SolidWorks 2011 Assemblies Bible - Matt Lombard [47]
Something to be careful about is that a degree-of-freedom analysis frequently predicts an over-defined mate scenario when SolidWorks does not in fact display any errors or warnings. For example, if one block is mated to another with the simple case of three face-to-face Coincident mates, and each Coincident mate ties down one translational and two rotational degrees of freedom, then the mating scenario ties down nine degrees of freedom, so the part is over-constrained by three rotational degrees of freedom. However, SolidWorks has a lot of forgiveness built in, so it commonly allows situations like this, where parts are severely overconstrained. When troubleshooting any over-constrained situation, you should not take this forgiveness for granted. If SolidWorks reports an assembly as over-constrained and the reason is not intuitively obvious, try reducing some of the degrees of freedom constrained. For example, instead of making two faces coincident, consider making them simply parallel, or mate a point to a face instead of two faces.
Best Practice
This may be an overly cautious approach, but it can mean the difference between an assembly that works and one where errors are frustratingly persistent. If you are careful to approach all parts with the degree-of-freedom analysis in mind such that any newly added mate does not duplicate any of the degrees of freedom that are already tied down, you will have fewer assembly mate errors and fewer problems with assembly motion.
This means that instead of having the traditional three face-to-face Coincident mates, you would have one face-to-face Coincident mate (one translational degree of freedom, two rotational degrees of freedom), one edge-to-face Coincident mate (one translational degree of freedom, one rotational degree of freedom), and one point-to-face Coincident mate (one translational degree of freedom). This accounts for three translational and three rotational degrees of freedom without over-defining any of them.
It is true that SolidWorks internally compensates for over-defined degrees of freedom, but relying on it to do so and then tempting fate by methodically over-defining all assemblies is a risk that you do not have to take, even though it is common practice.
Setting up successful motion
The best bet for creating motion in a SolidWorks assembly is to leave open a single degree of freedom. This means that there is only one way the part can move, back and forth, by translation or rotation. Computers in general do not respond well to ambiguity. Dragging an item that might move in several ways is more likely to cause jerky or hesitant motion.
A good example of this kind of problem with motion can be found in one of the sample assemblies that installs with SolidWorks. This example is included on the DVD for your convenience, and is shown in Figure 5.8. The filename for the assembly is Plunger.sldasm.
FIGURE 5.8
An assembly displaying best bet for motion
If you drag the assembly parts from the location shown in Figure 5.8, the performance varies. This is because when you drag the handle parts, for every position of the handle, there is only one solution for the rest of the parts. However, when dragging the plunger bar, for every position of the plunger bar, there are two possible positions for both the links and the handle (one possibility is as shown, and the other would be with the handle interfering with the base of the assembly). This kind of ambiguity causes problems in SolidWorks assemblies such as assemblies that have open degrees of freedom but will not move or only move in a jerky fashion.
Another example of difficulties related to open degrees of freedom and motion is shown in Figure 5.9. The grippers at the end of the arm move when the rest of the arm moves, but the grippers cannot be independently controlled by dragging. To fix this problem, you may want to either use the Fix/Float option (available through the RMB menu) or use configurations with mates suppressed or unsuppressed. Fix the part that you want to