This is a LEGO Mindstorms 3D printer I made with a 3D pen, and a custom g-code interpiter. Unlike other 3D printers made with LEGO Mindstorms, this one can make models like a conventional 3D printer. It uses four LEGO Mindstorms motors. Three on the motors are for the axis, and the fourth controls the extruder.

I chose the 3D I'm using because it took 1.75 mm filament in spool form, and was quite cheap at around $50 USD(Now It's $31.90 USD!).

The print quality isn't very good in comparison to a conventional 3D printer, but this printer is more of a proof of concept. Also, considering the printer is made with a plastic toy system, the print quality is quite respectable.

Items you'll need:

($349 USD)LEGO Mindstorms EV3:

http://www.lego.com/en-us/mindstorms/products/min...

($35.80 USD)3D Pen:

http://www.amazon.com/Soyan-Drawing-Printing-Print...

($50-$100 USD)Extra LEGOs:

http://brickowl.com

http://bricklink.com

(Free Dollars)G-PARS3:

http://github.com/cyruscuenca/g-pars3

Step 1: Building

The actual printer is just like any other cartesian 3D printer.

You must have three axis:

Z (Up and down)
X (Left and right)
Y (Forward and backwards)

And you need to have the fourth motor press the extrude button on the 3D pen.

Some building tips:

Reinforce corners with triangles when possible
Use smaller gears if you are running a platform along gear racks
Use the LEGO frames when you can

Step 2: Programming

I plan on making an instructional video, but if you know how to use a computer, you should be able to program your EV3 brick with text instructions.

How to install G-PARS3:

Go to ROBOTC.net and download the latest version of ROBOTC for LEGO MINDSTORMS.
Follow the install procedure and open the application.
Go to GitHub.com/cyruscuenca/g-pars3 and download the latest version of G-PARS3.
G-PARS3 will download as a .zip file, so next, you'll need to extract the file to your Desktop or some other folder. Just be sure to remember where it is.
Open the folder you extracted, and navigate to > programs > 3D Printer 2.c
Open the file in ROBOTC
Next click on "Robot" in the menu, click on "Platform Type", and select EV3.
Then, still in the "Robot: menu, click "Download EV3 Linux Kernel" and select the standard file.
Next, in the "Robot" menu, click "Download Firmware", and select the standard file as well.
Finally, click "Download to Robot".

Calibration:

Calibration instructions are regularly updated, so I keep them here.

How to print your model:

Obtain your .stl or other standard 3D model file, either by design, or by download. I would recommend thingiverse.com as a reputable 3D model source.
Open your file in a slicing program. I would recommend Slic3r. I've been using it, and it's given me no problems.
Scale your model to fit your build area.
Change the layer thickness and nozzle diameter to to .9.
Export as "gcode.txt" to your Desktop.
Plug your EV3 brick into your computor via USB.
Open "Robot" once again in the ROBOTC application, click "LEGO Brick", then click "File Management Utility".
Open the rc-data folder.
Once open, click download, then select your gcode.txt file and download it to the folder.
Now, click "Download to Robot", and click run.

NOTE: Once the free trial ends for ROBOTC, you can use the file management tool in LEGO's free programming environment.

Further Calibration:

If your 3D pen keeps getting stuck, increase your layer height.
If your print only has one perimeter, you can select "Spiral Vase" which will remove the layer change seam from when your printer moves along the X axis.