# How to create a small spherical surface on top of a large spherical surface

Hello there,

I am struggling with creating a proper spherical cap on top of a large spherical surface, as seen in the screenshot,
The little spherical has been created with a rail revolve, using a projected circle curve and an arc curve that should define the shape. It looks good in the front view, but not in the right view.
I’d appreciate any advise for this.

following a similar logic/method as how you’re attempting:

• use Line-> Normal to draw a centerline which is perpendicular to the larger sphere.
• Circle-> Around Curve to draw a circle perpendicular to that line
• Pull to pull the circle to the sphere (or maybe ExtrudeCrv then Intersect)
• Arc-> start,end,point on arc- using the quadrant snaps on the circle then a point on the perpendicular line to get the profile (amongst other methods)
• Revolve using the perpendicular line as the axis

i’m not convinced that’s the best way to do it but it’s similar to the way you’re already trying.

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Hi Ody,

It seems to me, you need to orient the axis different:

HTH
-Willem

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A projected circle onto the sphere does not always result in a circle. It will be a circle only if the projection line for the circle passes through the center of the sphere. In your case the resulting curve on the sphere will be an oval (ellipse to be more precise) so… you need to do very specific pre-arangements in order to have a small spherical bump on top of the big sphere.

The easiest way to acheive this is to boolean two spheres – a big and a small. Works great.

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Then you still need to align it to the surface though. these are necessarily exact spheres, but could be ellipsoids.

@adXok: yes it looks like the projected circle is not circle indeed. It sounds not practical to do the very specific pre-arrangements, but that looks like the way to do it then. I might consider using a boolean of 2 spheres then…

@Willem: you are right. I should orient the axis perpendicular. But I discovered that the projected circle is actually an ellipse…

@jeff_hammond: looks like the right steps… just did not do the Line-> Normal … will try out

Ody,
If you have an ellipse (as a projection) on the sphere surface then revolve is not going to work. Well you could do a certain “revolve” but it would be far too complex.
If you want to keep the ellipse as a basis for your bump sphere… (well, bump ellipsoid I mean) you have to make so many additional steps it wouldn’t be worth to.
So, I think you want to stick with a “sphere on top of a sphere”. Thus boolean the two spheres will do the job correctly.

But… you have to be careful. So, plan first how big R=? your big sphere is going to be.
Where on its surface you would want the small sphere (r=?) should be placed. maybe a little spherical bump? If so create your sphere from the center of the big one (it will save you later troubles). Then make a straight line form the center of the big sphere to the point where you want the bump’s center to be. Move the small sphere along the vector of that straight line (lock the move with the TAB button to match the vector). Works great here, 3 to 5 steps.

1. Line: Surface normal. BothSides.
2. Pipe, create a pipe along the line.
3. Split the big sphere with the pipe.
4. Sphere: 4 points. Near snap the first 3 points to the hole edge. Near snap the 4th point to the normal line to decide the radius of the small sphere.
5. Split the small sphere with the big sphere.

Hi Ody- I think I’d make this as a sphere to begin with.

1. Draw a Line Normal to the height of the desired ‘blister’.
2. Sphere, 2 point snapping the first point to the outer end of the normal and the second with ‘AlongLine’ or tab direction lock defined by the line.
3. Make the sphere as large as needed to intersect the larger surface by the right amount for your purposes. You can adjust the sphere by using Scale from the top end of the line. Intersect, with History, will let you keep track of the intersection curve.
4. When it looks right, trim or BooleanUnion etc as needed to remove most of the new sphere.

-Pascal

Thank you all for the advice.
Using a line normal to the surface worked out!

I used that line normal to ‘rail revolve’ an half arc along the projected circle (which is indeed an ellipse)

Cheers!