Design For Assembly

DFA

As with all parts of design, the design for assembly (DFA) must not be overlooked.  It’s not just clips, screws and adhesive.  Clips have to be very well thought out, screws must fit into bosses/holes and fail-safing must be used to stop components being assembled in an incorrect way.  Of course there is automated assemble, where the control is in the code, setup and mechanisms.  However when working with humans the control you have is in the design of the components and product.

Poka-Yoke

To prevent human error within product assemble a fail safe method must be used. For example if you needed to clip a cable with a connector into another part, to ensure that the connector is plugged in the way, an extra feature or rib could be used so that the two parts can only be clipped together in one way.  This term of fail-safing is not just used in the manufacturing and assembly Industry but also so we the users put our SIM cards for our phones in the correct way.  So we can plug our USB devices in only one way.  Or even draw runners that stop you from pulling the draw all the way out.  This method of fail-safing is called Poka-yoke.  A Japanese term first coined by Toyota’s manufacturing system. 

http://www.celmar.nl/admin/images/uploads/16/Poka_yoke_Sim_card.jpg

Clips, screws, adhesives and fasteners

As a basic rule, the less components, generally, but not always, the less time it takes to assemble.  The more components that make a product, the more parts to attach together. With plastic parts clipping and screwing them together is often used. 

Screws

• Are an extra component and cost.
• Take time to be put in.
• Must be the correct screw to use and put in correctly.  (Not at an angle, and to the correct torque).
• Easy to disassemble.

Clips

• Molded into component most often.
• Quality of the clip is controlled mostly be the designer and molding process.
• Not always easy to disassemble and can sometimes snap off.

The decision of whether to use screw or clips is a difficult one.  From the view of production clips means less components, most likely less time to assemble and less equipment needed.  Although it may be more difficult to rescue parts if a product is faulty/damaged and needs to disassembled to reuse the good parts.  Sometimes the use of both can be a good combination.

Assembly

A designer can tell how difficult an assembly process is by looking at the faces of assembly workers when they are assembling a product.  If they look as if they are struggling, there is your difficult part of the assembly process.  If there are multiple workers assembling the product look at which process is the bottleneck, which workers are standing around waiting for the others and who is constantly working to build for the next process.  By watching your assembly workers you can see the flaws in the design.  If you don’t have access to watching your assembly workers then you can try to assemble the product yourself.  Whether it is with real molded parts or 3D printed parts you will be able to get an idea of the difficult parts and processes.  Doing this can also make you think how you will assemble the product  and how you will want to break up the processes in jobs if multiple people are assembling the product.

 http://www.maclife.com/files/u220903/iPhone_factory_workers_380px.jpg

Hopefully this gives you a brief overview of DFA with a bias on plastic components.

Coming soon will be a series on Injection molding tooling design!