1 · Start with the idea
Design is a sequence of decisions, not a single clever shape. Think of planning a journey: first establish where you must arrive and what limits you face, then compare routes, travel, check progress and adjust. CAD represents a proposed solution; it does not define the problem for you.
2 · Why it matters in real machines
A strong component can still be a poor design if it cannot be installed, inspected, manufactured or maintained. A structured process keeps those constraints visible before time and money are committed. The output is both a physical idea and the evidence needed to decide whether it meets the need.
3 · Core concepts
Requirements → concept → analysis → prototype → test → iterate
- Requirements: define function, interfaces, environment, hazards and measurable acceptance criteria.
- Concept: compare more than one way to provide the function.
- Analysis: model load paths, relevant failure modes, manufacturing and lifecycle constraints.
- Prototype: answer specific questions with an appropriate model. A cardboard fit mock-up answers an access question, not a strength question.
- Test: follow an approved plan with instruments, conditions, acceptance criteria and records.
- Iterate: investigate differences, change the controlled design and repeat the affected checks.
Write requirements you can test
Replace “easy to assemble” with an observable interface or task. A useful statement has an identifier, a measurable requirement, its basis, a verification method and an owner. Separate mandatory pass/fail constraints from preferences. Do not hide an unknown requirement inside a guessed safety factor.
| ID | Requirement or input | Verification approach |
|---|---|---|
| R1 | Evaluate a static, centered tensile force of 1.00 kN [224.8 lbf]. | Document free-body diagram and load direction. |
| R2 | Use a 40 mm [1.575 in] wide, 6 mm [0.236 in] thick plate with a 10 mm [0.394 in] hole in the preliminary model. | Check the model against the stated geometry. |
| R3 | Document every unresolved lifting-specific requirement before any release. | Review record; release blocked while required checks remain open. |
Decision matrix: make the trade-offs visible
First reject concepts that fail mandatory requirements. Then compare feasible candidates using agreed weights and a consistent scale. In this classroom matrix, 1 means poor and 5 means strong. Scores are illustrative judgments, not measured product performance.
| Criterion | Weight | Flat plate concept A | Fabricated concept B |
|---|---|---|---|
| Manufacturing simplicity | 0.40 | 5 | 3 |
| Inspection access | 0.35 | 4 | 3 |
| Packaging flexibility | 0.25 | 2 | 5 |
| Weighted score Σ(weight × score) | 1.00 | 3.90 | 3.50 |
A ranks higher for these choices. That does not establish safety or capacity. If small, reasonable changes to the weights reverse the ranking, gather better evidence before committing.
Use reviews as decision gates
A requirements review checks that the problem is understood. A concept review checks alternatives and load paths. A detailed-design review checks calculations, interfaces, manufacturing and unresolved risks. A release review checks that required evidence exists and that open items have been dispositioned by authorized people. Record actions, responsibility and the evidence that closes each action.
4 · Worked example
Simple lifting-bracket study—analysis only. Model a flat plate loaded in its plane through one centered hole. Use the R1/R2 geometry. The example develops preliminary average stresses; it is not a lifting-device design, fabrication instruction or rated capacity.
For a plate width b = 40 mm, thickness t = 6 mm and hole diameter d = 10 mm, a section passing through the hole has a simple net area:
Anet = (b − d)t = (40 − 10) × 6 = 180 mm² [0.279 in²]
σnet,avg = F/Anet = 1000 N / 180 mm²
σnet,avg = 5.56 MPa [806 psi]
A simplified projected hole-bearing area is d × t:
Ab = dt = 10 × 6 = 60 mm² [0.0930 in²]
pavg = F/(dt) = 1000/60
pavg = 16.67 MPa [2417 psi]
Both come from average stress = force / area. The projected bearing area is not the curved hole surface area, and actual contact pressure is not uniform. At twice the force both average stresses double; this is a useful arithmetic check, not an overload-test instruction.
Decision: the calculations identify two demands, not an allowable load. Material grade/condition, edge distance, pin fit and strength, stress concentration, tear-out, bending from eccentricity, fatigue, connection details, dynamic loading and the applicable lifting requirements remain unresolved [VERIFY]. Do not choose a working load limit from these two values. Any real lifting device requires qualified design review and the applicable inspection/testing process.
5 · Common mistakes
- Starting detailed CAD before agreeing on requirements.
- Giving a concept a high weighted score despite failure of a mandatory constraint.
- Treating a successful fit prototype as a successful strength test.
- Calling average stress the peak stress near a hole.
- Changing a dimension without revisiting affected checks and drawings.
6 · In the field
A design review is stronger when it includes someone who will assemble, inspect or maintain the equipment. Ask what must be reached, measured, replaced and recorded. Keep observations generic in a portfolio and separate a teaching reconstruction from verified workplace evidence.
Lifting, guarding and stored-energy topics here are awareness-level. Follow employer procedures, obtain proper training and use qualified supervision; do not build or load-test a lifting bracket from this lesson.
7 · Quick checks
Does the highest matrix score automatically win?
No. Mandatory constraints come first, and uncertain scores may need more evidence. The matrix supports judgment; it does not replace it.
Does 5.56 MPa prove this bracket is safe?
No. It is one average-stress demand under a simplified static load. Capacity and other failure modes have not been established.
What should a test plan state before testing?
The question being tested, setup and boundary conditions, measured quantities, instruments, acceptance criteria, responsibilities and controls required by the approved procedure.
8 · Free resources & references
- MIT OpenCourseWare: Product Engineering Process ↗
Free course materials on structured product development.
- OpenStax: stress and strain ↗
Free background on force, area and deformation.
9 · Mini-project
Create two paper or cardboard bracket silhouettes and a one-page requirements sheet. Compare outline size, hole access and ease of inspection on a tabletop. Use a printed circle to represent a pin; do not hang anything from either model. Score both concepts, then change one weight and check whether the ranking changes. Submit a photograph or sketch, the matrix and a list of unanswered questions—not a load rating.