SolidWorks 2011 Parts Bible - Matt Lombard [147]
Using comments
Notice the comment to the right of the first equation in Figure 10.5. Comments can be very useful for annotating equations for yourself or others. Two important reasons to annotate are to remember the significance of variables or dimensions and to add special notes about the logic of the equation that may not be obvious.
You can make comments for equations by using a single quote after the end of the equation, or by clicking the Comment button in the Add Equation dialog box. In the following example, the comment, “This must be solved first,” is applied to the equation using the single quote before the comment.
“Spacing@LPattern1” = “Length@Sketch1” / (“Instances@LPattern1”) ‘This must be solved first
To expand on the earlier discussion about projected changes to the Equation interface, several standard selection functionalities do not work in the Edit Equation dialog box. These include triple-clicking to select all (although double-clicking works to select a single word) and pressing Ctrl+A to select all.
Tip
You can make general comments for the model in the Design Journal, a Microsoft Word document that is embedded into the SolidWorks file. The Design Journal is found in the Design Binder folder near the top of the FeatureManager.
On the DVD
You can find the part used in this section on the DVD with the filename Chapter 10 Equations.sldprt.
Using driven dimensions
Sometimes it is more convenient to use a driven (reference) dimension in an equation. This is particularly true when using geometry is the best way to calculate a number. For example, if you are manufacturing a helical auger in 90-degree sections from flat steel stock, then you need to design the auger in 3D but begin to manufacture it in 2D.
What is the shape of the auger when flat? The best way to figure this out (aside from lofted bends, which are discussed in Chapter 21) is to use a little high school geometry, a construction sketch, and some simple equations.
Figure 10.7 shows a 90-degree section of an auger blade. The outside diameter is 12 inches, and the blade width is 3 inches. The overall height is 4 inches. In this case, the auger is represented as a surface because the thickness is ignored. Surface features can be useful in situations like this (used as construction geometry) and are discussed in Chapter 20.
FIGURE 10.7
A representation of the auger
On the DVD
You can find the part for Figure 10.7 on the DVD with the filename Chapter 10 Auger.sldprt.
With this information, you can calculate the lengths of the 3D edges using a sketch and a simple equation. In Figure 10.8, the hypotenuses of the triangles represent the helical edges of the inside and outside of the auger. By making the triangles the same height as the auger section, and by making the horizontal side of the triangle the same length as a quarter of the inside or outside diameter by using simple equations, the geometry and sketch relations automatically calculate the flat lengths of the inside and outside edges of the auger (length of triangle side = diameter of circle × pi / 4). In this way, the triangle is used to simplify the calculation, and give it a visual result.
FIGURE 10.8
Triangles calculate the length of the helical edge.
From this point, you can calculate the flat pattern again, using SolidWorks' sketch-solving capabilities as the calculator. Think of the auger as being the cardboard tube inside a roll of paper towels. If you examine one of these tubes closely, you see that it is simply a straight and flat strip of cardboard that has been wound around a cylinder. What was the flat, straight edge of the original board is wound into a helix. This method simply reverses that process.
This example requires the little-used arc-length dimension to drive the size of the arc. The hypotenuse dimensions are shown by driven or reference dimensions, and these are used to drive the arc-length dimensions, as shown in Figure 10.9. Remember that you can create arc length dimensions by using the Smart Dimension