Working with Reference Geometry

REFERENCE PLANES:

Beyond X-Y, X-Z, Y-Z (the default coordinate planes):
  - Plane through 3 points
  - Plane offset from a face
  - Plane tangent to a curve
  - Plane angled from another plane

Use cases:
  - A face that's at 30° from horizontal.
  - A plane through a specific feature on a part.
  - A plane for cross-sections.

REFERENCE AXES:

Lines that aren't part of any feature.
Used for: defining rotational axes, alignment.

REFERENCE POINTS:

Single 3D points.
Useful for: alignment, snapping, dimensions.

CONSTRUCTION SKETCHES:

A sketch you use only for measurements + references.
Doesn't contribute to the 3D shape.

EXAMPLES:

A FAN BLADE:
  - Reference axis: rotation axis of the fan.
  - Reference plane: airfoil cross-section perpendicular to axis.
  - Build the airfoil sketch on that plane.

A BICYCLE FRAME:
  - Reference points: where each joint meets.
  - Reference axes: tube centers.
  - Reference planes: planes perpendicular to each tube's axis.

  Then: each tube is a swept circle along the reference axis between joints.

A SKEW CHAMFER:
  - Reference plane: a plane angled away from horizontal.
  - Sketch a triangle on it.
  - Sweep the triangle along an edge.
  - Result: an angled chamfer.

WORKFLOW:

In Fusion 360:
  Construct → Plane → choose method (offset, 3 points, etc.)
  Construct → Axis → choose method.
  Construct → Point → choose method.

In OnShape:
  Plane → Plane.
  Plane through 3 points.
  Various other plane types.

In FreeCAD:
  Part Design → Sketch on plane (any plane, including custom).
  Std Plane to create reference planes.

NAMING:

Name reference geometry: 'airfoil_plane', 'rotation_axis'.
Naming helps you find it later.

IMPACT ON FILE SIZE:

Reference geometry is computed; not stored as triangles.
Small file impact.

WHEN TO USE REFERENCE GEOMETRY vs SKETCHES:

Reference geometry:
  - For abstract relationships (axes, planes, points).
  - Not for actual geometry of the part.

Sketch lines / construction lines:
  - For 2D references within a sketch.

INTERSECTIONS:

Reference planes can intersect at axes/lines.
Useful for: rotating coordinates.

EXAMPLE: COMPLEX SOLID THROUGH 5 PARTS.

  Need: a curved tube that passes through 5 specific points.

  Construct reference points at each of the 5 positions.
  Sketch a spline through them.
  Sweep a circle along the spline.

  Without reference points: impossible to position the spline accurately.

DATUMS (engineering):

Reference geometry that are 'fixed' in space.
Used for: drawings, dimensions to reference, manufacturing.

In CAD: 'Datum' planes are often locked.
For your 3D printer designs: don't usually need formal datums.