Overview#
Loft creates a smooth transition between two or more profiles. The profiles can be sketches, planar faces, edges, or even points at the ends. Typical uses: bottles, funnels, ducts that change from round to rectangular, boat hulls, ergonomic handles, aircraft fuselages, and mouse shells.
Where Extrude keeps the cross-section constant, Loft lets it change shape and size. Where Sweep follows a path, Loft interpolates between sections you place yourself. Greater control comes from rails (curves that guide the surface edges) and a centerline (a curve that shapes the way profiles are spaced).
Where to find it#
- Workspace: Design.
- Tab / panel: Solid tab > Create > Loft.
- Shortcut: none listed in Autodesk's shortcut reference; press S and type "Loft".
- Surface tab: Surface > Create > Loft for surface lofts from open profiles.
Step by step#
- Create profile planes. Use Construct > Offset Plane, or sketch on existing faces. For a vase, make planes at 0, 50, 100 and 150 mm.
- Sketch each profile. Draw a closed shape on each plane: for example a 60 mm circle at the bottom, an 80 mm circle at 50 mm, a 40 mm circle at 100, and a 50 mm circle at 150. Finish each sketch.
- Open Loft. Create > Loft. The Profiles selector is active.
- Pick profiles in order. Click each profile in sequence from bottom to top. Order matters: Fusion lofts in selection order, and numbered tags appear on the profiles. The Profiles table has Reorder, Add and Remove buttons.
- Check the preview. Watch for twisting. If the surface twists, the start points of the profiles are mismatched (see Tips).
- Adjust end conditions. Use the End Condition selectors at the start and end profiles (Free, Direction, Tangent, Smooth, Sharp, Point Tangent; the available ones depend on the profile geometry) to control how the loft leaves each end.
- Add rails or a centerline if needed (set Guide Type to Rail or Centerline; select a profile label in the canvas to edit that profile's options).
- Set Operation and confirm.
Worked example: a round-to-square duct#
- Sketch a 50 mm circle on XY. Create an offset plane 80 mm above. Sketch a 50 mm square centered there.
- Loft: profile 1 = circle, profile 2 = square. The preview should be smooth. If twisted, edit the sketch so a square corner is vertically aligned with the circle's seam point.
- Click OK.
For a hollow duct, use Shell afterward and remove the end faces.
Options reference#
| Option | What it does | Notes/default |
|---|---|---|
| Profiles | Ordered list of sections to blend. | Can be sketch profiles, planar faces, edges, or a single point at an end. |
| Guide Type: Rail | 2D or 3D curves that affect the loft shape between sections. | You can add any number of rails. They must intersect each section, terminate on or beyond the first and last sections, and be tangent continuous. |
| Guide Type: Centerline | A type of rail to which the sections are held normal, behaving like a sweep path. | Same criteria as rails except it need not intersect the sections; only one allowed. |
| End Condition: Free | No end condition is applied. | |
| End Condition: Direction | Angle measured off the sketch plane. | Only for 2D sketch profiles. Uses Takeoff Angle and Takeoff Weight. |
| End Condition: Tangent | G1 tangent off the profile. | Needs body edges or faces. Uses Tangency Weight. |
| End Condition: Smooth | G2 curvature continuity off the profile. | Needs body edges or faces. |
| End Condition: Sharp | Ends at a sharp point. | For sketch or construction points. |
| End Condition: Point Tangent | Tangent at a point end, giving a dome. | For sketch or construction points. |
| Takeoff Angle | Start angle of the transition from the profile. | Direction end condition only. |
| Takeoff Weight | Amount of influence the takeoff angle has along the loft. | Direction end condition only. |
| Tangency Weight | Weight of Tangent, Smooth or Point Tangent. | |
| Reorder / Add / Remove | Change the order of profiles; add or remove profile rows. | |
| Chain Selection | ON: adjacent edges are selected together as one profile. OFF: edges are selected individually. | |
| Closed | Connects the last profile back to the first. | Torus-like ring. |
| Tangent Edges | Merge merges tangent edges; Keep leaves them unmerged. | |
| Analysis tab | Zebra, Curvature Map or Isocurve analysis of the previewed result. | Select Bodies to analyze. |
| Operation | Join, Cut, Intersect, New Body, New Component. | New Component is Hybrid design only. |
Tips and pitfalls#
Tip: Control twisting by checking the start point (the seam) of each closed profile. Autodesk's reference says that when closed sections have matching numbers of corners, Fusion maps the matching corners automatically; this does not apply to open sections, profiles without matching corners, or centerline lofts. Autodesk documents no manual point-mapping control, so control correspondence by editing the sketches (placing split points and matching segment counts) or by adding rails.
Tip: Profiles with the same number of segments loft more predictably. A circle (1 edge) to a square (4 edges) works, but to control shape, split the circle into four arcs with Split tools.
Warning: Selecting profiles out of order produces an hourglass or self-intersecting shape. Remove and reselect in the intended sequence.
Warning: Rails must intersect every profile exactly. Floating rails make the loft fail. Use Project or Sketch Intersect to tie the rail to profile points.
- A point profile (select a vertex or sketch point) at one end produces a pointed tip, such as a bullet or cone.
- Using too many profiles with wild changes makes wavy surfaces. Use as few sections as possible, and add rails for control.
- Tangent, Smooth and edge-alignment conditions need body edges or faces as the profile; make the base shape first.
- Use Closed only when the last profile should connect to the first, as in a donut-shaped loop.
- Centerline lofts are strong for bent shapes such as a curved neck, with sections held normal to the centerline.
Planning a loft#
Most failed lofts are planned badly, not operated badly. Before opening the dialog, decide four things.
1. How many sections? Two sections give a straight blend; three give one bend; each additional section adds an inflection. Use the fewest that describe the shape. A bottle needs base, shoulder, neck and lip, which is four. Adding a fifth section to correct a bulge usually creates a wobble elsewhere; add a rail instead.
2. Do the sections have matching topology? Fusion blends corresponding edges. A circle (one edge) to a rectangle (four) works because it distributes, but a polygon of 4 edges to an octagon places the extra vertices arbitrarily. Where you need control, split the circle at four quadrant points so each arc corresponds to one rectangle side.
3. Where are the seams? Each closed profile has a start vertex. If seam vertices sit on opposite sides, the loft twists by up to 180 degrees. Align them by sketching a short construction line from the center to each seam, or by splitting both profiles at the same angle.
4. Which guide? Rails pull the surface edges to follow specific curves; use them when you know the silhouette. A Centerline holds each section normal to a spine; use it for curved necks and hoses. Autodesk's documentation says only one centerline is allowed and that it need not touch the sections, whereas rails must intersect all sections.
Worked example: a bottle with numbers#
- Planes: XY at 0, offset planes at 20, 90, 120, 150 and 170 mm.
- Profiles: base 60 mm circle at 0, body 70 mm circle at 20, a 66 mm circle at 90 to bring the shoulder in gradually, shoulder 28 mm circle at 120, neck 24 mm circle at 150, lip 30 mm circle at 170.
- Loft with Chain Selection on and Tangent Edges set to Merge, so tangent edges are merged.
- Shell inward 2 mm, removing the top face.
- Volume check: the body between 20 and 90 mm is roughly a cylinder of 68 mm diameter, 70 mm tall: pi x 34^2 x 70 = about 254 cm3 (254 ml). Read the interior volume after shelling from the body's properties.
Rails: the silhouette method#
Draw a spline on the front plane that describes the silhouette, with its start and end points exactly on the first and last profile planes. Then sketch each profile so that the point where the rail passes through the plane is a vertex or on the profile (use Project or Intersect to tie it on). If a profile does not lie on the rail, the loft fails. Because rails must be tangent continuous, break a sharp-cornered silhouette into two lofts joined at a shared section instead of using one rail with a kink.
End conditions in depth#
- Direction: Takeoff Angle is the start angle of the transition, measured off the sketch plane, so 90 degrees starts the loft perpendicular to the plane (a reasonable reading of "measured off the sketch plane"); try smaller and larger angles and watch the preview. Takeoff Weight sets how far the angle's influence carries along the loft; larger values are more pronounced and can overshoot.
- Tangent / Smooth: when the profile is a body edge, the loft meets the face with G1 or G2 continuity. As general modeling practice, a high Tangency Weight tends to bulge the loft near the join; watch the preview.
- Sharp / Point Tangent: available when the profile is a sketch point or construction point; Sharp transitions to a sharp point, Point Tangent applies tangency at the point to create a dome.
Joining a loft to an existing body#
Select an edge loop of the body as the first profile and set the end condition to Tangent so the loft flows out of it; for a mounting boss with a blended neck, pick the boss top edge and a sketch circle above.
Failure modes and fixes#
| Symptom | Cause | Fix |
|---|---|---|
| Hourglass or bow-tie body | Profiles picked out of order | Reorder in the Profiles table |
| Spiral twist | Seams on opposite sides | Align seam vertices or split the profiles |
| "Loft fails with rails" | Rail floats or is not tangent continuous | Constrain rail endpoints onto profile geometry; split at kinks |
| Loft self-intersects at tight sections | Very small neck next to a large body | Add intermediate profiles or reduce Takeoff Weight |
| Flat spot at joins | Tangency Weight too low | Raise the weight and watch the preview |
| Shell fails after loft | Surface curvature tighter than wall thickness | Shell thinner, or lower curvature with larger neck |
| Loft cannot select a profile | Sketch not closed or profile overlaps | Check the sketch for gaps |
Practice exercise#
Design a computer mouse shell blank, then rebuild it two ways and compare.
- On XY draw a 60 x 100 mm rounded rectangle (corner radius 20 mm).
- Make an offset plane 20 mm up and draw a 50 x 85 mm rounded rectangle (radius 18 mm), then another plane at 32 mm with a 25 x 50 mm ellipse.
- Loft the three profiles with the top End Condition set to Direction, Takeoff Angle 90 degrees and a Takeoff Weight of 1. Preview and then try other angles such as 60 and 120 degrees; note how the top changes.
- Draw a side-profile spline on the YZ plane from the base edge to the top of the ellipse, with a peak at the rear, and use it as a Rail. Rails need to pass through each profile, so project the sections' side points onto the spline first.
- Shell inward 1.5 mm, removing the bottom face. Measure wall thickness at the top.
- Rebuild the same body with a Centerline (a curved spline in the YZ plane) and compare the two bodies with Inspect > Zebra Analysis.
- Break it: swap the selection order of profile 2 and 3 and describe the result, then fix it with the Profiles table.
Expected result: a domed shell body with a hollow interior of 1.5 mm walls, a clear sense of how Direction angle and weight shape the top, and a documented example of selection-order failure.
Related#
- Sweep (constant section along a path)
- Shell (hollowing a lofted body)
- Construction geometry (profile planes)
- Fillet and chamfer (finishing edges)