How do I model Curved Surfaces / Objects?

I am using SU15 make. I have a number of plugins relating to curved surfaces like curviloft, vertex tools, & engineering toolbox which I think will help me do what I need to do but I have no idea where to start or how to use them.
My problem is transitioning one shape into another. I can create a round tube and a rectangular tube with 2 round corners, but transitioning from one to the other @ 90° other than butting them up to the other is a total mystery. These parts will have a fan blowing into them so it would be better not to do a simple butt up.
Then I must do something similar at the bottom.
Is there some tutorial that gives me more than installing and starting? r there other tools for this task (free of course-its a hobby).

the basic tools in SU are really powerful on their own…
modelling plugins aid productivity, but cannot do anything SU can not do on it’s own…
if you first concentrate on how to use the native tools and follow their tutorials, you will have a better understanding of a plugins potential…
many wasted hours struggling with plugins taught me how to use native tools, now I use plugins only when appropriate…
start simple, intersect a cylinder and a cube, then progress…


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Plugins are of no use if you lack basic modeling skills.

Learning SketchUp — SketchUp Knowledge Center

SketchUp Video Tutorials — YouTube

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By this do you mean that you are trying to go around a corner at the same time as transitioning from the rectangular to the round cross-section? If so, that is a very complex shape that is going to be hard to create regardless of what tool you use. Also, it is nothing you are likely to be able to procure at your friendly HVAC supplier!

If you mean a straight transition, here’s doing it with curviloft. If you look at the edges before I click to accept the geometry, you can see what curviloft did, and you could manually draw the same edges using built-in tools.

If you’re trying to turn a corner and transition smoothly from a circle to a square (for example) the morphing option in the Spirix plugin will do this for you. As an example, start with a template of the circle and square. In this case, construct the circle using 24 segments and group the result. Create the square using four lines and group it as well. Position the groups in the X/Z plane along the x-axis at the radius of curvature you want (18" in this case):

In order to use the plugin to morph one shape into another, the profiles must be created in sequence and made into groups using the Create Group option in the Spirix plugin. To create the circular profile, create an arc using 12 segments from C1 to C3 with the height at C2. Then create a second arc from C3 to C1 using C4 as the height. Use the Spirix Create Group and name this “circle”. Now create a line from S1 to S2 and then use the right-click context to divide it into 6 segments. Create a second line from S2 to S3 and divide it as well. Continue for two more lines ending with S4 to S1 divided. Use the Create Group function and name this one “square”. Now run the Create Surface option with the following:

The Spirix plugin can be found here: spirixcode

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True … Transition Ell.skp (234.0 KB) via native tools