A Guide to Surface Modeling with Sweep1, Sweep2, and RailRevolve in Rhino

What you’ll learn in this guide

In Rhino, the Sweep1, Sweep2, and RailRevolve commands offer different ways to create surfaces from curves. This guide explains when to use one rail versus two, how to select cross-section curves, and which key options affect the surface’s behavior. It also covers aligning seams on closed curves, checking cross-section direction, and setting the axis of revolution in RailRevolve.

Start by choosing a command based on how you want to define the surface boundaries. Use Sweep1 when one rail defines a surface edge, or Sweep2 when two rails define two edges. RailRevolve offers a different approach when a profile needs to revolve around a rail. With all three commands, the order and position of the input curves affect the result.

Requirements

This guide is based on the commands described in McNeel’s Rhino 8 help documentation. Modeling requires Rhino and curves suitable for the command; the sources do not specify account, user account, or hardware requirements. No specific conditions should be assumed in these areas.

• Sweep1: One rail curve and profile curves that define the surface cross sections.

• Sweep2: Two rails that define the surface’s edges, plus cross-section curves.

• RailRevolve: A profile that defines the surface shape, a rail, and the start and end points of the axis of revolution.

Select cross sections in the order you want the surface to pass through them. Using closed curves may require additional seam settings. For Sweep2, it helps to position open cross-section curves near corresponding ends of the rails, and to align their endpoints with the rail ends or matching edit points on the rails. Reviewing the curve structure before modeling also makes it easier to choose the right options.

1. Create a surface along one rail with Sweep1

First select the rail curve, then select the cross-section curves in the order you want the surface to follow. Sweep1 creates a surface by carrying the cross sections along the rail. When multiple closed cross sections are selected, an additional step lets you adjust their seam points. Drag the seams along the curves to align them, and use Flip to reverse curve directions. Automatic attempts to match seam points and directions automatically, while Natural preserves their positions from when the command started.

McNeel’s documentation notes that a better sweep surface may result when the cross-section curves have matching structures. This can be the case when the curves share the same structure—for example, if they were copied from the same curve. If their structures differ, the Rebuild cross sections or Refit cross sections options let you use copies of the curves that have been rebuilt or refitted before the operation. Do not change cross sections leaves the cross sections unchanged.

Frame Style determines how the cross section rotates along the rail. Freeform is the default style; the cross section rotates to maintain its angle relative to the rail. Roadlike uses an axis to calculate the rotation, and Set axis lets you define the axis direction. If the rail is a surface edge, Align with surface makes the cross section rotate along with that surface edge. The documentation notes that if the cross section is tangent to the surface, the new surface may also be tangent.

Other options can also change the surface shape. Global shape blending creates a linear transition between cross sections, allowing the surface to taper from one end to the other; when disabled, the change occurs more toward the middle. Closed sweep is available only after selecting at least two cross sections, and continues the surface from the last cross section back to the first. Untrimmed miters keeps component surfaces untrimmed even when the sweep creates kinks.

2. Create a surface between two rails with Sweep2

In Sweep2, first select the two rails, then select the cross-section curves in the order you want the surface to follow. The rails define the surface’s two edges, while the cross sections define its shape. Select open cross sections near corresponding rail ends. For closed curves, align the seams; check that the cross-section endpoints line up with the rail ends or edit points on the rails. Rhino’s documentation explains that you can use edit points and Point object snaps to find matching edit points.

It is recommended that the rails have matching degrees and structures. Cross-section curves may have different structures; in that case, the surface takes on the structure of the most complex cross section. When using a single cross section, the surface spans the full length of the rails regardless of the settings. The documentation also describes a way to control where a sweep using the same cross section ends: use Orient commands to create a scaled copy, then position its ends at the desired points on the rails.

When edge tangency or curvature is matched, the surface may deviate from the cross-section curves. Preserve first cross section and Preserve last cross section force a match with the first and last cross sections, respectively. Maintain height separates height scaling from width scaling; by default, cross sections are typically scaled in both directions together. Add Slash adds extra alignments that guide how the surface forms between cross sections. When using these options, decide which cross sections need to be preserved and how you want the shape to transition between the two rails.

3. Revolve a profile around a rail with RailRevolve

In RailRevolve, first select the profile curve that defines the surface shape, then select the rail curve. Next, specify the start and end points of the axis of revolution. Pressing Enter lets you use the CPlane’s Z-axis for the axis direction.

ScaleHeight stretches the profile along the axis of revolution as well as revolving it. If the rail is planar and lies in a plane perpendicular to the axis, the result looks the same even when this option is not used. If the rail does not meet these conditions and you want part of the profile to remain stationary along the axis as it revolves, consider using ScaleHeight. The position of the axis start point matters, since it also determines the start of scaling.

If the rail is a closed curve, the seam should be where the profile touches the rail for a good result. When SplitAtTangents is enabled, the output surface is split at tangent points between segments of the profile and rail curves. When the option is disabled, these points are revolved as kinks in the surface.

Common mistakes

• Selecting cross sections in an arbitrary order: Sweep1 and Sweep2 require cross sections in the order the surface should pass through them. Check the selection order against the surface’s start and end.

• Overlooking seams on closed curves: Align the seam points and, if necessary, use Flip to match directions.

• Ignoring the positions of rail and cross-section endpoints in Sweep2: Review the rail structure and how the cross sections relate to their endpoints before modeling.

• Not realizing that cross sections may be modified: Sweep1 and Sweep2 offer different options for preserving or rebuilding cross sections. Check what each option does.

• Enabling ScaleHeight for every revolution: Whether you need this option depends on the rail’s plane and the axis of revolution. Also account for how the axis start point affects scaling.

Using these commands in architectural and visualization workflows

In architecture, interior design, and archviz, these commands can help you choose an approach for creating surfaces defined by curves. Consider Sweep1 for a surface using one rail and multiple cross sections; Sweep2 for a surface whose two edges are bounded by rails; and RailRevolve when a profile needs to revolve around a rail. This makes it clear from the outset which curves define the model’s geometry.

In practice, check the cross-section order, the endpoints of the rails and cross sections, seams on closed curves, and the selected frame or scaling options. In Sweep2, the relationship between the cross sections and matching points on the rails is especially important. The sources do not specify hardware, license costs, or project compatibility requirements, so it is not possible to draw general conclusions about these topics. Choose commands according to the geometry in your project workflow, then inspect how the resulting surface matches the cross sections and rails.

Next steps

First decide whether the surface is defined by one rail or two, or whether a profile will revolve around a rail. Then check the curves’ order, direction, and seam positions on closed curves. If you get unexpected rotation, review the Freeform and Roadlike settings in Sweep1; if the surface deviates from the cross sections, consider the Sweep2 options that preserve them. If the RailRevolve result moves along the axis unexpectedly, review ScaleHeight and the role of the axis start point.

Alongside Sweep1 and Sweep2, McNeel’s documentation lists SubDSweep1, SubDSweep2, Loft, and NetworkSrf as related surface-creation methods. Exploring these options as a next modeling step can help you compare which method best suits your curve layout.

Sources and license

This guide has been adapted into Turkish from McNeel’s Rhino 8 help documentation for Sweep1, Sweep2, and RailRevolve. The source records provided identify these documents as license=free. Sources: McNeel Rhino 8 help documentation, Sweep1, Sweep2, and RailRevolve.