Trim a surface

Trims a surface by removing (actually hiding) any part of the surface bounded by curves-on-surface. This lets you create complex edges and holes in NURBS surfaces. You can also split up (divide) a surface into multiple surfaces.

Trimming a surface involves trimming back to existing curves-on-surface. You can either project curves on the surface(s) while inside the Trim surface tool (see workflow below), or create curves-on-surface before using the Trim surface tool.

You can trim more than one surface at a time, using one or more projected curves.

Trim or divide a surface

Choose Surface Edit > Trim > Trim Surface ❒.
In the option box, turn on 3D Trimming. (In this example, we’ll also turn on Project Normal so we can pick the curves in the Perspective window).
Select all the surfaces you want to trim. Hold down the key to pick more than one surface, or use a pick box.
Select the curves you want to project onto the surface to create trim curves (curves on surface). If the desired curves-on-surface are already present, you can skip this step.

NoteThe surfaces and curves can be selected in any order, but the first object selected must be a surface.

As you select surfaces and curves, the curves are automatically projected onto the surfaces, and the corresponding trim curves are displayed on the surfaces.

NoteBy default, the direction of projection is determined by the view vector of the window where you selected the curves ( that is, perpendicular to the window). To project normal to the surface, turn on the Project Normal option.
Click down on the regions of the surfaces that you either want to keep or discard. (Regions are areas delimited by curves-on-surface).

Crosshairs appear on the selected regions.

If those region selectors appear too large or too small on your geometry, you can adjust their size by using the Region Selector U Size and V Size sliders in the control window.

TipYou do not have to click a “visible” part of the surface (such as an isoparametric curve). Clicking anywhere inside the surface edges will also work.
Press one of the buttons: Keep, Discard, or Divide, depending on the type of operation you want to perform.
Keep: Keeps the regions selected in step 4, and discards the rest.

Discard: Discards the regions selected in step 4, and keeps the rest.

Divide: Divides the selected regions from the others (making separate trim surfaces) but keeps all the regions.
After the trim operation has been performed, you can undo it by pressing the Revert button.
You are left within the tool with all your selections intact, so you can adjust your selections and trim again.

NoteA successful trim curve is shown with a green marker where the ends of the curve join.

NoteAn unsuccessful trim curve — as in this case, where a gap exists — has the two ends of the curve highlighted in yellow to call attention to the gap region.

What if...?

I can’t trim a surface because the curves-on-surface are outside tolerance?

To change the tolerances governing trim operations, choose Preferences > Construction Options and open the Curves On Surface/Trim section.

Trim Curve Fit controls the accuracy of the trim boundaries created using the trim tool.

Max Gap Between Curves is the maximum gap allowed between the endpoints of two curves-on-surface (or a curve-on-surface and a surface edge) to consider them closed when defining a trim region.

A successful trim curve is shown with a green marker where the ends of the curve join.

An unsuccessful trim curve — as in this case, where a gap exists — has the two ends of the curve highlighted in yellow to call attention to the gap region.

See Troubleshooting trimmed surfaces