I wanted to make something that is easy to print and easy to create in a 3D modeling program. Most 3D (and even 2D) programs will let you use a reference image to trace an object. That's the basic idea how this was created.

Step 1: Set Up Reference Image & Trace Object

I used Solidworks but all the 3D programs out there like Inventor, 3DS Max, 123D etc can do this. Set up your image on a plane (I used tools, sketch tools, sketch picture). I used reference lines so I would know how big to scale this. I traced the bell using a spline. Make sure to use a horizontal line for the bottom so that your object will stick easily to the build plate.

Step 2: Revolve Sketch & Shell

I next revolved my sketch around the vertical axis I drew. Not all programs out there call this revolve (ie Max calls it lathe). I used the shell feature to hollow this out but you could have also just offset your sketch to create the wall thickness.

Step 3: Add Ridges

The Liberty Bell had some decorative features on it. I made them ridges since that will print better than grooves. I drew circles along the edge of the bell and then revolved that around a center axis.

Step 4: Make a Hole for String

I drew a circle parallel to the bottom of the bell. I then extrude cut that through the bell. On some programs you can do a Boolean operation to subtract this.

If you only want a bell, you are done. If you want a "Liberty Bell", we need to add a crack.

Step 5: Make a Crack

I hide what I previously modeled so that I could see my reference image to trace the crack. You can also play around with your view shading. I traced the crack & then used extrude cut to create this feature.

Step 6: Optional Step, Add Text

The Liberty Bell has text on it. If you want your model to be more realistic, you can add that. It won't be as easy to print. You can create a sketch with the text you want & then use the wrap feature to emboss it normal to the surface.