Engineering Design Process for Mechanical Engineers
How real mechanical designs progress from vague 'I want to make this' to fully fleshed out hardware. Focus on decisions, iteration, and real-world constraints that junior engineers face moving from theory into actual practice.
Most guides to the engineering design process paint a clean and linear picture. Reality is far from it. Design starts with a hazy understanding of requirements, making key decisions before all of the relevant information is available. It then iterates over time, gradually producing something that works, can be produced, and will actually stand up under use.
⚙️ What the Engineering Design Process Actually Is
The engineering design process is not something you complete and put in your trophy case. Instead it is an ongoing decision process that evolves with your project as you learn new things. While it is desirable to work on idealized problems where everything can be defined perfectly at the beginning, most real-world problems require revisiting prior decisions as new information comes to light. This information might come from a test not previously performed, a supplier that is unable to meet a specified requirement, or manufacturing realities that were previously unknown but are revealed only when prototype hardware is inspected.
Your design goal is not the "perfect design" that equals ideal performance in all characteristics. In fact, the better goal is for a design that works as needed, is practical to build, and safely withstands the conditions under which it will be used. There are always trade-offs required involving performance, cost, schedule, manufacturability, and reliability. What you want to do is make these decisions on a rational basis rather than by luck.
Key Insight: Iteration is your friend, not the enemy—it is the default mode for product design and iteration is not a sign of failure. Your initial design decisions will invariably be made with incomplete information, and subsequent testing, analysis, and supplier input will then be used to iterate on those initial decisions to arrive at the best final solution. Design Process is how you manage your ignorance before arriving at a solution.
🎯 Start With the Problem, Not the CAD Model
⚠️ Common Mistake: Opening CAD before understanding the problem you are trying to solve. Geometry can be seductive, looking very active in producing something which appears to be progress, but it is not until much later that you realize you have redesigned your solution to the wrong problem around the wrong parts—thus redesigning the same thing three times in total.
Before any sketches, answer these questions:
💡 Example: "Make it lighter" is worthless to tackle. However, "Reduce weight by 20% while maintaining current stiffness and keeping cost under $500 per unit at 10,000 units per year" is something that you can design to and is a guard rail against misdirected effort.
📋 Turn Requirements Into Engineering Targets
Now that we have the problem well articulated, let's turn it into some engineering targets before the scope creeps out of control and design goes off the rails.
One of the challenges of writing specification is having a response ready for the inevitable "why A not B?". However, the best response is simply to cite the requirements. They can also define what finished means, a question usually answered by schedule pressure rather than engineering judgment.
💡 Generate Concepts Before You Optimize Anything
⚡ Critical Point: The largest gains in design come from selecting the initial architecture rather than continuing to tweak a mediocre design.
A common pitfall that many engineers fall into is screening too few concepts and subsequently spending too much time optimizing a design that had little chance of success.
- Sketch multiple approaches—compare architectures, not tiny variations
- Evaluate different load paths, mechanisms, attachment methods
- Use rough calculations: free-body diagrams, envelope checks
- Check packaging conflicts, manufacturing feasibility early
- Assess standard part availability and major risks
Fast but accurate screening of design alternatives is more valuable than false precision in detailed FEA models. For example, we can quickly determine that a cantilever beam design will not work, but a supported beam design will, without ever creating a full FEA model. Therefore, it is beneficial to make a directionally correct design decision as quickly as possible, saving the high resolution FEA for once the design has been steered into a promising direction.
🔨 Build the First Feasible Design
Once we have a concept direction, we need to build the first actual design. This design should have rough sizing done, and the key load paths and interfaces defined out so that other components can start development. We select candidate materials and manufacturing strategies based on what is really available to us.
- Build and test rough prototypes
- Expose problems early
- Get concrete feedback
- Iterate based on learning
- Perfecting before testing
- Endless CAD tweaking
- Optimizing too early
- Assuming success
The first feasible design for your product is not the end of the story, but rather the beginning. You will gain far more insights from testing a simple but viable prototype than from refining many parameters in CAD software.
📊 Move From Concept to Detailed Design
As the design matures, convert rough layouts into controlled geometry. Lock key interfaces and critical dimensions so manufacturing can quote the job and other teams can design mating parts.
Gradually add geometric detail and firm up key interfaces and dimensions. Lock down the critical dimensions, so manufacturing can determine a quote for the part, and other parties can design for fit with your engineering's components.
Develop an understanding of what needs to be controlled most closely and what does not. Over-tolerating everything can often be expensive for no good reason. This is the time to define your design intent and make it usable for review. See Engineering Drawings for documentation standards. Tolerances & Fits explains how to specify control where it matters.
✅ Validate the Design Before You Trust It
⚠️ Validation is not the same as decoration, validation is how you find out that your design actually works before you waste money producing production tooling.
Tests can contradict your model. Often this means that your assumptions and/or your model were wrong. Investigate the cause of failure before moving on. See How Things Fail for some general principles on failure analysis.
🔁 Iterate Without Losing Control
Projects always involve some iteration. After testing you find problems. Production uncovers issues that were hiding in your CAD model. Suppliers can't supply components to the specifications you assumed they could. But uncontrolled iteration devolves into chaos.
- If test results change, update the information — Don't ignore the results!
- Make individual changes one at a time — Limit effects to single revision
- Re-check key assumptions — After meaningful revisions
- Stop Iterating — No impact on key decisions? Then stop.
If changing a fillet radius from 3mm to 3.5mm makes no difference to strength, cost or manufacture, stop worrying about it. There will come a point where you have to release the design to production.
⚠️ Common Failure Points in the Process
Most failures in the design process are predictable. Most failures in design come from following established patterns in process. Even though failures are predictable, most designers fail to notice them because the failures fit within an established paradigm that feels effective.
Your best chance to catch these mistakes is during the design process. Take advantage of it.
🏁 What "Done" Actually Means
"Ready enough?" I asked. Turns out, "ready enough" means different things at different times for different releases. And in the end, the test of readiness is always whether there's enough documentation, whether it's "ready enough" for end users. A prototype release is never "ready enough" for end users to read documentation for it, but a production release is always too early to be "ready enough" to release unless it has enough documentation.
✅ Quick Process Checklist for Junior Mechanical Engineers
💡 Use this checklist throughout your design process
Why these questions: 1. These questions force you to think design critically instead of just following motions. 2. People (especially management) can use these questions to understand your design decisions and you can use them to deflect questions with "why did you do it that way?"
Other Related Topics: Understanding Load Cases & Assumptions * Failure Modes * Design for Failure * Design for Manufacturing (DFM) while your design is still cost effective to make changes.