The Boundary Surface tool allows you to make surfaces between profiles. It produces very high-quality and accurate surfaces that are useful for creating complex or organic shapes for product design. It is highly used in the aerospace, automobile, and shipbuilding industry.

The Boundary Surface feature is similar to the Lofted Surface tool. It lets you create surfaces that are tangent or have a continuous curvature in all directions. A Boundary Surface can be used instead of a Filled Surface for a more controllable transition between faces. In most cases, Boundary Surface delivers a higher quality result than the Loft or Surface Fill tool.

To Create a Boundary Surface go to Insert -> Surfaces -> Boundary Surface or select Boundary Surface from Sketch Toolbar.

In the PropertyManager, select sketch curves, faces, or edges that you want to use to make your boundary surface in Direction 1 and Direction 2. Use Selection Manager to select segments of a curve. (Right Click in the graphics area and select Selection Manager.)

Direction 1 and Direction 2 are fully interchangeable. You achieve the same results whether you select entities in Direction 1 or Direction 2.

Tip: You can also select a portion of each curve to be used for Boundary Surface by dragging the endpoints.

If the boundary surface is twisting between curves unintentionally, then the connectors that join the profiles are misaligned. Right-click in the graphics area and select Flip Connectors to try to fix them. If it fails, right-click on the sketch entity whose connector you want to flip in the Direction Box and select Flip Connectors.

Dir Curves Influence

There are a total of 5 types of curve influence options available for Dir Curves Influence but the availability of these options depends on the type of the curves you select for the direction. The Curve Influence option you select for a direction is applied to all the curves in that direction.

1. Global: If selected, the curve will have an influence on the entire boundary feature.
2. To Next Curve: The curve’s influence will be limited to the next curve only.
3. To Next Sharp: The curve’s influence will stop at the next sharp corner of the profile. (A sharp corner is made by any two continuous sketch entities that do not have a common tangent or an equal curvature relation with respect to each other.)
4. To Next Edge: The curve will have an influence up to the next edge only.
5. Linear: The curve will have an influence uniformly over the entire boundary feature, similar to a ruled surface. This option helps to avoid excessive curvature effects such as pocketing by a highly deformed guide curve on surfaces where curves in a single direction are coincident with each other.

You can experiment by changing the curves’ influence options in one or both directions to achieve the boundary surface design you need.

Tangent Type

This option is used to define the tangency of a selected curve.

1. Default: It uses a parabolic shape for tangency between the first and last profiles. This creates a more predictable and natural surface. (Available when you have a minimum of three curves in the direction.)
2. None: Tangency constraint is not applied (i.e. zero curvature).
3. Normal to Profile:  Applies a tangency constraint that is normal to the profile. (This option is only available when the curves are not attaching the boundary feature to existing geometry.)
4. Direction Vector: It will use the entity that you select as a direction vector for applying a tangency. You can select a linear edge or axis, face or plane, or a pair of vertices to set the direction vector.
5. Tangency to Face: It makes the boundary surface tangent to adjacent faces. (Only available when you are attaching the boundary surface to existing geometry.)
6. Curvature to Face: It makes the boundary surface curvature continuous to adjacent faces. It makes a very smooth and visually appealing surface at the selected curve. (This option is also only available when you are attaching a boundary feature to existing geometry.)

Align With

It controls the alignment of the iso parameters, which controls the flow of the surface. Experiment with this option until you get the Boundary Surface shape right. This option is available only for single-directional cases.

Single-Direction Boundary is the one in which you use only one Direction to create the Boundary Surface. In Bi-Directional Boundary Surface, you use both Direction 1 and Direction 2 to create the Boundary Surface Feature.

Draft Angle

This option applies a draft to the start or end curve. If required, click the Reverse Direction icon to reverse the draft angle.

For Single-Direction Boundary features, the draft angle is available for all Tangent Types. Draft Angle is not available if you are connecting a Bi-Directional Boundary Surface to an existing geometry that already has a draft because the system automatically applies the same draft to the boundary feature at the intersecting curve.

Tangent length

This helps you to control the amount of influence Tangent Type has on the boundary surface. It can be increased or decreased. The effect of tangent length is limited up to the next section. If necessary, click the Reverse Tangent Direction icon. This option is not available when None is selected for Tangent Type.

Notice how the boundary surface changes as we apply a draft of 45 degrees in the previously created shape.

Tangent Influence

It increases or decreases the curve’s influence on the next curve. Higher values extend the tangency’s effective distance. Useful for making very rounded shapes.

Only available when Global or To Next Sharp is selected for Curves Influence and with curves in both directions. Not available with Tangent Type set to None or Default.

Apply to all: Displays one handle that controls all the constraints for the entire profile. Clear this option to display multiple handles that allows you to control individual segments. Drag the handles to modify the tangent length. (Available only for single-directional cases.)

Direction 2

Options are the same as Direction 1 above. The two directions are interchangeable and give the same results regardless of whether you select the curves as Direction 1 or Direction 2. It is not necessary to use both the Directions to create the Boundary Feature. You can use only one Direction and leave the other Direction Box empty according to your requirements.

Options and Preview

Close Loft: Creates a closed body along the boundary feature direction. This connects the last sketch and the first sketch automatically.

Trim: It trims the surfaces by direction when curves do not form a closed boundary.

Drag Sketch: This feature is similar to the Instant3D tool. When enabled, you can drag sketch segments, points, or planes from the 3D sketch from which contours have been used for the boundary feature. You can also add dimensions to the 3D sketch using dimensioning tools. The boundary feature preview updates as soon as the drag ends or when you edit the 3D sketch dimensions. To exit drag mode, click Drag Sketch again.

Show Preview: Displays shaded previews of the boundary feature.

Create Solid: This feature is used to create a solid body from your boundary surface if your surface encloses a closed volume.

Curvature Display: You can select Mesh Preview, Zebra Stripes, or Curvature Combs to better understand the flow of the surface.

Zebra Stripes is a very powerful tool that can be used to quickly understand the quality of your surfaces. Zebra stripes simulate the reflection of long strips of light on a very shiny surface. With Zebra Stripes you can easily see any defects, imperfections, or unevenness in the surface and can also instantly identify the type of boundary that exists between two adjacent surfaces. The Zebra Stripes option is present in the Evaluate tab and also in View -> Display -> Zebra Stripes.