11.2 Axes, Points & Coordinate Systems

11.2 Axes, Points & Coordinate Systems

Quick Answer: Use temporary axes (from cylindrical faces) or Insert → Reference Geometry → Axis as the revolution center for Circular Pattern and related features. Use Reference Point for explicit locations, and Coordinate System when Mass Properties / CoM must be reported relative to a hole center or sheet origin—not only the default part triad. Show Temporary Axes and reference geometry as needed; hide them when the view is cluttered.

Reference planes place sketches. Axes and points place patterns, mates (later in assemblies), and measurement frames. On CSWA, the two highest-stakes uses are (1) correct axis for a circular hole/boss pattern and (2) coordinate system for center-of-mass answers after materials are applied (Ch10). Getting either wrong can zero a multi-point item even when solid geometry looks plausible.

Temporary axes

Every cylindrical or conical face (and related revolution geometry) has an implicit temporary axis along its centerline. Temporary axes are not always visible.

Show them: View → Hide/Show → Temporary Axes (or Heads-up View → Hide/Show Items → Temporary Axes). They appear as dashed centerlines through cylinders—boss extrudes, holes, fillets of cylindrical shape, Hole Wizard holes, etc.

Use them without creating a feature: In Circular Pattern, click the temporary axis of the hub or shaft as the pattern axis. No Reference Axis feature is required if the temporary axis is selectable and correct.

Limitations: Temporary axes clutter complex models; some selections are hard to pick when many cylinders exist. If picking is unreliable, create an explicit Reference Axis and select that feature in the PropertyManager.

Reference Axis feature

Insert → Reference Geometry → Axis creates a named axis in the FeatureManager. Common definition methods:

Name axes (Axis_Hub, Axis_BoltCircle) so Circular Pattern and mates stay readable. Axes are construction only—they do not add mass.

Selecting axes for circular patterns (critical)

Circular Pattern needs an axis of revolution—the center of the bolt circle—not the axis of one seed hole.

Correct: Temporary or reference axis of the central hub, shaft, or large bore that defines the pattern center.

Wrong (classic trap): Temporary axis of the seed hole you are patterning. Instances then orbit around that hole, placing copies on a circle that does not match the drawing’s bolt-circle diameter; mass and hole positions fail.

Workflow:

1. Create seed hole/boss at the correct radial distance from center.
2. Show Temporary Axes or pick Axis_Hub.
3. Circular Pattern → Features to Pattern = seed → Pattern Axis = hub axis.
4. Instances (including seed) + Equal Spacing 360° (or specified angle) as in Ch8.
5. Hide temporary axes if needed; verify instance count and diameter in a Top view.

Reference Point

Insert → Reference Geometry → Point places points used as parents for planes, axes, measurements, and some pattern centers.

On CSWA parts, points often mark a hole center you will use as the origin of a Coordinate System, or a point through which an Offset/Parallel plane must pass. They do not appear in mass; they only guide geometry and output frames.

Coordinate System (mass properties link)

Default Mass Properties report mass, volume, and center of mass relative to the part origin (intersection of Front/Top/Right) and the default axes orientation—unless you specify another coordinate system.

Insert → Reference Geometry → Coordinate System:

1. Set Origin (vertex, point, arc center, sketch point, etc.).
2. Set X, Y, and/or Z axis directions by selecting edges, axes, or points that define orientation (follow PropertyManager prompts).
3. Flip axis directions if needed so the triad matches the drawing’s assumed positive directions.
4. Name it (CS_HoleA, CS_Corner).

Mass Properties dialog: Choose the coordinate system from the dropdown (Output coordinate system) so X, Y, Z of CoM are measured in that frame. Ch10 covers materials, density, and CoM traps in depth—this section is the geometry that makes those numbers exam-correct.

Exam scenario: CoM coordinate system at hole center

Sheet: "Report center of mass relative to the center of the Ø12 hole on the top face" (or the exam’s numeric CoM fields imply that origin). Workflow:

1. Ensure the hole exists (Hole Wizard or cut) so a circular edge/center is available.
2. Create Coordinate System with origin at the hole center (arc center or reference point at center).
3. Align axes with model edges as the views suggest (often X along length, Y along width, Z out of face—or as the problem states).
4. Apply correct material (density).
5. Mass Properties → select CS_Hole → read CoM components to the required decimal places (often 2 decimals, MMGS).

If you leave the default origin, CoM numbers can be "right" for the part but wrong for the graded origin. Always match the problem’s measurement frame.

Exam scenario: hole pattern axis

Flange with six holes on a bolt circle:

1. Revolve or extrude hub; create seed hole at correct radius from hub center.
2. Show Temporary Axes; select hub cylinder axis (not seed hole axis).
3. Circular Pattern, 6 instances, Equal Spacing 360°.
4. Optional: Reference Axis on hub if temporary axes are hard to pick under exam stress.
5. Mass Properties after material—pattern errors dominate mass more than tiny fillet mistakes.

Show / Hide discipline

Right-click a reference feature → Hide / Show. View → Hide/Show toggles classes of items. Hidden reference geometry still exists for FeatureManager selection and parent-child relations.

Two planes → axis; axis → plane relationships

• Intersection of Front and Right is the default vertical axis direction of the origin triad—useful mental model when building a CS.
• Axis from two planes is robust when a cylinder is not present (rectangular bosses with a logical centerline defined by midplanes).
• A plane perpendicular to an axis through a point places end-cap sketches on shafts—pair with §11.1 plane methods.

Assembly preview (pointer)

In assemblies (Ch12–13), concentric mates and pattern-driven components also need clear axes. Building clean reference axes in the part makes assembly mates faster. For CSWA part items, focus on pattern axes and mass coordinate systems first.

Integrated exam traps

Workflow summary tying §11.1 and §11.2

1. Planes position sketches (offset, mid, angle, through entities).
2. Axes position circular repetition and measurement spines.
3. Points anchor CS origins and plane-through-point cases.
4. Coordinate systems reframe Mass Properties for graded CoM.
5. Show/Hide only what you need to pick; keep the tree named and ordered.

When a multi-feature part combines an offset-plane seed hole with a circular pattern and a CoM-at-hole question, work in that order: plane → seed → axis/pattern → material → coordinate system → Mass Properties. Each reference entity is simple; the exam score depends on choosing the right one at each step.