Fit and Fasten Assemblies

Overview

Today students learn to assemble multiple OnShape parts into a single model, focusing on how mates (constraints) determine placement. Students will connect this skill to constructing and understanding geometric relationships in CAD drawings.

Learning intentions

• Students will be able to create an assembly in OnShape and add multiple parts.
• Students will be able to apply mate constraints to position parts correctly.
• Students will be able to predict whether a set of measurements/constraints will determine a unique configuration, multiple configurations, or none.
• Students will be able to use tools in OnShape (measure, rotate, section view) to check accuracy.

Success criteria

• I can start an assembly and import or create at least two parts in OnShape.
• I can use mates to align parts in a controlled way (using planar/axis/point references).
• I can explain what happens when constraints are missing or conflicting.
• I can verify my assembly by measuring key distances/angles and checking fit.

Curriculum links

• Geometry — drawing geometric shapes from given conditions and recognizing unique vs multiple vs no triangles (7.G.A.2).
• Geometry — using scale drawings to compute actual lengths/areas and reproduce drawings at a different scale (7.G.A.1).
• Geometry — plane sections of 3D figures (informal check via section view) (7.G.A.3).
• Technology/geometry application — solving real-world problems involving area/volume/surface area when objects are built from geometric parts (7.G.B.6).
• Mathematical practice — using appropriate tools (ruler/protractor/technology) to explore and check solutions (MP5).

Lesson structure (45 minutes)

1. 0–5 min · Hook. Teacher displays a “mystery fit” image of two separate CAD parts and asks: “If you only know two placement facts, do you get one exact placement or many possible placements?” Students quick-write 1–2 predictions on a mini whiteboard.

2. 5–12 min · Mini demo (OnShape). Teacher demonstrates creating an OnShape Assembly, adding two parts, and applying 2–3 mates (one planar mate, one offset/angle, one distance/limit if available). Students watch and follow along on a blank “Sandbox Assembly,” not yet required to match the final model.

3. 12–20 min · Guided practice: first mates. Teacher provides a step-by-step checklist on the board:

• Import/select Part A and Part B
• Mate A’s face to B’s face (flush)
• Rotate/mate orientation to match the intended “front view” Students work in pairs to apply the first two mates, stopping after each mate to call the teacher for a quick check.

4. 20–30 min · Guided practice: constraints that lock (or don’t). Teacher runs a short “constraint reasoning” example: show a scenario where two parts are aligned but still can rotate freely, then add a second mate to remove that freedom. Teacher asks: “Is it unique, more than one, or impossible?” Students apply a third mate to fully position the parts, then test by attempting to drag/rotate—if motion remains, students identify which mate is missing.

5. 30–37 min · Accuracy check with tools. Teacher demonstrates measuring key distances/angles in OnShape and using a section view to confirm correct internal alignment. Students measure at least two specified values (teacher-provided) and record whether each is “within tolerance” or “needs adjustment,” using correct units.

6. 37–43 min · Troubleshooting sprint. Teacher assigns a “common issue” card to each group: wrong mate type, conflicting constraints, flipped orientation, offset sign error, or missing mate reference. Students fix one issue and then write a 1-sentence explanation: “My assembly was not unique (or not possible) because…”

7. 43–45 min · Exit ticket. Students complete a quick check:

• Question 1: “My assembly is ___ unique / could be multiple / is impossible.”
• Question 2: “The mate that made it lock was: ______.” Teacher collects or reviews responses at desks.

Resources

• Devices with OnShape access (student accounts logged in)
• Student handout: “Assembly Checklist: Mate → Lock → Measure → Verify”
• Two pre-made part files for the day’s assembly (Part A and Part B) or a teacher-prepared template workspace
• “Common Issues” cards (one per pair)
• Measuring/recording sheet with two target distances/angles and a tolerance range
• Timer and mini whiteboards for the hook predictions

Assessment

• Formative: teacher circulates during mate placement checks at 12–20 minutes
• Formative: observation of whether students can explain “unique vs multiple vs none” during the troubleshooting sprint
• Exit ticket: quick classification and identification of the mate that locks the assembly

Differentiation

• Support for beginners: provide a printed mate checklist with icons/labels (Planar mate, Offset/Angle, Distance) and pre-selected mate targets (face names or highlighted regions).
• Sentence starters for reasoning: “My assembly could be multiple because ______.” “It became unique after I added ______.”
• Grouping: pair students intentionally—one student reads the checklist while the other applies mates; rotate roles after 5–7 minutes.
• Extension (for fast finishers, kept within time): challenge them to add a third part (Part C) and decide if they can fully constrain it with the least number of mates while still matching the measurement targets.