Xsection GCDs
Geometry extraction for the Stretch Forming process group extracts two special GCDs that represent cross-sections of the part. These are listed as Xsection GCDs in the Geometric Cost Drivers pane, and are displayed in the Viewer in purple.
The properties of Xsection GCDs help the cost model determine feasible stretching directions for the part, as well as the required pull force (see Stretch Form Process Feasibility and Required Pull Force for Stretch Forming).
Both Xsections are positioned based on a heuristic designed to avoid locating them so that they cross cutouts in the blank. In the absence of cutouts, both Xsections pass through the middle of the blank’s SER.
One Xsection is aligned with the direction of maximum radius of curvature, and the other Xsection is orthogonal to it. (If there is no unique direction of maximum radius of curvature--as with, for example, a portion of a sphere--then the Xsections are aligned with the blank’s SER length and width.)
Many of the important Xsection properties are defined in terms of the following geometric objects associated with each Xsection (these objects are not explicitly represented as GCDs):
• Xsection curve: the intersection of the Xsection with the surface of the part
• Xsection hull: the shape described by a string aligned with the Xsection curve and tightened around one side of the part (the side that results in the longest hull overlap—see below).
• Xsection hull overlap: portion of the Xsection hull that is in contact with the Xsection curve. Note that the overlap might consist of multiple, non-contiguous segments of the hull.
Following are some of the important Xsection properties:
• Hull Overlap Length: total length of the Xsection hull overlap.
• Hull Avg Curvature: reciprocal of the average radius of curvature of the hull overlap (that is, average radius of curvature of the portion of the Xsection hull that is in contact with the Xsection curve).
• Hull Min Curvature: reciprocal of the radius of curvature of the flattest point of the hull overlap. If there is a completely flat segment of the overlap (that is, if the overlap crosses a plane), this value is set to 0.
• Hull Max Curvature: reciprocal of the radius of curvature of the sharpest point of the hull overlap. For sharp corners, this value is set to 2 divided the part thickness.
• Hull direction: this is UP when the direction from the hull to the chord connecting its endpoints is the ram direction. This is DOWN when the direction from the hull to the chord connecting its endpoints is the opposite of the ram direction.
• Curve Length: length of the Xsection curve
• Curve Length Extended Aligned: distance, in the direction of the Xsection, across the Xsection-aligned bounding box of the flattened blank
• Curve Length Extended Orthogonal: distance, in the direction orthogonal to the Xsection, across the Xsection-aligned bounding box of the flattened blank
• Rectangle Lower Left and Rectangle Upper Right: these provide coordinates of opposite corners of the smallest enclosing rectangle of the Xsection curve. The coordinates use a coordinate system whose origin is the intersection of the two Xsections and whose y-axis is tangent to the Xsection curve at the origin.
