Extrude Along Path

A very powerfull modeling tool is the "Extrude along path" technique. It consist in creating a surface by sweeping a given profile along a given path.

Both the profile and the path can be a Bézier or a NURNS curve.

Let's assume you have added a Bézier curve and a Bézier circle as separate objects to your scene (Figure 24).

Play a bit with both to obtain a nice 'wing-like' profile and a fancy path (Figure 25). Please note that, by default, Bézier exists only on a plane and that are 2D objects. to make the path span in all three directions of space, as in the example, you must press the '3D' button in the Curve EditButtons (F9) (Figure 26).

Now look at the name of the profile object. By default it is "CurveCircle" and it is shown on the EditButton toolbar when it is selected. You can change it by SHIFT-LMB on the name, if you like (Figure 27).

Now select the Path. In its EditButton locate the one named 'BevOb:' and write in there the name of the profile object. In our case 'CurveCircle' (Figure 28).

The result is a surface defined by the Profile, sweeping along the path (Figure 29).

To understand the results, and hence obtain the desired effects it is important to understand the following points:

• The profile is oriented in such a manner that its z-axis is tangent (i.e. directed along) the Path and that its x-axis is on the plane of the Path; consequently the y-axis is ortogonal to the plane of the Path;

• If the Path is 3D the "plane of the path" Is defined locally rather than globally and is visually rendered, in Edit mode, by several short segments perpendicular to the Path (Figure 30);

• The y-axis of the profile always points upward. This is indeed a source of unexpected results and problems, as it will be clarified later on.

	Tilting
	You can modify the orientation of the local Path plane by selecting a control point and pressing TKEY. This done, moving the mouse changes the orientation of the short segments smoothly in the neighborhood of the control point. LMB fixes the position, ESC reverts to previous state.

Being the y-axis constrained to be upwards gives unexpected results when the path is 3D and the profile, in its extrusion comes to a point where the path bends so that the y-axis of the profile should point downwards. If this occur there is an abrupt rotation of 180° of the path, so that the y-axis is upward again.

Figure 31 Show clearly the problem. On the left there is a Path whose steepnes is such that the normal to the loacl Path Plane is always upward. On the right a Path where, at the yellow circled point, such a normal begins to pint down. The result of the extrusion presents a abrupt turn there.

The only solutions to this problems are: To use multiple - matching - paths, or to carefullt tilt the path to assure a normal always pointing upwards.

	Changing profile orientation
	If the orientation of the profile along the curve is not the expected one and you want to rotate it for the whole Path length there is a better way than tilting all Path control points. You can simply rotate the Profile in Edit mode on its plane. This way the profile changes but its local reference doesn't.