Symmetry, Mirror and Circular Patterns in Form

Overview#

Most organic objects are symmetric about a plane or around an axis. Form lets you model only a half, quarter or single petal and have Fusion maintain the rest. This saves time, guarantees symmetry, and keeps the cage clean at the symmetry plane. There are two quite different ideas, and confusing them causes most trouble:

• Internal symmetry (Mirror - Internal, Circular - Internal): the symmetry exists inside one T-Spline body. You pick matching faces on the source and mirror sides, and edits on one side are carried to the other.
• Duplicate (Mirror - Duplicate, Circular - Duplicate): creates a mirrored or circular copy of a body across a plane or around an axis. According to Autodesk's help, changes to one copy are reflected in the other, so the copies stay linked. An optional Weld with a Weld Tolerance joins the bodies where they touch and embeds the symmetry in the body.

Where to find it#

• Form tab > Symmetry panel (Design > Form > Symmetry): Mirror - Internal, Circular - Internal, Mirror - Duplicate, Circular - Duplicate, Clear Symmetry and Isolate Symmetry. All six names match Autodesk's T-Spline symmetry help.
• Many primitive dialogs have a Symmetry section where you can enable symmetry as you create the body.
• Symmetry indicator: when internal symmetry is active, green symmetry lines are drawn on the body, and edits on one side are carried to the other. Faces excluded with Isolate Symmetry are drawn in red.

Step by step#

Mirror - Internal#

1. Build a body whose cage is already laid out symmetrically (for example a Box with an even number of faces across the centre).
2. Choose Symmetry > Mirror - Internal.
3. Select a face on the source side and the corresponding face on the mirror side. There is no plane picker in this command: the pair of faces defines the symmetry.
4. If symmetry could not be detected from the faces, pick symmetrical edges and then vertices on either side, and choose from the Possible Symmetries drop-down.
5. Click OK. A green symmetry line appears, and Edit Form now moves matching vertices on the other side as you drag. A single body can have multiple mirror symmetries.

Circular - Internal#

1. Choose Symmetry > Circular - Internal.
2. Select two faces that set the position of the symmetry (optionally two edges, then two vertices). Autodesk's overview describes selecting a face and then specifying the number of symmetry lines.
3. Choose from Possible Symmetries: Fusion calculates the possible areas of symmetry from the selected face and the body (for example 5 for a five-petal flower).
4. Click OK. Edits to one sector are repeated around the axis.

Mirror - Duplicate and Circular - Duplicate#

1. Choose Mirror - Duplicate (or Circular - Duplicate).
2. Pick the T-Spline Body and the Mirror Plane (a face or plane) for mirror. For circular, pick the body and the Axis, then set Angular Spacing (Full for 360 degrees, or Angle with a Total Angle) and the Quantity of copies.
3. Tick Weld and set the Weld Tolerance if the copy touches the original and you want one body with built-in symmetry.
4. Click OK. The copy is linked: changes to one copy are reflected in the other.

Clear Symmetry#

1. Choose Symmetry > Clear Symmetry.
2. Select the body or bodies (the optional Separate Bodies setting creates new bodies for disjointed surfaces). The symmetric constraints from Mirror - Internal, Circular - Internal, Mirror - Duplicate and Circular - Duplicate are removed. Use this when you want to add asymmetric details after the symmetric base is complete.

Setting symmetry at creation#

1. In a primitive dialog such as Box, set the Symmetry option (Mirror with Length, Width and Height Symmetry; Circular with Symmetric Faces on Cylinder, Sphere, Torus and Revolve; Quadball and Plane offer Mirror only).
2. Enable the directions you want, create the primitive, and Edit Form respects the symmetry immediately.
3. If you need a different kind of symmetry afterwards, add it with Mirror - Internal or Circular - Internal.

Options reference#

Tips and pitfalls#

Tip: Lay out the cage so the symmetry seam falls on an existing edge loop (an even face count across the centre), not through the middle of a face. Cage faces that straddle the seam cause visible kinks.

Tip: Model with Mirror - Internal until the form is 90 percent done, then clear symmetry and add one-sided details such as a logo recess, a button, or an asymmetric vent.

Tip: Use Circular - Internal with an odd number of sectors only if your base shape is built for it. A circular cage has a pole at the axis where all sectors meet; make sure the pole is a clean star point.

Warning: Dragging a vertex that sits exactly on the mirror plane across the plane breaks the body into overlapping halves. Check the seam after moving vertices near the plane (for example with soft modification) to make sure it is still closed (general guidance).

Warning: Symmetry is applied to the Form body. If an edit is blocked or behaves asymmetrically, use Isolate Symmetry on the affected elements or Clear Symmetry.

Warning: Duplicate variants stay linked in Fusion's Form: changes to one copy are reflected in the other (Autodesk's symmetry help). If you want independent copies, Clear Symmetry on them, or copy the body with Solid tools after Finish Form.

Tip: You can also use the Solid Mirror and Circular Pattern after you finish a Form body. That is the right choice when the mirrored part should remain a separate body or when you want to pattern a single finished boss many times.

Choosing internal symmetry or duplicates#

Worked example: a symmetric mouse shell#

Start with a Box 60 x 35 x 20 mm with 4 x 4 x 2 faces. An even count across the width puts an edge loop on the centre line. Apply Mirror - Internal by selecting a face on the left half and the matching face on the right half. Select the left-hand top ring and pull it up 5 mm: the right ring rises by the same amount. Now suppose you want a scroll-wheel slot on the centre line only. Use Isolate Symmetry on the central faces (they turn red), cut the slot there, and the rest of the body stays symmetric. Finally Clear Symmetry before adding a logo recess on one side.

Seams, welds and tolerances#

• With Mirror - Duplicate, set Weld when the copy abuts the original. The Weld Tolerance is the value used to close gaps between edges. Illustrative numbers: if the two halves are 0.2 mm apart, a tolerance of 0.5 mm should close the gap, while 0.05 mm would leave it open and you would keep two bodies.
• Weld Vertices (on the Modify panel) offers Vertex to Vertex, Vertex to Midpoint and Vertex to Tolerance modes if you need to close a seam manually.
• If you see a visible line down the centre after finishing, the seam vertices are not coincident. Reopen the form, switch to Box view and weld or move them onto the plane.

Common failures and fixes#

Practice exercise#

Model a five-petal flower head and test isolation.

1. Create a Cylinder with 5 Diameter Faces and 1 Height Face. Circular - Internal needs a cage that divides evenly, so 5 faces around is required for 5 sectors.
2. Choose Circular - Internal, select two faces that are one sector apart, and pick 5 from Possible Symmetries.
3. With Edit Form, pull the outer vertices of one sector outward 25 mm to a petal tip and raise the centre by 8 mm. Confirm the other four petals follow.
4. Use Insert Edge to add detail to the petal and crease the petal tip edge.
5. Use Isolate Symmetry on one petal and tilt it 10 degrees. The other four stay symmetrical.
6. Clear Symmetry and, if desired, add one more small asymmetric detail.
7. Finish Form and check with Inspect > Measure that the petal tips are the same radius (apart from the tilted one).

Expected result: a smooth five-petal shape with one deliberately irregular petal. If tilting moves every petal, the petal was not isolated before editing.

Related#

• Form primitives
• Edit Form
• Finish Form and conversion