How does Two Rail Sweep work in Aspire, and how do I fix common problems with direction, start nodes, and corners?

Overview

Two Rail Sweep creates a 3D component by sweeping one or more cross-section profiles between two drive rail vectors. The shape flows from one rail to the other, following the rail curves and the cross-section's profile. It is the primary tool for making sign borders, picture frames, leaf shapes, organic flowing shapes, and any 3D form that has a consistent cross-sectional profile traveling along a path. This entry covers how the tool works and how to fix the common problems: twisted or upside-down sweeps, start nodes, rail direction, stretched corners and profile height.

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The Core Inputs

Two Drive Rails: open or closed vectors. Rail 1 (red) and Rail 2 (green) define the width direction at any point along the path.

Cross-Section(s): open vector(s) defining the profile of the shape as you slice across the rails. Cross-sections must be open vectors. The ends of the cross-section attach to the two rails.

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Step-by-Step Workflow

1. Draw two rail vectors (for example an outer oval and an inner oval for a sign border).
2. Draw the cross-section profile (the side-view shape of the molding).
3. Open Two Rail Sweep from the Modeling tab.
4. Select both rail vectors in the 2D view, then click Use Selection.

• The first rail turns red, the second turns green.
• Green squares show each rail's start node.
• Arrows show the direction the sweep will travel.

5. Check that both rails run in the same direction (arrows both clockwise, or both counterclockwise). If not, right-click the incorrect rail and choose Reverse Rail.
6. Check that start nodes are at the same relative position on both rails (for example both at the top of the oval). If not, fix them (see below).
7. Select the cross-section vector, then click the rail to attach it. A check mark shows a valid place to attach it. A colored node at one end shows which way the cross-section is "hung" across the rails.
8. Two cross-sections are always created when you attach the first one: one across the start nodes of the two rails and one across the ends.
9. Click Apply. The 3D sweep appears.

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Understanding Rail Colors and Direction

• Red rail (selected first) and green rail (selected second).
• Arrows show which direction the sweep will travel along each rail.
• Cross-sections hang "across" from one rail to the other, starting at the colored node end.
• Swap Rail Order swaps red and green, causing cross-sections to hang in the opposite direction. Use it when the profile is flipping the wrong way, for example curving inward instead of outward.

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Problem: Sweep Is Twisted, Distorted or Upside-Down (Start Nodes and Direction)

The most common source of problems: the green start nodes on each rail are at different positions, or the rails run in opposite directions. If the start node on the outer oval is at the top but the start node on the inner oval is at the right side, the sweep twists and distorts instead of flowing cleanly.

Fix checklist:

1. Look at the green start-node squares and the direction arrows on both rails in the 2D view.
2. If the arrows point in opposite directions, right-click one rail and choose Reverse Rail.
3. On a closed vector, place the cursor over an existing node, right-click and choose Make Start Point. Or right-click anywhere on the vector and choose Insert Start Point to create a new start node there. Make both rails' start nodes sit at the same relative position.
4. Make sure both closed rails travel the same way (both clockwise or both counterclockwise).
5. Use Swap Rail Order if the cross-sections hang in the wrong direction.

For offset shapes (inner and outer ovals made with Offset): the offset copy can have its start node at a different position, so always check and align start nodes before sweeping.

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Problem: Frame Corners Are Stretched or Bulging

By default the sweep connects points at the same proportional distance along each rail. On a frame, the corners of the inner and outer rectangles are at different proportional positions, so the cross-section is stretched around the frame instead of connecting the corners.

Option A, automatic (requires the same number of nodes on both rails):

1. Add the first cross-section.
2. Right-click the cross-section's preview position on the rail.
3. Select Add To All Rail Nodes. The same cross-section is placed at every node pair on both rails.
4. Apply. Corners are now handled. If the node counts differ, use Node Editing to make them match first.

Option B, manual (any rails):

1. Add a cross-section at one corner node on Rail 1.
2. Drag the preview position on the other rail to align with the matching corner node on Rail 2.
3. Repeat for each corner.

Option C, separate sides: split both rails at every corner using Node Editing, then run Two Rail Sweep on each matching pair of segments with the cross-section. This gives one sweep component per side that meet at the corners.

Tip: when attaching cross-sections at corners, keep the ends of the cross-section on the same line so the profile does not tip as it sweeps around the corner.

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Problem: Profile Facing the Wrong Direction

Symptom: the bead, ogee or quarter-round curves to the wrong side (inward instead of outward, or the reverse).

1. Click Swap Rail Order to make the cross-sections hang in the opposite direction.
2. Check the colored node on the cross-section, which shows the direction it is hung, and redraw or flip the cross-section vector if needed.
3. Click Apply to see the corrected result.

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Problem: Profile Changes Height Along the Sweep

Cause: the Scale cross sections with width option is ticked. It changes the height of the cross-section in proportion to the distance between the rails, so the shape is higher where the rails are further apart and lower where they get closer.

• Intentional: tapered shapes such as legs or spires.
• Not intentional: untick the option to keep the exact shape and height of the cross-section everywhere. Rails that are offset copies of each other also keep the spacing constant.

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Multiple Cross-Sections (Morphing Profiles)

You can attach different cross-section shapes at different positions along the rails. Aspire blends smoothly between them:

• Select a cross-section vector, then click the position on the rail where you want it attached.
• Add as many cross-sections as needed. Each uses a different colored indicator node.
• The sweep blends from one profile to the next. Example: a leg that is a square profile at the top and a round profile lower down.
• The same cross-section vector can be used at several positions.
• Sweep between spans becomes active only when all the cross-sections have the same number of spans and nodes. The shape then flows from a node or span in one cross-section to the same one in the next.
• Smooth or sharp transitions: right-click the end node of a cross-section at its attachment point and toggle Smooth on or off. Smooth (the default) gives a flowing transition through the profile.

Repositioning: click and hold one of the nodes at either end of the cross-section preview (the ones on the rails) and drag it to a new position on the curve. Do not drag a cross-section past another one. To change the order, remove the cross-section and insert the same shape at the new position. To replace a cross-section with a different open vector, select the new vector, then click the end node of the cross-section you want to change. To remove one, right-click it and choose Delete Cross Section. Right-click a part of the curve with no cross-section for Remove All Cross Sections. Dragging the preview end nodes moves the cross-section along the rails and does not edit its profile; edit the vector with Node Editing for that.

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Fill Center of Inner Closed Vector Rails

When both rails are closed vectors (like a plaque with an outer and inner boundary), this option finds the height the cross-section forms on the inner boundary and fills the interior to that height. It turns a molding border into a complete plaque or base with a filled center surface. Ideal for sign blanks and decorative bases.

Concave dish example: Rail 1 is a small circle (the center of the bowl), Rail 2 is a large circle (the outer rim), and the cross-section is a curve that dips below the zero line and rises back up. Tick Fill center of inner closed vector rails to fill the center to the height the cross-section forms on the inner boundary.

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Cross-Section Rules

• Cross-sections must be open vectors (not closed).
• The ends of the cross-section attach to the rail points, so both ends should be at the height you want the rail to sit at (usually zero).
• A cross-section that dips below its end points can make a channel or recess instead of a raised shape. Preview it in the 3D view before committing.
• The cross-section's width is stretched to span the gap between the rails. Its height stays as drawn unless you tick Scale cross sections with width.
• Scale to Exact Height: tick it to scale the calculated shape so its maximum height is the value you enter in the Height box. Use it to match a material thickness or a neighboring component.

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Two Rail Sweep vs. Moulding Toolpath

For most decorative molding work Two Rail Sweep is more flexible, because you can combine the component with other elements, sculpt it and adjust its height.

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Related Entries

For picture frames and full sign faces, see the entry on using Two Rail Sweep to create a picture frame and a sign face. To trim a sweep to a precise outline such as a leaf, see the entry on Two Rail Sweep plus Clear Outside.