Trim Surface

3D Trimming

If this box is checked, you can trim the surface along projected curves and cross sections, or you can select a surface and generate a trimming curve automatically from the intersection with the target surface. By default it is unchecked.

Method

Displays when 3D Trimming is selected.

• Project – Creates trimming curves by projecting the selected curves onto the target surface.
• Intersect – Creates a trimming curve from the intersection of the selected surface and the target surface.

You can use the Project or Intersect method in a single trimming operation by toggling back and forth between them.

Create History

If this box is checked, the projected curves have construction history. Editing or transforming the curves and surfaces causes the projection and trim operations to be re-executed.

This option is only available if 3D Trimming is turned on.

Extend

Extend a curve that does not project entirely on the patch, either automatically or manually, and continue the trim operation without leaving the tool.

Auto

When the trim fails, any curves-on-surface that are within the set Tolerance of another curve-on-surface or boundary are extended and the trim is attempted again.

Selected

A trim extend arrow appears on the curve that needs extending. Clicking the arrow snaps the end of the curve to the patch edge.

Vector Options

These options are only available if 3D Trimming is turned on.

X, Y, Z

Specifies a projection vector along that axis.

View

Specifies a vector normal to the current view. The vector is not drawn in the view windows.

If the current view is changed, click Refresh View Vector to update the vector.

Picked

Lets you specify the name of an existing vector in the Picked Vector field, or pick the vector in the view.

Normal

The curve is projected onto each point on the surface along the normal at that point, instead of along a single vector direction.

Picked Vector

This text field only appears when Picked is selected. It displays the name of the selected vector object. Alternatively, you can type in the name of a vector object.

Refresh Vector

This button only appears if View is selected. Click it to update the vector if the view has been modified

Retain Vector

Click this button to create a vector construction object in the view windows.Otherwise, the tool uses the vector direction you specified, but you are not able to see and re-use the vector.

Create Vector

Click this button to create a projection vector.

Click in the view to place the start point of the vector, or type the position and press . Use the vector manipulator to position the vector then click Accept.

Limit Projection

Use this option to limit the projection distance of curves along the specified projection vector (Vector Options) so that the curves do not automatically get projected through the target object. When trimming closed objects, this allows you to control the distance from the surface or depth of the trim, rather than trimming through both walls of the target object.

Adjust the Positive Distance and the Negative Distance values to specify the projection distance from the current position of the projected curve. Use the in-canvas arrow manipulator or enter values for Positive Distance and Negative Distance in the set measurement units.

Display Options

Show Intersections

If this box is checked, green locators indicate the intersections between the trim curves, and between the trim curves and trim surfaces. Yellow locators indicate trim curve endpoints that do not intersect any other curve-on-surface or trim surface edge.

Show Preview

If this box is checked, a preview of the trimmed surface is highlighted yellow.

Region Selector Length

Adjust the value if the selectors (crosses) appear too small or too large on your model. Displays trim locators based on pixel size rather than based on the size of the surfaces to be trimmed.

Note: The Shrink Surface option has been replaced by a plug-in called shrinkToTrim which can be loaded through Utilities > Plug-in Manager . shrinkToTrim shrinks the UV parameters of the underlying surface to cover only the visible (non-trimmed) parts of the surface. This functionality is useful when applying label-style textures to the surface.

Control Options

Chain Select

If this box is checked, selecting a curve to project also selects all other curves that are tangent continuous with it.

Subdiv Select

Turn this on when trimming the surface of subdivision objects. When on, the Trim tool behaves the same way with subdivision objects as it does with NURBS surfaces. Subdiv Select is on by default.

Trim Divide History

If this box is checked, trim divided surfaces are linked together with history so that subsequent modifications to one of the divided surfaces are reflected in the other.

Mouse Defines Action

If this box is checked, the mouse buttons control the trim mode. The left, middle, and right mouse buttons are mapped to Keep, Discard, and Divide, respectively. Click the trim region with the appropriate mouse button and press the spacebar to complete the trim.

Show Last Selectors

If this box is checked, the last trim region selectors are saved and restored after untrimming. If after untrimming you want to make changes and retrim, you only have to add or move the selection locators for the changes you are making. You do not have to reclick all the original ones.

Continuity Check

Adds a surface continuity locator to the trimmed surfaces, providing a pass or fail indication on continuity and showing any discontinuities. This option is available when the 3D Trimming method is set to Project.

Continuity Check Type

When Continuity Check is on, you can choose to check for a specific level of continuity. Choose an option from the Continuity Check Type list, which includes G0 Position, G1 Tangent, or G2 Curvature.

Trim or divide a surface by projecting curves on surfaces

1. Shift-select the Trim Surface tool.

2. In the control window, turn on 3D Trimming and choose a Vector Option.

   If you select Normal, the curve is projected onto each point on the surface along the normal at that point, instead of along a single vector direction.

   Note: If you are trimming or dividing subdivision surfaces, ensure that Subdiv Select is turned on.
   

3. Select all the surfaces you want to trim. Hold down the Shift key to pick more than one surface, or use a pick box.

4. 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.

   

   Two surfaces and one curve selected.

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

   

   After selecting the second curve

   As you select surfaces and curves, the curves are automatically projected onto the surfaces, and the corresponding trim curves are displayed on the surfaces. The direction of the projection is determined by the set Vector Option.The default View, which sets the direction of projection based on the view vector of the window where you selected the curves ( that is, perpendicular to the window). To project normal to the surface, you must select the Normal option.

5. To limit the distance of the projection, turn on Limit Projection. This option is useful when projecting curves onto closed NURBS and subdivision objects. For example, setting a projection distance stops the projection from passing through both walls of the closed object.

   When this option is on, an arrow manipulator appears in the viewport attached to two representations of the projected curve at both ends. Drag the manipulator the set the projection distance based on the current position of the curve. You can also enter values for Positive Distance and Negative Distance.

   

6. Click on the regions of the surfaces that you either want to keep or discard. (Regions are areas delimited by curves-on-surface). If the trim regions are spread across multiple surfaces, you can box-select them.

   

   Crosshairs appear on the selected regions. To move a crosshair, click it and drag.

   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.

   Tip: You 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.

7. At any time, before performing the next step, you can change the vector option in the control window to change the direction of projection.

   The curves-on-surface update.

   Note: If you choose Picked, you must then pick a vector object along which to project.
   Note: If you choose View, you can project different curves along different view vectors. However, if you then click Refresh View Vector, or select a different vector option, all curves-on-surface update to match the current projection vector.

8. Click 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 5, and discards the rest.

   Discard: Discards the regions selected in step 5, and keeps the rest.

   Divide: Divides the selected regions from the others (making separate trim surfaces) but keeps all the regions.

   

   In this example, Keep was used.

   Tip: Turn on the Mouse Defines Action option in the Trim Control window to map the left, middle, and right mouse buttons to Keep, Discard, and Divide, respectively. When this option is turned on, you can still use the Keep, Discard, and Divide in-canvas buttons to choose a trim operation.

9. After the trim operation has been performed, you can undo it by clicking the Revert button.

   You are left within the tool with all your selections intact, so you can adjust your selections and trim again.

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

   

   Note: 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.
   Tip: Use the mouse buttons to control the trim mode by selecting the Mouse Defines Action option under Control Options in the Trim Control window. The left, middle, and right mouse buttons are mapped to Keep, Discard, and Divide, respectively. Click the trim region with the appropriate mouse button and press the spacebar to complete the trim.

Trim or divide a surface along the intersection of another surface

1. Shift-select the Trim Surface tool.

2. In the control window, select 3D Trimming and choose Intersect as the method.

   Note: If you are trimming or dividing subdivision surfaces, ensure that Subdiv Select is turned on.

   

3. Select all the surfaces you want to trim. Hold down the Shif key to pick more than one surface, or use a pick box.

4. Select the intersecting surfaces you want to use to create the trim curves (curves-on-surface).

   As you select surfaces and intersecting surfaces, the trim curves are automatically created and displayed on the surfaces.

   Note: If you box-select several surfaces, and some of them are already selected for trimming, Alias picks the unselected surfaces and intersects them with the selected surfaces.

5. Click on the regions of the surfaces that you either want to keep or discard. (Regions are areas delimited by curves-on-surface). If the trim regions are spread across multiple surfaces, you can box-select them.

   Crosshairs appear on the selected regions. To move a crosshair, click it and drag.

   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.

   Tip: You 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.

6. Click 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 5, and discards the rest.

   Discard: Discards the regions selected in step 5, and keeps the rest.

   Divide: Divides the selected regions from the others (making separate trim surfaces) but keeps all the regions.

7. After the trim operation has been performed, you can undo it by clicking the Revert button.

   You are left within the tool with all your selections intact, so you can adjust your selections and trim again.

   Note: A successful trim curve is shown with a green marker where the ends of the curve join.
   Note: An unsuccessful trim curve — as in this case, where a gap exists — has the two ends of the curve highlighted in red to call attention to the gap region.
   Tip: Use the mouse buttons to control the trim mode by selecting the Mouse Defines Action option under Control Options in the Trim Control window. The left, middle, and right mouse buttons are mapped to Keep, Discard, and Divide, respectively. Click the trim region with the appropriate mouse button and press the spacebar to complete the trim.

Trim or divide a surface using cross sections

1. Shift-select the Trim Surface tool.

2. In the control window, check on 3D Trimming.

   Note: If you are trimming or dividing subdivision surfaces, ensure that Subdiv Select is turned on.

   

3. If you are trimming a surface with its own cross-sections, choose Normal under Vector Options.

4. Select the surface to trim (hold down the Shift key to add more surfaces to the selection).

5. Select the cross sections to use as trimming curves.

6. Click on the regions to keep or discard.

7. Click the Keep, Discard, or Divide button.

   

   Trimming both a regular surface and a trimmed surface with their Y-sections.

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.

Split (divide) a surface into trimmed surfaces along curves-on-surface

How to create multiple trimmed surfaces from the trim boundaries of a surface.

1. Select the Trim Surface tool .

2. Click the surface you want to split.

3. Click an area you want to separate. You can click more than one area of the surface to separate. Crosshairs appear on the selected areas.

   Tip: You 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.

4. Click the Divide button in the window.

   The tool trims each selected area from the original, and creates a new trimmed surface of the separated area.

5. After the trim divide operation has been performed, you can undo it by clicking the Reset button.

If you are applying an additional trim operation to a primary surface that has been automatically trimmed when creating a secondary surface, you may see the following alert message:

You may also see this message if you have performed a trim operation on curves created by projecting or intersecting curves or surfaces. The usual cause of the problem is that one of the initial surfaces (or curves) used to create the trim curve or secondary surface has been moved, resulting in a trim curve on one of the initial surfaces that no longer isolates an area that can be trimmed, and this has been diagnosed when you are attempting to create a new trim on the surface.

For example, this diagram shows a fillet created between two flat surfaces. One surface was moved, resulting in an incomplete trim surface. When attempting to create an additional trim on the surface (the circular hole), the problem with the now-incomplete trim curve was discovered.

You can alter the visible shape of a surface by trimming away parts. Since NURBS surfaces are intrinsically four-sided and do not allow holes, you need a way to visually simulate irregular shapes and holes when using NURBS. The answer is trimming.

Trimming lets you visually cut or divide a surface along a curve-on-surface so it appears to have holes or missing pieces. The trimmed surface, however, is not actually cut. It exists in a hidden form that does not render or affect modeling. You can recover the trimmed part of a surface using the Untrim tool.

Creating curves-on-surface and then trimming is the most common way to combine NURBS surfaces in industrial design.

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

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

You can create the bounding curves-on-surface while inside the Trim Surface tool, by projecting curves on the surfaces(s) or you can automatically create a trimming curve-on-surface from the intersection of a surface with the target surface(s).

You can also create curves-on-surface before using the Trim Surface tool.

Cross sections are created using Windows > Cross Section Editor.