also covered a little bit here:
http://wiki.makerbot.com/how-to-print#toc4
Step 1B: Clean Your Model!
It's pretty easy to create a non-printable 3D model, especially if you're using a design you found on the internet. Being able to clean up a model and fix it to be printable is a very valuable skill to possess. There are a couple tools that are great for model cleaning.
Cleanup with Blender
A few UI tips:
•the 'A' key toggles between select all and select none
•the right key is how you select objects
•the tab key will take you between object and mesh mode
•holding middle click will allow you to move your view. scrolling it will zoom.
•the num pad keys will give you good default views if you get lost
•the 'X' key will allow you to delete objects, like the default cube when you open blender
Import / Export
Blender supports a TON of import and export formats. You'll want to export your final object as STL for printing though.
Remove duplicate vertices
1. Hit Tab to enter Mesh Edit mode
2. Hit A to select all nodes (everything goes yellow and purple)
3. Type W6 or select Mesh -> Vertices -> Remove Doubles
Find non-manifold points
Non-manifold points are points that just don't make sense in the real world. These can be hanging points, internal surfaces, holes, zero-thickness walls, etc. Unfortunately there is no way to automatically fix them in software. The best you can do is to identify the problems and then attempt to fix them.
1. Hit Tab to enter Mesh Edit mode
2. Hit A to deselect all nodes (everything goes gray and pink)
3. Select Select -> Non-Manifold
If there are non-manifold points, they will be selected. If nothing happens, you're golden. If there are problems, then you can see what's wrong and attempt to fix it.
