Trim

The Trim command cuts and deletes selected portions of an object at the intersection with another object.

Trimmed Surfaces

A trimmed surface has two parts: a surface that underlies everything that defines the geometric shape, and trimming curves that either trim away the outside portion of the surface or cut holes in its interior.

Those trimming curves exist on the underlying surface. The underlying surface may be larger than the trim curves, but you won't see the underlying surface because Rhino doesn't draw anything for the part of the surface that is outside the trim curves.

Only the underlying surface defines the actual geometry of the shape. The trim curves do not define a surface -- they only mark which part of the surface is to be considered trimmed away.

If you have a trim curve that runs diagonally across a surface, the trim curve itself doesn't have any real relationship to the control-point structure of the surface. You can see this if you select such a trimmed surface and turn its control points on. You'll see the control points for the whole underlying surface, which doesn't necessarily have any connection with the trim curves.

You can remove the trim curves and get back to the underlying surface using the Untrim commands to remove holes or outside boundaries.

When you have a trimmed surface whose underlying surface is much larger than the outside trimming boundary, you can use the ShrinkTrimmedSrf command to shrink the surface back so that it is only large enough to hold the trimming boundaries and doesn't have a large extra unused area.

The Untrim command removes trim curves and surfaces joined at trim curves from a surface.

 Crease splitting enabled

Command-line options
KeepTrimObjects	Determines whether or not the original trimming objects and joined geometry are separated and retained or deleted. KeepTrimObjects=Yes (left); KeepTrimObjects=No (right).
AllSimilar	Removes all trimming curves on the edge of a trimmed surface, or if a hole edge is selected, all holes on the same face will be deleted. Original object (left), AllSimilar=No, (center), AllSimilar=Yes (right).

 Crease splitting enabled

The UntrimAll command untrims all trimming curves on an object, both edges and holes in one operation.

The UntrimBorder command untrims the exterior border leaving holes trimmed.

The RemoveAllNakedMicroEdges command removes very small single naked edges; that is, edges that fold or loop back on themselves and have no matching edge to which they can be joined.

See also

Split and trim curves and surfaces