Finish Form and Converting Between Form and BRep

Overview#

Form geometry is a smooth approximation computed from a cage. It is not yet a regular Fusion body that you can fillet, shell, or dimension. Finish Form exits the Form editing context and generates a BRep (boundary representation) body: a solid if the form is closed, or a surface if it has open edges. After that, Solid and Surface tools work normally.

The Form feature remains in the timeline, so you can double-click it and edit the cage; changes are recomputed when you finish again. Autodesk notes T-spline operations are not recorded in history the way solid operations are.

Where to find it#

• Form tab > Finish Form (at the end of the Form toolbar). It returns you to the top level of the Design workspace.
• To re-enter: double-click the Form feature in the timeline, or right-click it > Edit Feature.
• Convert (Form > Utilities > Convert): converts between body types. Its Convert Types are T-Splines to BRep, BRep Face to T-Splines and Quad Mesh to T-Splines. In direct-modeling designs it is also at Solid > Modify > Convert and Surface > Modify > Convert. For general meshes use Mesh > Modify > Convert Mesh.
• Repair Body (Form tab, Utilities).

Step by step#

Finish a Form#

1. Check the form in Smooth display mode and use the Inspect analyses (for example Zebra Analysis) to confirm continuity. In Box or Control Frame display, check that edges do not intersect: T-spline edges cannot intersect for conversion to BRep.
2. Run Repair Body if you suspect problems.
3. Click Finish Form. In a history (parametric) design the conversion happens automatically at this point.
4. A closed form becomes a solid body; an open form (for example a Plane) becomes a surface body. Check the body icon in the browser.
5. If conversion errors occur, problem faces are highlighted in the canvas and listed in a dialog, and the body with errors is marked in the browser if you continue without resolving them. See the repair steps below.

Repair a failing form#

1. Reopen the Form by double-clicking the Form feature.
2. Choose Utilities > Repair Body and select the body. It shows error star points (red star), error T points (red T) and free edges (open edges); clicking a star converts it to a T-junction and vice versa, and AutoRepair attempts to fix error star points, error T points and free edges (with an optional Weld Tolerance).
3. If that does not work, look for (general guidance): faces with zero area, overlapping or coincident vertices (use Weld Vertices), non-manifold edges (three or more faces on an edge), flipped normals, or a single vertex shared by two sets of faces.
4. Delete the offending faces or edges, then rebuild with Bridge, Fill Hole or Merge Edge.
5. Try Finish Form again.

Convert a BRep to a Form body#

1. In the Form tab, choose Utilities > Convert.
2. Set Convert Type to BRep Face to T-Splines and set the Selection Filter to BRep Face. Select the solid or surface faces.
3. Set Length Spacing and Width Spacing (Uniform with a Faces count, or Curvature with a Deviation) and the Operation (New Body or New Component). Click OK.
4. T-spline faces are fit to the selected face; the modified faces are automatically combined with the BRep when you complete the tool. You can now edit them with Edit Form.

The conversion fits T-spline faces to the BRep, so it approximates the original: curves may not match exactly and sharp edges may soften (general guidance; check with Inspect).

Convert a mesh to a Form body (Organic)#

1. In the Mesh tab, use Modify > Convert Mesh with the Organic method and the Base Feature operation (see the mesh conversion page). Autodesk's help states that an organic conversion as a Base Feature creates a T-Spline Form in the timeline. Organic conversion is part of the Fusion Design Extension. This is the way to turn a scan into editable freeform geometry.
2. For a mesh made mostly of quads (for example a quad OBJ), you can instead use Form Utilities > Convert with Convert Type Quad Mesh to T-Splines and the filter set to Mesh Body. You get a warning if there are too few quad faces, and mostly-triangle meshes will not convert successfully; for those, create a T-spline body and use Pull or Object Snap to snap it to the mesh.

Convert a Form to a Mesh#

1. Use Mesh tab > Create > Tessellate on the finished solid or surface body (Refinement High, Medium, Low or Custom; optional Create Quads, which lets the mesh convert directly to a T-spline). You can also save bodies as 3MF, OBJ or STL directly.

Options reference#

Tips and pitfalls#

Tip: Finish early and often during development. Seeing the BRep result lets you test Fillet, Shell, and Combine before you've polished the form.

Tip: If you intend to 3D print or mill, finish the form and then use Solid features to add the exact mating geometry. Form is great for shape, not for tolerances.

Warning: Once downstream features (fillets, holes, sketches on form faces) reference faces of the finished body, changing cage topology (adding or removing faces) can break those references. Move vertices freely; avoid structural topology changes after you've built features on top.

Warning: Autodesk states that bodies with intersecting edges or self-intersecting faces cannot be converted to a solid. If your body looks fine in Smooth mode but fails to finish, switch to Box or Control Frame display and look near thin parts and tight creases.

Tip: A form that finishes as a surface (instead of solid) usually has an open edge. Repair Body's Free Edges option highlights open edges; close them with Fill Hole.

Warning: Converting from BRep to T-Spline can create a dense or irregular cage. Prefer rebuilding the shape manually with primitives if you need long-term editability.

Tip: Because Form results are BRep, they are also good bodies to hand over to CAM, simulation and drawings once finished.

What Finish Form actually does#

Finish Form evaluates the T-spline surface and builds BRep faces on it. A closed cage becomes a solid body, and a cage with any open boundary becomes a surface body. Three details are worth knowing.

• The Form feature survives. After Finish Form the timeline holds a Form feature. Double-clicking it reopens the cage. Features that come later in the timeline are recomputed against the new shape when you finish again.
• Creases become edges. A creased edge becomes a real edge between BRep faces, which you can fillet or chamfer in Solid.
• Star points split the surface. Autodesk notes that when a T-spline is converted to BRep it splits into separate surfaces at each star point.

Solid vs surface: closing the form#

If you need a solid but get a surface, find the opening. Switch to Box display (or turn on Free Edges in Repair Body) and look for edges drawn as boundary edges (they sit at the edge of a hole). Typical culprits (general guidance) are a deleted face you forgot, a Bridge that did not include every edge, or a seam between two bodies that was never welded. Fix with Fill Hole, Bridge or Weld Vertices. A form meant to be a thin shell can instead be finished as a surface on purpose and then thickened in Solid with a dimensioned thickness.

Repair Body and the usual topology defects#

Run Repair Body when Finish Form fails or warns. The defects it targets, and how you spot them by hand, are:

If Repair Body does not clear the problem, binary-search with the timeline: roll the Form feature back through its recorded operations until the failure disappears, then redo the last operation more carefully.

Worked failure: pear with a hole at the top#

You model a pear from a Quadball with 3 faces, then delete the top face to start a neck. After Finish Form the body is a surface, not a solid. You wanted a solid, so you roll back inside the Form, extrude the open edge loop upward twice to form the neck, and use Fill Hole with Reduced Star on the final opening. Finish Form now returns a solid. If you instead wanted an open neck, you would keep it open, finish as a surface, and use Solid Thicken 2 mm. The two paths produce different body types for the same cage, which shows why checking the icon in the browser after finishing is a habit worth having.

Re-entering and editing after features exist#

Once Solid features reference faces from the finished body, you can still move vertices safely. Changes that add or remove cage faces (Subdivide, Insert Edge, Delete, Bridge) may change the face identities that later fillets, holes and sketches refer to, so those later features can fail or move. Mitigations:

1. Finish with the topology you need before adding references to individual faces.
2. Reference construction planes and origin geometry instead of form faces when possible.
3. If you must change topology, edit it, finish, then check the timeline for warning icons and repair each downstream feature in order from the earliest.
4. Duplicate the design before the change so you can compare.

Moving between mesh, Form and BRep#

Common failures and fixes#

Practice exercise#

Practice finishing and continuing in solid mode, and provoke one failure deliberately.

1. Make a Quadball with 3 faces, 80 mm across. Pull the lower faces down 25 mm to make a pear shape and Finish Form. Check the browser icon: it should be a solid.
2. In Solid, use Modify > Shell with 2 mm thickness, selecting the top face to remove it so the pear is a hollow vessel.
3. Double-click the Form feature, pull the pear's belly out 5 mm, and finish again. The shell should update to follow.
4. Re-enter the Form, delete one top-side face to open the cage, and Finish Form. Note that the body is now a surface and the shell may fail.
5. Undo that step, reopen the Form, and instead use Subdivide on the belly. Finish Form and check whether the Shell still works. Note any warning icons in the timeline.

Expected result: a hollow pear-shaped vessel that updates when you edit the cage, plus first-hand experience of the difference between a vertex move (safe) and a topology change (may break downstream features). Write down which operation caused any warning.

Related#

• Subdivide and refining Form topology
• Converting a mesh to BRep
• Reverse engineering workflow