Untrimming a surface creates a face part of the surface's underlying geometry, or it replaces the original geometry with the untrimmed geometry. Untrim is useful to see the history of a surface, which may not be clear as a result of further modeling of the surface.
For example, if a cylinder that was extruded from a freeform profile is stamped by another freeform profile, the original cylinder boundary may be cut away. Untrim can retrieve the original boundary of such a face.
Normally, Untrim creates a new face part and the original part remains unchanged. You also have the option to replace the original geometry with the untrimmed geometry. Modify a face in this way, and only the face's underlying geometry is changed, without visible effect on the model. Situations where this can be useful are:
• You want to modify a blend on an imported part, but after the blend is removed, the faces don't intersect.
• You want to Cut a face or a feature from a model, but not enough underlying geometry exists.
You can also restrict the direction of untrimming. By default, the face is untrimmed in all directions, but you can select any of four directions in which to untrim the surface. This is useful, for example, when you want one surface to meet another.
You can untrim parts or face parts either created in Creo Elements/Direct Modeling or imported from an external source.
The figure shows how a curved face is untrimmed from the curved surface it was used to create.
Untrim can regenerate implied surface geometry that was not visible even when it was originally created. For example, a blended cylinder edge becomes a torus when untrimmed.
To untrim a face,
1. Click 3D Geometry and then, in the 3D Surface Tools group, click Untrim. The Untrim dialog box opens.
2. Creo Elements/Direct Modeling automatically selects a new face part name for the untrimmed surface. You now have two options:
◦ To change the face part name, enter a new name in the Face Part data entry field.
◦ To specify that the untrimmed geometry should replace the original underlying geometry, click Replace Geometry on.
3. Click the face you want to untrim.
4. If you select By Direction, you can set the following options:
◦ The direction(s) to untrim the face (+u, -u, +v, -v).
◦ If you want to extend the untrimmed geometry, enter a value in the Extension field. Set the Extension to determine how much the surface grows. The Extension is the ratio between the enlarged (linear) size and the original size of the surface.
◦ Use the Facebox option to restrict the boundaries of an untrimmed face to an area slightly larger than the underlying surface. Switch off the Facebox option if you want to view the entire face used to create the surface you are untrimming. The default behavior is to restrict the size of the untrimmed face to an area slightly larger than the boundaries of the surface.
◦ Select an Enlarge Type. Each of these types will continue the curvature of the original surface:
▪ Linear: Continues the surface in a linear manner.
▪ Cv Const: Continues the curvature by appending circular arcs that have the same curvature radius as the original curve at its end points. This type of enlargement is limited to a full circle.
▪ Cv Diminish: Begins with the curvature of the original surface and gradually straightens until it becomes almost linear at the outer edges.
5. If you select Along Edges, you can set the following options:
◦ Select the edge or edges along which you want the part untrimmed. Edge 1 is required and Edge 2 is optional. The edge must attach to a corner of the face surface smoothly.
◦ Select Flat End to make the edge opposite the original surface straight.
6. Click to complete the operation.
Untrim tips
Numbers to use for Extension:
• Numbers smaller than 1 are ignored.
• An extension of 1 will not do anything to the surface.
• An extension of 2 will double the surface in linear size, 3 will triple the surface, and so on.
• Do not use a number higher than 3 when using the Cv Const and Cv Diminish methods.
The original size of the surface depends upon Facebox:
• If you are working with an untrimmed surface, Facebox does not make a difference.
• If you are working with a trimmed surface and Facebox is checked, the original size of a surface is the size of a box that "fits" the trimmed surface.
• If you are working with a trimmed surface and Facebox is not checked, the original size of a surface is the size of the untrimmed surface.
Set the Enlarge Type to specify the curvature of the enlarged surface:
Settings
Result
Extension parameter is 1.
Facebox unchecked.
Creo Elements/Direct Modeling expands the face to its full, untrimmed geometry.
Extension parameter is 1.
Facebox is checked.
Creo Elements/Direct Modeling performs a partial untrim of the face. The face expands to a rectangle that contains the original face, which is often smaller than the full, untrimmed geometry.
(If a face is not trimmed)
Extension parameter > 1.
Facebox unchecked.
Creo Elements/Direct Modeling produces something bigger than the original face. Enlarge Type invents a new surface that contains the original geometry, but is larger than the original.
Enlarge Type:
• Linear - Linear enlargements usually are curvature discontinuous because they are created by tangentially appending straight lines to the surface in the enlargement directions.
• Cv Const or Cv Diminish - Curvature enlargements smoothly continue the curvature of the original surface. When you perform a surface analysis, there are no visible seams where the original surface ends and the enlargement begins.
Limitations
• When you enlarge along edges, the face must contain a freeform (bspline) surface. You will get an error message if you select a face that contains a different kind of surface.
• When you enlarge along edges, the edges selected must attach smoothly to a corner of the surface contained in the face. If not, you will get an error message that the enlargement is not possible with the selected edges.
• When you enlarge along edges and select two edges, the direction of the edges has to agree—both must continue the surface in the u direction or the v direction. Mixed enlargements are not supported.
Untrim. The Untrim dialog box opens.
to complete the operation.