8.3 Mirror Feature & Pattern Strategy

8.3 Mirror Feature & Pattern Strategy

Quick Answer: Insert → Pattern/Mirror → Mirror reflects features or bodies across a plane or planar face. Select every feature that defines the half you modeled, pick the true midplane, and combine Mirror with Linear/Circular Pattern in an order that matches the drawing’s symmetry. Wrong plane or incomplete seed selection flips CoM or leaves half the part blank.

Many CSWA parts are symmetric: brackets with left/right ribs, plates with mirrored cutouts, housings with identical side bosses. Modeling both sides manually doubles sketch work and doubles the chance of a dimensional mismatch. Mirror enforces design intent: one side drives the other, so a modification to the seed half updates the mirrored half automatically.

Mirror PropertyManager essentials

Plane or planar face

Valid mirror references:

• Front, Top, Right (or equivalent) default planes when the part is built about the origin
• A reference plane you created (midplane between faces, offset plane, angle plane)
• A planar model face that truly sits on the symmetry plane (for example, a center web face)

Exam trap — outer face as “mirror”: Mirroring features about a side face of a block produces a second set of features outside or in the wrong place, often creating multi-body mess or wrong volume. If the drawing’s centerline is the symmetry line, the mirror plane must contain that centerline in 3D (the midplane), not a boundary face.

Features vs bodies

Features to Mirror (default associate habit)

Select the boss, cuts, fillets, holes, and other features that exist only on the seed side and should appear on the opposite side. The mirrored results typically remain part of the same body when the seed features were added/removed on that body.

Select all that belong to the half: If you mirrored only the Extrude but not the subsequent cut and fillet that finish that side, the opposite side is incomplete. Mass will be wrong and the visual will show missing pockets or rounds.

Bodies to Mirror

Use when you intentionally built a body that should be reflected as a solid copy. Watch merge options: without merging, you may get two bodies; with merging, you need coincident faces that can knit into one solid. Multi-body surprises change Mass Properties dialog body selection and can confuse CoM if you measure the wrong body.

Mirror vs sketch Mirror Entities

Do not confuse tools:

When to sketch-mirror: Symmetric profile of a single extrude (both halves in one sketch) can be fully defined with sketch mirror + centerline. When to feature-mirror: After building complex one-sided feature stacks (ribs, hole sets, local cuts) that would be painful to duplicate in one sketch. Both can appear on the same exam part at different stages.

Pattern strategy: Mirror then Pattern vs Pattern then Mirror

Symmetric CSWA parts often need both arrays and reflection. Order is a design-intent decision.

Strategy A — Model quarter or half, Pattern, then Mirror

Example: A plate with a 2×3 hole grid that is also symmetric about the centerline, or a hub with holes only modeled on one side of a plane plus mirror.

1. Build base body centered on planes.
2. Create seed hole(s) on one side.
3. Linear or Circular Pattern to complete the array on that side (or the unique set).
4. Mirror the pattern feature (and related features) across the midplane if the drawing’s other side is a reflection of that set.

Works well when: The pattern itself is not centered as a single full-circle feature, or when left and right are reflections of a patterned cluster.

Strategy B — Mirror first, then Pattern

1. Build one feature on the left.
2. Mirror across midplane to get the right-hand twin.
3. Linear Pattern both (select both features or pattern a feature that already includes symmetry—depends on structure).

Works well when: You need left-right pairs first, then repeat that pair along a length.

Strategy C — Full circular pattern only (no mirror)

For a complete equal bolt circle about the center, Circular Pattern alone is usually enough. Adding Mirror can duplicate holes on top of existing instances if you already patterned 360°.

Strategy D — Half circular pattern + Mirror

Sometimes a drawing’s dimensions define holes only on one semicircle with a centerline, implying mirror. You can circular-pattern three holes on 180° and mirror—or pattern six on 360°. Prefer the approach that matches how dimensions are written and which modification is likely (count vs symmetry).

Double geometry trap: If mass is roughly double what you expect and the graphics show overlapping features, you likely mirrored a full pattern that already crossed the plane. Delete or edit the redundant mirror; keep one associative definition.

Common exam traps (CoM and mass)

1. Wrong mirror plane flips CoM

Symmetry about Right versus Front is not interchangeable. Mirroring across the wrong default plane can:

• Place cuts on the wrong side of the part
• Create a model that looks plausible in one orthographic view but fails another
• Shift center of mass even when volume is coincidentally similar

Defense: Identify the drawing’s centerline in 3D; map it to Front/Top/Right or a custom midplane. Spin the model in isometric and compare to the exam isometric. Run Mass Properties and glance at CoM coordinates for sanity (near 0 on axes of symmetry when the part and origin align).

2. Missing seed features

Mirror only what you select. Forgotten fillets, chamfers, or hole wizard features on the seed side mean the mirrored side lacks them. Check FeatureManager: the Mirror feature’s feature list should include every half-side operation that is not already symmetric by construction.

3. Mirroring the base body incorrectly

If the base extrude was already sketched as a full symmetric profile, you do not mirror the base again. Mirror only the asymmetric additions (side pocket, side boss). Mirroring an already-full base can create overlapping bodies or zero-thickness failures.

4. Plane from a face that moves

If you mirror about a model face that later disappears in a modification (cut removes the face), the Mirror feature can fail. Prefer stable reference planes (default or reference geometry) for exam durability.

5. Units and material still apply

Mirror does not set density. After symmetric features are complete, apply material and measure. Symmetric geometry about the origin often yields CoM near zero on two axes—if CoM is unexpectedly far off, suspect wrong plane or missing mirrored cuts.

Workflow: half-part modeling under time pressure

1. Orient the part so the symmetry plane is a default plane when possible.
2. Build the base as full width or true half-body (if half-body, plan Bodies to Mirror + merge carefully—many candidates find full-width base + feature mirror easier).
3. Add all unique one-side features; fully define sketches.
4. Mirror across the midplane; inspect both sides.
5. Apply patterns that are still required (or pattern before mirror per strategy).
6. Edge treatments that should be symmetric: either include in mirror feature list or apply after on both sides with care.
7. Mass Properties.

Modification example: “Increase the mirrored side pocket depth from 5 mm to 8 mm.” Edit the seed pocket feature (not a frozen independent copy). Rebuild; mirror updates; re-measure mass/CoM.

Combining all three tools (Linear, Circular, Mirror)

A realistic intermediate part might include:

• Revolved hub (prior chapter tools)
• Circular Pattern of bolt holes
• Side bracket geometry Mirrored left/right
• Linear Pattern of slots along a rail

Order recommendation for stability:

1. Base solid(s)
2. Centered circular patterns about permanent axes
3. One-sided local features
4. Mirror for bilateral symmetry
5. Linear patterns along rails that are already full-length
6. Fillets/chamfers that need complete edges
7. Material + Mass Properties

Adjust when the drawing’s dimensions make a different order clearer—but always avoid double-creating the same hole with two pattern tools.

Mirror checklist for CSWA

1. Identify symmetry plane from the drawing centerline → pick matching plane/face.
2. Decide features vs bodies scope.
3. Select complete seed feature set for the half.
4. Preview; confirm no double copies with existing patterns.
5. Prefer stable reference planes over fragile faces.
6. After edits to the seed half, rebuild and re-check the mirrored half visually.
7. Mass Properties and CoM—wrong plane is a CoM killer even when “something” mirrored.

Mirror, Linear Pattern, and Circular Pattern together are the associative toolkit that keeps CSWA parts editable under modification steps. Use Circular for radial arrays, Linear for straight grids, and Mirror for true planar symmetry—and choose sequence so each feature appears once in the design intent tree with the correct parent parameters driving mass.