Fitting Blends to Polygonal Mesh-Data. Mesh-Nurbs. Reverse Engineering. Alias.

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Here’s an interesting question sent in by a Brazilian student ( Gustavo):

Model by Mathew Tan (Malaysia)

Gustavo: Q)  Generally speaking, when I build transition surfaces (like blends, cant rail and pillars for example) I build them already putting the desired continuity with the master surface (g1,g2,etc..) and then balance the parameters to get the surf close  to the mesh, or should I first snap the surf to the mesh and then check continuity (provided the clay model data already solved the desired transitions between the master surfaces).

graham : (NB: for  master surface   please use Parent Surface)

General ans:
Use the FFB tool to achieve the desired level of crvtr cont.
An in depth explanation of how a model is built in an industrial environment:
Ans) Remember! that you are not always going to be copying the mesh data. You copy it where it flows smoothly and has good HL’s, but where the mesh can be improved upon, then you call the designer and advise him/her that, to improve the surfaces you need him to agree to you deviating from the 1.0mm mesh tol. So to ans. your Q, I can only tell you how I work: Cant-Rail, use FFB (Free-Form Blend) with crvtr. restraints set to suit the mesh condition eg. The cant-rail to theoretical side-glass  may require G2, BUT often the boundary with the *roof will be a panel joint with a 10mm wide plastic trim inset (even the Audi A5 is constructed thus), so here you could use C0 or G1, the choice is design dependent. Also just what level of srfc cvtr cont is demanded will vary as the project progresses from the first few months and starts to mature, so *C0 could be acceptable early on as long as the designer is happy with the look of the cant rail and then further down the line engineering constraints may be imposed forcing the modeler to achieve say G1.
So not a straight forward ans, but we have to grasp that the exterior starts life driven by Styling and then later on it has to conform more and more to Engineering specs.
Finally we must remember that not all boundaries have to be G2 eg: where there is a break like a panel joint or parting line G2 isn’t really relevant. A lot depends also on what your Modeling Manager demands, as he is the guy who has to sign-off your work. I can tell you at Audi during the first month of a project you have to work very quickly to develop the model and there isn’t always time to achieve pretty srfc boundary locators “Green Flags”. The designer will accept this to get te model into Milling, just so long as the surface flow (Highlights) look really good.
If I have time to model both edges of the Cant-Rail to G2, then I will, but often when there is a really tight deadline to transfer my data to Milling
Then I will model to C0 at the roof boundary and G1 at the theoretical Side Glass bound.

TIP: Often to get the cant rail to fall within mesh-tols, I will slide the srfc boundary of the roof using Extend with direct hist. on the blend.

As you can see srfc modeling is an ART and you are working in a close relationship with the designer.

kind regards

graham

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