Prusa Mendel Reprap 3d Printer
There is a lot of information concerning 3d printers available on the net, making it a little hard to make a choice.
The Prusa Mendel is based around open source, and seemed fairly easy to understand. Its open construction makes access easy, and it is much cheaper than most alternatives, making it a good starting point and learning platform.
These are are few brief notes on what was needed to make a pre-assembled unit work, written by a total “newbie”. I had to follow a few blind alleys before finding what worked for me.
I used a Linux platform to run this, but no doubt Windows would allow a very similar installation.
The unit needs 12volt, at a bit less than 20 Amp total. A strip LED power supply is fine. Most of the current is used to power the two heaters.
The Prusa was available preassembled and delivered , including power supply for ~$500 Aus (us $450). The quality of the assembly was good and careful. No instructions or data of any sort were included.
There seem to be many subversions and variations of the Mendel.
The main control PCB was a “Melzi Arduinissimo”. This seems a fairly common control board. In my case no firmware was installed.
To program the Melzi an Arduino “toolchain” with a “Sanguino“ add-on is needed.
I downloaded “arduino-1.0.5-linux32.tgz” which unpacked and ran without issues.
The Melzi card cannot be selected without installing the Sanguino add-on. To do this download “Sanguino-0101r1.zip” and unzip it into the unpacked directory “arduino-1.0.5/hardware/”. Having copied Sanguino it's possible to select the Melzi board by clicking tools->board->Sanguino w/ AT-Mega1284p 16 Mhz.
The firmware for the Prusa Mendel is called “Marlin” , which can be loaded from github as “Marlin-master.zip”. Unzip this into the Arduino sketchbook directory which appears after running arduino.
Once this firmware is installed it can be compiled by clicking File->Sketchbook->Marlin-master-Marlin. The top left icon (a “tick”) will do a test compile. To download the firmware the power is removed and two jumpers must be moved on the Melzi PCB. This is described at the Melzi-Reprap Wiki and GeeeTech . Once the two jumpers are in place the firmare is installed by clicking on the second icon on the top left (right arrow).
Next step was to install a control program on the PC, in this case “pronterface.py” (aka Printrun). For debian/Ubuntu dependencies: apt-get install python-wxgtk2.8 python-serial, and make sure users are in group ”dialout”.
The Melzi PCB is unshielded, and therefore susceptible to radio frequency interference. In my case the card would lose communication when a neighbouring drink fridge (very necessary for this sort of project!) turned on or off. A single print run can take hours, so any electrically noisy equipment must be kept away or turned off.
Pronterface is fairly straightforward. All the limit switches were connected normally open on my Prusa, and needed to be swapped by moving the spade connectors on the switches. One stepper motor ran the wrong way and needed a winding connection reversed.
After that the X,Y,Z positioning worked well, but the Z scaling was incorrect. The extruder heater and associated temperature sensor worked correctly. The heat bed would not activate, and a temperature of -50C was shown. A pot of hot water brought the indication up to 0C, at which point the heater would activate.
The answer to these two issues was to edit Configure.h using the arduino program as above, and then reprogram the Melzi.
The thermistor configuration table selection needed to be changed, as did the number of steps per millimetre for the Z-Axis leadscrew (e.g. 200 steps/turn * 16 microsteps / 1.25mm pitch =2560). It looks as if different builds of the Prusa-Mendel have a mix of thermistors, leedscrew pitches and XY pulley tooth pitches. All these things are settable in Configure.h.
Next step was to try printing something. Slic3r is an stl file to .gcode converter which is very easy to use. The gcode files can be viewed using tatlin.py (presumably named after the artist, by Denis Kobozev) . First attempt was with black ABS filament. The result was a bit squishy, probably because the bed was a bit too hot.
Two types of fibre are commonly used, ABS (Acrylonitrile-Butadiene-Styrene copolymer) and PLA (Polylactic acid) . ABS has a higher melting point, is not easily degradable, and has good electrical properties. PLA is biodegradable, and quite strong. PLA does not dissolve easily in non-chlorinated cold solvents like alcohol,acetone or xylene. It will very slowly dissolve in cold caustic soda.
Slic3r fills solid volumes with a honeycomb structure, which is quite robust.
The next attempt used PLA filament, which melts at a much lower temperature, and would'nt burn the cats' noses (it was hard to keep them out of things). If changing from ABS to PLA it is important to drive PLA filament through the extruder at the ABS temperature to clear all remaining ABS from the head, or it will clog.
The first layer is the main challenge, the bed temperature must be correct or the layer will not stick. I tried the suggestion made on the reprap blog to paint diluted PVA on the bedplate , and found it very good.
Preparation of .stl files: This is probably the commonest format for describing 3D objects for manufacture. These are files of surfaces described by wire basket meshes with a triangle as the primitive shape. Many ready objects can be found on the net for testing (see thingiverse). They can be viewed and scaled using meshlab. There are a number of free CAD programs available. I have tried freecad and brlcad. Both are quite good. Freecad is a bit easier to use, and has a good python interface.