Primary surfacing (NURBS) questions (archtop guitar top)

Kyle - Thank you! I will check it out.

I’m old school (trained when the Dead Sea was sick) and have been trying to approach it as follows:

Just finding the commands to use is challenging me right now. Just discovered arc-start, end,point on arc - a lifesaver. I’m approaching this the way that I build them - lay out arches and create surfaces from them - still get baffled by the number of control points some of the commands generate, and just learning about locking outer control points so my edges don’t curl up like potato chips when trying to change the topology. I thought I would project the “J” to the guitar surface once the tail area is done, so the bottom of the “J” will allow me to have 4 sides in the lower quarter to use EdgeSrf .
Then I want the neck end to have a flat or very shallow arch. It facilitates the filler piece under the fretboard, if needed.
To me the killer area is where the 2 cutaways are, and I keep scratching my head on how to break it up. I like Pascal’s sketch. Hi Pascal! As a builder I think of cutting the plate close to finished size, then shaping all the arching, rather than surfacing a rectangle and then profiling, Yeah? I wear 3 hats - designer, maker, and dog walker (red Carhartt!) :rofl:
Once I get one side done, I’ll mirror it, and keep my fingers crossed for tangency and curvature.

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the corner is creased, you can simply remove that crease and the curvature returns, or leave it, convert to nurbs and fillet…

Would I be able to accomplish my model with SubD, or stick with NURBS?

I was not talking about the presence of a crease. I was talking about the curvature adjacent to a crease, Two very different concepts.

(Non-SubD friendly) NURBS allows Curvature to be non-zero adjacent to a crease.

SubD always results in zero curvature adjacent to a crease. This is inherent in Rhino SubD.

To retain the crease but change the curvature adjacent to the crease requires conversion to NURBS surface followed by modification of the NURBS surface.

Example of the difference between zero curvature and non-zero curvature adjacent to a crease, using the edge of Kyle’s guitar.

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Something hand finished, and hand assembled, will this make a difference? You are correct with what you showed above, but my point is, for this project, who cares?

does it impact the final part? Does it save time, does it make your life easier? these are questions that will determine if Nurbs or SubD (or some combo of the two) is the way to go.

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Hi Carl, can you get a 3d scan of your guitar? Some universities have 3D printers or maybe a scanner. There also might be someone in a business that might scan it for you.
Looks like nice work.—-Mark

Highly polished lacquer finishes are a tough test for surface quality :slight_smile: There is always some bench time to make the surface ready for finish. I would want to build a Florentine cutaway without a flat in the curve. In David’s example. NURBS gets the vote, and I would rather spend more time getting the model right. The guitars I build have no fillets along the top and back - there are 2 ledges cut to take the purfling and binding, which are rectangular in cross section


When people are paying thousands for a guitar, they do care, I hope…

At this point I feel comfortable with NURBS, have watched various videos on SubD (one was a nice rubber duck), and do not know enough about SubD

Thanks!

Scanning is a great idea, but does that give a huge mass of point clouds? I think I’d rather start with a few vectors. Please correct me if I’m wrong.

I’ve also thought of digitizing points along the surface, using my CNC router Y gantry and a drop indicator, but I already have a drawer full of arching templates I can scan and trace in Rhino, as I have in the Rhino screenshot.

Carl, I think a scanner will give you a mesh. I guess that is a point cloud with polygons connected to the points. It could help in showing how to build surfaces though. Probably with some trial and error. Even better would be to use a plug-in like 2Surface.
Over the years this community has seen some reverse engineering tools in plugins like TSplines or Shape Modeling. Don’t need to go there though.
Your idea of using your patterns would sound like the best way to go. Simple in house solution.—-Mark

scan>mesh>quadremesh>tosubd>tonurbs

will get you from scan to polysurface.

“Highly polished lacquer finishes are a tough test for surface quality”
Yes they certainly are. I’ve spent countless hours refining arched top and back plates. One Rhino tool I find helpful is running “contour” through the 3d surfaces in both x and y axis to identify thickness anomalies. This also gives me a network of curves to analyze and pinpoint trouble areas before carving. When I carve with CNC I cut the outside first and run a uniform thickness extra border - about 6 mm thick and 8 mm wide- around the perimeter so I can vacuum seal tightly into a nest for the inside cut. I also know some builders who cut the inside first to avoid needing a vacuum seal but I get too much flutter on the outside cut this way. Modeling is the first step, then the cutting, then your hands take over :slight_smile:

Thanks!

Looking it up , it looks like a good command to know about. My first 2 guitars were done that way - phew!

Yes I shape the outside first, then do the inside, clean the inside, brace it, then do the recurve once the top and back are glued to the rim. The sketch I posted is for a ES335 style guitar, and the inside of the plate will probably not be as meticulous as a full hollow body with traditional bracing and will be more of a chambered effect.

I will try out contour.