Sweep 2 rails bulge

I’ve a two rail sweep that I want to be flat on it’s top but I get a bulging problem. The top of the profile curve is not higher than the top rail ( as best as I can determine) but all the same it’s creating an uneven surface. There’s pic and I’ve attached the file if anyone might want to check.

ArgoFlex.3dm.zip (474.7 KB)

This is a deceivingly difficult form to model. It’s going to take careful planning and multiple curves. As you have it now you are going to get a pinched looking form. That just won’t look right in my opinion. This is how I would do it.

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Attached is what you get if you add the curves I suggest in my first post, maintain the radius tangent to the flat top surface all the way around, and extend a tangent from those radiuses to the second rail curve. The result has some pretty wonky topology (areas with very tight iso lines) but tweaking the section curves will help a lot with that. Comes down to your design intent.

ArgoFlex_Mod.3dm (2.2 MB)

That looks great. So as I understand it you made a total of six profile curves instead of my three. It is a more complex problem than it appears as per Stratosfear. Did you notice if the profile curves I used were tangent or not to the top surface?

In any case thanks to both of you.

For the sake of variety - it looks like your concept is straight lines and tangent arcs - when that is the case, and you want to maintain the rigid geometry. consider using planes and fillets among them. If I use your fillet radii directly the result falls away a but from your inputs but you can find a lower radius that allows you to hit that a little better.
ArgoFlex_PG.3dm (2.3 MB)

Also, the reason for the bubble, your original question, is that Sweep2 has to rotate and scale the profile to accomodate the farther-away lower rail:


I see, the profile is rotated and extended to reach the lower rail. I though the “maintain height” would prevent that.

Maintain Height forces scaling in 1d as opposed to 3 - stretching the profile, but it cannot do anything about the profile needing to tip as it goes around. The ‘height’ here is the height of the surface relative to the two rails, and not in world or CPlane space.



I created a lower curve set, upper curve set and drew a line to act as a guide to draw the intermediary curve set. I then lofted between the bottom and intermediary curve set. I then created some offset construction lines and created the inset curve set at the top of the model. I then used blend to create the surface (in green) and used the add shapes option to align the blend at the corners. Since it is a symmetrical model I only modeled one half and mirrored to create the full model. Here is the result:

That looks great! Would you mind showing me the two ‘sets’ you speak of?

Hello JKayten,

I did a little more tweaking with the curves, nothing major other that the blend looks a little bit cleaner. The curve sets are attached.

“Top” is your reference, Core Line Set is the blocking I created from scratch to mimic your guide curves and Corner Markers were for fleshing out the corner curve areas. Wall Guides were created so I would have the lines required to create my Blend curves, and finally the G2 Corner Curves are the corner curves themselves. To create the solid I Lofted the bottom set of Wall Guides and G2 Corner curves to the middle curve set. I then created a planar surface using the top curve set and then ran the Blend command making sure to Add Shapes at the corner junctions to direct the blend. I then mirrored the one half of the model to make a completed surface and did a little cleanup work making the top and front/back surfaces single surfaces instead of joined.

You could experiment with tweaking the curve proportions and parameters to get things just right, and if you had an engineering drawings you could make things exactly. In this case I didn’t have those so I took a little creative license.

I can make a short video of the process but am unsure if you can link to Vimeo pages on this forum. Hope this helps.

AeroNew01.zip (30.1 KB)

I really appreciate your help! I’ll study the elements and learn much. If you’re so inclined, by all means make a video - the learning that would facilitate would be of service to many more than just myself. Happy new year!

I’ve not yet become adept at creating the proper G2 curves that you’ve made for the corners. That is how do you correctly scale the G2 curves to make a smooth transition

But the technique is quite nice!


Glad it helped a bit, the model is looking good… As far as curve creation goes I find it’s good to create the reference lines / curves to create the corners, in this case the wall curves in green. I’ve kept the blended curves in Blue for reference only:

Then run the BlendCrv command and select the pair of lines or curves you’d like to blend between:

Remember to hold down the Shift key while dragging the control handles to enable symmetry (which in this case I did). If you need to change the shape curve angle you can hold down the Alt key:

In order to get close to the approximate curvature of the middle corner curves I lofted the bottom corner curve to an offset construction curve at the top and trimmed the loft at the middle curve height. It’s not usually a good idea to blend from trimmed surfaces if at all possible, it results in really dense and ugly control point distribution that makes further modification very difficult. I can include that in the video if you like. I don’t think I can get around to it before the end of the year but I’ll work on it then.

This is very helpful. I’m in the stage where I’m developing a work process - (relatively new to Rhino) I was accustomed to being able to select items at snap points like for instance ‘midpoint’ and then being able to drag and snap to another point on a different item. That doesn’t seem to be an option in Rhino. What’s a bit frustrating so far is the constant need to check if lines have actually snapped to the points they need to be at despite the osnaps display. I’ve had to repeatedly project curves to a cplane to make sure they’re going to relate properly. I moved the whole process from where the item will actually live so I could utilize the construction planes more easily. I find it difficult to work with anything ‘up in space’.

Anyway, thanks again and I look forward to anything you have to offer.



Rhino has the ability to snap, you just need to make sure to turn on the varied snapping options. On the PC the check boxes are found at the bottom left of the screen.

In my opinion Rhino is a bit lacking when it comes to filleting, blending and manipulating surfaces. I use the VSR (now Autodesk) Shape Modeling plugin for Rhino and it really makes these sort of modeling tasks much easier but I don’t think it’s an option on the Mac and the plugin is north of $1300.

I also regularly use MoI (whose CEO, Michael Gibson, created Rhino) for the vast majority of my line work since I can move and align more quickly in MoI and then I am able to directly copy and paste back and forth between MoI and Rhino to take advantage of the strengths of each application. I should have a video on the steps I used for this model up in January, just search for OSTexo on the Vimeo site.

When I found out that Rhino was to be ported over to the Mac I was one of the most excited men in the states. I’d spent years using Sketchup thinking it would eventually become a professional-level program. It hasn’t. There was an auto cad program for Mac, but I was put off by the learning curve.

Rhino was intimidating at first, but one can get some confidence fairly soon and that makes the learning easy to pursue.

There are some features of SU that I miss in Rhino however; For instance, in SU I can hover over a line or edge of an object and the smart snap function will show logical points like mid point or center. When one clicks on the point the item is ‘grabbed’ and can be moved around in space to where ever it needs to be placed. The object stays highlighted and manipulated by the chosen point until its clicked again. This makes for easy placement to other logical points on other line or objects which of course also show up when hovered near or over. Maybe this sort of interface function is possible in Rhino and I just haven’t realized it yet.

I did a complete new build using your techniques as best as I can surmise and the object is very much improved, but still there’s a smoothness issue as per below.

@JKayten That functionality exists in Rhino, though perhaps not as an exact clone of the how SketchUp works. In Rhino it’s called OSnaps and the user can select which, if any, snaps they want to use. I use Rhino for Windows rather than Rhino for Mac but I would be extremely surprised if OSnaps are not available in Rhino for Mac since they are such an important feature of the interface. Try this simple exercise (based on Windows interface but the same should be possible with the Mac interface).

Turn OSnaps on.

Activate all the available OSnaps.

Create a circle.

Create a rectangle.

Start the move command and select the circle as the object to be moved, then Enter.

Request for Point to move from: should appear.

Move the cursor around over the circle. The cursor will jump to various location on the circle or the center of the circle. These are the OSnaps. (In the Windows version the name of the selected OSnap will appear next to the cursor if the appropriate option has been set in properties.) Select one of the OSnaps as the point to move from.

Move the cursor around over the rectangle. Various OSnaps will appear. Select one and the circle will move.

Repeat using the Copy command.

Experiment with other types of Objects.

Read the help section about OSnaps, and then try “one-shop” OSnaps.

Thanks much for your help. I’ve been staying in the gumboil mode mostly and haven’t bothered with the move tool. I’ll have to change some habit here and make a quick key to bring up the proper move tool. Yes, it’s different in SU as any object automatically displays its snap points whether or not the move tool is selected. It’s been tedious unlearning habits and perceptions developed over the last 4+ years of SU. This forum is absolutely brilliant.