Overview#
Two commands bridge the solid and surface worlds:
- Fillet rounds an edge. Fusion has a Fillet for solid edges (Solid > Modify) and a Fillet for surface body edges (Surface > Modify). Fillet is the first thing to try before building a manual blend patch.
- Replace Face swaps one or more faces of a solid with a different face, usually a surface, so the solid reshapes to match. It is the quickest way to give a prismatic solid a freeform top.
Where to find it#
- Fillet: Solid > Modify > Fillet, or press F. For surface edges, Autodesk documents Surface > Modify > Fillet (and Chamfer): select the edges, then use the manipulator or the dialog.
- Replace Face: Solid > Modify > Replace Face (confirmed in Autodesk's help; it is not on the Surface > Modify panel).
Step by step#
Fillet on surface edges#
- As general guidance, stitch the two surfaces first so the edge between them is a shared edge.
- Start Fillet (Surface > Modify > Fillet) and select the edge.
- Enter a radius; pick a Radius Type (Constant, Chord Length, Variable or Asymmetric) as needed.
- Set Continuity (Tangent G1 or Curvature G2) in the dialog.
- Click OK. If the surfaces are free (unstitched) or the radius is too large for the surface extents, the fillet fails.
Replace Face#
- Build the solid (for example, a rounded box) and a trimmed surface that represents the new top.
- Start Replace Face.
- Select the solid faces to be replaced (the old top). These are the Source faces.
- Activate Target Faces and select the face or faces that mark the new location. Targets can be planar or nonplanar solid faces, surface patches or workplanes.
- Click OK. The solid's side walls extend or trim to meet the surface.
Requirement (Autodesk): the target faces must fully intersect and encompass the portion of the body being trimmed or extended. Extend the surface first if needed.
Options reference#
Fillet (most-used options)#
| Option | What it does | Notes/default |
|---|---|---|
| Type | Fillet, Rule Fillet or Full Round Fillet | Fillet is the usual choice |
| Selection set | Edges, faces or features, each with its own Radius Type, Radius and Continuity | Add rows with + |
| Tangent Chain | Selects tangentially connected edges | Uncheck to pick single edges |
| Radius Type | Constant, Chord Length, Variable or Asymmetric | |
| Radius | Fillet size in design units | Must fit available geometry |
| Continuity | Tangent (G1) or Curvature (G2) | G2 gives smoother reflections |
| Corner Type | Rolling Ball or Setback | Where fillets meet at a corner |
Replace Face#
| Option | What it does | Notes |
|---|---|---|
| Source Faces | Faces of the solid to remove | Multiple allowed |
| Target Faces | Faces that give the new location | Planar or nonplanar; solid faces, surface patches or workplanes |
| Timeline | A Replace Face feature appears | Edit it there |
Tips and pitfalls#
Tip: Make the target surface 10 to 20 percent larger than the solid face so the walls definitely meet it.
Tip: For a variable-width blend that Fillet cannot make, build the blend as a Patch with tangent edges or use a rule/variable fillet, then stitch.
Warning: Fillet fails most often on edges with changing angle or when the radius is larger than the adjacent face. Reduce the radius, split the edge, or fillet in a different order (larger radii first).
Warning: Replace Face removes the original face's geometry and re-trims neighbors. If features (holes, bosses) sit on the old face, they may disappear or fail to regenerate.
Tip: For complex tops, an alternative to Replace Face is to split the solid with a surface using Split Body and delete the unwanted half. This is more tolerant of small mismatches.
Fillet: choosing the options with numbers#
The Fillet command (F, Solid > Modify > Fillet) rounds edges with a radius. For surface work the questions are which edges, which radius, and which continuity.
Worked example: a 6 mm blend between two walls#
Two walls meet at a 120 degree angle along a 50 mm edge. A 6 mm constant radius fillet gives a circular arc of 6 mm radius, tangent to both walls. The length of the arc is r x angle, where the angle turned is 180 minus 120 = 60 degrees = 1.047 radians, so the arc is 6.28 mm long. The fillet consumes tangent lengths of r x tan(angle / 2) = 6 x tan(30 degrees) = 3.46 mm back from the edge on each wall. If either wall is narrower than 3.46 mm at that location, the fillet fails.
Compare options:
| Option | Effect | Use |
|---|---|---|
| Constant radius | One radius along the edge | Default; most edges |
| Chord length | You specify the chord instead of the radius | When the visible width of the blend matters |
| Variable radius | Different radii at points along the edge | Tapering blends such as a handle |
| Tangent (G1) | Standard circular blend | Most manufactured parts |
| Curvature (G2) | Smoother blend, curvature continuous | Visible, glossy transitions |
| Setback corner | Blends rounded corners into adjacent faces | Corners where fillets meet |
The fillet "rolling ball" model means a radius larger than the shortest adjacent face width fails. A 10 mm radius cannot fit along an edge whose neighboring face is only 8 mm wide.
Replace Face: what it is for#
Replace Face lives in Solid > Modify. It takes one or more faces of a solid body and replaces them with a target surface, trimming or extending the body to meet it. It is the clean way to shape a solid face to a freeform surface: you build a surface, then replace a flat face with it, rather than trying to cut with a boolean.
Worked example: a curved top on a block#
- Model a 60 x 40 x 20 mm block.
- Create a surface with a 10 mm rise across its 60 mm width (for example, an arc surface extruded across the block), extended wider than the block.
- Choose Replace Face, pick the top face as the source, pick the surface as the target. Fusion trims or extends the block to the surface.
- Hide the surface. The block's top is now the arc. Check height at the middle with Measure: it should read 30 mm if the surface rises 10 mm above the original top.
The target must fully cover the face region, so extend it generously. A target that falls short leaves the command unable to find a complete replacement, and a target that crosses the faces more than once produces the wrong region.
Failure modes and fixes#
| Symptom | Cause | Fix |
|---|---|---|
| Fillet fails on a long edge | Radius is larger than the adjacent face allows, or neighbors are tangent | Reduce the radius, split the job, or fillet the larger radius first. |
| Fillet fails near a corner | Three edges meet and the radii conflict | Use a setback corner, or fillet in order: largest first. |
| G2 fillet looks flat | Continuity was set to Tangent (G1) | Switch Continuity to Curvature (G2) and compare with Zebra. |
| Replace Face reports no replacement | Target does not cover the face region | Extend the target surface beyond the face boundaries. |
| Replace Face removes more than expected | Target intersects the body twice | Trim the target to a simple, single crossing. |
| Result has sharp edges at the boundary | Neighbors not tangent to the target | Add a fillet after the replacement. |
| Command not on the Surface tab | It is a Solid command | Use Solid > Modify > Replace Face. |
Order of operations#
- Make the large shape (Replace Face, loft, patch).
- Add the large fillets.
- Add the small fillets last.
- Shell or thicken only after fillets are done, so wall offsets are consistent.
Rounding the corner before shaping the surface leaves the fillet tied to the old geometry, and Replace Face afterwards often removes it.
Fillets on surface bodies#
Autodesk documents Surface > Modify > Fillet for surface body edges. If a given edge cannot be rounded, convert the surfaces to a solid with Stitch, round the solid, or build the blend as a separate surface using Patch or Loft with G1/G2 edge conditions.
Practice exercise#
- Extrude a 60 by 40 mm rectangle 20 mm as a solid.
- Create a gently domed surface above it with a Loft or a Patch and extend it beyond the box footprint.
- Replace the box's top face with the surface.
- Fillet the top edges with a 2 mm radius, Continuity set to Curvature (G2).
- Check the result with Zebra Analysis.
Expected: a box with a domed top, rounded edges, and smooth stripes through the fillet.
Stretch challenge#
Start with a 60 x 40 x 20 mm block. Fillet the four vertical edges at 5 mm, then use Replace Face on the top face with an arc surface that rises 8 mm at the center. Fillet the new top perimeter at 2 mm. Expected result: the top surface rises to 28 mm at its midline, the 2 mm fillets run around the curved perimeter without failing, and a Zebra analysis shows smooth stripes crossing the fillet. If the fillet fails, try 1.5 mm or inspect for tiny faces left over from the replacement.
Related#
- Stitch and Unstitch
- Trim and Untrim
- Continuity G0 to G3