Patching Method

Before any patches can be generated, all of the defining boundaries should be constructed.The patch boundaries can be any 3D curve, such as plines, knuckles, sections, waterlines,FOB, FOS, . There are several basic rules which should be followed when constructing patch boundaries:

The patch size used will differ with the amount of curvature variation. Areas of the design which contain high curvature and/or rapidly changing curvature will need to use small patches to maintain accuracy. Areas such as the FOS, FOB and other flat patches can use larger patches and still retain the desired accuracy.

Patches should not cross the FOS, FOB, knuckle lines, . because the generated patch is not able to accurately describe abrupt changes in curvature. The FOS, FOB and any knuckle lines should be used as patch boundaries.

Plines should be constructed so that they follow zones of 'similar' shape, such as inflection points and regions of similar curvature.

Any intersections of constructed plines with the FOB, FOS, knuckle lines . should occur at a station or waterline, hence defining a patch boundary corner. If this is not possible then another station/waterline may need to be generated at the intersection ordinate, or alternatively, a further pline may be constructed.

The ends of the FOS and FOB should also lie on a station. If they do not, either create a station at the end point or generate a pline which ends at the same position.

Where straight line segments, arc segments, ., exist in the patch boundary curves, intersecting patch boundaries must be created at the ends of these segments.

All patch boundaries must intersect to form a closed loop.

Generally, patch boundaries are FOS, FOB, Knuckles, Stations, Waterlines and Plines. The plines are usually constructed by creating clines in buttock view and then intersecting them to produce 3D plines which lie on the surface.

The suggested procedure for this patching method is described in the following sections: