SolidWorks 2011 Parts Bible - Matt Lombard [97]
As mentioned earlier, relations in 3D sketches are not projected like they are in 2D sketches. For example, an entity in a 2D sketch can be made coincident to an entity that is out of plane. This is because to make the relation, the out-of-plane entity is projected into the sketch plane, and the relation is made to the projection. In a 3D sketch, Coincident means Coincident, with no projection.
As a general caution, keep in mind that solving sketches in 3D is more difficult than it is in 2D. You will see more situations where sketch relations fail, or flip in the wrong direction. Angle dimensions in particular are notorious in 3D sketches for flipping direction if they change and go across the 180-degree mark. When possible, it is advisable to work with fully defined sketches, and also to be careful (and conservative) with sketch relations.
For example, the sketch shown in Figure 6.17 cannot be fully defined without also overdefining the sketch. The main difficulty is that the combination of the tangent arc and the symmetric legs of the end brace cannot be located rotationally, even using the questionable reliability of 3D planes that are discussed next.
FIGURE 6.17
Three-dimensional sketches may be difficult to fully define.
Using planes in space
It is possible to create planes directly in 3D sketches. These planes work like regular planes starting in SolidWorks 2010, where they are defined by constraints and selections rather than selecting a type of method to define a plane. Sketches can be created on these planes, and move with the planes. Having planes in the sketch also enables planar sketch entities such as arcs and circles in 3D sketches.
Unfortunately, there is a lot to watch out for with 3D planes, as they are called. The first thing to watch out for is that they do not follow their original definition like normal Reference Geometry type planes. Figure 6.18 shows the PropertyManager interface for creating 3D planes; however, keep in mind that the plane does not maintain the original relation to these initial references. The parent-and-child relations that SolidWorks users are used to are suspended for this one function, or work in the reverse from what you normally expect.
3D planes cannot be fully defined unless there is some sketch geometry on the plane that is in turn related to something else. Limited types of sketch relations can be applied directly to the plane itself, Horizontal and Vertical relations cannot be applied directly to the plane to orient it. Horizontal and Vertical relations of entities on the plane are relative only to the plane and not to the rest of the part, and so making a line horizontal on the plane does not mean anything when the plane rotates (which it is free to do until it is somehow constrained to prevent this).
Beyond this, when a plane violates a sketch relation, the relation is not reported, which severely limits the amount of confidence that you can place in planes that are created in this way. The biggest danger is in the plane rotating, because that is the direction in which it is most difficult to fully lock down. The best recommendation I can make here is reference sketch lines given some relations to something stable, preferably outside of the 3D sketch.
FIGURE 6.18
The 3D planes PropertyManager
If you choose to use these planes, to activate the plane for sketching, you can double-click the plane with the cursor. The plane is activated when it displays a grid. You can double-click an empty space to return to regular 3D Sketch mode. The main thing that you give up with abandoning 3D sketch planes is the ability to use the dynamic drag options when all loft or boundary sketches are made in a single 3D sketch, which I have never used except to demonstrate the idea once.
Using planar path segments
Some path segments that are allowed in 3D sketches can only be used if they are sketched on a plane. These entities include circles and