Creating Lofted Surfaces

This task shows how to create a lofted surface.

You can generate a lofted surface by sweeping one or two planar section curves along a computed or user-defined spine. The surface can be made to respect one or more guide curves.

Open the Loft1.CATPart document..

  1. Click the Loft icon  .

The Lofted Surface Definition dialog box appears.

  1. Select one or two planar section curves.

These sections (two at maximum) may be tangent to support surfaces, provided they are not parallel.
Closed section curves can have point continuity at each closing point.

  1. If needed, select one or more guide curves.

  1. In the Spine tab page, select the Spine check box to use an automatically computed spine or select a curve to impose that curve as the spine. 

  1. It is possible to edit the loft reference elements by first selecting a curve in the dialog box list then choosing a button to either:

Remove the selected curve
Replace the selected curve by another curve.
Add another curve.

More possibilities are available with the contextual menu and by right-clicking on the red text or on the object. For example, it is possible to remove and replace tangent surfaces and closing points.

  1. Click OK to create the lofted surface.

The surface (identified as Loft.xxx) is added to the specification tree.

You can impose tangency conditions onto sections and/or guides, by specifying a direction for the tangent vector (selecting a plane to take its normal, for example). This is useful for creating parts that are symmetrical with respect to a plane. Tangency conditions can be imposed on the two symmetrical halves.
Similarly, you can impose a tangency onto each guide, by selection of a surface or a  plane (the direction is tangent to the plane's normal).  In this case, the sections must also be tangent to the surface.
You can create lofted surfaces between closed section curves. These curves have point continuity at their closing point.
This closing point is either a vertex or an extremum point automatically detected and highlighted by the system.
By default, the closing points of each section are linked to each other.

The red arrows in the figures below represent the closing points of the closed section curves. You can change the closing point by selecting any point on the curve. 

The surface is twisted

A new closing point has been imposed
to get a non-twisted surface

The Relimitation tab lets you specify the loft relimitation type. .
You can choose to limit the loft only on the Start section, only on the End section, on both, or on none.
when one or both are checked: the loft is limited to corresponding section
when one or both are when unchecked: the loft is swept along the spine:
if the spine is a user spine, the loft is limited by the spine extremities
if the spine is an automatically computed spine, and no guide is selected:
the loft is limited by the start and end sections 
if the spine is an automatically computed spine, and guides are selected:
the loft is limited by the guides extremities.

Loft relimitation option checked
 on both Start and End section

Loft relimitation option unchecked
 on End section only
Use the Planar surface detection check button (Canonical Surfaces tab) to automatically convert planar surfaces into planes.

Coupling

This task presents the two kinds of coupling during the creation of the lofted surface:
coupling between two consecutive sections
coupling between guides

These couplings compute the distribution of isoparameters on the surface.

Open the Loft2.CATPart document. To perform  the following scenario you will need to get some geometry locate

Coupling between two consecutive sections

This coupling is based on the curvilinear abscissa.

  1. Click the Loft icon  .

The Lofted Surface Definition dialog box appears.

  1. Select the two consecutive sections.

  1. Click OK to create the loft.
To create a coupling between particular points, you can add guides or define the coupling type.

Coupling between guides

This coupling is performed by the spine.
If a guide is the concatenation of several curves, the resulting loft will contain as many surfaces as curves within the guide.

Several coupling types are available, depending on the section configuration:
Ratio: the curves are coupled according to the curvilinear abscissa ratio.

Tangency: the curves are coupled according to their tangency discontinuity points. If they do not have the same number of points, they cannot be coupled using this option. 

Tangency then curvature: the curves are coupled according to their tangency continuity first then curvature discontinuity points. If they do not have the same number of points, they cannot be coupled using this option. 

Vertices: the curves are coupled according to their vertices. If they do not have the same number of vertices, they cannot be coupled using this option. 

 

 
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