Fighting with computers

Computers are not always friendly.

Thursday, December 27, 2012

On extrusions speeds

I have been losing a fight with my latest extuder/hotend combination. It works ok as far as I do not want it to work too fast. I want to keep the PG35L motor not too hot or it will break. So I am powering at 500mA, which seems to be well beyond its maximum rated current, so it gets hot. But I've added a passive cooler to it so won't get dangerously hot. Should I used a higher current, as someone suggested, maybe my mileage might vary, but at the expense of needing a fan blowing directly to keep the motor from frying itself.

One of the things that was bothering me was the maximum printing speed I could use with this new setup on my Prusa i3. Extrusion speed on Pronterface software (the host I use) is user selectable but measured in millimeters per minute, while travel speed is usually configured in the slicing software in millimeters per second. But doing the apparently simple math of dividing by 60 will not help here, among other things because the extrusion speed is the 3mm filament input speed.

As the hotend nozzle is much thinner than the 3mm stock filament, a much longer distance is expected at the output of the nozzle. A simple calculation can be done (assuming the volume is preserved through the extrusion process). If we consider that both input and output filaments are cylinder shaped, then, for a 0.5 mm output nozzle I can measure a 0.6mm diameter for my extruded filament output. Each millimiter that enters the hotend is 7.07 cubic millimeters of plastic, that for a 0.6 mm output diameter will turn into 25 mm of length (yes, that's right, every input millimeter turns into 25 mm of filament at the output).

Equipped with this data, we can finally do the math. For example, if the maximum speed you can get an steady flow of plastic is 180 mm/min, then that means you can lay output filament (without stretching it) at 180 * 25 / 60 = 75 mm / sec.

Of course this calculation needs to be adapted whenever input or output diameters of filament differ from these above. Please let me know if I am getting the facts wrong.

Tuesday, December 04, 2012

Building a Prusa i3

I am very happy with the Prusa I built, and so I am with the Printrbot, which much smaller but it does a nice job too. In fact, most of the parts (on different shades of blue) of my new Prusa i3 have been printed with the Printrbot, but the black ones, which are ABS parts, have been printed with my Prusa.

Having two working 3D printers seems to leave you in a difficult position to explain your wife why in hell you're building yet another one, but somehow I managed. I was curious to learn how this new version from Josef Prusa would do compared to what I was familiar with.

I've done a short picture album of the process with some comments in them whenever something different or creative was done there.

My first impressions are that this printer can work really fast and it seems very stable at high speeds. It feels much sturdier than the previous Prusa's and more stable than Printrbot. To its advantage has the reduced mass of the new compact extruder which I have not mastered in its tuning. At the moment high extrusion speeds are not yet working for me, but I guess it is a matter of grip, current and maybe springs (which I do not have yet in the extruder idler).

On the cons side (or not, depending how you look at it) is that when moving it to another location the whole thing does not feel as solid as the older models. But the advantage is that you can easily turn it into two separated pieces you can carry around or fit in your suitcase. A list of changes I have done to the original design:

  1. Instead of Aluminium my frame is made of acrylic GS (Prusa letters are gone from the top to keep the frame stronger). I could not find an Alu frame in Europe. Link to design file.
  2. Y-axis bed is made of acrylic, so I've chosen a more squared shape.
  3. Acrylic frame holes are tapped for M3 bolts, that worked nicely, no nuts required.
  4. The compact extruder motor I bought had a 9mm long shaft, which won't align well with the Mk7 drive gear and the filament guide hole of the compact extruder. So I made some changes to the compact extruder to make all fit nicely. I also adapted the 16mm hole in the base for fitting an extruder I bought from a fellow reprapper. 
  5. Though I bought 48mm long motors in the hope they will keep my frame perfectly vertical, I was wrong. The steppers still need 3mm for the frame to be vertical. I ended up taping a bit of cardboard to the bottom of the motors.
  6. Prusa nozzle is not yet available so I have used another hotend, but maybe I will give that a shot soon.
  7. I am powering the extruder at 350mA and that seems to keep it for overheating quickly. I used initially 1A and that will set it to a too high temperature in a couple of minutes.
  8. I made a mistake when printing the x-carriage so I could not use the teeth as they do not match my belt. So I used a zip tie to held together the two ends of the belt and insert the result into the belt slot which fitted nicely (there is a picture of this).