Procedural Flow for Mechanical Design (Part 2)
For this series, I have created a PCB mock-up. Typically, this is provided by the electrical engineer designing the board. Most modern PCB design software like Altium has an automatic model export function which makes it easy to get a 3D model of the board. I put two connectors on it (C1 and C2) and two BGA chips (MCU and stepper drive). These features are important for design since the location of the interface and heat generated by the chips can heavily effect the enclosure.
Conceptual designs phase is meant to draft a couple top level designs that you can choose from. Some engineer likes to quickly draft their ideas on paper (back of the napkin sketches) while others likes to create some basic blocks in CAD software. There is no right or wrong choice but I normally chose the method according to the application.
One of the best book to reference is “Assembly Automation and Product Design (Manufacturing Engineering and Materials Processing” by Geoffrey Boothroyd (ISBN-13: 978-1574446432). It is an old book but has a lot of interesting and relevant information for today.
Things to Consider
During the design phase, there are couple of things to keep in mind which include:
- Design for Manufacturing
- Design for Assembly
Quantity is the most important thing you should determine first. The reason for this is that this number effects everything downstream. Economy of scale is one of the things an engineer should keep in mind. Whether the quantity is high (10K+) or low (below 10K) dictates the assembly and manufacturing process especially since the margin and economical viability can make or break a product. For instance, at low quantities, die casting makes very little sense since the die is usually very expensive. Therefore, alternative methods such machining or sheet metal stamping should be explored.
Material is important because it can effect how the sub-assembly is built and manufactured. For instance, if the application requires a specific plastic (such as medical industry), then sheet metal and die casting makes very little sense. Also, if it is plastic, methods of joining such as snap fits can be explored rather than using screws.
Design for Manufacturing
Design for manufacturing usually refers to how parts are made. One you know the quantity and material, it’s usually obvious which manufacturing method should be selected. Once you know the method, you should be aware of limitation of the process when you are designing. I have listed a couple common manufacturing method and limitation below.
- Undercuts should be avoided
- Organic shapes are complicated and expensive
- Higher reject rates at high quantity
- Draft angle is required
- Material can be limited
- The most faces that have features, the more expensive it’ll become (for instance, a three piece mold is required for features on multiple faces)
- Shrinkage of material makes tolerance hard to hold so multiple runs is required to tune the parameters on the machine and mold
- Material can be limited
- Post processing is usually required for important features
- Bad surface finish for most material
A good habit to get into when decided on different concepts is to create a matrix. As shown below, this a matrix to decide which process is appropriate.