Extrapolating Surfaces

In this task you will learn how to extrapolate a surface, that is add an extra surface to the initial one while taking continuity constraints into account.

Open the FreeStyle_Part_51.CATPart document.

  1. Select a surface boundary.
    A manipulator is displayed.

  2. Click Styling Extrapolate .
 

The Extrapolation dialog box is displayed.
  • Element: Select the element to be extrapolated.
  • Limit Type: Two options are available:
    • Length: Specify the length for extrapolation.
    • Up To: Specify the element up to which you want the input element to be extrapolated. You can select a curve, plane or surface.
  • Length: This field is displayed if Length option is selected in the Limit Type list. Enter the extrapolation length.
  • Up To: This field is displayed if Up To option is selected in the Limit Type list. Select the limiting element.
    You can also create limiting plane element using the contextual menu command. Following commands are available:
    Create Plane
    XY Plane
    YZ Plane
    ZX Plane    .
  • Type: Two options are available:
    • Tangential: the surface is extended along the tangent to the surface on the selected boundary and of a given distance (G1 continuity).
    • Curvature: the surface is extrapolated by the specified value, complying with the surface curvature (G2 continuity). No manipulator is displayed.
  • Exact: Select this check box for analytical computation of resulting surface. If this check box is cleared, the resulting surface is created using geometrical computation.
 

  1. Select Tangential option and pull on the manipulator till you reach 60 in the Length field of the Extrapolation dialog box.
    The extrapolation is pre-visualized and dynamically evolves as you modify the extension length.

You could just as well enter the value directly in the Length field.
 

  1. Click OK.
    The extrapolated surface is created.
    Here are two views of the extrapolated surface, illustrating the tangency constraint on the selected edge.

Similarly, if an edge is selected, and the Curvature option active, the surface is extrapolated according its initial curvature:

With Curvature option

With Tangential option
  • To extrapolate a set of surface boundaries, you need to select these, using the Ctrl-key, before clicking the Extrapolate icon .
  • The curvilinear length between the middle of the selected original surface edge and the middle of the extended surface is displayed.
  • Whether in Tangential or Curvature mode, select the Exact option if you want to switch to the exact mode. The figures below show a surface extrapolated when Exact mode is checked and unchecked.
    • when the Exact option is cleared, computation is analytical and the resulting surface presents the same degree as the initial surface.
      Since you chose this constraint, there may be no possible result in some cases.
    • when the Exact option is not checked, the extrapolation is computed according to the geometric mode.

Exact mode cleared

Exact mode selected