SolidWorks 2011 Parts Bible - Matt Lombard [112]
FIGURE 7.20
The sweep profile follows the path and is controlled parametrically by guide curves.
On the DVD
The part shown in Figure 7.20 is on the DVD with the filename Chapter7 Bottle.sldprt.
The PropertyManager for the Sweep function includes an option for Show Sections, which in this case creates almost 200 intermediate cross-sections. These sections are used behind the scenes to create a loft. You can think of complex sweeps with guide curves or centerlines as an automated setup for an even more complex loft. It is helpful to envision features such as this when you are troubleshooting or setting up sweeps that are more complex. If you open the part mentioned previously from the DVD, you can edit the Sweep feature to examine the sections for yourself.
In most other published SolidWorks materials that cover these topics, sweeps are covered before lofts because many people consider lofts the more advanced topic. However, I have put lofts first because understanding them is necessary before you can understand complex sweeps, as complex sweeps really are just lofts.
Using a Pierce relation
The Pierce sketch relation is the only sketch relation that applies to a 3D out-of-plane edge or curve without projecting the edge or curve into the sketch plane. It acts as if the 3D curve is a length of string and the sketch point is the hole in the center of a bead where the string pierces the hole in the bead. The Pierce relation is most important in the Sweep feature when it is applied in the profile sketch between endpoints, centerpoints, or sketch points and the out-of-plane guide curves. This is because the Pierce relation determines how the profile sketch will be solved when it is moved down the sweep path to create intermediate profiles.
Figure 7.21 illustrates the function of the Pierce relation in a sweep with guide curves. The dark section on the left is the sweep section that is sketched. The lighter sketches to the right represent the intermediate profiles that are automatically created behind the scenes and are used internally to create the loft.
FIGURE 7.21
The effects of the Pierce relation
Figure 7.21 shows what is happening behind the scenes in a sweep feature. The sweep re-creates the original profile at various points along the path. The guide curve in this case forces the profile to rebuild with a different shape. Pierce constraints are not required in simple sweeps, but when you start using guide curves, you should also use a pierce.
Tip
If you feel that you need more profile control, but still want to create a sweep-like feature, try a centerline loft. The centerline acts like a sweep path that doesn't touch the profiles, but unlike a sweep, you can use multiple profiles with it.
Figure 7.22 shows a more complicated 3D sweep, where both the path and the guide curve are 3D curves. I cover 3D curves in Chapter 8; you can refer to these sections to understand how this part is made.
On the DVD
The part shown in Figure 7.22 is on the DVD with the filename Chapter 7 3D Sweep.sldprt.
This part is created by making a pair of tapered helices, with the profile sketch plane perpendicular to the end of one of the curves. The taper on the outer helix is greater than on the inner one, which causes the twist to become larger in diameter as it goes up.
FIGURE 7.22
A 3D sweep
To make the circle follow both helices, you must create two pierce relations, one between the center of the circle and a helix, and the other between a sketch point that is placed on the circumference of the circle and the other helix. This means that the difference in taper angles between the two helices is what drives the change in diameter of the sweep.
Using a cut sweep with a solid profile
The Cut Sweep feature has an option to use a solid sweep profile. This kind of functionality has many uses, but is primarily intended for simulating complex cuts made by a mill or lathe.