By Rajesh Chakravarty
Working with imported CAD models in ACIS
The more advanced modeling operations customers intend to perform, the more preprocessing of the ACIS model is required. For example, a visualization application would need less preprocessing than a CAD/CAM/CAE application. The figure below gives the typical preprocessing needed on a translated ACIS model for CAD/CAM/CAE applications.
In comparison, a visualization application may just need to import the CAD model into ACIS without any preprocessing.
The “Healing Workflow” improves CAD models imported from other modeling systems into ACIS in order to make them most usable in ACIS. Typically, models created in some other modeling system and translated into an ACIS model may have
- Geometrical inaccuracies
- Topological inaccuracies
The workflow addresses these issues in an efficient way to create an optimized model in ACIS. This is the recommended workflow for healing ACIS models which supersedes the deprecated Automatic Healing functionality.
Healing Workflow Steps
The workflow involves the three steps of “Stitch”, “Simplify” and “Tighten gaps”.
Step 1: Stitch
The stitch (“Tolerant Stitch”) functionality builds topology, if not already present or partially present, in the translated ACIS model by stitching or sewing separate faces together to make a sheet or manifold solid body that is topologically complete.
Is there a need to Stitch?
The stitch step is required if the customer application requires surface connectivity. If the
incoming model is already stitched, then the stitch step can be skipped.
How to customize Stitch?
The Tolerant Stitch functionality allows a tolerance parameter to be passed for stitching the
separate faces having gaps less than this parameter value. For details on this refer to
For more information on customizing the stitch functionality refer to
Step 2: Simplify
The simplify step converts spline data in the incoming model to corresponding analytic forms (for example, plane, cone, torus, or sphere) wherever possible. It results in reduction of data size and speed improvements in geometric operations.
- Is there a need to Simplify? The simplify step can be skipped if:
- The customer application does not require changes in the model geometry
- The customer application does not want the gaps in the model to increase (up to simplification tolerance)
- How to customize Simplify? The simplification functionality works with a user defined simplification tolerance. Users can set
this tolerance value to either:
- The source modeler tolerance in which the data was created. (Best choice)
- If the above information is not available, then users can use their manufacturing tolerance (i.e. how big gaps user wants to permit)
For more information on customizing the simplify functionality refer to http://doc.spatial.com/qref/ACIS/html/classsimplify__options.html
Step 3: Tighten Gaps
The “Tighten Gaps” step tightens the gaps (inaccuracies) in the input models by building geometry as necessary. In this phase, series of geometric operations are performed to improve the precision of face, edge, and vertex data. Geometry building essentially removes the gaps between adjacent surfaces by recomputing the intersections. Some examples of tightening gaps:
- Is there a need to tighten gaps? The tighten gaps step can be skipped if:
- There is no requirement on model tolerances being below a particular limit
- There is no plan to do complex modeling operations like blending
The tighten gaps step changes the model geometry and is usually associated with increased data size.
- How to customize Tighten Gaps? Users can pass in a "desired_gap_tightness" parameter to enable the functionality to tighten only
those gaps in the model that are greater than this value. The "desired_gap_tightness" parameter should be same as the users manufacturing tolerance which is usually bigger than ACIS resolution SPAresabs.
- 10-5 is a good value to use by default for CAD & CAM
- 10-4 may be sufficient for CAE
The workflow has been designed to make model healing in ACIS very efficient. However, presence of some issues in the translated ACIS model may prevent the workflow from not working effectively. It is preferred that these issues are addressed prior to calling the workflow to improve its effectiveness.
1) Presence of check errors in the translated ACIS model.
The "Healing Workflow" does not repair topological and geometric invalidities. It is expected that the input model to the workflow checks good at level 30 with the option check_discont enabled.
The "3D Interop (with Healing)" step attempts to ensure that the translated ACIS model input to the workflow checks good. The healing of 3d Interop addresses the geometrical and or topological check errors as much as possible during the translation process ensuring precise and valid translation.
2) Presence of spurious/sliver features in the translated ACIS model.
Users should check for spurious/sliver features in the models translated into ACIS and remove them before calling the “Healing workflow”.
For details on removal of spurious/sliver features from the model, refer to Healing Workflow Notes.
If it is not possible to do the removal before the workflow, then it can be alternatively done after the Stitch step in the “Healing workflow”.
3) Presence of coincident faces in the translated ACIS model.
The translated ACIS model may contain duplicate or partially coincident faces (face pair that are "back-to-back").
Coincident faces usually signify the presence of duplicate and/or construction faces in the translated ACIS model. If the user has control of the origin of the coincident faces, then it is advised that the user removes them appropriately before calling the “Healing workflow”.
If it is not possible to know the presence of coincident faces in the model beforehand, then the user can take necessary corrective actions on the coincident faces reported by Stitch. For some guidelines on how to handle coincident faces, refer to Tolerant Stitching.
Note: Presence of coincident faces in the incoming model is an illegal configuration for tolerant stitching. Tolerant Stitch has been setup to report such faces when they are found, but it is not guaranteed to detect all coincident faces.