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combining nurbs surfaces to form solid


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I'm trying to model a solid comprised of 4 nurbs surfaces which I want to subtract from another solid. It is essentially a cylinder which fillets at the bottom onto a curved base.

Any guidance as to the error of my ways and how I could successfully model this shape would be greatly appreciated.


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Didn't make the radius of the can & rim same as your original, but this produces an OK model. The fillet is a bit uneven where it meets the can. Fiddle with the radius and options in the Create Fillet Surface to see what works best for you.

Not sure what your version, but VWX file is 2016 ported back to 2011.



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  • 2 weeks later...

Hi Benson,

Having a few problems with your directions.

As a bit of background, I'm building a keyless soprano saxophone (see screenshots). The toneholes are not cylindrical but are undercut (they flare out where the hole meets the instrument's bore). Hence the solid I'm trying to model to subtract.

I'm using VW2014.

Working through your directions:

(a) The project tool doesn't seem to project a circular surface (red).

(b) The bore is not cylindrical so I've made a tapered arch. Now the project tool doesn't work at all.

© I managed to create a lower surface (red) from tapered arch. Surfaces refuse to fillet.

(d), (e) and (f) Extracting lower edge is again not circular in plan view resulting in a misaligned solid.

Really appreciate your time and understanding.


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That tapered arch is not same as the shape of the sections of the saxophone. Don't try to subtract the fillet. Just cut the hole and fillet the edge.

Make your sections of the sax by any means - Sweep, NURBS & Shell, etc.

This assumes base shape of hole is a cylinder, meaning it would be drilled w/ a standard drill prior to any fillet operation. If the hole is conical or other shape, subtract a that other shape instead of the straight cylinder, then fillet.

1. Locate the hole:

Rotate the section so that axis of hole is vertical in front view.

Place a 3d vertex at entry point of the hole.

Switch to Top view and draw a 2d circle, center at 3d locus, diameter as desired.

2. Cut the hole:

Extrude the circle.

Select the extrude and sax.

Model>Subtract Solids

3. Fillet the hole:

Engage the Fillet Edge tool.

Open the prefs dialog. Deselect the Faces option.

Click the edge(s) of the hole where fillet is desired.

Click the green check mark.

OIP can adjust fillet radius as needed.

OK post back if more help needed.




Edited by Benson Shaw
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Great. That seems to work.

One more thing - I assume the facetted appearance of the left section, created by subtracting multiple extrudes, is only a rendering issue and that the printed result will be a smooth curve like the sweep (centre and right).

I've had a problem subtracting from a sweep - I'm either told that the geometry is wrong (centre) or my solid is just two circular walls (right).



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Hi, John. Looks like you are getting there.

Couple things in your file that should help with the faceting:

1. Set your OpenGL options upward. For this file I think Very High for Display and enable Anti Aliasing. Show Edges and Shadows at your option. These changes only help views in the OGL render mode.

2. I opened your file in v2016, so there might be a conversion problem, but . . . The profile/source objects for your sweeps are unconnected line segments. Edit each sweep and redraw the source object as a closed polygon or polyline - trace right over the original, then delete the original. Adjust the attributes to suit, then exit the sweep.

A. The taller figure on the right has a profile of 12 line segments. Don't bother to compose them. Just trace a new polygon or polyline with 4 points, then delete the original segments

B. The shorter sweep in the middle has a profile formed by 4 unconnected lines. Either compose them, or trace a new, closed polygon or polyline (then delete the originals). The solid subtraction created from the extruded circle and this altered sweep is not faceted. The fillet tool identifies a single edge at the "bottom" of the hole, rather than several segments. The resulting fillet object is not faceted.

3. Note about Sweeps - Segment angle is adjustable in the OIP or creation dialog. Smaller segment can make a smoother appearance.

Your Sweeps have 5.62° segment angle. This seems OK in this file with the above changes. So probably no need to use smaller segment angle.

Generally, smaller segment angle (1° or even less) smooths a sweep, but increases complexity. This can be cause of laggy navigation, bigger file size and longer renders.


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  • 3 weeks later...

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