⚙️ CAD · Intermediate · Mechanism Design

Mechanism
Concept Sprint

A Concept Sprint is a structured session where your team generates multiple mechanism ideas, models the top candidates quickly in CAD, and picks the best one with evidence — not gut feel.

// Section 01
The Concept Sprint Method
Top teams never build the first mechanism they think of. They generate at least three different approaches, model them quickly in CAD, compare them against criteria, and pick the best one. This is the EDP Brainstorm → Select cycle done properly.
Mechanism sprint workflow sketch A sketch B sketch C 3 concept sketches CAD model A CAD model B top 2 modeled in Onshape Selected decision matrix picks winner 1 concept moves to build
⏱
Sprint timeline: 45-minute session. 10 min: define what the mechanism must do. 15 min: sketch 3 concepts on paper. 15 min: rough CAD of top 2. 5 min: fill in the tradeoff matrix. Output: one selected concept with documented reasoning.

What the Mechanism Must Do — Define This First

Before you brainstorm, write down the requirements. Vague requirements lead to vague designs. Specific requirements let you evaluate whether a concept actually works.

📝 Mechanism Requirements — Fill Before Brainstorming

Why CAD Before Build?

A physical prototype takes 2–4 hours. A CAD concept takes 20–40 minutes — and you can evaluate it without buying parts.

  • CAD two or three concepts before building any of them
  • Use the tradeoff matrix to score each concept against your criteria: weight, complexity, height, reliability
  • Show both concepts to your strategist — the highest-scoring mechanism and the simplest reliable one are often different
  • Document both in the notebook: what you chose and what you rejected, with reasoning
⚙ STEM HighlightEngineering: Concurrent Design & Design Reviews
Professional engineering teams use design reviews at the concept phase before committing any resources to building. Boeing, NASA, and SpaceX all hold Preliminary Design Reviews (PDR) and Critical Design Reviews (CDR) before physical fabrication begins. The goal is identical to a Concept Sprint: generate multiple approaches, model them analytically, review as a team, and select based on evidence. Skipping the concept phase is what leads to expensive redesigns late in a program — the same way it leads to rebuilds at VRC competitions.
🎤 Interview line: “Before we built our intake, we ran a Concept Sprint — we modeled three different designs in Onshape, evaluated each against our requirements, and selected the best one using a tradeoff matrix. That decision is documented in the notebook. The mechanism we built on the first try was competition-ready because we evaluated it in CAD first.”
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// Section 02
Generating Concepts
The rule for brainstorming: generate first, evaluate second. Do not kill an idea during the brainstorm phase. Quantity first, quality second — the best ideas often come after the obvious ones.

Paper Sketch Rules

🔬
Look at reference designs, then close the tab. It is fine to research what other teams have built. It is not fine to open Onshape with a reference photo and copy-model it without understanding why it was designed that way. Look at references for inspiration, understand the principle, then design your own version from scratch.

Common Mechanism Types for VRC

🏅 Roller Intake
Motorized rollers that pull game elements in on contact. Fast cycle time. Works well with round or tumbling game elements. Can be single-roller or counter-rotating pair. Simple to build and repair.
🔗 Claw / Gripper
Mechanical gripper that closes around game elements. More precise pickup, slower cycle. Best for irregularly shaped objects or when you need to hold during movement. More complex mechanically.
✅ Belt / Conveyor
Continuous belt or chain with paddles. Moves elements through the robot continuously. Used when elements need to travel a longer path (from pickup to scoring height). Good throughput, more parts.
📦 Scoop / Bucket
Passive scoop that collects elements by driving into them. Zero motors used. Works only when elements are stationary. Fastest possible pickup but very limited in where/how you can score.
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// Section 03
Tradeoff Analysis
Once you have 3 concepts, evaluate them against the same criteria. The one with the best score across your team’s specific priorities wins. This is your decision matrix — and it goes in the notebook.

Interactive Decision Matrix

Score each of your three concepts on each criterion (1–5). The one with the highest total wins. Adjust criterion weights to reflect your team’s priorities.

🎯 Design Decision Matrix
Weighted Totals
📝
Take a screenshot of this matrix and paste it into your notebook. This is exactly what the RECF rubric means by “multiple detailed diagrams with pros/cons and research backing.” A decision matrix with three concepts, weighted criteria, and a documented winner is Expert-level evidence for the “Select Best Solution” EDP step.
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// Section 04
Mechanism Packaging in CAD
“Packaging” means fitting your mechanism into the available space on the robot without conflicting with the drivetrain, electronics, or other mechanisms. This is where most student designs fail.

Packaging Checklist — Check Every One

⚠️
Weight distribution: Mechanisms mounted high or far forward move the center of gravity forward and up. A robot that tips during fast movements loses points. In Onshape you can estimate CG by using the “Mass properties” tool — it will show you the CG location for the full assembly. Keep it as low and centered as possible.
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// Section 05
Select & Document
The sprint ends with a decision and a notebook entry. Both are required. A decision without documentation is not engineering — it is just building.

What Goes in the Notebook

🏆
Expert-level notebook evidence for “Select Best Solution” is exactly this: multiple detailed designs with a scored decision matrix and a written rationale that references specific data from your testing or analysis. If your notebook shows you always built the first thing you thought of, judges know you skipped this step.

What’s Next

🔧
CAD to Build Handoff
Turn your selected design into a build document
 📝
Engineering Notebook
EDP, decision matrices, judging rubric
 🔬
Testing System
Test the mechanism you just designed
🔬 Check for Understanding
You run the weighted decision matrix and the scores are 8.2 vs 8.0 for two concepts. What should you do?
Re-run the matrix with adjusted weights until one concept scores clearly higher
Select the higher score automatically — that is the purpose of the matrix
Document both scores and write out what tipped the final decision — a close result means your reasoning matters more than the number
Present both options to judges without making a selection
Correct. A close matrix result is actually good notebook evidence — it shows you genuinely evaluated real tradeoffs. Write one sentence explaining what factor made the difference. Judges specifically look for decision reasoning, not just the winning score. This goes on a Purple slide.
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For Coaches & Mentors
Sketching vocabulary, discussion questions, and the official PTC/Onshape Creating Custom Components unit guide.
🛠
What students build in this unit: Two custom parts to mount a distance sensor to the front of their robot — modeled in-context inside the assembly. This is the bridge from "inserting pre-made parts" to "designing your own." Prerequisite: complete Intro to Onshape and Assemble a Drivetrain first.

Sketching Vocabulary

This unit introduces 17 new terms. Cover the most critical ones before students open Onshape — the rest will appear naturally as students work through the lesson.

Sketch
Lines, curves, and points drawn on a plane, governed by dimensions and constraints. The foundation of every part.
In-Context
Editing a part while seeing the rest of the assembly around it. Lets you design parts to fit perfectly without guessing dimensions.
Constrained
Fully controlled — every line and point has defined behavior. A fully constrained sketch turns black. Unconstrained geometry is blue.
Dimension
Adds specific lengths or angles to sketch geometry. Drives the sketch so changes update automatically.
Extrude
Adds depth to a sketch region to create a solid part. The most common feature in part modeling.
Fillet
Rounds sharp interior or exterior edges with a defined radius. Reduces stress concentrations and mimics manufactured parts.
Hole
Creates holes at sketch points using ANSI or ISO standards. Use this instead of extruding a circle for proper hole sizing.
Construction
Sketch entities used as references or guides but not turned into solid geometry. Shown as dashed lines.
Mirror
Reflects selected entities across a mirror line or plane. Halves the work when designing symmetric parts.
Use (Project)
Projects edges or geometry from existing parts onto the active sketch plane. Key to in-context design — lets you reference what's already there.
View Normal
Views straight on, perpendicular to a specific plane. Use before sketching so you're looking directly at the face you're drawing on.
Origin
The point where all three planes (Front, Top, Right) intersect — the absolute zero of the model space.

Lesson Breakdown & Discussion Questions

Sketching In-Context
Students learn to sketch while seeing the surrounding assembly — the core skill of in-context design.
  • How do you know you are in-context when modeling?
  • What can be seen when a context is turned on or off?
  • How do you turn a context on or off?
Creating the Parts
Students build out the sensor mount features using sketching tools and best practices.
  • What other parts on our robot would benefit from in-context design?
  • What does it mean for a sketch to be fully constrained? What color will it turn?
Final Assembly
Students add their custom parts into the assembly, mate everything, and place hardware for a complete model.
  • What are mate connectors and why do they matter?
  • Why is it important to add hardware to your final assembly?
  • What is the difference between a Fastened mate and a Revolute mate?

Official PTC/Onshape Resources

📚 Unit Guide — PTC / Onshape
Creating Custom Components
Full instructor guide with vocab, preparation notes, lesson breakdown, and discussion questions.
🔗 PTC Resource Center →
🛠 Official Onshape Document
Creating Custom Components
Each student needs their own copy. They'll build two custom sensor mount parts from scratch inside the assembly.
🔗 Open in Onshape →
🎓 Learning Center
In-Context Design Courses
Managed In-Context Design + Editing in the Context of the Assembly — self-paced courses for advanced students.
🔗 Start Course →
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Related Guides
🚗 First Drivetrain → 🔧 CAD to Build → ✂️ Custom Parts →
▶ Next Step

Mechanism selected and modeled. Now package the design into a BOM and handoff document for the build team.

🔧 CAD to Build Handoff →
▶ OFFICIAL ONSHAPE TRAINING — LEVEL 3: BUILDING FROM A CAD MODEL
CAD for VEX Using Onshape
Level 3 — Building from a CAD Model
This site is an informational reference. RECF EN4 prohibits AI-generated content in engineering notebooks and programming code. Rewrite everything in your own words. About EN4 →  ·  References & Sources →
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