Modelling a UFO

I am a new user of SketchUp. I need to create a cube inside of a sphere where the points of the cube touch the inner surface of the sphere.

I know how to make both a sphere and a cube, but I cannot make them concentric and the correct dimensions.

Share what you’ve got so we can see. If I were doing this sort of thing I would model everything in place centered on the model origin.

I made this real quick if you want to use it :smiley:

saucer.skp (243.3 KB)

Start with a cube and draw the diagonals:


Construct a circle in the plane of the diagonals:


Use the circle to make a sphere:


The radius of the sphere is the sin(60 [deg]) times the length of a side of the cube. For a 10 cm cube, that would be 8.660254 cm.


As a little challenge I did check that via Geogebra on my phone. In SketchUp it’s much easier to do though.

This is what I did based on @jimhami42’s method.


I would probably do it this way, note that I am keeping the circle aligned to the red axis and ‘guessing’ the touch point. If you snap the circle to the corner of the cube the subsequent sphere created will miss the other corners.
Cube sphere


I don’t understand. Why would it miss the other corners?


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Make it into a sphere and see what happens.

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It touches all of the corners.

The very nature of a circle in sketchup makes it miss.
GIF 29-05-2022 3-39-07 PM

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I get your point now. I was using 96 segments so there wasn’t much of a miss. Here’s another approach:


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I think both methods are equally not quite accurate, but close enough.
Your four pies, mmmm pies, won’t match up properly with a circle, close but no cigar.

My method is only as close as you can guess it.
In the grand scheme of things it’s two imperfectly skinned cats that have no skin.

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scaling is pretty accurate:

Used a line for snapping the cube when scaling about centre.
Afterwards, one can resize the model with the Ø of the sphere or side of the cube

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for comparison in Geogebra working in 3D:

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sphere_cube.skp (281,9 KB)

This is all very interesting in a theoretical sense, but none of it is much use when it comes to 3d printing. More info is needed from the OP to actually give an appropriate solution, there needs to be thickness somewhere for this to work and some concept of what is air and what is material.
I’ll leave you all to the mathematic gymnastics.

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Agreeing with that.

That is what I’ve been avoiding with Geogebra. And with the correct method in SketchUp. And some coffee of course!
(create cube side=10 > midpoint opposite corners > create sphere using midpoint and corner > show radius)

You made a good point with:

@jimhami42’s method with four arcs on the cross section gets four endpoints on the facetted “sphere”.
To also get the other four endpoints on that “sphere” the process of four arcs needs to be repeated for the circular path on the ground, to ensure all eight endpoints on the final spherical surface.

SketchUp doesn’t have an entity for spheres, it’s just faces and edges with no radius or Ø. Thus it will always be an approximate. Drawing the circle with a higher number of segments will increase accuracy, though. Rotating the sphere might work, but then it will become more difficult to work with.
Building an UFO isn’t rocket science, there is to much that is unknown😀