> Example: Create a guided loft with birail curves

Example: Create a guided loft with birail curves
Birail curves define the paths for the profiles to follow and are interpolated by the created surfaces. Use birail curves to add the surface to an existing part or to create a more complex shape.
When using birails, choose from these options:
None: No scaling is applied. The number of birails to be used is 1.
Isotropic: Scaling occurs in all directions within the profile plane and with respect to the spine. The number of birails to be used is 1 or 2.
Unidirectional: Scaling occurs in a specific direction depending on the number of birails. The number of birails to be used is 1 or 2.
One birail: The direction is determined by the distance between the spine positions and the corresponding positions on the birail.
Two birails: The direction is determined by the two points where the profile plane intersects the birails.
Bidirectional: Scaling occurs in two directions. The first direction is determined by the normal field, chosen by the Orientation Option. For each point on the spine, the normal field defines a direction that is orthogonal to the spine tangent at this point. The second direction is orthogonal to the first direction and lies within the profile plane. The number of birails to be used is 2.
Check the Exact box to create surfaces that approximate the birails within the part accuracy. This will create a large amount of data.
Uncheck the Exact box to create surfaces that approximate the birails within a tolerance of 0.1. This will create a lower amount of data and a less expensive surface.
Choose a continuity option for a smooth transition between surfaces.
Example using one birail
Create a guided loft using a spine (orange), profile (blue), and birail (white).
A guided loft without selecting a birail, or by selecting a birail and the None option.
Using the Isotropic option, the profile was scaled and rotated to fit the birail when swept along the spine.
Spine (orange), profile (blue), birail (white)
Using the None option, the profile is scaled along the birail, not the spine.
Using the Isotropic option, the profile is scaled along the spine in all directions within the profile plane.
Using the Unidirectional option, the profile is scaled along the spine in the direction that is determined by the distance between spine positions and corresponding birail positions.
Example using two birails
Birails can be used to integrate a surface into a given model. The profiles are translated, rotated, and scaled in order to make the surface fit to the birail curves.
Create a guided loft surface with birails to integrate a new surface into an existing surface.
The surface is integrated using the spine (orange), profiles (blue), and birails (white).
Spine (orange), profiles (blue), birails (yellow and white), guided curve (yellow). The left birail is chosen as a guide curve under Orientation and defines the first scaling direction. The second birail lies in a plane orthogonal to the plane defined by the first birail in order to get a symmetric result.
Using the Isotropic option, the scaling leads to an asymmetrical result.
Using the Bidirectional option, the scaling leads to a symmetrical result.
Conditions for birails
The maximum number of birail curves is 3.
The birail curves have to touch each profile exactly at one of the vertices of the profile edges in order to be interpolated by the surface. If they are not at the vertices, they have no influence on the shape of the surface.
If the spine touches each profile at one of the vertices of the profile edges, it automatically becomes a birail and will be interpolated by the created surface. In this case, only one additional birail can be specified.
The birail curves can be longer than the spine but not shorter; each plane defined by a position and corresponding tangent on the spine has to intersect the birail curve.
The spine and the birails can be a chain of curves with a tangency connection.
Although birails can be used to create complex shapes, they are not the best choice when working with a fine resolution (1e-6mm). The system would have to create surfaces with a high amount of data in order to interpolate the birails. Decrease the resolution of the birail from 1e-6mm to 1e-4 or 1e-3mm.
The Unidirectional Scaling option is available if two birails are selected to scale a profile(s) to the birails.
Limitations
Birails are not supported if the profiles are not planar or orthogonal to the spine.
Birails cannot be used to create a surface with a tangential connection to existing surfaces.