What is design for assembly?
Design for assembly is a product design approach where the designer focuses on ease of assembly for the product.
The goal is to simplify assembly through various methods, such as reducing parts used, thereby reducing costs and production time.
DFA complements design for manufacturing and should usually be performed in-depth first. The first priority is usually to find ways to make assembly easier (which means faster, better, and overall much cheaper). Then, an in-depth DFM analysis helps optimize the ways to make the needed components.
The classic example of the IBM Proprinter
These days, the high-volume consumer electronics and home appliances that are successfully “reshored” from Asia to North America and Western Europe have, for the most part, been re-engineered based on the DFA guidelines. That’s what allows for assembly in a high-wage country to be affordable.
We should look back at the IBM Proprinter for inspiration. That model was launched when Japanese printers were much cheaper, and yet it was assembled automatically in the USA (at a time when automated processes were much more limited than they are today) thanks to a great design:
How did DFA originate?
A common DFA approach was developed in the late 70s by Professors Geoffrey Boothroyd and Peter Dewhurst at the University of Massachusetts Amherst. This approach was based on the premise that reducing the number of parts used in a product would reduce assembly costs.
To simplify, they developed three criteria that demonstrate if a component could be removed from the product design or at least combined with something else:
- Is there relative motion between this part and all other parts already assembled?
- Must the part be made of a different material or be isolated from all other parts already assembled?
- Must the part be separated from all parts already assembled because necessary assembly or disassembly would otherwise be impossible?
These criteria would be teamed with tables relating assembly time to various design factors influencing part grasping, orientation and insertion, and could be used to estimate total assembly time and to rate the quality of a product design from an assembly viewpoint. For automatic assembly, tables of factors could be used to estimate the cost of automatic feeding and orienting and automatic insertion of the parts on an assembly machine. (Source).
What does a product designer focus on with DFA?
A product designer will attempt to achieve the following goals when following a design for assembly approach:
- Reduce the number of parts used. The criteria above are a common-sense approach here when looking at the complete list of parts. Is part X truly necessary to create this product, or can it be removed or combined with another?
- Pursue the use of standard ‘off-the-shelf’ parts or materials where possible. Custom parts increase quality risks and production errors. Using proven components, say, that already work in similar products makes assembly easier and cheaper as they do not need to be developed piece by piece.
- Only design products that can be assembled within the factory’s process capabilities and machine’s tolerances. Doing so should reduce waste and production time as the designer provides the factory with a product that they can manufacture without needing to adopt new or enhanced processes and that is well within tolerances (so minor variations won’t be detrimental to the product’s functionality).
- Assembly instructions can be provided for the operator to assure that they have the information required to avoid handling errors which will cost time. The designer will indicate the order to be used of the parts in the assembly process.
- Choose parts that are easy to handle for operators and can be easily fitted without the operator needing to take a long time to work out which way they fit, etc. By the way, that’s why products that are designed for automated assembly (DFAA) tend to also be very easy for manual workers.
- Design products with mistake-proofing in mind. For example, unique shaped parts or slots may be used that indicate how to fit the part to operators so they don’t make a mistake.
- Avoid fasteners that will take a long time to screw. For instance, instead of a nut and bolt, an adhesive could be used. (However, with the recent ‘right to repair‘ trend, the use of adhesive, should often be avoided if possible. Same thing for ultrasonic welding and other such assembly methods that make repairing much harder or impossible.)
Overall, these goals aim to reduce your costs, lead times, and help support the manufacturer’s productivity by making their lives easier. Easier work is also done at a much higher quality.