Design Process (Part 1)

Design Process (Part 1)

Purpose

This write-up is part of a series (1 out of 6). The purpose of this series is to show and explain the procedure flow for a Mechanical Engineer. For this series, I will be focusing on how to design an enclosure for a stepper motor control board. As trivial this might sound, there are a lot details that must be considered. The most important advice to always keep in mind, is that hardware is manufactured for keeps. Typically, you have only one shot and any changes or defects downstream can be really expensive, enough where it can kill a product. Therefore, make sure use all the tools within your repertoire.

This series will also go over how Computational Fluid Dynamics (CFD) and Finite Elements Analysis (FEA) to reduce the iterations time for prototypes and to catch any design mistakes. For this particular series, I will not go over how to setup the boundary condition but I will be discussing the results from the simulations to modify the design appropriately. There will be another series in the future to teach you the in and outs of CFD and FEA and how to validate your design and develop an intuition.

This series will cover the following:

  1. Defining the Design Scope
  2. Conceptual Designs (DFM and DFA)
  3. Detailed Design
  4. Using FEA and CFD to Improve the Design
  5. Rapid Prototyping
  6. Drawings and GD&T

Design Scope

It is very important to determine the scope of the project. Typically, the mechanical engineer will wear the project engineer hat for this particular part. By defining the design scope clearly upfront, you can mitigate the dreaded scope creep and also mitigate the risk of any miscommunication. Once the scope is defined, the respective team-lead should sign off on it to finalize the document.

Design Scope for Enclosure

This is my example of a design scope for this stepper motor driver enclosure. This particular example was created in Lucid Charts. Notice the electrical interfaces and how it can tie into the electrical schematic (such as connector C1). This creates a consistency throughout all teams and allows an engineer to see the deliverable of the project.

Block Diagram

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Design Block Diagram

You can define the design scope with different methods such as a formal word document. However, I personally believe a design block diagram is the clearest and simplest method.

To create a design block diagram, the best software to use is Microsoft Visio. The functionality and ease of use is the best out of all the ones I have tried. Representing the design thoughts and defining the sub-assembly is simple with a couple clicks. However, the biggest problem is that it is very expensive. Therefore, many managers and companies are reluctant to provide this software.

Free alternatives include Draw.io and Lucid Charts. I used to use draw.io for most of design block diagrams but I have noticed that it has been behaving unpredictably these past year which makes the whole process really frustrating (but it’s free!). Lucid Chart is better in terms of stability but some of the functions are paid to use. What you decide using depends on your budget and the time you want to spend playing with the software to make it work.

How to Create a Design Block Diagram

Creating a good block diagram takes some practice. If you keep some pointers in mind, it will make the whole design process faster and more comprehensive.

Define the sub-assemblies

Always define your sub-assemblies upfront. It might not always be possible to get it to 100% at first, but try your best to define the top level sub-assemblies you can think of. This is helpful for everyone since it is obvious what needs to be done for each section. This method also pairs well with CAD that have design tree capacilities such as Solidworks Design Treehouse

Example of Solidwork's Treehouse Source

List the Interfaces

Try to list all the mechanical and electrical interfaces. The interfaces are important because it is usually where most of the mistakes happen. Also, any mistake that happen within the interfaces are catastrophic because it can prevent all the sub-assemblies coming together. Also, do not forget about the electrical requirements. For instance, maybe for the electrical design, an enclosure ground is required. If this is the case, you have to be careful how you coat the enclosure. Processes such powder coating and anodizing can prevent you from getting a good contact for the ground which will require post processing such as sanding or using special mounting hardware such as star lock washers.