Cubit
15.8 *User
Documentation*

Curves are created by specifying the bounding lower-order topology (i.e. the vertices) and the geometry (shape) of the curve (along with any parameters necessary for that geometry). There are several forms of this command:

- Straight
- Parabolic, Circular, Ellipse
- Spline
- Copy
- Arc Three
- Arc End Vertices and Radius
- Arc Center Vertex
- Arc Center Angle
- From Vertex Onto Curve
- Offset
- From Mesh Edges
- Close_To
- Surface Intersection
- By Projection
- Helix

**1. Straight:** The
first form of the command creates a straight line or a line lying on the
specified surface. If a surface is used, the curve will lie on that surface
but will not be associated with the surface's topology.

Create Curve [Vertex] <vertex_id> [Vertex] <vertex_id> [On Surface <surface_id>]

Straight curves can be created using an axis. The syntax is as follows:

Create Curve Axis {options}

The length of the axis must be specified. Go to Location, Direction, and Axis Specification to see the axis command description.

Additionally, several connected straight curves can be created with a single command. The syntax for the polyline command is as follows:

Create Curve Polyline Location {options} Location {options} ...

Notice that two or more locations are used to define a polyline. See Location, Direction, and Axis Specification for the location command description.

**2. Parabolic, Circular, Ellipse:** The
parabolic option creates a parabolic arc which goes through the three
vertices. The circular and ellipse options create circular and elliptical
curves respectively that go through the first and last vertices.

Create Curve [Vertex <vertex_id> [Vertex] <vertex_id> [[Vertex] <vertex_id> [Parabolic|Circular|ELLIPSE [start angle <val=0>] [stop angle <val=90>]]]

If
**'ellipse'**
is
specified, Cubit will create an ellipse assuming the vectors between vertices
(1 and 3) and (2 and 3) are orthogonal. v1-v3 and v2-v3 define the major
and minor axes of the ellipse and v3 defines the center point. These vectors
should be at 90 degrees. If not, Cubit will issue a warning indicating
the vertices are not sufficient to create an ellipse and will then default
to creating a spiral.

The angle options will specify what portion of the ellipse to create.
If none are specified, start**
angle** will default to 0 and stop
**angle** to 90 and the ellipse will go from vertex 1 to vertex
2; if the vertices are free vertices they will be consumed in the
ellipse creation. Start **angle** tells Cubit where to
start the ellipse -- the angle from the first axis (v1 - v3) specified.
Stop **angle** tells
Cubit where to end the ellipse -- the angle from the first axis. The angle
follows the right-hand rule about the normal defined by (v1 - v3) X (v2
- v3).

**3. Spline:** The spline form of the
command creates a spline curve that goes through all the input vertices
or locations. To create a curve from a list of vertices use the syntax
shown below. The **delete** option will remove all of the intermediate
vertices used to create the spline leaving only the end vertices.

Create Curve [Vertex] <vertex_id_list> [Spline] [Delete]

Additionally, spline curves can be created by inputting a list of locations. Where the spline will pass through all of the specified locations. The syntax is shown below:

Create Curve Spline {List of locations}

See Location, Direction, and Axis Specification to view the location specification syntax.

**4. Copy:** This command actually copies
the geometric definition in the specified curve to the newly created curve.
The new curve is free floating.

Create Curve From Curve <curve_id>

**5. Combine Existing Curves:** This command
creates a new curve from a connected chain of existing ACIS curves.

Create Curve combine curve <id_list> [delete]

** 6. Arc Three:**
The following command creates an arc either through 3 vertices or tangent
to 3 curves. The

Create Curve Arc Three {Vertex|Curve} <id_list> [Full]

** 7. Arc End Vertices
and Radius:** The following command creates an arc using two vertices,
the radius and a normal direction. The

Create Curve Arc Vertex <id_list>

Radius <value> Normal {<x> <y> <z> | {direction options} [Full]

Go to Location, Direction, and Axis Specification to see the direction command description.

** 8. Arc Center Vertex:**
The next form of the command creates an arc using the center of the arc
and 2 points on the arc. The arc will always have a radius at a distance
from the center to the first point, unless the

Create Curve Arc Center Vertex <center_id> <end1_id> <end2_id>

[Radius <value>] [Full]

[Normal {<x> <y> <z> | {direction options}]

Go to Location, Direction, and Axis Specification to see the direction command description.

**Note:** Requires 3 Vertices - first
is the center, the other two are the end points of the arc. A normal direction
is required when the three points are colinear. Otherwise a normal direction
is optional.

** 9. Arc Center Angle:**
This form of the command creates an arc using the center position of the
arc, the radius, the normal direction and the sweep angle.

Create Curve Arc Center {<x=0> <y=0> <z=0> | {location options}

Radius <value>

Normal {<x> <y> <z> | {direction options}

Start Angle <value=0> Stop Angle <value=360>

Go to Location, Direction, and Axis Specification to see the location and direction command descriptions.

** 10. From Vertex Onto
Curve:** The following command will create a curve from a vertex onto
a specified position along a curve. If none of the optional parameters
are given, the location on the curve is calculated as using the shortest
distance from the start vertex to the curve (i.e., the new curve will
be normal to the existing curve).

Create Curve From Vertex <vertex_id> Onto Curve <curve_id> [Fraction <f> | Distance <d> | Position <xval><yval><zval> | Close_To Vertex <vertex_id> [[From] Vertex <vertex_id> (optional for 'Fraction' & 'Distance')]] [On Surface <surface_id>]

**Note:** Default = Normal to the Curve

** 11. Offset:** The
next command creates curves offset at a specified distance from a planar
chain of curves. The direction vector is only needed if a single straight
curve is given. The offset curves are trimmed or extended so that no overlaps
or gaps exist between them. If the curves need to be extended the extension
type can be

Create Curve Offset Curve <id_list> Distance <val> [Direction <x> <y> <z>] [Rounded|EXTENDED|Natural]

**Note:** Direction is optional for offsets
of individual straight curves only

In all cases, the specified vertices are not used directly but rather their positions are used to create new vertices.

** 12. From Mesh Edges**:
This commands creates a curve from an existing mesh given a starting node
and an adjacent edge.

Create Curve From Mesh Node <id> Edge <id> [Length <val>]

The adjacent edge indicates which direction to propagate the curve.

The curve will be composed of mesh edges up to the specified length.

If no length is specified the curve will propagate as far as the boundary
of the mesh. Figure 1 shows a example of a curve generated from the mesh.

**Figure 1.
Example of curve created from mesh**

The underlying geometry kernel used for this command is Mesh-Based geometry. The new curve will also be meshed with the edges it was propagated through. A related command for assigning mesh edges directly to a mesh block is the Rebar command. See Element Block Specification for more details.

Note: Full hexes or full tets must be used to propagate the curves through the interior of volume.

**13.
Close_To** This option takes two geometric entities and creates the
shortest possible curve between the two entities at the location where
the two entities are the closest. The two entities may NOT intersect.
If two vertices are given, the command will create a straight line between
the two vertices.

Create Curve Close_To {Vertex|Curve|Surface|Volume|Body} <id_1> {Vertex|Curve|Surface|Volume|Body} <id_2>

**14.
Surface Intersection** The following command creates curves at
surface intersections. Multiple curves can be created from a single
command.

Create Curve Intersecting Surface <id_list>

**15. By Projection **
The project command projects curves, or the curves of a surface
another a single surface or multiple surfaces of a volume or body. The command syntax is
as follows:

Project { Curve <id_list> | Surface <id_list> } Onto Surface <surface_id> [Imprint [Keepcurve] [Keepbody]] [Trim]

Project { Curve <id_list> | Surface <id_list> } Onto {Body <id> | Volume <id>} [Target_surface <id_list>] [Imprint [Keepcurve] [Keepbody]]

The first form of the command takes a list of curves or surfaces, and a projection surface. If a list of curves is given, the result will be the creation of a set of free curves on top of the projection surface. If a list of surfaces is given, the result will be the same as selecting the curves of the surface (i.e. a group of free curves on the projecting surface).

The second form will imprint the list of curves (or curves of the surface(s))
onto the surfaces of the specified bodies or volumes. The **Target_surface**
option helps when the projection is ambiguous, for example projecting curves onto
a thin-walled volume where the projection could be to either side, as shown
in Figure 2.

**Figure 2.
Example of projecting curves to specific side of thin-walled volume.**

The imprint option will imprint the resulting projected curves onto the projection surface. If this option is NOT given, the new curves will lie coincident to the surface, but will not be part of the surface. Imprinting changes the topology of the projection surface. Keepcurve option retains the new curves as both free curves, and curves in the projection surface. The keepbody option retains the original body under the new imprinted body. When projecting curves, the trim option will cause the curve to be trimmed to the target surface.

16. Creating a Helix: This command will create a helical curve. The command syntax is as follows:

Create Curve Helix { axis <xpoint ypoint zpoint xvector yvector zvector> | xaxis | yaxis | zaxis } location (options) thread_distance <value> angle <value> [RIGHT_HANDED | left_handed]

axis = axis about which to create the helix

location (options) = starting point of the helix

thread_distance = distance between each 360 degree segment of the helix

angle = number of degrees in rotation of the helix

handedness = right-handed or left- handed threads