# How to make a double sided funnel

Trying to make a double sided funnel to replicate this poster:

So far what I've tried is making two funnels using the XYZ math surface mesh, rotating one on the x-axis 180, and then joining them together.

The only problem I have is that you can see where they both got joined together, and that the inner curve is more like a sharp angle than something smooth. How can I fix both of these issues?

Select and delete the "unwanted" faces":

After this, you can scale or use proportional editing to get the exact shape you want:

Once you have it ready, add a Subdivision Surface modifier and shade smooth to make it perfectly "curved":

Lots of types of shapes can be easily made this way:

This is a good opportunity for combining a curve with a screw modifier.

I start with a Bezier curve and edit it to the shape of the outer edge of the desired double funnel (hour glass):

The key features are to have the central control point exactly on one of the axis, and its handles parallel to the axis that forms a plane with the first axis. For the example, I picked the Y and X axis, respectively.

The distance along the Y axis sets the narrowest part of the curve.

Then I add a screw modifier:

Since I was lazy and left the Bezier curve on the XY plane and since I picked the Y axis for the center point, the screw should apply on the X axis:

Here's another view, after editing the curve:

The upside of this approach is that it can be very fast and it is non destructive. Only 3 control points and a modifier that you don't have to adjust once you've set it up.

The downside is that if you want symmetry you need to be careful in how you handle the two outside control points. They have to have the same Y coordinate and the X coordinate of one must be the negative of the X coordinate of the other.

But if you start with a curve that already has those properties that's easy enough to do.

• I would recommend adding a "Mirror" modifier with clipping turned on if you need symmetry. When I used mirror before screw with merge turned on viewport shading had a discontinuity problem at the neck of the funnel, but that went away with switching the modifier order or using clipping without merge. (using 2.93.3) Feb 26, 2022 at 19:29

You can model this shape with a math function. The two sided funnel is an hourglass, and that technically is modeled as a Hyperboloid of one sheet The equation is

$$x^2/a^2 + y^2/b^2 + z^2/c^2 = 1$$

The Wikipedia page has examples of parametric forms that would be suitable for Blender's equations.

$$x = a * cosh(v) * cos(u)$$ $$y = b * cosh(v) * sin(u)$$ $$z = c * sinh(v)$$

Here's an example with $$a = b = c = 1$$:

Here is the result, with a subsurf applied for smoothing:

You can create a cylinder, create a loop cut at its middle (CtrlR), scale it (S):

Then bevel (CtrlB), you can tweak the parameters in the Operator box: