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06 Assembly

Full Assembly Workflow

Ground your first part, stack on joints, check for interference, and run motion studies on a full robot.

Assembly Fundamentals

An assembly combines multiple parts and sub-assemblies into one coordinated model.

Top-Down vs. Bottom-Up Assembly
  • Bottom-Up: Design each part separately, then insert and constrain in an assembly. Best for reusable components.
  • Top-Down: Create parts inside the assembly, referencing existing geometry. Best for custom adapters conforming to neighbors.
  • Hybrid: Import standard components bottom-up; design custom brackets top-down. The professional robotics workflow.
Grounding the First Component

Every assembly needs one grounded (fixed) component as its anchor -- typically the chassis or base frame.

  • The grounded component's origin becomes the assembly origin — align it to the robot's center-of-rotation or center-of-footprint.
  • Only one component should be grounded; multiple grounded parts prevent correct motion simulation.
1 Experience
2 Reflect
3 Theorize
4 Apply

DOF in Assembly Context

Recall from Module 3: a free body has 6 DOF (3 translations + 3 rotations), and each constraint removes some. In a full assembly, DOF management becomes critical:

Assembly-Level DOF Rules
  • Under-constrained parts retain DOF intentionally for moving components, but indicate a mistake for structural parts.
  • Fully constrained parts have 0 remaining DOF and are correct for all structural members.
  • Over-constrained parts have conflicting redundant constraints that cause solver errors -- remove the extras.
  • Counting DOF: Total DOF = (6 × moving parts) − (DOF removed by all constraints).
  • Robot capability: A 6-DOF arm reaches any point at any orientation; a 3-DOF planar robot is restricted to a flat plane.

Constraint Types

Module 3 introduced Coincident, Concentric, Distance, and Parallel constraints. Full assemblies also use:

◯
Tangent

Forces a curved surface to touch another at one point or line. Use for cam-followers, bearing races, and wheels on ground planes.

∠
Angle

Locks a specific angle between two faces or edges, removing 1 rotational DOF. Use for linkage sweep angles, camera-mount tilt, and angled brackets.

Constraint Strategy: Ask "What DOF should this part retain?" then apply only the constraints needed to remove unwanted DOF. Over-constraining creates solver conflicts.

Joint Types

Module 3 covered Revolute, Prismatic, Cylindrical, and Rigid joints. Full assemblies also use:

Additional Joint Types
Ball Joint — 3 rotational DOF (removes 3 DOF)

Rotates freely around all three axes with a fixed center point. Used in spherical wrists, Stewart platforms, tie-rod ends, and gimbal mounts.

Planar Joint — 2 translational + 1 rotational DOF (removes 3 DOF)

Free sliding across a plane plus rotation around its normal. Used in holonomic drive bases, XY gantry stages, and pick-and-place surfaces.

Joints vs. Constraints: When to Use Which
  • Use joints for connections that move -- they carry axes, limits, and simulation data.
  • Use constraints for positioning parts with no relative motion.
  • In practice: Many engineers use joints for all connections since they're easier to modify later.
  • Software differences: SolidWorks says "mates"; Fusion 360/Onshape say "joints." Same concepts.

Assembly Workflow

1
Ground the Base Component

Insert your chassis first and ground it. Align its origin with the robot's center of rotation.

2
Insert Components & Sub-Assemblies

Work outward from the base. Import standard components from manufacturer libraries and group related parts into sub-assemblies.

3
Apply Joints & Constraints

Constrain each part immediately after inserting. Match joint type to the real connection and set travel/angle limits.

4
Test Motion

Drag components to verify joint behavior. Confirm the kinematic chain propagates correctly and limits engage properly.

5
Check Interference

Run interference detection at multiple positions throughout the full range of motion, not just home.

6
Create Exploded Views

Separate parts along assembly directions. Export as images or animations for build guides.

Interference Detection

Catching collisions in CAD is free. Catching them after fabrication is expensive.

  • Static interference: Parts overlap in the default position due to dimension errors or wrong-scale imports.
  • Dynamic interference: Parts collide only during motion at specific joint positions — test across the full range.
  • Clearance violations: Parts too close for manufacturing tolerances, thermal expansion, or cable routing.
Pro Tip: Set a minimum clearance distance (e.g., 1 mm) in your interference settings to account for tolerances and coatings.

Motion Studies

Motion studies animate joints over time to catch problems invisible at a single position. Drive joints with constant velocity, keyframes, or math functions.

Common Motion Study Checks
  • Drivetrain: Spin wheels and verify no frame interference through full suspension travel and steering range.
  • Manipulator arms: Run the full pick-and-place cycle and confirm all positions are reachable without link collisions.
  • Intake/scoring: Cycle the complete acquisition-to-scoring sequence and verify clearances at speed.
  • Linkages: Animate four-bar/six-bar linkages through their full cycle, watching for dead points and path deviations.
Assembly Golden Rule: Ground your chassis first and build outward from most constrained to least. This minimizes solver errors and mirrors physical assembly order.

Ball Bearing -- assembled with concentric and revolute constraints.

Robot Chassis — the grounded base component that anchors every assembly.

Stage 2 Pause and Reflect
✓ Your reflections are saved automatically
Stage 4 Apply What You Learned

You have a two-wheel differential drive robot. Plan the assembly constraints needed.

  • Identify which component should be grounded (the chassis)
  • List the joint types needed for each wheel (revolute? cylindrical?)
  • Plan how to attach the caster wheel with the right DOF
  • Determine how the motor would be rigidly fixed to the chassis
  • Check: does your planned assembly have any unintended DOF?
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