Building a 3D Printer from Scratch
Most 3D printers on the market today are built around designs and technology that is derived from the RepRap project. This is very good for those looking to build their own 3D printer from scratch as the community that has developed around the RepRap is like no other. In addition to the support available from the community, the entire RepRap project is open source, meaning that all of the design files, hardware bills of materials, and software are free for anyone to download, modify, and use as they wish.
What design do I choose? - This is something you will need to research thoroughly as each model is unique in its own way and as such, each model has it's pros and cons. Personally I am a big fan of the Mendel design, more specifically the Mendel Max variant. I like this design because it robust reliable and easy to set up and tune. With RepRap as long as you stick with something of the Mendel variant or one of the Prusa designs, you will always be able to find help in forums or IRC chat.
Below we take a look at some of the different RepRap designs and will list out their bill of materials. Keep in mind these are not the only 3D printers you can build from scratch, they are just my personal preference. After being part of the RepRap community for several years now, I feel more than confident in these recommendations.
RepRap Mendel - The RepRap Mendel is the second generation 3D printer from the RepRap project and is a wedge design. It's small enough to fit on your desk, but with a print volume large enough for you to make big things. The machine is made up of parts sourced from local suppliers and online. Some parts will need to be manufactured by you. Like most kits we are going to talk about, you will need to acquire some 3D printed parts to complete the build. The bill of materials can be found here. More info can be found on the RepRap Mendel page of RepRap.org.
Prusa Mendel V2 - The Prusa Mendel is by far the most popular of the RepRap derived 3D printers because the build is simplified and uses less parts. Prusa Mendel improves on a previous design by being more streamlined for manufacture. The Prusa Mendel is a simpler remix of the original Mendel. By default, it uses printed bushings instead of regular bearings, though options to substitute inexpensive LM8UU linear bearings or other types of bearings or bushings are available. You can find the Prusa Mendel's bill of materials here. Additional information can be found on the Prusa Mendel page on RepRap.org.
Mendel Max - My personal Favorite, the Mendel Max (MM) is an Open Source RepRap 3D printer designed by Maxbots in December 2011. It is a true RepRap using printed brackets, but instead of using threaded rod for the structural elements, it uses inexpensive aluminum extrusions. This gives a huge increase in rigidity for a minimal extra cost (self-sourcing will cost about $80 more than a standard Prusa Mendel).
Mendel Max is based on the Prusa Mendel and keeps its best parts. The Prusa Mendel kept the frame of the Sells Mendel, but fixed the problems in the X, Y and Z axes. Mendel Max 1.0 builds on that by largely keeping the current XYZ axes, but completely redesigning the frame.
In addition to the increased rigidity, the printer is much easier to assemble than a standard Prusa. Even an inexperienced builder should have no trouble building the whole frame in an evening, two at most. And along with the easy assembly comes easy hackability. Almost any part on the bot can be removed with just a few screws, so swapping out literally any part on the bot is now a trivial operation. I have listed the bill of materials here and more information can be found on the Mendel Max page on RepRap.org.
Prusa i3 - The Prusa i3 (iteration 3) is the newest and current 3D printer design by RepRap Core Developer Prusajr. The i3 incorporates lessons learned from the previous two Prusa designs, as well as other popular modern RepRaps designs. While this design is refined, it is still in its infancy and is actively being developed by many members in the community, as well as large companies such as long spot who's new to TAZ 3D printer appears to be based off the i3 design. To be honest, I am really unfamiliar with this design and would recommend you do research before deciding to build your own. The bill of materials can be found here, and you can find out more by visiting the Prusa i3 page on RepRap.org.