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option for either a straight transition or a smooth transition. This works like the two-profile lofts that were mentioned earlier in this chapter. The names may be somewhat misleading because both transitions are smooth. The straight transition goes in a straight line, from one size to the next, and the smooth transition takes a swooping S-shaped path between the sizes. The difference between these two transitions is demonstrated in Figure 7.37.

FIGURE 7.37

Straight versus smooth transitions of a variable radius fillet

Recognizing other uses for the variable radius fillet

Variable radius fillets use a different method to create the fillet geometry than the default constant radius fillet. Sometimes using a variable radius fillet can make a difference where a constant radius fillet does not work. This is sometimes true even when the variable radius fillet uses constant radius values. It is just another tool in the toolbox.

Using face fillets

Face fillets may be the most flexible type of fillet because of the range of what they can do. Face fillets start as simply an alternate selection technique for a constant radius fillet and extend to the extremely flexible double hold line face fillet, which is more of a blend than a fillet.

Under normal circumstances, the default fillet type uses the selection of an edge to create the fillet. An edge is used because it represents the intersection between two faces. However, there can sometimes be a problem with the edge not being clean or being broken up into smaller pieces, or any number of other reasons causing a constant radius fillet using an edge selection to fail. In cases like this, SolidWorks displays the error message, “Failed to create fillet. Please check the input geometry and radius values or try using the ‘Face Fillet' option.”

Users almost universally ignore these messages. In the situation shown in Figure 7.38, the Face Fillet option suggested in the error message is exactly the one that you should use. Here the face fillet covers over all the junk on the edge that prevents the fillet from executing.

FIGURE 7.38

A face fillet covering a bad corner

Face fillets are sometimes amazing at covering over a mess of geometry that you might think you could never fillet. The main limitation on fillets of this type is that the fillet must be big enough to bridge the gap. That's right, I said big enough. Face fillets can fail if they are either too small or too large. Figure 7.39 shows a complex fillet situation that is completely covered by a face fillet.

FIGURE 7.39

A face fillet covering complex geometry

On the DVD

The model used for this image can be found on the DVD, with the filename Chapter 7 Plastic Cover Fillets.sldprt.

Using continuous curvature face fillets

Curvature continuity refers to the quality of a transition between two curves or faces, where the curvature is the same or continuous at and around the transition. The best way to convey this concept is with simple 2D sketch elements. When a line transitions to an arc, you have non-continuous curvature. The line has no curvature, and there is an abrupt change because the arc has a specific radius.

Note

Radius is the inverse of curvature, and so r = 1/c. For a straight line, r = ∞, in which case c = 0.

To make the transition from r = ∞ to r = 2 smoothly, you would need to use a variable radius arc if such a thing existed. There are several types of sketch geometry that have variable curvature, such as ellipses, parabolas, and splines. Ellipses and parabolas follow specific mathematical formulas to create the shape, but the spline is a general curve that can take on any shape that you want, and you can control its curvature to change smoothly or continuously. Splines, by their very definition, have continuous curvature within the spline, although you cannot control the specific curvature or radius values directly.

All this means that continuous curvature Face fillets use a spline-based variable-radius section for the fillet, rather than an arc-based constant radius. Figure 7.40 illustrates