Improved quality

No major new developments with the printer. I received white PLA from mendel-parts.com and have been using that after the green color all over my apartement started to hurt my eyes.

Green and white seem to behave pretty similar (which is not guaranteed!) but lately I had issues with the printquality being lower than with the previous extruder. For some reason the plastic was not always sticking to the layers below it, but tended to curl up a bit, leave a small void, and than deposit a blob a bit further instead of making a nice line. I tried cleaning the extruder hot-end, varying the temperature, varying the extrusion speed, but nothing gave major improvements. Last weekend I changed for the first time ever the layer thickness from 0.34 to 0.25 mm. This was a major change! Of course you get better details because of the layer thickness, but the plastic does not get the chance to escape anymore to curl up and it will always stick to the layer below.

Some pretty nice quality in the pictures below:

There is a drawback: both running skeinforge and printing takes much more time than before, but for now I can live with that. The remaining unwanted jitter in the surfaces which should be flat is due to many different small factors like: z-axis wobble, belt tension, x and y stepper gear train play etc.

Extruder rebuilt

The extruder worked fine for a long time, but started to become unreliable. Sometimes for no apparent reason the stepper motor missed a few steps; the force needed to drive the filament through was increasing. I increased the temperature a bit, but this did help only for a while. I did not want to risk damaging the rest of the extruder, so a rebuilt was needed.

Though the old extruder worked fine there were some parts which needed improving.

• An aluminum bar was keeping the hot end in place; this leads to loss of energy causing long warm-up times and heating of the extruder's plastic part in the places where the bolts are attached (see here).
• The hot end was very short making the placement of the thermistor difficult. basically the thermistor was taped with kapton on the nichrome wire which is not the best place.

I already bought a new hot end v4  form mendel-parts last month and now was time to take it into use. The new hot end has a machined piece of PEEK to keep it into place which solves the slow heating-up problem and it is longer to help in placing the thermistor in a better place. In addition the PTFE part is screwed into the metal tube and not the other way around which make the chance of leaking plastic much smaller (though I never had that problem, others seem to suffer from that).

Because the old hot end was kept in place with a aluminum bar with m3 bolts and the new PEEK bar has m4 holes in different positions, an adapter piece (copper) was made to make it fit.

 The old thermistor and new nichrome wire attached again (old nichrome wire had turned white and the insulation was falling off when taking it off the old hot end).

 Since the new hot end is a bit longer I decided to insulate the it, just like the original makerbot extruder (see here). The insulation tape was bought locally in Finland from www.robomaa.com.

The temperature of the extruder seems to be much more constant. It seems the actual temperature is always about + or - 1degree of the set temperature. The old hot end had + or - 5 degrees.

The extruder with the rebuilt hot end has been printing all day and seems to be OK again. There are some small issues with precision of placement of plastic (belts?) but that is a worry for later.

3D print from start to finish

Though similar videos exist on the internet, I wanted to document the full printing cycle of my repstrap printer. A 40 minute print condensed into 4 minutes:

The final object is here:

From the video a few issues with printing can be seen which I hope to work on improving in the future:

• Speed. The printer is not very fast. Increasing the speed is certainly possible, but I need to make sure I have enough spare parts to repair the printer if I break it before I attempt to do that.
• PLA dripping out after the extruder motor stops. It can be observed that after the test extrusion, when I have removed the extruded plastic hanging from the nozzle, that melted PLA drips out. This results in blobs on printed objects which should not be necessary.
• The first few centimeters of printing did not work. This could be related to the previous issue: plastic has been dripping out and pressure has to be build up before there is a constant flow of plastic.

This object can be found at thingiverse here and is my favorite test object which has been printed quite a few times in different materials and different sizes.

What is this?

I printed this thing:

Documented on thingiverse: here.

Filament spool

With the printer becoming more and more reliable, longer prints are made and a filament spool was needed. The spool was made using a metal tube with printed PLA endpoints glued to it and some cardboard. The filament is kept resting on some M6 threaded rods.


The spool is hanging in a huxley frame. I may replace the frame with something else if I need to finnish the huxley, for now the rapstrap is working fine.

Today a 2 hour print and a 5 hour print were done without any help in the middle. Should order more PLA now...

Z opto endstop

The new z-axis is very stable and I am confident enough to take the z-axis opto endstop into use for calibrating the height automatically at the beginning of a print. A very simple bracket to hold the opto endstop was printed and fixed to a vertical rod. The endstop is mounted with bolts with springs to allow for easy calibration. Three nuts are trapped in the plastic. The bracket looks like this:

The final endstop installed looks like this:

The printing procedure now is to set the extruder in the middle of the area where an object should be printed, with height close to zero (0-2 cm high). The extruder than will be lowered 2 cm, but the movement will stop when the opto endstop is activated. This is the point where the height is calibrated to zero and the extruder can move up 2 cm to warm up and do a test extrusion.

Extruder crontol

The original makerbot MK4 extruder worked well until it broke, but like most extruders the starting and stopping of the plastic flow was not very accurate. When printing simple objects this is not a problem, a few loose strings here or there are easily removed with a sharp knife. The extruder motor did reverse a bit after extruding to try to reduce oozing out of plastic, but the gear motor was not really suited for that purpose. I read that now in the the latest version of ReplicatorG the reversing is not switched on by default to reduce the strain on the motor. One of the reasons for building the new extruder based on Wades's design a while back was exactly the extra control over the flow of plastic which a stepper motor would provide. The new extruder has been working fine so far, but one thing I had not done yet: the new extruder was running all the time using a separate Arduino processor and was not controlled at all by the printing software. This works ok, but of course it produces a large amount of strings between the different parts of a model, like in this part of the new Z axis support:

The part is fine after some cleanup with a knife as can be seen here.

The current standard software for the mendel includes support for a stepper motor in the extruder, but the control circuit which is used by default is a double motor steering on the extruder mother board which is not very good for this purpose. I think there are several pieces of software being developped which look promising for the near future (e.g. here), but I don't have a standard solution right now. So in order to get control over my extruder now, I hooked up the output of the extruder mother board which used to go to the MK4 extruder gear motor to my arduino board which is controlling my stepper motor. Since the extruder board is sending 12V signals I reduced the signal voltage with some diodes and resistors so it is safe to connect to the input of the arduino.

The arduino will now detect the extruder motor state from the 2 signals and determine if the motor should go forward, backward or stop. There are some limitations, but it works!

I printed this nice puzzle, a Philippe Dubois burr from thingiverse, which consists of 6 different parts. All parts where printed at the same time and the extruder stopped and started during moves between the different parts for each layer (first layer 1 for all parts is printed, then layer 2 for all parts etc.). As you can see the amount of strings is very little; strings are not completely gone, but no comparison to the z-axis picture above!

The puzzle looks like this when assembled:

The puzzle is much more difficult than I imaged. There is a good website describing the different forms of the puzzle and also includes animations of the solutions. This particular puzzle is what is described on the website as a level 6.4 burr puzzle. This means it takes 6 (!) moves before you can remove the first piece, after that it takes 4 moves before you can remove the second piece. After this the other pieces fall apart.