Surfaces > Swept Surfaces

To sweep a curve along one or two paths

Create a surface by sweeping or blending one or more generation curves along one or two path curves (rails).

The Rail Surface tool takes its name from its use of one or two “rail” curves. The generation curves maintain contact with the rails at the same points through the sweep, like a train traveling on a track.

The Rail Surface tool is similar to the Extrude tool, but has many more options and much more power.

The appearance of the Rail Surface icon depends on the Generation Curves and Rail Curves settings in the Rail Surface Options window.

Icon
Generation Curves	1	1	2/2+	2	2+
Rail Curves	1	2	1	2	2

To sweep one generation curve along one rail curve

The procedure for using the Rail Surface tool has several optional steps, depending on the settings in the Rail Surface Control window.

Double-click the Rail Surface icon, or choose Surfaces > Swept Surfaces > Rail Surface ❒ from the tool palette.
The Rail Surface Control window appears.
Make sure Create History is checked.
You almost always want construction history on. This allows you to change the settings after you sweep the curves.

This example uses the following Advanced Tab settings.

When you are satisfied with the new surface, you can delete the history information with Delete > Delete Construction History .
Click the generation curve. This is the curve that is swept along the rail curve to create the surface.
The generation curve can be a free curve, isopam curve, curve on surface, or trim edge.
If Curve Segments is on, use the locators to place the start and end edit points of the generation curve.
If Sweep Pivot is On Curve or Off Curve, use the locator to set the pivot point of the generation curve.
- Click the locator and drag to move the locator along the curve (On Curve) or in space (Off Curve).
- Type a parameter (On Curve) or 3D coordinate (Off Curve) to set the pivot exactly.
- Click Go to continue.
Click the rail curve. This is the path along which the generation curve travels.
The rail curve can be a free curve, isoparametric curve, curve on surface, or trim edge.
If Curve Segments is on, use the locators to set the start and end edit points of the rail curve.
If Sweep Pivot is On Curve or Off Curve, use the locator to set the pivot point on the rail curve.
- Click the locator and drag to move the locator along the curve.
- Type a parameter to set the pivot exactly
- Click Go to continue.
If the Sweep Mode is Spine, click the spine curve. This curve controls the orientation of the generation curve as it travels along the rail curve.

To sweep one or more generation curves along two rail curves

Double-click the Rail Surface icon, or choose Surfaces > Swept Surfaces > Rail Surface ❒ from the tool palette.
The Rail Surface Control window appears.
Make sure Create History is checked.
You almost always want construction history on. This allows you to change the settings after you sweep the curves.

When you are satisfied with the new surface, you can delete the history information with Delete > Delete Construction History .
In the Rail Surface Control window, set Rail Curves to 2.
Set Generation Curves to the number of generation curves to use:
- Choose 1 to sweep one curve along the rails.
- Choose 2 to blend from one generation curve to another along the rails.
- Choose 2+ to blend a series of generation curves along the rails.
Click the first-generation curve.
The generation curve(s) can be a free curve, isoparametric curve, curve on surface, or trim edge.
If Generation Curves is 2, choose the second-generation curve.
If Generation Curves is 2+, pick all the generation curves in order, then click Go.
Click the first rail curve, then click the second rail curve.
The generation curve(s) must intersect both rail curves.

The rail curves can be free curves, isoparametric curves, curves on surface, or trim edges.

To create a helical shape

Create a generation curve and a rail curve. The distance between the generation curve and rail curve is the radius of the helix.
Double-click the Rail Surface icon, or choose Surfaces > Swept Surfaces > Rail Surface ❒ from the tool palette.
In the Rail Surface Control window:
- Set Generation Curves to 1.
- Set Rail Curves to 1.
- Set Sweep Mode to View.
- Set the Sweep Projection to the plane to which the generation curve is parallel.
- Set the Sweep Pivot to Off Curve.
- Turn on Auto. Recalc.
Click the generation curve.
A 3D locator appears and you are prompted for the generation curve pivot. Use magnet snapping to move it to the starting point of the rail curve. Then click Go.
Click the rail curve.
A curve locator appears for the rail curve pivot. Move the locator to the starting point of the rail curve, then click Go.
The generation curve’s pivot point moves along the rail curve. Since the pivot is offset from the actual generation curve, the generation curve sweeps the new surface at an offset from the rail curve.
In the Rotate Transform field, enter the degrees of rotation you want as the generation curve travels around the rail curve.
For example, if you enter 360, the generation curve makes one full revolution as it travels the length of the rail curve.

helix curves helix curves active

To create a funnel shape

Create a generation curve and a rail curve.
Double-click the Rail Surface icon, or choose Surfaces > Swept Surfaces > Rail Surface ❒ from the tool palette.
In the Rail Surface Control window:
- Set Generation Curves to 1.
- Set Rail Curves to 1.
- Set the Sweep Pivot to Off Curve.
- Turn on Auto. Recalc.
Click the generation curve.
A 3D locator appears and you are prompted for the generation curve pivot. Use magnet snapping to move it to the center of the generation curve. Then click Go.
Click the rail curve.
A curve locator appears for the rail curve pivot. Move the locator to the starting point of the rail curve. Then click Go.
In the Scale Transform field, enter the factor by which you want the generation curve to grow as it travels along the rail curve.
For example, if you enter 2, the generation curve grows twice as large as it travels the length of the rail curve. If you enter 0.5, it shrinks by half.

To edit the construction history of a Rail surface

Pick the surface you want to edit.
Click the Rail Surface icon, or choose Surfaces > Swept Surfaces > Rail Surface from the tool palette.
The Rail Surface Control window appears.
Use the curve modification tools (in the Transform, Curve Edit, and Object Edit palettes) to reshape the curves used to create the surface, and use the Rail Surface Control window to change the surface creation options.

Tips and notes

The Spine sweep mode can be very tricky. If you have trouble with it, try the following:
- Create a copy of the rail curve to use as the spine curve.
- Turn on Create History and Auto Recalc in the Rail Surface Control window.
- Reshape the spine curve in small steps and observe the effect on the surface.
After you start a surface in Natural, Parallel, or View mode, you cannot convert it to Spine mode.
You cannot create a rail surface along a rail curve with multi-knots or weighted CVs.
Turn on the Rebuild option for all rail curves to remove multi-knots and set the weight of all CVs to 1 before creating the surface.
It is the generation curve pivot that travels along the rail curve. If the pivot is not on the generation curve, the new surface will be offset.
Changing the settings after you have clicked the generation curve does not affect the rest of the procedure for using the Rail Surface tool.
For example, if you turn on the Curve Segments option after you have already picked the generation curve, you will not be asked to define curve segments. However, after the surface is built, text boxes will appear in the Rail Surface Control window where you can change the start and end parameters for the curve segments. You can also click the Next button and start again.
The Rail Surface tool does not support rail curves with sharp corners (cusps).
Use curve snapping to make sure the generation curves intersect the rail curves.
The Rail Surface tool tests whether the curves intersect using the Curve Fit Distance value in Construction Options. The default tolerance depends on your Alias product. If the curves are within the default tolerance of each other, they are seen as intersecting.
To change the Curve Fit Distance option, choose Construction Options from the Preferences menu.
When using two or more generation curves, after you pick the first generation curve you can unpick it by clicking it. You can then pick a different first generation curve.
The Rail Surface tool can produce poor surfaces with long generation curves.
In these cases, try reversing the choice of generation and rail curves, and choose the shorter, simpler curves for the generation curves and long, winding curves as the rails.

Rail Control window

Generation Curves: 1 – Sweep one generation curve along the rail curves.

2 – Blend between a start and end generation curve along the rail curves. Use the Gen. Blend Value slider to control the midpoint of the blend.

2+ – Blend between a series of generation curves along the rail curves.
Rail Curves: The number of paths to sweep the generation curve(s) along: either 1 or 2. The default setting is 2.

Setting Rail Curves to 1 is similar to using the original Swept tool (in Alias 9.5). Setting Rail Curves to 2 is similar to using the original Birail tool.

Continuity table

The Continuity Table displays one row for each boundary curve involved in the rail operation.

Use the pop-up menu next to each curve to set the level of continuity you want across that curve.
Use the Continuity pop-up menu above the table to set the continuity you want for all of the curves at once.
Click the checkboxes at the end of each row to rebuild the curves to reduce data and improve parameterization.
Use the Rebuild pop-up menu to set rebuild on or off for all the curves at once.
For example, before creating the surface, rebuild the first rail curve, second rail curve, first generation curve, and/or last generation curve. (To rebuild interior generation curves, use the Rebuild Interior Gen. Curves option below.)

This can improve the parameterization and reduce the complexity of the new surface when the curves are complex.

Rebuilding curves can also remove multi-knots and reset the weight of all CVs in the rail curve(s) to 1. This is necessary for rail curves with multi-knots or CV weights.

Continuity: Position – Only keep positional continuity. This is the default.

Implied Tangent – Try to keep tangency with an implied surface that shares this edge. The implied surface is the surface that would be created by mirroring the new surface.

Implied Curvature – Try to keep curvature continuity with an implied surface that shares this edge. The implied surface is the surface that would be created by mirroring the new surface.

Implied Tangent and Implied Curvature are powerful features. They let you model one half of a symmetrical surface (such as a car body), and maintain continuity at the seam. When you duplicate the surface to create the other half, the seam will already be continuous.

NoteFor this to work across a symmetry plane, you must make sure the ends of the curves are tangent or curvature continuous across the symmetry plane. (For tangency, this means the tangents are perpendicular to the symmetry plane).

Tangent Angle – Try to keep tangency at an angle with a surface that shares this edge.

The Rail Surface tool calculates the tangent angle at both ends of the common edge. If the two angles are different, Rail Surface blends between them to determine the angle to keep at every point along the edge.

A tangent angle of 0.0 (or 180, -180, 360) is equivalent to Tangent continuity.

Tangent – Try to keep tangency with a surface that shares this edge.

Curvature – Try to keep curvature continuity with a surface that shares this edge.
Rebuild Interior Gen. Curves: Rebuilds all interior generation curves. This option is only available when Generation Curves is 2+.

This option is only available either when Rail Curves is 1 or when Rail Curves is 2 and at least one rail curve is being rebuilt.

Sweep Mode

Proportional – Rebuild the rail curves to create a surface with proportionally spaced spans and smooth parameterization based on the first rail curve.

This option is only available when Rail Curves is 2 and at least one rail curve is being rebuilt.

Natural – As the generation curve sweeps along the rail, it pivots to maintain the same angle relative to the rail curve.

This option is only available when Rail Curves is 1. It has the same effect as the Tube option in the Extrude tool..

Parallel – As the generation curve sweeps along the rail, it maintains its original orientation.

This option is only available when Rail Curves is 1. It has the same effect as the Flat option in the Extrude tool.

View – As the generation curve sweeps along the rail, it only rotates in one plane (choose the plane with the Sweep Projection pop-up menu). This maintains the visual angle between the generation and rail curves.

Spine – Use a spine curve to control the orientation of the generation curve as it sweeps along the rail.

Sweep Projection

This option controls the plane in which the generation curve is allowed to rotate when the Sweep Mode is View.

This option appears when Sweep Mode is View.

Active – Take the rotation plane from the current view window.

XY XZ YZ – Use a global plane (XY, XZ, or YZ) as the rotation plane.

User – Type a 3D vector in the View Plane text boxes to define a plane.

Sweep Pivot

This option is only available when Rail Curves is 1.

Sweeping involves two pivot points:

The point on or off the generation curve (the Generation Pivot) that stays on the rail curve as the generation curve sweeps. The generation curve rotates around and scales from this point.
The point on the rail curve (the rail pivot) that corresponds to the generation curve before it is swept. In other words, the part of the surface corresponding to this point on the rail curve will have the exact cross section of the generation curve.

You can have Rail Surface set the pivot points automatically (as in Extrude), or set them manually for the generation and rail curves.

Closest – Set the pivots to the closest points on the two curves.

This option is the default. It works best when the generation curve is close to the start or end points of the rail curve.

On Curve – Set the pivots to specific parameters on the two curves.

Use the Gen. Pivot Parameter and Rail Pivot Parameter sliders to set the parameters for each curve.

Off Curve – Set the pivots to a specific parameter on the rail curve and a 3D point in space for the generation curve.

Use the Rail Pivot Parameter slider to set the parameter for the rail curve.
Use the Gen. Space Pivot text boxes to set the 3D pivot point for the generation curve.

Transform Control

This option controls how the Rail Surface tool modifies the generation curves to stay on the rail curves during the sweep. It is only available when Rail Curves is 2.

Scale – Scale the generation curve proportionally, to the size necessary for the curve to stay on both rails.

Non-Prop Scale – Scale the generation curve non-proportionally, along the vector connecting the rail curves.

Rotate & Trim – Rotate the generation curve around the intersection with the first rail curve, so that the generation curve stays on both rails. Trim away any part of the new surface that goes beyond either rail curve. The generation curve is not scaled.

This option only applies when Generation Curves is 1.

Rotate No Trim – Rotate the generation curve around the intersection with the first rail curve, so that the generation curve stays on both rails. The generation curve is not scaled.

This option only applies when Generation Curves is 1.

Blend Control – Enable the Gen. Blend Value slider to control the midpoint of the blend between the two generation curves.

This option appears when Generation Curves is 2.

Gen. Blend Value – The percentage of the distance (from 0 to 1) along the rails where the influence of both generation curves is equal.

For example, a value of 0.5 (the default) blends the two curves equally, so that the halfway point of the blend occurs at the midpoint of the rails.

A value of 0.25 places the halfway point of the blend 25% of the distance along the rail curves, hence giving more influence to the second generation curve. Conversely, a value of 0.75 would give more influence to the first generation curve.

The values 0.0 and 1.0 still blend the two curves a minimum amount.

Fixed Curve

This option determines which of the curves will be moved in order to make the two pivot points coincide before the sweep. This in turn controls where the new surface will be created.

This option is only available when Rail Curves and Generation Curves are both 1.

GEN. – Keep the generation curve fixed and move the rail. The new surface is built beginning at the generation curve.

RAIL – Keep the rail fixed and move the generation curve. The new surface is built along the rail curve.

Rotate Xform

Available only when Rail Curves and Generation Curves are both 1.

The degree of rotation as the generation curve sweeps along the rail curve.

For example, if Rotate Transform is 45, the generation curve will rotate 45 degrees as it sweeps along the length of the rail curve. Use negative numbers to rotate in the opposite direction.

Scale Xform

Available only when Rail Curves and Generation Curves are both 1.

The scale factor as the generation curve sweeps along the rail curve.

For example, if Scale Transform is 2, the generation curve will double in size as it sweeps along the length of the rail curve. If Scale Transform is 0.5, the generation curve will shrink by half as it sweeps.

Curve Segments

This option only applies when Generation Curves is 1.

Specify sections of the generation and rail curves to use in the sweep.

When this option is on, you will be prompted during the sweep procedure to click start and end edit points on the first rail curve and possibly (depending on the other settings) on the second rail and/or the generation curve.

After the surface is built:

Use the locators (in each corner of the surface) to change the start and end parameters of the generation and rail curves.

or

Use the Gen. 1 Segment and/or Gen 2. Segment text boxes to change the start and end parameters of the generation curve(s). Use the Rail 1 Segment and Rail 2 Segment text boxes to change the start and end parameters of the rail curves.

Explicit Control

Turn on this option to open the Explicit Control Options section that allows you to explicitly specify the degree and number of spans of the rail surface in both the U and V direction.

Explicit Control Options

These options are available only when Explicit Control is on.

Rail Degree (U): Degree of the rail surface in the U direction (along the rail curves).
Gen. Degree (V): Degree of the rail surface in the V direction (along the generation curves).
Rail Spans (U): Number of spans of the rail surface in the U direction.
Gen. Spans (V): Number of spans of the rail surface in the V direction.

Continuity Options

Max. New Spans: Specify the maximum number of spans that can be added to the surface (in both the U and V direction) when attempting to maintain the requested levels of continuity.

If this number is too small, continuity may fail along some of the edges.

Max. New Spans is not available when Explicit Control is turned on.
Insert at Midpoint: On – Insert extra edit points at the midpoint of the span with the largest continuity deviation. This is the default, and results in a better distribution of the isoparametric curves.

Off – Insert extra edit points at the location of the largest continuity deviation.

Insert at Midpoint is not available when Explicit Control is turned on.

Colinear Options

Gen.1/Gen.2/Rail 1/Rail 2: Check the boundaries across which you want the isoparametric curves of the new surface to line up with adjacent surfaces.

This is similar to turning off Skews in the Align tool.

Control Options

Create History

Save the history of the new surface for later editing. If you turn Create History on, you can modify the curves that were used to create the surface, and the surface will update.

Auto Recalc

Update the new surface automatically as you change the values in the Rail Surface Control window.

Boundary Labels

Label the generation, rail, and spine curves in the view windows. The labels also show

the kind of continuity wanted,
whether the continuity failed, and
which tangents are implied.

Continuity Check

Display the surface continuity locator at the boundaries between the rail surface and adjacent surfaces. The locator is persistent and will remain after you exit from the Rail Surface tool. To remove it, use Pick > Locator to pick the locator, then select Delete > Delete Active , or toggle the checkmark off when entering Rail Surface tool again.

Buttons

Recalc: Recalculate the surface with the current values in the Rail Surface Control window.
Next: Finish the current surface and prompt for new curves.