How to create a pyramid with 2 curved sides

[Lee B.]

Hi, Just learning CAD and wanted to know how to make the attached pyramid in AutoCAD. Currently when I create pyramids I create the base polygon. Then add center point, which I raise along the z-axis. I then loft the polygon base with the elevated point. I'm not sure how to get two of the sides to curve like the image. Any help is appreciated. Thanks. FYI this is a surface model not a solid.

[lrm]

Assuming that surface A is flat and that  vertex 3 is above the plane containing vertices 1 and 2 you could create a UCS using point 1, 2, 3 and then create a straight line from 2 to 3 and a spline from 1 to 3. CPolar array the line and spline about 3 by 180 degrees.  Finish by lofting from the spline to the line.  Loft again from the spline to the next line. Polar array the two surfaces.

 

[ammobake]

Barring any unseen complex geometry with the faces (assuming they are flat) the way I would draw this is with 3d solids.

Personally, I would draw 4 separate closed polylines to create the 4 different faces of the pyramid. 

So this would start with drawing out the 2d geometry/linework.

 

Some of these polylines will include arcs but it's all good.  you can still use pedit to convert the arcs to a polyline and then join them all into one closed polyline that you can extrude.

 

The way I would draw the arcs is to first draw a circle centered on the edge of the square.

From there you can use the circle as a guide for ensuring your arcs are tangent.  Or you could also trim the circle off at the centerpoint - resulting in an arc.

A few ways to do this..

 

Once all 4 polylines are drawn and closed, I would extrude all 4 to the required height.

 

From there would use the corners and the top of the pyramid to "slice" each face off at the correct angle.   And you would be done at that point.

One could also merge all of them together into one object at the end for posterity.

 

Resulting in (1) 3d solid of the required dimensions.

 

-ChriS

[ammobake]

I was able to do this myself. 

The slice command at the end works best in 2d wireframe mode with the 3 point option - for ease of snapping to the intended points.

Personally I always keep both sides after slicing, then delete the one I don't want. 

 

Just seems easier for me I dunno.

The resultant object is a pyramid.  but if you look at the sides it is half of a circle. 

 

This is like some MC Escher stuff.  lol.

 

-ChriS

[Dadgad]

@ammobake, well done, and yes, an interesting Escheresque 3D Solid! 

[lrm]

Here's another approach, solution?

 

You can start with a half cylinder, extrude 2 polylines and slice or Boolean subtract  an incline on their bottoms and then subtract them from the half cylinder.  Boolean union the result with a half cylinder.

 

 

[Lee B.]

Hi Ammobake, Thanks for the advice. It looks like you have a half moon because two of the sides extend past the other sides when you sliced them. Those sides should actually be concaved to follow the curve. 

 

Hi lrm, Using your technique I was able to successfully create the shape I wanted. The one difference - Instead of using a spline for the curved side I aligned a circle on the proper UCS and used trim to get the curve. I attached the resulting shape. 

 

Thanks everyone for your help with this. Excellent first experience. 

 

Lee

[ammobake]

lol nice! And i thought my brain hurt already!

Happy Friday everyone.

 

-ChriS