Rules for Defining a Trajectory
A constant section sweep can use either a trajectory sketched at the time of feature creation or a trajectory made up of selected datum curves or edges. As a general rule, the trajectory must have adjacent reference surfaces or be planar.
When you define a sweep, the system checks the specified trajectory for validity and establishes normal surfaces. A normal surface is the surface whose normal is used to establish the y-axis of the trajectory. When several surfaces could possibly be the normal surface, one surface is selected by default. You can change this surface by clicking the Next button until the desired surface is selected to be the normal surface.
Depending on the type of chain selected as a trajectory, the following occurs:
• All chain segments reference edges—The normal surfaces are the adjacent surfaces of the edges. If the edges are two-sided, click the Next button until the arrows indicate the surfaces and normals you want.
• All chain segments reference entities that belong to a datum curve, created by referencing surfaces (for example, by using the Projected option)—The normal surfaces are reference surfaces of the curve. If the curve references two sets of surfaces, click the Next button until the arrows indicate the surfaces and normals you want.
• All chain segments reference a sketched datum curve—the normal surface is the sketching plane of the curve.
• The chain of edges/curves is planar (other than a straight line).
• The normal surface is the plane defined by the chain.
Consider the following special cases:
• If a datum curve and its adjacent surfaces were bent by a toroidal bend feature, you can use that curve as a trajectory.
• If you extend the chain with Trim at Reference or Extend to Reference in the Chain dialog box, the system accepts that chain if it is planar.
A sweep might fail if:
• A trajectory crosses itself.
• You align or dimension a section to fixed entities, but the orientation of the section changes when it is swept along the three-dimensional trajectory.
• An arc or a spline radius is too small relative to the section, and the feature intersects itself traversing around the arc.