Free-form best practice - Eges or curves?

[SiW979]

When you are useing the free form tools, it is obviously best practice to be consitent when picking geometry. However whayt I wold like to ask, and I guess it's a fairly elementry question, is if you have some good quality splines etc, then you obviously would start the sheet creation by using these splines, however what about when you create a sheet adjeacent to aother, is it best to pick the sheet edge as the primary/cross string, or would you pick the original spline?

Cheers

Si


Best regards

Simon NX 7.5.4.4 MP8 and NX 8.5 (native) - TC 8

http://www.jcb.com

 

[Xwheelguy]

My preference has been to have the sheets overlap and then Trim them to each other to have a common edge. I realize in some instances, that adds a step to creating the sheets, but I've ran into issues trying to make sheet edges butt up against each other - you can run into situations where you have to increase your Sew tolerance and I prefer to keep my tolerances the same from beginning to end of model.

Tim Flater
NX Designer
NX 7.5.4.4 MP8
WinXP Pro x64 SP2
Intel Xeon 2.53 GHz 6GB RAM
NVIDIA Quadro 4000 2GB

 

[JohnRBaker]

I've always try to select an EDGE even if there might have been an identical curve coincident with that edge. I think it just makes it 'cleaner' and if my intention was that the second surface was SUPPOSED to match-up with the edge of the first surface then by selecting the surface's edge I've just provided that much more 'design intent'. Otherwise it might look like it was just of coincidence that the CURVE I selected matched the EDGE. If I had picked the EDGE then there would be no doubt what my intentions were.

John R. Baker, P.E.
Product 'Evangelist'
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Siemens PLM Software Inc.
Industry Sector
Cypress, CA
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To an Engineer, the glass is twice as big as it needs to be.

 

[Toost]

This question is a bit deep,
As soon as there is approximation involved there will be some form of deviation,
Lets assume that i have a clean spline, when i create a sheet using the spline the surface needs to approximate the spline, the resulting edge is within the NX modeling distance tolerance. ( assume that the spline is a degree 5 single segment and that the sheet is degree 3, since degree 3 cannot create the same shape in a single patch NX needs to increase the number of patches , keeping the sheet within the modeling distance tolerance to the spline.)
Also since / due to the approximation a completely different type of edge might appear. ( "Intersection curve" and /or "Tolerant curve") Quite soon the resulting edges starts to be "un clean"
Now for the next step, sheet 2, the previous edge might both be poor input and also a bit "off" in location.
If we would build repeatedly on the previous resulting edge we would sooner or later start seeing the edge being quite different to the spline.
Per the above, i would say "original spline 100%".

Now the deep part,...
As John noted, If i want to match a second surface to the first, using the first's edge might simplify the matching since the edge of the second might be closer...

Regards,
Tomas

 

[Xwheelguy]

Tomas' explanation concerning approximation is why I've avoided using Edges and prefer to overlap and Trim back to faces or sheet bodies. There isn't anything "wrong" with taking the route you wish, but the approximations that Tomas mentions can lead to issues down the road. If you're not working with splines or tolerant edges, then you may not have a big issue with the approximations.

Tim Flater
NX Designer
NX 7.5.4.4 MP8
WinXP Pro x64 SP2
Intel Xeon 2.53 GHz 6GB RAM
NVIDIA Quadro 4000 2GB

 

[kapmnit123]

Hi Simon,
Sorry i may be going a bit (or may be more than that ) astray with the discussion but could you please share the test case you are inspecting.Just out of curiosity due to my past (and short stint) Class-A surfacing background...
I remember when we did lot of iterations with the primary patches (first creating one section and one guide curve ...creating a patch surface ... checking it's reflection pattern and then if there is any discrepancy...we extracted the extremities curves of this patch and delete the patch...then again made the patch using the extracted curves (so this time you have 2 section and 2 guide curves to manipulate) (although we ensure the quality of the curves is good and as per design intent for ex.. Constantly accelerating (in most of the cases to facilitate good transition down the line) or with a hump as required) we create the patch once again...this methodology was followed to keep a tight control on the patch quality as the extremity curves defines it completely (rather than the surface based approach where user use x-form or any other surface manipulating method directly).We avoided considering edges directly in those cases (trimmed edges as the section/guide curves used to be a strict no for us.).This at least ensured that the feature lines we get in terms of two primary patches intersections are closest to the design intent(as per required comb structure and smoothness).
Best Regards
Kapil Sharma

 

[SiW979]

Hi Kapmnit

I dont' have a specific example, we don't really do 'A' class surfacing here either, it's 99% mechanical free-form for castings and plastic moulding for inside a vehicle cab that have textured surfaces, so we don't need to have full blown 'A' class surfaces thank goodness. I was just looking for a rule of thumb really I have attached a part I use when training people and I if you have a look at thru curves mesh 13, you will see it has been given G1 continuity to the two extruded surfaces (11 and 12)these extruded control surfaces are based on two bridge curves so when I create the thru curves mesh, I'm basically asking is it better to select the bridge curves as my cross strings or the edges of the sheet bodies which I am using for the control surfaces?

Thanks for the input guys.

Si

Best regards

Simon NX 7.5.4.4 MP8 and NX 8.5 (native) - TC 8 www.jcb.com