software is an analysis tool that enables design teams to predict the cost effects of early design decisions on product configurations, manufacturing costs and, subsequently, their relation to production volumes. A major tenet of DFMA is the promotion of product simplification by parts integration.It is well known that once the overall size, material and manufacturing process for a particular product are selected, 70 to 80 percent of the part costs have been set. Additionally, the detailed design of the part must be carried out to suit the chosen process. For example, the features of a plastic injection-molded cover can be very different from those in a cover made by sheet-metal stamping, even though both covers fulfill the same functional requirements. Consequently, once the detailed part design has been carried out, it becomes increasingly difficult and costly to go back and transition to a more economic material and process combination.
We feel the cost prediction procedures in a DFMA analysis can give you realistic estimates of processing costs before detailed part design, optimizing the final material and process combination selection. Some processing methods have the advantage of allowing a manufacturer to create a part with multiple features in only one step, reducing costs and possibly avoiding quality problems related to the interfacing of separate parts. This type of evaluation is another major tenet of the DFMA process.
The overall goal of DFMA analysis is to search for the best design and manufacturing process for the product at the required production volumes-without being restricted to a single material or process. Open-minded examination of the available processes can optimize both design features and manufacturing costs.
Breaking It Down
Provided that the functional requirements of strength, wear resistance and so on are met, there are often several possible material and process combinations that may be suitable for a given part. Imagine that you are designing a steel combination spur gear part, as shown in the screenshot above, with an overall diameter of 1.5". You cannot readily make it as a single piece by machining because the conventional gear-cutting processes (hobbing, shaping, etc.) cannot produce the teeth on the smaller gear up to the step face of the larger gear. Instead, you must machine the part as two separate gears, and then assemble it by press fitting, or a similar process.
Alternatively, you could make the gear as a single piece by powder compaction and sintering. Since powder metal parts are inherently slightly porous, some small reduction in strength would occur, compared to a gear produced by machining from steel bar stock. Typically, this is less than a 20 percent reduction, depending on the sintered density. The initial tooling investment, however, would be much higher, so it is unlikely that the choice of powder metal would be economical for smaller overall quantities.
Another material/process combination for the lightly loaded gear is to use injection molding on a material like glass-filled nylon. If the part is molded to the proportions shown in the screen capture, the relatively thick sections would result in longer cycle times and possible quality problems due to solidification shrinkage.
Furthermore, modifying the design of the part to reduce the maximum section thickness would increase tooling costs because the die cavities would be more complex, but the benefit of an overall reduction in manufacturing times would be realized.
Putting It Together
Given all the aforementioned information about materials choices, we've designed DFMA to facilitate your final decision by calculating the effect of volume on overall costs for each method of production. A graph illustrates the comparison of the estimated overall costs of four different manufacturing choices that could include the following;
Making a combination gear by machining it as two separate gears assembled together. Producing the gear as a single piece by powder metal compaction and sintering. Two methods of gear production by injection molding-one from solid nylon, one from cored.Through the graph, it becomes clear that an overall quantity of less than 3,000 parts would make machining the most economic decision. However, above this quantity, the powder processing route is the least expensive. Additionally, once the production volume reaches around 40,000, injection molded gears become the least expensive. These estimates include the initial tooling costs that could be spread over the quantity being produced.
The cost-estimating operations available with DFMA analysis enable cost comparisons such as this for a wide range of material and process combinations. We feel this software can help design teams make the most appropriate processing choices before carrying out detailed design of the individual part.
Dr. Winston A. Knight is the vice president of Boothroyd Dewhurst Inc., a Wakefield, RI-based company focused on software tools and services that allow design teams to develop products, while managing costs.