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Design for Assembly:
Countering the reappearance of old-school views on product design

By Brian Rapoza, Boothroyd Dewhurst

In the early 1980s, a product design methodology called Design for Assembly (DFA) began to gain popularity. It focused on achieving a high overall structural efficiency for a product by evaluating the amount of assembly labor required for manufacture. Since that time, the Design-for-Assembly methodology has been adopted with much success by more than 850 corporations.

Nonetheless, in the past 10 years there has been a significant resurgence in a different design philosophy -- a very old and outdated one -- that is skewing the productivity gains companies achieved through DFA.

This outdated philosophy promotes product-cost reduction through an isolated focus on the cost of individual parts within the product, rather than on the role a "system" of integrated parts plays on overall cost and performance.

Truth is, assembly-labor reduction continues to deliver larger savings and superior products when compared to engineering strategies that emphasize a narrow focus on part-cost reduction and single-part design.

The assembly, manufacturing, and overhead "cost pie"
Although the exact percentages can vary, sometimes pretty substantially, the overall cost breakdown of a typical manufactured product, based upon years of user surveys, appears much like the one shown in Fig. 1.

Figure 1: Cost breakdown of a typical manufactured product.

 

 

Here, the overwhelming majority of the manufactured product cost is tied up in materials and process-related outlays. The cost of direct assembly labor usually makes up the smallest proportion. Overhead is still significant, but that also represents a much smaller proportion of the total costs.

Design for Producibility vs. Design for Assembly
Around the early 1950s, an engineering practice called Design for Producibility (DFP) began to take hold. Design managers, seeing a steady increase in manufacturing costs, worked more formally with production to make designs better suit shop-floor capabilities of the day. This sort of approach would invariably simplify parts but create a much more complex total product structure through an increase in the number of parts required within the product. One such Design for Producibility guideline is shown in Fig. 2, where it is recommended that designers separate complex parts into a series of simple shapes and then join them together during final assembly.

Figure 2: Design for Producibility guideline published by General Electric in 1960.

 

 

Based on the typical breakdown of costs for a manufactured product shown in Fig. 1, the practice might seem to make some sense on its surface because it's the parts themselves that make up the majority of the total product cost, and the cost of assembly labor is often small. However, more than 30 years of experience shows that it is much more cost effective to produce the design that uses the single, more complex, multifunctional part than the series of simple shapes all joined together.

Perhaps what is less understood is that the majority of the cost reduction that results from using the more complex (functional) shape does not come specifically from a reduction in the assembly cost. The largest savings actually come most often from a reduction in the part manufacturing costs that result from the DFA exercise in assembly-labor reduction! This is because it is often less expensive to use modern manufacturing processes to make the single, more complex part than it is to make a greater number of simple parts.

However, the most easily recognizable characteristic that distinguishes between the two designs shown in Fig. 2 is the amount of assembly labor required. Dr. Geoffrey Boothroyd used this realization to develop the Design-for-Assembly methodology, which uses assembly-labor reduction as a guide to help designers develop more cost-effective products at their "global," integrated level -- as a holistic system of tightly interrelated functions accomplished using the fewest parts.

In order to illustrate what happens during redesign when the focus is correctly placed on the assembly labor, consider a thought experiment where the three-part product shown on the right side of Fig. 2 is redesigned to reduce the amount of assembly labor as much as possible. That goal would tend to lead toward the less expensive single-part design shown on the left, where no assembly labor is required. The broader outcome of using DFA for assembly-labor reduction is that a larger number of simple, single-function parts are combined into a smaller number of more complex, multi-functional parts. The redesign that results delivers a double cost reduction because the parts themselves are less expensive to manufacture and the cost of assembly labor is also reduced. This is a key point, often overlooked by industry in its rush to market.

To see what happens during redesign when the focus is placed exclusively on the cost of each part, consider yet a second thought experiment where the single-part product shown on the left side of Fig. 2 is redesigned to minimize the individual part costs. That goal could very easily lead the designer to break up the complicated, multifunctional part into a series of simple shapes so that the more expensive design shown on the right is produced.

Experience has shown that this is exactly what happens when designers focus on cost reduction of individual parts, especially when they have little accurate feedback on part manufacturing costs during initial design.

In the past 10 years, DFA experts, myself included, have noticed an increased interest in design methods that concentrate heavily on the exclusive cost reduction of individual parts within a product. Evidence that there is resurgence in this old-and-outdated design philosophy include:

• The opening of new consulting companies (many of which also sell part-costing software) that promote a narrow methodology that treats the cost of individual parts as the sole savings method. They mistakenly view that section of the cost "pie" (Fig. 1) as if it were removed from the influence of part-integration approaches provided through both design and modern manufacturing. Their insistence on this separate-part design practice is happening even as 3D printing and additive manufacturing (AM) companies, along with advanced casting/molding/machining manufacturers, demonstrate the advantages offered by more complex, unified part geometries.
• University-level coursework is currently being taught based on this outdated design philosophy, and university professors have stated that their curriculums will be "modernized" with this approach. Class projects focus on part costing and cost reduction centered on individual parts. For example, the professor might take apart a bicycle in front of the class and ask for cost-reduction ideas, one part at a time. With many engineering schools receiving the latest software for integrated digital design, manufacturing, and shape optimization, shouldn't there also be a more interdisciplinary-driven interest in DFA product simplification and costing?
• Newer users of Design for Manufacture and Assembly (DFMA) commonly focus on part costing as an up-front product design tool instead of first focusing on Design for Assembly to simplify product structures. For example, I recently spoke with an overseas manufacturer of sewing machines and they were fairly close-minded about Design for Assembly, opting for Design for Manufacture (DFM) instead. All of their assembly work is done in China at a very low hourly labor rate, so the direct cost of assembly is insignificant to them. As a result, they want to focus on the parts because they believe that is where the costs reside.
• Newer projects with Boothroyd Dewhurst's customers reveal that they, too, are attracted first and foremost on reduction of part costs, even when the projects are called "Design-for-Assembly projects." This might be a credit to the ease of using DFM Concurrent Costing software, but it shows a disengagement by design from system-wide cost strategies that only they can implement within the organization.

This reappearance of the outdated design method that focuses on the cost of the parts is likely due to a lack of knowledge and understanding about what Design for Assembly actually does. In the 1990s, DFA knowledge was somewhat common, but over time the engineers and designers with that knowledge have moved on or retired, and the education of their younger replacements has become fragmented. The problem may also be indicative of how far companies have yet to go in their efforts to work concurrently with other disciplines -- manufacturing, purchasing, etc. -- that can provide valuable feedback on total cost and offer best-in-class strategies.

A lamp assembly and the "big picture" of product requirements
As an example of how DFA can lower cost and improve functionality, consider this redesign of an original rear lamp assembly, as shown in Fig. 3. This product mounts to the rear of a piece of heavy equipment and is made from 37 individual parts. There is a lamp bracket made from a mounting block that is welded to a cantilevered mounting plate. There are four rubber isolation mounts that are secured to the lamp bracket with nuts and washers. A lamp "guard" is then mounted to the other end of the four rubber isolation mounts, also with nuts and washers. The lamp and pivot bracket are then mounted inside the lamp guard with a screw and washer. Many of the parts are made from 1/4-in.-thick mild steel, and the total weight of this assembly is just about 18 lb. This design has been in production for more than 20 years.

Figure 3: Rear lamp assembly -- original design.

 

 

The Boothroyd Dewhurst DFA software was used to derive an estimate of 13 minutes to assemble this original design, including the time for all of the welding and weld dressing required. The DFM Concurrent Costing software was also employed to estimate the manufacturing cost for all of the individual parts within this design. The total cost to manufacture, assemble, and install this design of the lamp assembly was estimated to be $88. Feedback received from the manufacturer indicates that the estimate is within 10 percent of the price they pay for the product. So the DFMA estimate of total cost for the original design is reasonable.

The company identified this original design as a possibility for cost reduction and redesigned this product in order to reduce its cost. The cost-reduced redesign that resulted is shown in Fig. 4.

Figure 4: Rear lamp assembly -- cost-reduced redesign.

 

 

In order to generate this cost-reduced redesign, the lamp guard and the rubber isolation mounts for the lamp have all been removed from the product by the manufacturer. The lamp bracket used in this design is now made from just 1/8-in. mild steel instead of the thicker material used in the previous design. Each of the parts in this cost-reduced redesign still serve only a single function, and the functionality of the lamp bracket in the design is certainly debatable. The DFMA software was used to estimate that this redesigned lamp assembly costs $42 to manufacture, assemble, and install on the equipment. Feedback received from the manufacturer indicates that estimate is again within 10 percent of what the company pays for this design of the lamp assembly, so the estimate is also a fair benchmark.

Clearly, the design philosophy this company has taken in regard to their lamp assembly is directed at the cost of the individual parts rather than on the efficiency of the whole product as a system of integrated components. In order to satisfy their target, the company has removed the functionality of the lamp guard and the rubber lamp mounts. The remaining parts have been cheapened through a reduction in their thickness and their strength. The practice of functionality removal to save money is quite unusual today because, normally, functionality in a product is fixed by the final customer's expectations or by the functionality of similarly classed products from competitors.

However, it is very common for design teams or cost-reduction teams to cheapen parts within a product through strategies like reducing their thickness and strength, changing to less expensive but less effective materials, and sacrificing aspects of the product's performance to gain a cost reduction. Each of these approaches to achieving savings is tied to a single-part/single-cost view of the product -- and each results in an outcome that the end customer will see as compromised and of lower overall quality.

The original design of the lamp assembly is located on the vehicle, just above the transmission cover casting as shown in Fig. 5. That is the optimal location for the lamp because it is unlikely to suffer an impact significant enough to cause damage, and it can provide light where needed on the rear of the equipment. This situation presents an opportunity to use Design-for-Assembly principles to reduce cost through the integration of the lamp mounting bracket and lamp guard into the transmission cover casting. This DFA redesign changes the simple, single-function transmission cover into the multifunctional part shown in Fig. 6.

Figure 5: Location of rear lamp assembly original design on vehicle.

 

 

Figure 6: DFA redesign of lamp assembly.

 

 

In order to determine the portion of the cost for the multifunctional part that is attributed to the lamp assembly, the cost of the original, single-function transmission cover was estimated and then subtracted from the estimate for the redesigned cover that includes the integral lamp guard. This results in a cost estimate of $48 for the DFA redesign of the lamp assembly and includes the amortized cost of the pattern and core box required to make the aluminum sand casting. The estimate for the DFA redesign also includes a cost of just over $4 for the rubber isolation mount and associated hardware to mount the lamp inside the lamp guard.

Discussion of case study results
The single-part/single-cost engineering method that the manufacturer initially used in redesigning the lamp assembly led to a noteworthy 52.3 percent cost reduction. However, that reduction was generated at the expense of the functionality and usefulness that the end customer pays for when they buy the lamp assembly. This illustrates the largest problem associated with such a design philosophy: It leaves the design team with few options to satisfy cost-reduction goals other than to sacrifice the very things that the customer wants to buy, such as the functionality, durability, and performance of the final product.

If this design method were applied to redesign the remainder of the vehicle, it is very likely that the end customer would find less value in the vehicle and make their purchase from a competitor instead.

The Design-for-Assembly method, which concentrated on finding assembly labor efficiency as a core strategy, resulted in a very similar 45.5 percent cost reduction. Yet, it sacrificed none of the original design's functionality. In fact, a knowledgeable customer is likely to view the DFA redesign as an improvement in the product rather than a cost reduction because the DFA redesign is likely to handle vibration better than the original. It will tend to be more reliable during use due to the robust lamp guard integrated into the casting.

However, the DFA redesign in this case does cost $6, or 14.3 percent, more than the manufacturer's cost-reduced redesign. More than two-thirds of that difference can be attributed to the $4 needed to rubber-mount the lamp, a feature that was removed during the OEM-driven redesign in order to save money.

Upon presentation of this DFA redesign to the company, the designer said that he would never take this approach with the design because the redesigned transmission cover casting would cost too much to produce and would require a core during manufacture. These statements from the designer really demonstrate this resurgence that is occurring right now in the old design philosophy that focuses on the cost of the parts in a product, even to the point that the functionality and usefulness that the customer buys is removed in order to save money.

While it is true that the multifunctional casting used in the DFA redesign costs more to produce than the original transmission cover casting, the multifunctional casting also eliminates the need for a separate lamp bracket and lamp guard. When all of this is considered, the DFA redesign is quite cost competitive, especially if the functionality and usefulness of the design are taken into account.

The designer at this company also indicated that it would be much more cost effective to add a guard and a rubber isolation mount to the cost-reduced redesign. This option was investigated, and the DFMA software determined that this design would cost just over $60 to manufacture, assemble, and install. This means that the least expensive option that incorporates all of the original design's functionality is the DFA redesign that utilizes the multifunctional transmission cover/lamp guard casting to minimize the amount of assembly labor required!

It should be noted that this feedback we received from the designer did occur before the designer saw the cost estimates for each design alternative. After the cost estimates were disclosed, the managers at the company thought it was well worth the extra $6 to provide the customer with a guard and a rubber-mounted lamp.

Figure 7: Total cost to manufacture, assemble, and install each design alternative.

 

 

Conclusions
The methodology adopted during product redesign and cost-reduction efforts will largely determine the type of product design that results. The old and outdated redesign methodology that focuses on the cost of single parts means that product designs will be composed primarily of single-function parts that are all joined together. Cost-reduction goals will then be met through the use of short-term tactics, such as the elimination of functionality or features from the product, the lessening of strength and factor-of-safety margins associated with choosing lower performing materials, and a reduction in the usefulness and durability of the final product.

All of these approaches present the end customer with a compromised design. For that reason, the redesign "default" methodology that focuses only on the individual cost of each of the parts -- rather than on the "system" of parts that form sub-assemblies and whole products -- is not sustainable from a business perspective. When the former approach is carried out on a large proportion of a product, then the customer will leave and buy a competitor's product instead.

The Design-for-Assembly redesign methodology that focuses on assembly-labor reduction means that product designs will be composed of a smaller number of multifunctional parts. Cost-reduction goals will be met through the simplification of the product structure -- and that results in products that the customer doesn't view as "cost reduced." This is because, in most cases, the products that are redesigned with this methodology not only generate cost reductions, but they also generate improvements in performance, durability, and reliability. Thirty years of experience proves that Design for Assembly is a superior method to generate cost reductions without sacrifices to the product that the customer really wants to purchase.

Learn more at www.dfma.com.

Published July 2016

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