Curve and Surface Analysis

[dgs]

So, as my eyes glaze over with all this talk of beziers and vectors

, I have a basic question.

Pro|E can produce 'curvature continuous' transitions in three areas (that I'm aware of), in sketcher, in a boundary blend edge condition and in ISDX. In understand that in each case, you get G0 just by having things joined. Also, in each case, you get G1 by specifying tangent. What do I get if I specify 'curvature continuous', G2 or G3? I assume it's G2, if so, is it possible to get G3? How?

Oh, and here's another, what's the difference between Cx & Gx?

 

[mgnt8]

Here is how they define it on wikipedia:



The continuity is defined using the terms
<ul>[*]G0

 

[design-engine]

To fit into a definition of a Class A Surface one must be able to:

1. Design the form with a light type of construction. Cant make it more complicated than the form needs to be. ie. don't create more spans (extra knots or edit points) in a spline for example
2. The Geometry meets the level of continuity or tangency required for the form. G2 if it's an auto interior for example.
3. the light reflections meet your form constraints or your Gaussian analysis meets g2 like you want it.


Edited by: design-engine

 

[jsantangelo]

Pro|E can produce 'curvature continuous' transitions in three areas (that I'm aware of), in sketcher, in a boundary blend edge condition and in ISDX.

How do you do CC in sketcher?

 

[dgs]

ISDX explicitly sets "curvature" but, looking at curvature graphs, it is (and Jeff's spline constraining formulas are) capable of this definition and previous posts' definition of G3.

So, ISDX skips G2 and goes straight to G3? Or does it do G2 that can be manipulated to approximate G3? Or what?

In our everyday work, G2/G3 (or most times, G1/G2/G3) doesn't really matter, but I am interested in knowing more in case ...

 

[mgnt8]

I think there may be a difference in nomenclature. From Pro/E help:
<LI ="kadov-p-CTopic-Text-Bulleted">
Position (G0)

 

[mgnt8]

To fit into a definition of a Class A Surface one must be able to:

Does Alias featurize class A surface creation, or is it like Pro/E where you establish connections, arrange CV's, analyze geometry, etcuntil you get it right.

 

[dgs]

How do you do CC in sketcher?

Someone posted that here before, but I can't recall how to do it. Was that you Jeff?

 

[jeff4136]

jsantangelo,
In Sketcher you can define the end point curvature (and therefore make
it continuous) of a spline by
(1) Creating a radius of curvature dimension (Help: Sketcher -> ...
-> Dimensioning a Spline).
(2) Constraining it equal (=) to a line or an arc.
(3) By appropriate placement of Control Vertices (as shown in previously
attached examples, if you're using SE I should be able to dig up some
pics to post).
_ _ _ _


Re Class A, Bart's explanation illustrates a characteristic that's worth
noting: subjectivity that can not be quantified, beyond a few widely
accepted norms, except by the person (customer / boss / ?) defining it on
a per instance basis.
_ _ _ _


Be back later ... ;^)

 

[design-engine]

I would much rather talk about surfacing than RSD... That sh*t is kicking my ass.Why does it have to be so hard?I wonder of an electrical engineer with Mentor Graphics experience would find RSD hard to figure out.

I cant seam to link up a block/symbol that I create and get it into the catalog!
Edited by: design-engine

 

[mgnt8]

You can dimension radius of curvature. Set tangeny at the endpoint where your spline meets the arc, then dimension it by selecting the edpoint once. Change the resulting curvature dimension to equal the radius of the arc. Otherwise constrain the polygon segments like in Jeff's 'nonat_bezier_andbasis_splines' part.


Jeff have you created any "typical" curves in sketcher?

 

[jsantangelo]

jsantangelo,
In Sketcher you can define the end point curvature (and therefore make
it continuous) of a spline by
(1) Creating a radius of curvature dimension (Help: Sketcher -> ...
-> Dimensioning a Spline).
(2) Constraining it equal (=) to a line or an arc.
(3) By appropriate placement of Control Vertices (as shown in previously
attached examples, if you're using SE I should be able to dig up some
pics to post).

Thanks for the clarification. Is it possible to create a CC constraint like the tangent constraint off an existing feature similar to what is possible in SW? How would you create a CC condition in this:

 

[jsantangelo]

You can dimension radius of curvature. Set tangeny at the endpoint where your spline meets the arc, then dimension it by selecting the edpoint once. Change the resulting curvature dimension to equal the radius of the arc. Otherwise constrain the polygon segments like in Jeff's 'nonat_bezier_andbasis_splines' part.


Jeff have you created any "typical" curves in sketcher?

Thanks. You posted as I was posting.

 

[mgnt8]

Thanks. You posted as I was posting.

Hot topic - can't get them in fast enough

 

[jsantangelo]

Thanks. You posted as I was posting.

Hot topic - can't get them in fast enough

Ok here's a quickie. How do you create the spline end point to be tangent with out using a construction line that is tangent and collinear? Currently to make the spline end point tangent to the circle in the above pic I had to create a short construction line that was tangent to both the circle and spline. Is there another way?

 

[design-engine]

Don't try to force continuity in a sketch curve!Do it with curve thru points instead!Or better yet ISDX.I think you double click on the end but its a bad technique.


Edited by: design-engine

 

[mgnt8]

Good question - I've run into this problem also with the conic. The only thing I can suggest is a center line running thru the circle center thru the intersection point, then add a 90 degree dimension. Still utilizes a construction feature, however.

 

[jeff4136]

Re Cn continuity:
Digging thru my notes I believe (will have to try to digest it myself) there
are definitions I can relate to and practically apply ...
[url]http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1990001 2238_1990012238.pdf[/url]
(if the link is no longer any good I can post the document).


Re the rest, it'll take some time to catch up .... ;^)

 

[jeff4136]

> CC constraint like the tangent constraint off an existing feature


That is the equal (=) contraint. It works with (circular) arcs and lines only.
Sketch an arc and disjoint line. Bridge the two with a spline. Apply the (=)
constraints. You'll notice that two constraints (T & C) are actually added.
If the reference curve isn't an arc or line it will be necessary to create a
Datum Evaluate feature and use a radius of curvature dimension (assuming a
parametric relationship is desired).



> created any "typical" curves in sketcher?


I can't say I have.
If the description can't be satisfied by something relatively simple, like(regarding the vectors)
Mv = v + (M2v - v) / 2 and
Mv bisects the included angle between v & M2v
then it's probably over my head.
Edited by: jeff4136

 

[mgnt8]

then it's probably over my head.

Tell me about it. That's why I'mwondering if programs like Alias or ICEM crunch these numbers to make their class A surfaces, otherwise its beyond my inclinations to force it into Pro/E.