Splines: Fit Point, Control Point, and Conic Curves

Overview#

Not everything is a line or an arc. Ergonomic grips, aerodynamic outlines, cam profiles, and surface rails all need free-form curves. Fusion provides three families of free-form sketch curves:

• Fit Point Spline: the curve passes through points you place.
• Control Point Spline: the curve is shaped by a control frame of points; the curve generally does not touch them except at the ends.
• Conic Curve: a curve driven by two end points, a vertex point and a Rho value.

Understanding the difference helps you pick the right tool for the shape you want, and for how you plan to edit it.

Where to find it#

Press S to search for a command by name.

Step by step: fit point spline#

1. Choose Create > Spline > Fit Point Spline.
2. Click to place the first end point, then click each point the spline should pass through.
3. To finish, click the Check icon. To close the spline into a loop, click the start point.
4. Select the spline to see its curvature handles.
5. Drag the end points of a curvature handle to adjust the curvature of the spline (right-click the spline to Activate or Deactivate Curvature Handle). With the Move/Copy tool active, selecting any spline point shows or hides all of the manipulator handles.
6. Dimension the points or constrain them to other geometry so the shape is repeatable.

Behavior notes.

• Snapping to existing geometry adds the logical constraints, such as Coincident.
• Click the end point of any line, arc or spline and drag the pointer away from it to create a tangent constraint between the spline and that geometry; it appears once the spline is complete.
• Right-click the spline for Open/Close Spline Curve and Insert Spline Fit Point (fit point splines only).
• A fit point spline is stable and intuitive but tends to wobble if you place points unevenly; a few well-placed points give a smoother curve than many.

Step by step: control point spline#

1. Choose Create > Spline > Control Point Spline.
2. In the Sketch Palette, choose the degree: Degree 5 (the default, recommended for general modeling) or Degree 3 (recommended in the sculpt environment).
3. Click to place the first end point, then keep clicking to define the control frame.
4. Click the Check icon to complete the spline.
5. To edit, right-click the spline and use Insert Spline Control Point (control point splines only), or drag the control points.

The curve is shaped by the points but not forced through them, apart from the ends.

Degree 3 versus degree 5#

Per Autodesk's Spline degree control reference, a higher-degree spline is typically smoother but needs more control points. Degree 5 splines ensure G4 continuity internal to the curve and suit primary surfaces and transition surfaces; degree 3 splines usually need fewer control points and are easier to work with, but are only guaranteed G2-continuous internally (a degree 3 spline with many control points can show bumps in the curvature comb). G2 is smooth enough for most uses; surfaces from G2 curves still have smooth highlights. With a Control Point Spline selected, Change Spline Degree in the Sketch Palette increases the degree (adds control points) or decreases it (fewer control points, which alters and simplifies the shape).

Fit point versus control point#

Handles, tangent, and curvature#

Selecting a spline shows its curvature handles; dragging their end points adjusts the curvature. A Tangent constraint on a spline end point to an adjacent line or arc fixes the direction. A Curvature constraint (Sketch > Constraints > Curvature) creates smooth G2 continuity between a spline and another sketch curve, which can be a spline or a regular curve. Select the spline and the chained curve. Use it for junctions that will be lofted or swept into visible surfaces.

The curvature comb is a diagnostic overlay. Right-click the selected spline and choose Toggle Curvature Display; in the Setup Curvature Display dialog adjust the Density and Scale of the comb. A smooth curve has a smooth comb, and kinks or sudden jumps show up immediately.

Conic curves#

A conic curve (Create > Conic Curve) is driven by end points, a vertex and a Rho value:

1. Click the first end point.
2. Click the second end point.
3. Click the vertex point.
4. Drag the Rho manipulator handle or specify an exact Rho value.

Rho controls how full the curve is. As general geometry (not stated in Autodesk's help), a conic section is an ellipse arc for Rho below 0.5, a parabola at 0.5, and a hyperbola above 0.5. Conic curves are a way to blend two lines with a smoother transition than a circular fillet.

Note: not confirmed in Autodesk's documentation: which conic-curve values can be dimensioned.

Options reference#

Tips and pitfalls#

Tip: Use fewer points. A common beginner error is placing eight points when four would do. Extra points introduce wobble that the curvature comb reveals immediately.

Tip: Draw the rough spline first, then add dimensions and constraints. Lock the end points with Coincident and Tangent, and leave interior points free so that the curve shape is determined by a handful of explicit parameters.

Tip: When you need the curve to hit exact points (for example, tracing a canvas image or a measured profile), use a Fit Point Spline. When you need beautiful flowing curves, use a Control Point Spline.

Warning: As general guidance, avoid constraining every point of a spline; it is easy to over-constrain it.

Warning: A spline whose end point is near but not on the neighboring geometry creates a gap that prevents the profile from closing. Use Coincident constraints on the ends rather than relying on visual proximity.

Warning: If the spline changes shape unexpectedly after adding a Curvature (G2) constraint, undo and check whether the geometry is compatible.

Practice exercise#

1. In a sketch, draw a horizontal line from the origin to (40, 0) mm, and a vertical line from (80, 30) up to (80, 70).
2. Add a Control Point Spline from the end of the first line to the bottom of the second, using four control points. Apply Tangent constraints at both ends.
3. Turn on the curvature comb (right-click the spline > Toggle Curvature Display) and drag interior control points until the comb is smooth with a single gentle peak.
4. Add a Curvature (G2) constraint at one end and see how the comb changes there.
5. Repeat using a Fit Point Spline for comparison and note the differences.
6. Close the sketch with straight segments and extrude the profile.

Expected result: a smooth curved transition between two straight edges; you can articulate why the control-point curve is smoother with fewer edits.

Related#

• Constraints (02-sketching/constraints)
• Lines, arcs, and points (02-sketching/lines-and-arcs)
• Modify tools (02-sketching/modify-tools)
• Troubleshooting sketches (02-sketching/troubleshooting)