The ultimate guide to DFA (Design For Assembly)

The Concept of Assembly

Assembly refers to the assembly of components into a product that can achieve the quality of a variety of products. The concept of assembly can be derived from the meaning of assembly:

a) Put the components together;

b) Realize the corresponding functions;

c) Achieve great quality of the product.

The most important thing in assembly is to achieve the product functions requirement and product quality requirement, not simply locking the screw or putting together the components. Therefore, assembly is a way to realize product function and product quality.

A product needs to go through the process of assembly before it can be manufactured and become a product. Products contain anywhere from a few to millions of parts. A stapler has dozens of parts, a cell phone has hundreds of parts, a car has tens of thousands of parts, and an airplane has more than a few million parts. Assembly is an important part of the product manufacturing process, and the assembly process has a great impact on product quality, product cost, product development cycle, and so on.

There are seven steps in the typical assembly process: 

Step-1  Place the base; The base of a part does not necessarily refer to the fixture bench, it could also be a part. For example, in the winding of the stator, relative to the enameled wire, the stator rack is a part serving as a base.
Step-2 Identify the parts; Identifying parts is also important in the assembly design process. For example, if the color of the O-ring and the parts are all black, it is difficult to recognize different parts in terms of the vision system.
3) Grab the parts; The difference between manual assembly and automated assembly needs to be considered. If the part is too soft, or too slippery, it’s difficult to grasp it for assembly.
4) Move the part to the assembly position; Parts are best assembled once in place with no interference along the way.
5) Adjust the parts to the correct position; Sometimes we need to consider adding guide features to assist the adjustment. 
6) The parts are fixed; The parts of each process are preferably fully constrained, and then proceed to the next process.
7) Inspection; This is the end of a process.

The process of manual assembly and automated assembly will be slightly different. But all assembly design requirements stem from these seven basic steps, so it’s important to remember.

Mechanical designers can generally understand the assembly process based on these seven steps, but assembly process engineers need to know more, such as the location of incoming materials, the arrangement and efficiency of fixture benches, and the specific assembly methods of production line workers. Therefore, after the mechanical engineer completes the DFA (Design for Assembly), it is necessary to discuss it with the assembly engineer.

The Difference Between A Good Assembly Design And A Bad Assembly Design

Good Assembly Process Bad Assembly Process
Parts are easy to recognize Parts are difficult to recognize
Parts are easy to grab and placed in the right position Parts are difficult to grab and easy to fall
Parts can be easily adjusted to the right position Parts need workers to keep adjusting in order to find the right position 
Parts have a single right assembly point Parts can be assembled at multiple assembly points and it’s difficult to tell which point is right. Parts can be even assembled at the wrong point. 
Very few screws. Parts can be assembled quickly. Many screws and many types of screws.
No tool or fixture is needed in the assembly process Tools or fixture is needed in the assembly process
Very few adjustment is needed in the assembly process Consistent adjustments are needed in the assembly process
The assembly process is easy to study execute The assembly process is difficult to study and execute

Design For Assembly (DFA)

Design for assembly (DFA) refers to making the product have good assembly ability in the product design stage, ensuring a simple assembly process, high assembly efficiency, high assembly quality, low assembly defect rate, and low assembly cost. The purpose of doing Design for assembly (DFA) is to figure out the best assembly process of every single component or part by optimizing the product design. 
Design for Assembly (DFA) should be considered at all stages of the design process, especially in the early stages of design. As the design team considers multiple options, the ease of assembly of the product or part needs to be carefully considered. Design teams need a DFA tool to efficiently analyze the ease of assembly of a product or part. Design tools should provide results quickly, and be simple and easy to operate. It shall ensure the coherence and completeness of the product assembly feasibility assessment. It should also eliminate subjective judgment in assembly design, allow free association, allow easy comparison of different designs, ensure scientific evaluation of the final solution, determine the scope of assembly problems, and be able to provide multiple alternatives to simplify product mechanical design, thereby reducing the cost of manufacturing and assembly. Through the application of DFA, the communication between manufacturing and design can be improved, and the ideas, reasoning, and decisions made during the product design process can be well documented for future reference.
The DFA approach attempts to achieve these goals in the following ways:
① Provide a tool for those designers or teams to ensure that their consideration of product complexity and assembly occurs only at the initial design stage. This can avoid the risk that the designer only focuses on the function of the product at the initial stage of design without sufficient consideration of the cost and competitiveness of the product.
② Guide designers or design teams to simplify products, thereby reducing assembly costs and parts costs.
③ Collect the experience data usually owned by experienced design engineers, organize these data, and provide them to inexperienced design engineers to study.

④ Establish a database that contains assembly time and cost elements under various design states and production conditions.

Through design for assembly (DFA), product development can achieve the following goals:

① Simplify the product assembly process.
②Reduce product assembly time.
③Reduce product assembly errors.
④Reduce product design modifications.
⑤Reduce product assembly costs.
⑥Reduce product assembly defect rate.
⑦Improve product assembly quality.
⑧Improve product assembly efficiency.
⑨Increase the utilization of existing equipment.


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