First Sketch Mastery

Overview

In this lesson (Day 5 of 15), students use OnShape sketch tools to draw a simple 2D sketch with correct angles and side lengths. They practice how three given measurements can determine (or not determine) a unique triangle, then recreate a sketch accurately using dimensions.

Learning intentions

Students will be able to:

• create a sketch on a selected plane using OnShape sketch tools
• add and edit dimensions to match a given design specification
• analyze whether a triangle is uniquely determined by given angle/side measures
• use appropriate tools (ruler, protractor/OnShape measuring, and dimension constraints) to check accuracy

Success criteria

Students can:

• produce a clear triangle-based sketch that matches all provided angle/length measures
• explain what makes their triangle unique, possible but not unique, or impossible based on the measures
• use dimensions in OnShape so the sketch does not “drift” after edits
• verify accuracy by measuring/checking key lengths and angles

Curriculum links

• Geometry — drawing and constructing triangles from three angle/side measures, including cases of unique, multiple, or no triangle - Geometry — using scale drawings to compute actual lengths/areas and reproduce at a different scale - Geometry — connecting 2D slice ideas to 3D figures (briefly as context for sketch planes) - Mathematical practice — choosing and using tools strategically ## Lesson structure (45 minutes)

1. 0–5 min · Hook (triangle challenge). Teacher displays three measure sets on the board (for example: three sides, or two sides + included angle), and asks: “Does this create one triangle, more than one, or no triangle?” Students quick-vote with thumbs up/down and justify in pairs.

2. 5–12 min · Mini direct teach (OnShape sketch workflow). Teacher models the exact steps: start sketch on a plane, choose sketch tools (line, polygon/triangle/arc if needed), then apply dimensions and constraints; teacher emphasizes that dimensions “lock” the shape. Students follow along on their own devices or observe while taking a 3-step checklist.

3. 12–17 min · Standards check: triangle determination talk. Teacher connects the hook to the day’s goal: “When we know three measurements, sometimes the triangle is fully determined, sometimes not.” Teacher reminds students to think about whether the measurements could all be true at the same time. Students write a one-sentence prediction for their assigned design set (unique / multiple / none).

4. 17–30 min · Guided build: Create your first sketch (round 1). Teacher assigns a worksheet spec card with either:

• 3 side lengths for a triangle, or
• 2 angles + 1 side, or
• 2 sides + included angle, plus required scale note (e.g., “dimensions are in mm; your OnShape model units are mm”). Students create the sketch in OnShape, add dimensions, and adjust until it matches the spec; teacher circulates with “dimension-first” reminders.

5. 30–37 min · Accuracy verification (round 2). Teacher demonstrates how to measure/check and how to fix over/under-defined issues (for example: if a dimension changes, ensure constraints are consistent). Students verify at least two key measurements (one side length and one angle) directly in OnShape, then re-check their triangle determination prediction.

6. 37–44 min · Quick share + exit ticket. Teacher prompts: “What tool helped you most—dimensions, snapping, or measurement?” Students do a 30-second partner share, then complete a short exit ticket:

• list one dimension you added
• name whether your triangle was uniquely determined, more than one, or none
• state one strategy you used to correct a mismatch

7. 44–45 min · Clean close. Teacher collects/monitors exit tickets and reminds students to save/export their sketch for the next lesson in the unit.

Resources

• Student OnShape accounts and devices (Chromebooks/laptops)
• OnShape sketch tutorial reference (teacher-made one-page steps)
• Triangle sketch specification cards (multiple versions for differentiation)
• Measuring/geometry reference sheet: “unique vs multiple vs none” triangle reasoning cues
• Pencil, eraser, and notebook paper for predictions
• Exit ticket slips or digital form
• Optional: printed grid paper for rough planning (only if needed)

Assessment

• Formative check during circulation: Is the sketch dimensioned (not free-drawn)?
• Targeted questions: “What are the three given measures?” and “Why is it unique/not unique/no triangle?”
• Exit ticket alignment: one accurate dimension + correct determination claim + one correction strategy

Differentiation

• Support: Provide sentence starters (“I think it is unique because…”, “My triangle didn’t match because…”) and a step-by-step checklist for the OnShape workflow.
• Support for math confidence: Use fewer/cleaner measure sets first (e.g., three sides that clearly form a triangle) and allow students to focus on dimensioning accuracy.
• Extension: Offer an additional “scale twist” on the spec card (e.g., “reproduce at half scale”) so students practice scale-drawing reasoning.
• EAL/SEN: Pre-teach key words (sketch, plane, dimension, angle, length) with visuals; allow oral responses for the exit ticket justification if needed. Keep directions to short chunks and display the workflow steps on the board.