This article is one in a series of Instructables articles about DIY technology. The full list can be seen here.
Step 1: Sand Castles
Imagine you have a tray and coat the bottom with a fine layer of sand. You then took some glue and drew a circle in the middle, creating a ring of glued sand. Now take some more sand, spread it evenly on top, and draw another circle with the glue, gluing a new ring of sand to the previous one. Keep repeating this process off adding a layer of sand and gluing circles and eventually there will be a glued sand cylinder inside the tray of sand.
Now, if you reach in to the tray of sand, you can pull out a hollow cylinder of sand. By choosing the radius of the circle and the number of layers you can get the exact dimensions that you want. If you made each circle a little bit smaller you would get a cone. Get creative and you can make any shape you want.
Step 2: Other Techniques
A different approach is Fused Deposition Modeling (FDM) which build models up by adding material instead of binding it. With this technique, plastic or metal is extruded from a nozzle that moves around on top of the model being built. Since these models don't have the other material to hold them up, they also use support material. It looks like scaffolding and when the model is finished it can be easily removed to leave the resulting piece.
Photo by fungus amungus from How to use Google SketchUp for Ponoko 3D printing
Step 3: DIY Options
Even though the industrial machines are way too expensive for the casual user, their services are easy to access. Two websites, Shapeways and Ponoko, provide a simple system for 3D printing. Just upload a file, choose a material, and they'll ship it to you when it's finished.
So whether you want to start out by printing a few pieces or jump on in and get a MakerBot, there are definitely some solid options for making your ideas become tangible objects without all the mess of making it by hand. That way you can worry more about the design and make things that you never thought were within reach.
RepRap - desktop 3D printer
Fab@Home - desktop 3D printer
Thingiverse - share 3D files with others