SolidWorks 2011 Assemblies Bible - Matt Lombard [55]
With this information, the layout sketch looks like Figure 6.3.
FIGURE 6.3
Adding information to the layout sketch
With this data, you have all of the critical point locations to design the actual frame. The first step in creating the frame is to place reference geometry (sketch planes) from which to make sketches for the extrudes or other features used to make the individual tubes of the frame. The frame will be a carbon fiber monocoque, but will still rely on tubular geometry, with smooth blends between the tubes to reduce stress concentrations. The layout with the planes and the initial tubes is shown in Figure 6.4.
FIGURE 6.4
Building the tubes for the frame
When all of the tubes are created in-place in the assembly from the assembly layout sketch, the top of the assembly FeatureManager looks like Figure 6.5.
You should notice two things right away from this FeatureManager. The first is that the BibleBikeFrame part listed in the tree displays a rebuild symbol (which resembles a traffic light), and the second is that every feature in the list has the -> symbol to the right of it. The rebuild symbol means that SolidWorks thinks there is something that needs to be recalculated in the part, although there is not. If you open the part in its own window, there are no rebuild symbols, but in the context of the assembly it seems there always are, even if you force a rebuild (Ctrl+Q). Many users who have experimented with in-context relations experience this kind of behavior with the software. This has a lot to do with why so many people have misgivings about in-context techniques — there just seem to be too many things that happen that are out of your control. Assembly layout sketches are a fantastic technique, but they are not without their quirks.
The -> symbol means that there is an external reference from the part to the assembly sketch. (External references are covered in more detail in Chapter 10.) Using the assembly layout sketch technique, an external reference is made every time a relationship is made from the part to the assembly sketch. Many users prefer to avoid external references, mainly because of the file management issues they cause, the difficulty in repairing them when broken, and rebuild speed performance issues. However, references to an assembly sketch are more stable than references between two parts in an assembly.
FIGURE 6.5
Examining the features built from the assembly layout sketch
When you use an assembly layout sketch for either the in-context part building or simply part positioning, the main advantage that it offers is to give you a single driving sketch that enables you to change the size, shape, and position of the parts. You can use as many layout sketches as you want, and you can make them on different sketch planes. This enables you to control parts in all directions.
Caution
When using layout sketches, it is assumed that the relationships are created such that the sketch drives everything else. However, nothing is preventing you from using other things in the assembly to drive the sketch. You should avoid this type of conflict, called a circular reference. It can create sketches that change with every rebuild and can seriously impact rebuild times. When using any type of in-context relations, you need to be careful to establish one or more driving entities, which are not in turn driven by other entities.
To take this a step further, it is best to avoid daisy chaining, where A drives B, B drives C, and so on. It is better practice to make A drive both B and C directly. This saves on rebuild times and troubleshooting.
One of the drawbacks of this technique is that you give up dynamic assembly motion. By creating the parts in the context of the assembly and creating relationships between sketches or features and the assembly sketch, you cannot move the