About Blend Tangent to Surfaces
The Blend Tangent to Surfaces functionality allows you to create a draft surface (blended surface) tangent to surfaces from an edge or a curve. You may need to create a parting surface and a reference curve such as a draft line, before using the Blend Tangent to Surfaces functionality.
The types of tangent draft surfaces are:
Curve-driven tangent draft surface—Creates a surface on one or both sides of a parting surface between a reference curve (such as a parting curve or a sketched curve) and selected surfaces of the reference part, tangent to these surfaces. The reference curve must lie outside the reference part.
Constant-angle tangent draft outside a draft surface—Creates a surface by following the trajectory of the reference curve and creating surfaces at a specified constant angle to the pull direction. Use this feature to add tangent draft to surfaces that cannot be drafted with the regular Draft feature. You can also use this feature to add tangent drafts to a rib with rounded edges and preserve tangency to the reference part.
Constant-angle tangent draft inside a draft surface—Creates a surface with a constant draft angle inside the draft surface. This surface is created on one or both sides of a reference curve (such as a draft curve or a silhouette curve) at a specified angle to the reference part surfaces and provides a rounded transition between the draft surfaces and the adjacent surfaces of the reference part.
When creating a tangent draft, you must select the draft type, the draft direction, and specify the pull direction or accept the default draft direction. Next, select a reference curve and define other draft references such as tangent surfaces or draft angle and radius, depending on the tangent draft type.
The optional elements of a tangent draft are:
Closing Surfaces—Lets you trim or, in some cases, extend the tangent draft up to selected surfaces. Use this element when adjacent surfaces are located at an angle to the surface being drafted.
A closing surface must always be a solid surface. A datum plane or a surface geometry cannot be a closing surface.
Spine Curves—Lets you specify an additional curve that controls the orientation of normals to the sectioning plane. Use this element if using the reference curve alone results in the geometry intersecting itself.  
Cap Angle—For one-sided curve-driven tangent drafts. Controls the draft angle for additional planes that are automatically created when a draft line does not extend to the surface borders and you have not specified the closing surfaces. If you do not specify a value, the system uses a zero angle.
Finally, you can edit the reference curve by using the Curves tab in the Tangent Surface dialog box. Select the reference curve segments to include in the draft line or exclude from the draft line. Use this functionality when the system has trouble creating the tangent draft, for example, when the reference curve intersects itself.