Theory Builder Title

Trimming in Alias

Many CAD systems modify a surface directly using a normal curve:

Direct trimming using a curve

In Alias there is an intermediate step required for trimming, the creation of a Curve-on-Surface:

Intermediate step of creating a curve-on-surface


A Curve-on-Surface is often referred to in the Alias interface as a 'CoS'. It works differently to a normal NURBS or Bezier curve defined in the x, y, z modelling space in the following ways:

Difference between a normal NURBS curve and a curve-on-surface

Tools for Creating Curves-on-Surface

On the Surface Edit Palette, there are three tools for creating CoS, of which the first two are the most commonly used:

Using Project and Intersect to create curves-on-surface

Note: There is also a freehand drawing tool on the Curve Palette. This requires you to position the Edit Points of the CoS directly onto the UV parameters of the surface, a method which is unpredictable. So for most situations, the projection method (above) is preferred as this allows you to control the shape more easily by CV manipulation.

Using New Curve-on-Surface to draw freehand

Tools for Using Curves-on-Surface

Accuracy of Curves-on-Surface and Trimming

There is a specific section in the Preferences → Construction Options called Curve on Surface / Trim where tolerances are set for trimming:

Construction Options for Trimming

Understanding Natural and Trimmed Edges

A surface can be cut by Detaching or Trimming, and it's important to understand the difference:

Detach Trim
Can only happen in the direction of the isoparms---Can have any shape - created by curves-on-surface
Will always result in a four-sided boundary---Can create any number of boundaries
Edges will be Natural surface edges---The Trimmed edges will have more complex parameterisation
The cut cannot be restored using Object Edit → Attach---The cut can be restored using Surface Edit → Un-Trim
ICEMsurf equivalent tool: Trim to Iso-Curves---ICEMsurf equivalent tool: Face

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