting. Tooling cost is about 60% of total manufacturing cost. The biggest contributing factor was mold devices, in this case one side pull included in the mold to produce two small holes that alternatively could be drilled after casting.

A major tenet of DFMA is the promotion of product simplification by parts integration. If two or more components of an assembly do not move in relation to each other, it is to the manufacturer's advantage to combine them into a single part with a combination of the shape features of the separate parts. Some metal processing methods have the advantage of allowing a manufacturer to create a part with multiple features in only one processing step, thereby reducing assembly costs and possibly avoiding quality problems related to the interfacing of separate parts. This advantage, together with the potential for large production volumes at low cost, would direct a designer toward selecting metals.

Moreover, the capabilities of metals manufacturing processes have improved in recent years. Powder metal processing can, for example, allow the manufacture of composite geared parts as one piece, where previously each gear had to be formed separately. New casting techniques allow the design of thinner sections and parts with more features than previously possible.

Metal injection molding allows the manufacture of intricate shapes similar to those that can be molded in plastic. For example, United States Surgical Corporation uses metal injection molding in materials such as titanium and stainless steel to manufacture small parts for surgical devices. The alternatives for creating those parts might be forging or investment casting, but metal injection molding is the most economical for the production volumes required.

A stacked bar chart from DFM Concurrent Costing shows total part cost for different processes for a life volume of 100,000 and a batch size of 10,000 for a small brass connecting rod. The most expensive process is investment casting and the least expensive is powder metal.

Even time-tested metal processing techniques have been improved in order to promote parts integration and reduce assembly costs. For instance, investment casting, an established process in ancient Egypt, now uses centrifugal force and vacuum casting to readily create small parts with intricate details. American Industrial Castings manufactures very intricate, small investment-cast parts that can replace multi-part assemblies with single net-shape cast components at substantial savings in time and cost. The castings are able to produce a similar range of detail as one would find in injection molding.

Developments in machine tools over the past twenty years, such as advanced spindle and slideway systems, allow metals to be cut at very high speeds. New cutting tool materials--polycrystalline, ceramic, and coated carbide tools--can sustain those speeds. With the resulting reduction in machining cycle times, it becomes economically viable to start with a block of readily machinable metal such as an aluminum alloy and cut an integrated, complex part as a single unit. Even without the savings in assembly costs from integration, reducing manufacturing to a single process makes high-speed machining a cost-effective manufacturing choice for some parts. Thirty years ago, this just wasn't so.

At Boeing, a DFMA analysis performed on the pilot's instrument panel of the Apache AH-64A helicopter resulted in a recommended design that used high-speed machining to integrate the 74 parts of the original design into nine complex, machinable parts. As a result, fabrication time dropped from 305 hours to 20 hours, and total manufacturing and assembly time was reduced from 697 hours to 181 hours. These numbers show that an awareness of new metal processes can dramatically lower manufacturing costs.

The future may bring an even wider range of processing options to the designer. At present there are a number of methods for rapid prototyping for visualizing and performing limited testing on assembly designs. Although none of these is yet an economical substitute for standard manufacturing methods, they will certainly mature and we may begin to see them compete with traditional methods, particularly for small volume production.

The goal of DFMA analysis is to search for the best design for both the product and the production run without being restricted to a single material or process. Ironically, factors such as the designer's own expertise often restrict that search. For example, an engineer experienced in die casting will likely decide on a die-cast part, even if the design provided was sketched on the back of an envelope with no process specifications whatsoever. The existing in-house processing equipment or supply chain can also limit a designer's choice of materials and processes.

On the other hand, an open-minded examination of the available processes can optimize both design features and manufacturing costs. For example, a small brass connecting rod could be made by a variety of processes including die casting, automatic sand casting, investment casting, and forging or powdered metal compaction. Integrated costing tools are available today to assess the various volume runs of the product and determine the economic process for different volumes. Engineers can review feature cost-optimization as well.

The greater an engineer's exposure to advances in metals processing, the easier the task becomes of selecting a cost-effective process. The expertise is out there: in journals and conference proceedings, in consultant services and in the subcontractors and suppliers who have taken the risk to make new metalforming technologies practical and cost-effective. A wide knowledge base with up-to-date resources on metals processing technology will afford designers the liberty to concentrate on the primary design drivers in DFA and in any optimized product: function, simplicity, and cost.

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