Single span projections & surface quality

Dear all,

After being ‘illuminated’ by @sgreenawalt on being mindful about surface quality through his YouTube videos, I decided to give it a shot and change up my workflow.

I mostly use rhino to model eyewear frames for injection molding. Even though the products themselves are quite small, we can still notice some tangency/curvature imperfections on high gloss finishes after molding. These may require a lot of time in postprocessing to correct and achieve a high quality product.
Secondly, simpler models would make it much easier to make minor adjustments to our models. Right now, we generally re-model the whole frame if we need to modify its design.

The biggest obstacle I have found in adopting the ‘sculpt and match’ workflow is that we heavily rely on the project command to define our geometry. A frame is built around the lens, which come in various specific spherical diameters. Therefore, we currently project our frame silhouette on surfaces equal in curvature to the lens.

Ideally, we would like to keep the frames’ front surface as a whole. This is easily achieved by projecting and trimming. While I have tried several patch layouts, I cannot seem to find a way to avoid 5-sided surfaces.

Any help/suggestions?

Single span projection.3dm (552.2 KB)

Hello - sorry, I do not understand the question yet…



I realize that the problem is probably clear only in my thoughts.

To cut it short, I would like to know how I should approach modelling this sketch with single span surfaces while keeping the front curvature (in both U and V) accurate to the industry standard measurements.

I would normally trim out the surfaces with the curves, but then the edges would not be single span anymore. Therefore extruding those edges would yield overly complex geometries which are hard to tweak and match.

What I would do is extrude the loose projected curves, not the trimmed edges. These will not be 100% on the surfaces but very close - then, presumably there will be a small transition surface along these edges anyway, and these will ‘bridge’ any over-tolerance gaps from the loose projected curve/extruded edge to the lens.
Does that make sense with what you are asking?


Interesting approach. That transition could be used in place of the usual fillet.
I tried this out using surface blend, and, as expected, it doesn’t yield a single span surface. Complexity isn’t too bad though. Do you suggest there is a better way to make this transition surface?

Hello - I would not be too obsessed with single spans - obsess with getting good clean geometry. Since these transitions are small and, presumably, added late in the modeling process, and probably not point -edited once in place for the most part, it seems like you could get away with a somewhat more complex surfaces here. You could always hand-build transitions of course, but there is no guarantee that the edges along the lens will be within tolerance without more points than the extruded surface - most likely it will take multiple spans either as more single span surfaces or a multi span. Either way what hat you’ve done is kick the complexity problem down the road to the smaller. less ‘important’ transition surfaces, which, I say, is a win.

@nicolocostan99 - see the attached - the red is a FilletSrf, the blue shares the points structure of the simple extruded surface - it cannot quite get to the lens surface.

Glasses-2 transitions.3dm (91.3 KB)

making that blue surface degree 5 in U will get things much closer.


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I agree this is a much better solution to what I was used to. You could always delete those transition surfaces, adjust the rest of the geometry and then rebuild them. Definitely a win.

I ask about single spans because those, of course, would be the ultimate solution.

As @pascal said: for small blends/radii single span is totally over the top, as there would be no visible difference in the final product (depending on real world dimensions and production methods).
So in these cases single span surfaces are only the “ultimate” solution for the anally fixated :laughing:

No idea what you mean by ‘anally fixated’ but don’t we all love some surface porn?

You need to read your Freud!
The specificic term would be “anally retentive personality” I think it is mostly a made-up theory not backed by empirical data, but it remains a shorthand for being compulsively orderly and tidy.

Dear old Sigmund really should have found a better name for it then…

It is quite descriptive of what he thought was at the bottom of this behaviour.
I think it tells us moreabout Herr Freud than human personalities in greneral.