Design for Manufacturing Methods Every Engineer Should Know

Designing a product that moves efficiently from concept to production requires more than just a good idea, it demands an in-depth understanding of how each component will be manufactured and assembled. At Epec, we work with engineers across various industries to optimize designs for manufacturability, whether it’s a custom cable assembly, rigid-flex PCB, battery pack, or fully integrated user interface.

This blog post explores Design for Manufacturing (DFM) and Design for Assembly (DFA) techniques that not only improve product performance but also reduce production costs and lead times. By applying these principles early in the design phase, our customers are able to create smarter, more cost-effective designs, especially when dealing with complex assemblies like overmolded cables, multi-layer flex circuits, or battery packs with embedded sensors.

What is Design for Manufacturing?

Design for manufacturing techniques are used to improve the efficiency of a design at the component level. The techniques are based around the question: How will each of the components of a design be manufactured?

Typically, the cost of components accounts for a large percentage of the total cost to build a product. Design for Manufacturing techniques are used to reduce the complexity of components, lowering the cost to manufacture them.

Here is a list of Design for Manufacturing techniques that engineers use when designing components in an assembly:

Use standard off-the-shelf components: Standard off-the-shelf components are much cheaper than custom-made components.
Use Standard Materials: Using ‘rare’ or expensive materials for components can significantly increase the cost of those components and can lead to long lead times due to limited material availability.
Use Easily Processable Materials: Materials that require special machines or tools to manufacture can significantly impact component cost.
Design Components with the Manufacturing Process in Mind: Every manufacturing process has different limitations and requirements. For example, injection molded parts typically require a few degrees of draft to allow them to release from their mold.
Simplify Components as Much as Possible: Component complexity has a significant impact on component cost. Remove unnecessary features on components to reduce component cost.
Specify Loose Tolerances when Possible: Tight tolerances on a part will increase manufacturing costs for the component and lead to more fallout during manufacturing.

Designing this bezel involved considering the tolerances and assembly of subcomponents such as keys.

Considering each of these techniques when creating a new design or modifying an existing design helps engineers avoid high component costs in their designs. However, these techniques are not all that goes into designing an efficient assembly because they do not consider what happens to the parts after they are manufactured.

The other critical factor in the cost of a design is the assembly process, where all the components are combined to create the final product. In addition to Design for Manufacturing techniques, engineers use Design for Assembly techniques to develop designs that are efficient to assemble.

What is Design for Assembly?

Design for assembly techniques are used to reduce assembly, repair, and rework times, lowering labor costs and delivery times. The techniques are based on the question: How will each component of a design be combined to create the assembly?

The cost of assembly accounts for another large percentage of the total cost to build a product. Design for Assembly techniques are used to reduce the complexity of the assembly process, lowering the time required for assembly, therefore lowering the total production cost.

Here is a list of Design for Assembly techniques that engineers use when developing a design:

Minimize the number of parts: The total number of parts in an assembly has a direct impact on both assembly time and total component cost.
1. Combine parts
2. Reduce or eliminate fasteners
3. Remove unnecessary components
Minimize the number of unique parts: Reducing the number of unique parts can reduce over-ordering due to MOQs, allow for bulk purchasing, reduce required tooling, reduce required storage space, and reduce the number of orders/part numbers to manage.
1. Use the same fasteners
2. Use mirrored components

Keys can use the same mold but change only the artwork.

Mistake-proof similar parts: Similar components should be easily distinguishable from each other. If components are too similar, they may be mistaken for each other, creating the potential for mistakes during assembly.
1. Use unique and distinguishable features: Color, size, shape, etc.
Use symmetrical components: Design parts with symmetry to reduce the number of incorrect orientations. This reduces the time required for an operator to orient the part during the assembly process.
Include orientation features on parts: When there are asymmetrical parts, make the asymmetry more obvious to simplify the orientation of the part.
Include self-locating features between parts: Include features for aligning parts to each other, reducing the need for an assembler to precisely align components.
Avoid blind holes for placement: Avoid blind placement inside holes in an assembly. Alignment features on a part should be visible during assembly.
Include features for gripping parts: Small or flat parts should have features that allow the part to be picked up more easily. Completely flat parts are very difficult to pick up off a flat surface.
Design to Prevent Nesting: Certain parts can become nested, making it difficult to separate them. Use design features that prevent nesting from occurring.
Design to Prevent Tangling: Avoid features on parts that allow them to get tangled with each other.

Considering each of these techniques when working on a design helps engineers avoid high assembly times for their designs. Once an engineer has created a design using these techniques, the design must be evaluated to confirm its efficiency. There are several methods that engineers use to evaluate a given design, some of which will be discussed next.

Design Evaluation Methods

The most obvious evaluation method is to do a comprehensive cost analysis of a design. This means estimating the cost of purchasing each component and estimating the cost of labor to assemble the design.

This evaluation method is the most accurate, but it is also a very time-consuming and therefore expensive process. This has led many engineers to consider other methods for evaluating designs that simplify the process by reducing the time and cost of evaluation. Many design firms will still conduct a complete cost analysis of a design towards the end of development when there is only one or two designs to evaluate, however, other firms choose to eliminate this process entirely.

Two of the most common methods used today are the Boothroyd-Dewhurst and Lucas methods. This blog will focus on these two methods, but some of the other methods include: the Assembly Evaluation Method (AEM by Hitachi), the GE Hitachi method, and the Westinghouse method.

Both the Boothroyd-Dewhurst and Lucas methods begin by evaluating each component in a design and determining if it is necessary or not.

The Boothroyd-Dewhurst method asks 5 questions:

Do the parts need to move relative to each other?
Do the parts need to be electrically or thermally insulated?
Do the parts need to be made from different materials?
Would combining any two parts prevent the assembly of other parts?
Does removing or combining the parts negatively affect the servicing of the assembly?

If the answer to all the questions is “no” for a given part, it is deemed unnecessary and should be removed from the assembly or combined with another part in the assembly. The Lucas method simply asks the engineer to decide if a part is essential for the product's function or not.

In both cases, this component analysis is used to define the ‘ideal’ design, which has only essential parts. This ‘ideal’ design is a theoretical design that uses the minimum number of possible parts in an assembly. Typically, this design is not the most effective because it does not consider the manufacturability of the components. Usually, engineers look for a middle ground between the original design and the ‘ideal’ design that effectively balances the manufacturing cost of each component and the assembly cost of placing each component.

After the component evaluation step, the two methods differ in their analysis. The Boothroyd-Dewhurst method evaluates a design based on assembly times for each component. It provides tables for estimating assembly times for different types of components, but these tables can be supplemented or replaced with real, known assembly times. The Lucas method is more abstract and uses a series of tables to evaluate each part in an assembly based on how easy it is to handle and how easy it is to fit into an assembly.

Boothroyd-Dewhurst Method

Boothroyd Dewhurst uses tables to estimate the assembly time of a design. For each part in a design, an assembly time is determined using a series of tables. The tables are broken down into handling times and insertion times.

Handling times consider part size, part symmetry, part flexibility, number of hands needed to handle the part, if grasping or optical tools are required to handle and see the part, and/or if multiple people or mechanical assistance is required to handle the part. Insertion times consider if the part is secured in place or not, if the part needs to be held in place during insertion, if the part has obstructions during insertion, if there is resistance during insertion, and/or if the part requires other operations such as screwing or riveting to be held in place.

After following the charts to determine handling and insertion time, the values can be added together to get the assembly time for each component. Alternatively, the values from these tables can be replaced with known assembly times by conducting a time study. Once an assembly time is determined for each component, the times can be added together to estimate the total assembly time for a design.

In this method, the efficiency of the design is calculated using the following formula:

(Design Efficiency) = 3 * (# of Essential Parts) / (Total Assembly Time)

This efficiency helps engineers understand how their design choices affect assembly times and can be used to compare the efficiency of two or more potential designs.

Lucas Method

The Lucas method starts by calculating the general efficiency or “functional efficiency” of a design based on the number of parts deemed necessary and unnecessary. The functional efficiency of the design can be calculated with the following formula:

(Functional Efficiency) = (# of necessary parts) / (Total # of parts)

Functional efficiency is used to eliminate grossly inefficient designs right off the bat, saving engineering time. Typically, engineers aim for a functional efficiency of 60% or higher to allow a design to move to the next stage of analysis.

Instead of estimating assembly times like the Boothroyd-Dewhurst method, the Lucas method evaluates parts using a ‘Handling Index’ and a ‘Fitting Index’. Each index is calculated for every component in a design using a table of values. The tables are based on how difficult it is to handle a part and how difficult it is to fit a part into the assembly. In general, engineers aim for the handling and fitting indexes of a given part to each be 1.5 or lower. If a component is determined to have a handling or fitting index greater than 1.5, a redesign of the part should be considered. The overall efficiency of designs can be compared using the handling and fitting ratios of the design.

These ratios can be calculated using the following formula:

(Handling/Fitting Ratio) = (Sum of Handling/Fitting Ratios for Each Component in a Design) / (# of Necessary Parts)

Comparing these ratios between the two given designs gives engineers an idea of which design will be easier to assemble. The larger the value of either ratio, means the design is less efficient.

Summary

When starting a new design, consider all the DFM and DFA techniques to try and make the initial design as efficient as possible. Once an initial design or series of potential designs are created, follow one or more of the design evaluation methods to determine the efficiency of the designs.

If the engineer determines that the design is efficient enough, it can be finalized and put into production. Otherwise, the results of the design evaluation should be used to modify the design to improve efficiency. Repeat the design evaluation on the modified design to determine its efficiency. Continue to redesign and re-evaluate until a satisfactory design is achieved. There is no set method for knowing when to stop the process, but each redesign is likely to have diminishing results. Once the efficiency of the redesigns starts to stagnate, that is generally a good place to stop.

It is important to understand that often DFM techniques and DFA techniques contradict each other. In these cases, the best option needs to be evaluated for a specific design. A simple example of this is adding an alignment feature to a part to make it easier to assemble. If the feature is included, the cost of manufacturing the part increases, but the time to assemble the part is reduced. The decision to add the feature can be determined by comparing the cost of adding the feature to the cost savings from reducing assembly time. Let’s say that adding the feature increases the manufacturing cost of the part by $0.50. If we assume the cost of labor is $20/hour, the feature will need to save a minute and a half on the assembly time to make up the cost difference. Carefully considering and evaluating each part in a design is the best way an engineer can improve the cost-effectiveness of a design.

Key Takeaways

Early DFM/DFA Integration Saves Time and Cost: Applying Design for Manufacturing (DFM) and Design for Assembly (DFA) techniques early in the design process significantly reduces production costs, shortens lead times, and minimizes the risk of rework, especially in complex assemblies like overmolded cables or multi-layer flex circuits.
Simplify Parts to Improve Manufacturability: Using standard materials, off-the-shelf components, loose tolerances, and simpler geometries make parts cheaper and easier to produce. Designing with the intended manufacturing process in mind helps reduce complexity and avoid costly revisions.
Design with Assembly in Mind: DFA principles like reducing part count, using symmetrical and self-locating features, and minimizing fasteners streamline the assembly process, reduce errors, and lower labor costs.
Evaluate Designs with Proven Methods: Tools like the Boothroyd-Dewhurst and Lucas methods help assess design efficiency by analyzing part necessity, handling difficulty, and assembly time. These evaluations guide improvements and ensure an optimal balance between manufacturing and assembly efficiency.
Balance Tradeoffs Between DFM and DFA: Manufacturing and assembly goals can sometimes conflict. Engineers must weigh the benefits of added features (e.g., alignment tabs) against their manufacturing costs to achieve the most cost-effective and practical design outcome.