Sweep and Bi-Rail Tools

The Sweep tool constructs a surface by moving a cross section curve along one or two rail curves.
 

Creating a tube

An 'S' shaped tube can be created by sweeping a circular cross section curve along an 'S' shaped rail curve:

1. Switch to the front view and create a closed NURBS curve.

2. Switch to the side view and create an 'S' shaped open NURBS curve which starts from the center of the first curve.
 

A closed 'cross section' curve and an open 'rail' curve

3. Select the cross section curve and the rail curve (in this order). As always, the control bar shows you the tools, which can be applied to the selected curves.

4. Click the Sweep tool.

There are two ways to sweep the circular cross section curve on the rail curve.

1. Orthogonal sweep. The direction of the cross section curve is rotated with the curvature of the rail.

2. Coplanar sweep. In this method the direction of the cross section circle remains the same.

Let's use the orthogonal sweep, so set this option in the control bar. Then click 'Accept'. Leaving the 'orthogonal' option unchecked in the control bar, results in a coplanar sweep.
 

A tube created by sweeping a closed curve along an 'S' shaped open curve
 
Tutorial project: 'tutorprojects\modeling\nurbs\sweep\tube'
 

Controlling the order of the surface

Let's assume you want to create a circular tube, which consists of straight cylinders. This can be constructed by sweeping a smooth circular cross section curve along a polygonal rail curve.

To create such a tube, just replace the sweeping curve of the previous example with a polygonal curve.

A cubic circle swept along a polygonal rail

The curves, from which the surface is constructed, determine the mesh properties. If you want to create a completely polygonal NURBS surface, use two polygonal curves.
 
Tutorial project: 'tutorprojects\modeling\nurbs\sweep\polygonal rail'

In addition to 'Order', also the 'Open/Closed' attributes of the mesh are determined by the corresponding state of the construction curves. A torus surface is closed in both directions. Such a surface can be swept by using two closed curves. Remember to start drawing the rail curve from the middle of the cross section curve.

A torus surface created by sweeping a circular cross section curve along a closed rail curve  

The weight and parametrization of the construction curves control parametrization and weights of the mesh. For example, if you increase the weight of a control point, the weight of the corresponding 'isoparametric curve' is changed. Or, if you use a Bezier curve as a rail, also the surface will be Bezier-parametrized in the corresponding direction.

A Bezier curve used as a cross section curve
 
 

Bi-rail

The Sweep tool supports also two rail curves. Bi-rail sweeping activates automatically when you select three parameter curves for the tool. The first selected curve is taken as a cross section curve and the remaining two curves are used as rails to move, rotate and scale the cross section curve.

The bi-rail tool: a Bezier curve swept by two rail curves
 
Tutorial project: 'tutorprojects\modeling\nurbs\sweep\bi-rail'