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If you want tighter control in "the middle" of the surface, use boundary surface. If you don't need tighter control, use fill.
If you are creating a 4-sided patch, use boundary. If you are creating a 3, or 5+ sided patch, consider using fill.
Fill can do non-4 sided patches in addition to the 4 sided patches. even with 3 and 2 sided patches. Edge conditions on a Fill are more like suggestions - they aren't necessarily followed very closely, and can be violated by the software without causing an error. Fill never makes a degenerate surface (all UV lines come down to a point). Fill can use a single point or a sketch that doesn't touch edges on either side as a constraint sketch, boundary can only use a single point as an end profile like a loft, and interior profiles must touch the outer edges.
Boundary gives better control with interior curves, and is supposedly more accurate in the c2 condition at the edges. Boundary can also do a closed loop, while Fill cannot. Boundary allows you to trim with direction 1 or 2, and also to manually pull back the ends of the profiles.
There are lots of differences, really. Mark B was always surprised when I asked the same question, saying that you would never use fill and boundary for the same surface, but I found that there was a fair amount of overlap in the application of the two tools.
I have your SolidWorks 2007 bible and am thinking of purchasing your SW surfacing book. Just read some review and it was said that your book is more for advance users than the beginners
Whats your thought about the review?
I'm not sure which review you're talking about. Amazon has several reviews. Some people get it and some obviously don't. I hesitate to say what kind of user the surfacing book is for because regardless of what anybody says about it, someone else has another very strongly held opinion.
The book is not written in the style of any of the typical CAD how-to tutorial books written by academics. Like the 2007 bible, it's a desk reference, not a tutorial.
The book has some introductory chapters that are meant to bring casual users up to speed on the concepts and terminology. SW tries to insulate users from both of those things, but you really have to understand the technology to be able to troubleshoot the tools. Make no mistake, most surfacing work is some combination of applying the underlying concepts and troubleshooting the tools.
After the introductory chapters, the book goes through several examples. A lot of the surfacing book is for what a typical SW user might consider "advanced". A Rhino user might find what I wrote to be for the casual user. Some people have remarked with disdain that the book is very heavy in theory. Well, yes, this is true. I think its a strength of the book. You cannot go very far in surfacing work without understanding the theory. The theoretical stuff is definitely applied, but not in the form of a series of you-cant-screw-this-up tutorials. The middle section is more of a look-over-my-shoulder-while-I-do-this set of examples. There are some step-by-step tutorials, but they are generally just there to demonstrate the workflow, because I don't think step-by-step tutorials teach anything beyond the relatively superficial.
You can't go far unless you know why certain things don't work, and you have to understand the theory to get that far. Much of the book is dedicated to workarounds. In SW surfacing, you have to know 4 ways to do everything because the first 3 may not work.
Once you start looking at SW as a surfacing program, your whole outlook on the software is different. Solid modeling is easy, automatic. Surface modeling is not.
I strongly recommend Matt's Surfacing book. Not only did I find it very helpful, I actually enjoyed reading it!
I have asked the same question, and I have begun to use a boundary surface whenever possible.
Sometimes I've used multiple boundary surfaces rather than a filled surface when the area I'm filling is complex and a filled surface doesn't give me results I like. Sometimes you can fill part of a complex area using a 4 sided boundary surface and finish it off with a smaller (and neater) filled surface.