Reference Topics > Modifying Blends: Handling and Restrictions
Modifying Blends: Handling and Restrictions
Blend hierarchy
When you modify blends, Creo Elements/Direct Modeling detects the blend hierarchy of the selected blends and the affected blends. After blend suppression, the (modified) blends are recreated in several steps according to the detected hierarchy. If a blend-recreate step cannot be performed, Creo Elements/Direct Modeling prompts you to accept a partial result. If accepted, Creo Elements/Direct Modeling provides feedback on the blends that have been lost.
If a lower hierarchy modification (a blend below a blend) creates additional edges, the blend on top of these edges may fail and lead to a partial result.
Local modification
The local modification technique is designed to minimize the number of affected blends. That means that only those blends that are directly affected by the modification will need to be suppressed/recreated. The affected blend faces do not spread out across all connected vertex regions and blend chains. This often results in a dramatic reduction of blends that need to be modified. This local modification applies to direct blend modifications as well as for modeling operations that have the redo blend functionality.
Blends that are affected by the modification are highlighted (right picture)
Variable radius blends cannot act as terminators
Blends containing internal features (for example, holes and pockets) cannot act as terminators
This local modify of blends cannot modify non-modifiable blends.
Recreating neighboring faces
The data structure of blending is such that it allows neighboring faces to be recreated in order to remove or modify another blend. The face is created form the original blend boundary and the tangent information along this boundary. This will allow the regeneration of most geometry like planes, cylinders and simple (swept) freeform geometry. In the following example, the red blend faces have been created using the rollover option and these have removed the original neighboring faces of the purple blends. If the red blends need to be removed, a valid solid model would not be possible without creating the cyan faces adjacent to the purple blends.
The newly created surfaces are created as a linear swept surface. They might differ from the original models shape.
Only the adjacent faces of still-existing blends can be recreated
For complex surfaces, the shape of the recreated surfaces may be different from the original shape.
Hierarchic blend suppression (Redo Blend)
In a situation with blend hierarchies and removed adjacent faces of the blends, a blend or model modification is likely to need hierarchic blend suppression. That is, the blends have to be removed in several steps in order to be able to recreate formerly removed adjacent faces of blends. These hierarchical sequences are transparent.
The second blend chain is created on top of the first one (hierarchically on top).
Modifying the lower hierarchy blend (purple) or (removing both blends) means that the former adjacent face (cyan) needs recreating before modification is possible.
In this example, offsetting the yellow face automatically triggers the following sequence:
1. Top blend is suppressed (green)
2. Faces are recreated (cyan)
3. Lower blend is suppressed (purple)
4. Face is offset (yellow)
5. Lower blend is recreated (purple)
6. Upper blend is recreated (green).
Convex/concave vertex regions
The system usually creates a vertex region in corners only when all edges meeting at the corner are blended. In other situations the blend surfaces are intersected against each other. For situations where two edges meet with different convexity (convex and concave), the intersection will not be successful. In earlier software releases, an error message was generated. With the current release, the system now creates a vertex region as shown in the following:
Even if the shape is not the desired one, the additional edge can be blended using insert blend and a regular vertex region will be created. This allows a more incremental way selecting edges and blending. The convex/concave vertex region is also good solution for temporarily removing blends since they act as a terminator to the affected blend calculation.
Limitations: Convex/concave vertex regions are only generated when exactly two edges of a corner are blended; one convex, one concave.
NURBS freeform blends
Blends surfaces that cannot be represented by analytic geometry are now represented as freeform surfaces that have an exact circular cross section. That means that freeform blends and analytic blends fit together much better. This improves the situation where a section and a reunite operation bring analytic and b-spline blends in contact.
Restrictions: blend on top of a rib
A rib is a form element consisting of the elements below:
Two chains of opposing side faces (blue)
A chain of top faces connecting the side faces (magenta)
Terminating Faces (green)
A slot is also considered as a rib (that is, a concave).
When removing or modifying a blend, the following rules apply:
The blend must be a constant radius blend. See example below. Variable radius blends are not considered.
The edges are equidistant.
The side faces are either planar, cylindrical or conic with the axis orthogonal to the top faces, and at the centerline the top faces are planar or cylindrical.
The resulting blend surfaces meet exactly in the centerline of the top faces.
Edges that connect opposing side faces cannot be blended at the same time.
The blend radius size is limited by the opposing blends size.
When blending additional edges from the terminating face, the blend creation may fail due to existing vertex region limitations.
Blends on full rib blends will hinder modification of the full rib blend.
The height of a rib is not changed by modify/delete full rib blend.
Only blends created with the current Creo Elements/Direct Modeling can be removed. Blends from the 11.0 versions cannot be removed.
All other restrictions of create/modify and delete blend still apply.
The following is an example of a fully blended rib.