Constraints in CAD

Overview

In this lesson, students learn how constraints control the size and shape of sketches in CAD (OnShape) and how the same constraints can determine a unique shape, multiple shapes, or no valid shape. This builds from earlier CAD basics (navigating OnShape, making sketches, and basic drawing tools) toward more precise geometric construction.

Learning intentions

• Students will be able to identify common constraint types (e.g., fixed, coincident, parallel, perpendicular, equal, midpoint, and dimension constraints).
• Students will be able to predict how constraints affect a sketch’s degrees of freedom.
• Students will be able to construct triangles or quadrilaterals using angle/side information and recognize when the conditions determine a unique figure, more than one figure, or no figure.
• Students will be able to use geometric tools (ruler/protractor, or OnShape measurements) to verify their CAD sketch.

Success criteria

• I can explain what a constraint “locks” in a sketch and why that matters.
• I can apply at least 3 constraints to stabilize a sketch in OnShape.
• I can state whether a given set of geometric conditions produces a unique figure, multiple figures, or no figure.
• I can measure/confirm my sketch dimensions and adjust constraints when the sketch overconstrains or fails.

Curriculum links

• Geometry: Draw geometric shapes with given conditions; recognize when conditions determine a unique triangle, more than one triangle, or no triangle.
• Geometry: Use tools (ruler, protractor, technology) to draw/construct shapes and verify results.
• Technology (CAD modeling practice): Solve building/validation problems by choosing appropriate tools and checking outcomes.

Lesson structure (45 minutes)

1. 0–5 min · Hook (Constraint challenge). Teacher shows two quick sketches: one “wobbly” and one stabilized with constraints, then asks, “What changed that makes one sketch stay put?” Students respond in pairs with ideas about what tools could “lock” geometry.

2. 5–12 min · Mini-lesson: Constraints as degrees of freedom. Teacher demonstrates in OnShape: draw a triangle with no constraints, then add constraints step-by-step while students watch how parts move less. Students sketch notes using a 3-column chart: “Constraint type,” “What it locks,” “What I notice changes.”

3. 12–22 min · Direct teach: Predict before you apply. Teacher states three scenarios and has students predict outcomes before clicking in CAD:

• Scenario A: enough information to make a unique triangle
• Scenario B: information that allows more than one triangle
• Scenario C: information that makes a triangle impossible Students complete a quick “predict” worksheet (or paper table) labeling each scenario as unique / more than one / no triangle, then justify with a sentence.

4. 22–35 min · Guided CAD practice: Build and stabilize a sketch. Teacher models one example: create a triangle in OnShape, then apply constraints (at least one of: fixed, equal sides/angles, parallel/perpendicular where relevant, and dimension constraints) until the sketch stops over- or under-constraining. Students follow a step-by-step task card:

• Use OnShape to draw a triangle or simple quadrilateral from given measures (use the same “unique / multiple / no” scenario from earlier)
• Add at least 3 constraints so the sketch becomes stable
• Use dimensions and measurement readouts to verify the sketch matches the given conditions
• If it fails, they must identify whether they created an impossible condition or an overconstraint conflict and try a corrected set

5. 35–42 min · Validation round (peer check). Teacher assigns partner roles: “Modeler” and “Inspector.” Students use a checklist to confirm: correct constraint use, sketch stability, and correct classification (unique/multiple/none) based on the given conditions.

6. 42–45 min · Exit ticket (quick geometric reasoning + CAD outcome). Teacher collects a short exit ticket:

• “One constraint I used was…”
• “My conditions were classified as…” (unique/multiple/none)
• “A reason my sketch moved (or didn’t) was…”

Resources

• Devices with OnShape access (one per student or shared pairs)
• CAD quick-reference card listing common constraint icons and names
• Printed scenario cards (A unique / B multiple / C impossible)
• Student task card with steps and a constraint requirement (minimum 3)
• Exit ticket slips or digital form
• Paper, pencils, ruler; optional protractor for non-CAD prediction
• Timer for transitions and partner validation

Assessment

• During mini-lesson and guided practice: listen for correct language (“locks,” “stabilizes,” “overconstrained,” “impossible condition”)
• Predict-before-build checks: look for correct unique/multiple/none classifications and reasoning
• Exit ticket: verify students can connect constraint behavior to geometric conditions and outcomes

Differentiation

• Support: Provide sentence starters (“A unique triangle happens when…,” “My sketch is stable because I applied…,” “My triangle didn’t form because…”).
• Support: Offer a “constraint bank” (word bank + icon bank) on the task card; allow students to use dimension-first or constraints-first depending on comfort.
• Extension: Students who finish early can try a second scenario (e.g., take their sketch and deliberately remove one constraint to observe regained movement, then reapply a different constraint type to restabilize).
• EAL/SEN: Use visual cues (color-coded constraint categories) and reduce writing load by allowing short responses or label-based answers on the worksheet.