SolidWorks 2011 Parts Bible - Matt Lombard [104]
I have already mentioned several sketch types, including closed loop and open loop. Closed loop sketches make solid features by default, but you can also use them to make thin features. Open loop sketches make thin features by default, and you cannot use them to make solid features. A nested loop is one closed loop inside another, like concentric circles. Self-intersecting sketches can be any type of sketch where the geometry crosses itself. SolidWorks also identifies sketches where three or more sketch elements intersect at a point by issuing an error if you try to use the sketch in a feature. Figure 7.4 illustrates these different types of sketches. Some of these examples are errors and some are just warnings.
FIGURE 7.4
Identifying different types of sketches in SolidWorks
Sketch contours
Sketch contours enable you to select enclosed areas where the sketch entities themselves actually cross or otherwise violate the usual sketch rules. One of these conditions is the self-intersecting contour.
Best Practice
SolidWorks works best with well-disciplined sketches that follow the rules. Therefore, if you plan to use sketch contours, you should make sure that it is not simply because you are unwilling to clean up a messy sketch.
When you define features by selecting sketch contours, they are more likely to fail if the selection changes when the selected contour's bounded area changes in some way. It is considered best practice to use the normal closed loop sketch when you are defining features. Contour selection is best suited to “fast and dirty” conceptual models, which are used in very limited situations.
There are several types of contour selection, as shown in Figure 7.5.
3D sketch
You can make extrusions from 3D sketches, even 3D sketches that are not planar. While not necessarily the best way to do extrudes, this is a method that you can use when needed. You can establish direction for an extrusion by selecting a plane (normal direction), axis, sketch line, or model edge.
When you make an extrusion from a 3D sketch, the direction of extrusion cannot be assumed or inferred from anything — it must be explicitly identified. The default extrusion direction from a 2D sketch is always perpendicular to the sketch plane unless otherwise specified.
Non-planar sketches become somewhat problematic when you are creating the final extruded feature. The biggest problem is how you cap the ends. Figure 7.6 shows a non-planar 3D sketch that is being extruded. Notice that the end faces are, by necessity, not planar, and are capped by an unpredictable method, probably a simple Fill surface. This is a problem only if your part is going to use these faces in the end; if it does not, then there may be no issue with using this technique. If you would like to examine this part, it is included on the DVD as Chapter 7 Extrude 3D Sketch.sldprt.
FIGURE 7.5
Types of contour selection
FIGURE 7.6
Extruding a non-planar 3D sketch
If you need to have end faces with a specific shape, and you still want to extrude from a non-planar 3D sketch, then you should use an extruded surface feature rather than an extruded solid feature.
One big advantage of using a 3D sketch to extrude from is that you can include profiles on many different levels, although they must all have the same end condition. Therefore, if you have several pockets in a plate, you can draw the profile for each pocket at the bottom of the pocket using planes offset at different levels, and extrude all the profiles Through All; as a result, they will all be cut to different depths.
Three-dimensional sketches also have an advantage when all the profiles of a single loft or boundary are made in a single 3D sketch. This enables you to drag the profiles and watch the loft update in real time.
Cross-Reference
Surfacing features are covered in detail in Chapter 20. Chapter 4 contains additional details on