VSS - justification

I'm starting to get my course notes updated for next semester and am again
at the Variable Section Sweep.

Every year I get to this section and come upstuck in finding good real life
example of its application - those endless bottles and abstract edge ribs
really aren't very convincing for why my students should learn this
function.

If anyone has a some good real life application which really show the power
of this feature they would be gratefully recieved.

As soon as I show any examples* the question is why don't you use a boundary
blend? - you have control over both end sections and you can 'fill' between
existing sections. As I understand it you will generally get a smoother
surface with a VSS - is the reason for this easy to explain to undergrads?

I tend to sell it very much as a base feature or [as alot of the tutorial
examples] a sweep along existing edges.

Thoughts on this in terms of convincing students its worth understanding
what is one of the more complex functionalities in the toolbox would be
gratefully recieved.

BTW - remember how rudimentary their knowledge is - I sometimes struggle
getting them to grasp tangency and normalcy!

Cheers, sean

--

*One of my ongoing frustrations with students and published tutorials is
innapropriate use of a feature creation method - in the current version of
the CadTrain Coach tutorials they have a revolved vase done as a VSS.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Sean Kerslake
Dept. Design & Technology
Loughborough University
Loughborough
LE11 3TU

01509 228317

 

: I'm starting to get my course notes updated for next semester and am again
: at the Variable Section Sweep.
:
: Every year I get to this section and come upstuck in finding good real life
: example of its application - those endless bottles and abstract edge ribs
: really aren't very convincing for why my students should learn this
: function.

One of the most exciting things about learning Pro/e was the discovery that there
were always manu different ways to model the same part. Even the starting points
could be quite different: start with a little piece of the part (one feature) and
add features? or start with a big block and starting cutting the features out of
it. Part discovering the rich variety of feature creation methods and styles was
realizing that even a simple 30mm x 50mm x 10mm block (sketch and extrude a
rectangle) can be made three different ways: sketch a 30x50 rectangle on the Top
plane and extrude 10mm; sketch a 10x50 rectangle on the Right plane and extrude
30mm, etc.
:
: If anyone has a some good real life application which really show the power
: of this feature they would be gratefully recieved.
:
: As soon as I show any examples* the question is why don't you use a boundary
: blend? - you have control over both end sections and you can 'fill' between
: existing sections. As I understand it you will generally get a smoother
: surface with a VSS - is the reason for this easy to explain to undergrads?
:
In addition, as I gained some sophistication in using the software and realized
that there were usually several ways to create a feature, I started experimenting.
If a feature could be made as a blend or a swept blend or an extrusion with a
bunch of cuts, I tried them all. I evaluated them by the results achieved, by how
easy they were to modify, by limitations in control of shape or attachment or
surface continuity, and by their complexity and the time it took to create the
feature. Along the way, I gained mastery of Pro/e's surface and curve analysis
tools. When you're trying to evaluate the results of different methods, you can't
rely on the unaided eye; you need 'porcupine' or 'highlight' analysis to see
discontinuity. And, by these means, you'll also see the influence of the curve
foundation on surface quality.

Because of the curves that boundary blend surfaces are built on, they are usually
the most troublesome. Part of it is the fact that boundary conditions and how high
a degree of continuity you can impose depends on the curves and their end point
tangency. So, building the curve framework that can support the highest degree of
surface continuity is an art in itself. It is compounded by two things: by the
number and size of the patches and by whether you *begin* with boundary curves.
You have probably noticed that when you have a series of patches to construct, the
first patch has no other surface to which to set 'surface curvature' boundary
condition. That was the reason for introducing, in the last few revs, the Datum
Ribbon. Before that, the best you could do was to set the surface at a boundary
normal to a plane. One or two could be helped this way, the rest were left free to
go where and how they would. Sometimes the problems didn't show up until the
adjacent surfaces were set 'surface curvature' to this first patch. Then you'd get
wrinkling or twisting. This type of problem was the basis for introducing another
aid to good, curvature continuous surfaces, namely ISDX, which carries boundary
blend surfaces to the next level. In any case, there are enough problems using
boundary blend surfaces that I pretty consistently use them only for patching or
blending between features.

The VSS has none of these difficulties or defects. With smoothly curving
trajectories and cross section curves, you produce the smoothest surface possible.
And with less underlying curve geometry and with just as much control of tangency
at the start and end of the swept geometry as with boundary blends and swept
blends.

: I tend to sell it very much as a base feature or [as alot of the tutorial
: examples] a sweep along existing edges.
:
: Thoughts on this in terms of convincing students its worth understanding
: what is one of the more complex functionalities in the toolbox would be
: gratefully recieved.

Again, the use of a variety of techniques, experimenting with different feature
creation methods will do it's own proving, its own arguing. The only general
argument that can be made for learning more technique(s) is that it makes choice
possible. Ignorance will make the choice for them and their "reasons" for not
using VSS or any advanced techniques, including boundary blends, will sound like
sour grapes. Choice means being able to weigh and compare based on experience and
they won't have any experience by avoiding learning VSSes. And the best you can do
is practice and teach the first rule in life and learning ~ don't panic, it's
probably not as bad or as hard as you think.

David Janes