You must have a trimmed surface or face of a solid available to untrim.
Use the Surface wizard or select Untrim surface from the surface toolbar flyout. All of the trimming loops are displayed.
Set whether to Create a new surface or Modify the existing surface.
Name the new surface for future reference (if you are creating a new surface).
To remove all trimming loops, click Untrim all.
To remove a single loop,
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Click Untrim selected loop,
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Click the Pick loop
button and select the loop to remove.
To remove a single edge,
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Click Untrim edge(s).
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The dialog box warps into a button labeled Untrim a trimmed….
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Select the trim curve(s) in the graphics window.
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Double-click on the button labeled Untrim a trimmed …. The dialog box appears again.
Click Finish or OK to complete the operation.