Tommelise AEM Extruder 1.0
Page Contents
Making the polymer pump
First off, you first ought to look at the Mk 2 FDM extruder over at RepRap so that you will have a point of departure for what I'm going to show you.
Right at the beginning of the documentation there is a little drawing that Vik Olivier did quite some time ago that shows how he thought a polymer pump that is using filament feed stock ought to work.
Dr. Adrian Bowyer, who designed the Mk 2, took the essence of Vik's idea.
Looking at that drawing you will quickly suspect that that if you tried to pump polymer there was going to be a reactive thrust in the opposite direction (towards the gearmotor). While the Mk 2, as built, takes that all into account not having a lathe or a lot of skill with metal working tools I tried to simplify the design using some of Vik's original concept. Here's what the Tommelise FDM Extruder 1.0 that eventually came out of my ham-handed efforts looks like.
Like Vik, I used a double nut with a lock washer (circled in red) to stop this thrust...
...against the brass plate bushing (circled in red in the pic just below)...
...that you see between the top of the pump and the motor. Since that brass plate served as a bushing for both axial thrust AND lateral thrust I didn't need the brass bushing at the top of the pump only the bottom.
I didn't have an afghan lathe worth the name to take threads off of the pump shaft and making a half round brass fitting of that size was darned difficult (I've since realised that I could have bought a brass bolt and cut the head off of it), so I just took a quarter inch thick piece of steel and drilled it to seat the quarter inch threaded rod I had and just left the threads on.
Those wear down so that I will have to replace the pump's quarter inch threaded rod, but that is no big deal since I had to buy three feet of the stuff to get that little piece that the extruder needs.
Anyhow, I used my dremel grinder wheel to shape that steel bushing after I cut it in half so that I could seat the rod. Once I got it about right I pressed it into the ABS pump body with my vise. That got me a good, firm fit.
The ABS plastic coupling that came with the parts set for the Mk from Adrian didn't quite fit the Solarbotics GM3 gearmotor .
It has a slightly larger drive shaft than the one that Adrian uses. As well, it was made to thread onto the machined end of a 5 mm threaded studding rod, not quarter inch American threaded rod (6.35 mm). I tried to open up the ends to fit with my Dremel tool but ruined it in the process. I then built my own coupling from a quarter inch nut, a lock washer and a quarter inch coupling nut that I sawed open with a hacksaw at the top to seat the GM3.
Housing that nut/lock washer/nut axial thrust collar, the brass lateral and axial bushing and that long coupling nut assembly required that I add some extra height to the Mk 2 polymer pump. I basically just used wood spacers to make that happen.
You will also notice that the M3 (3 mm) studding that the Mk 2 uses to hold things together has been replaced with American #4-40 machine screw studding. It's just about the same size and can be had in American hardware shops of the better sort without a lot of drama or over-the-internet purchasing.
Looking a bit more closely at this pic you will notice that while I use a nut/lock washer/nut connector at the bottom of those long #4-40 pieces of studding I don't try to lock them down at the top. There is method, or experience, in that piece of seeming madness.
If you don't manage to lock down that quarter inch nut/lock washer/nut thrust collar really, really tight on the threaded rod used as the pump it can break loose when the pump is in operation. Because you are creating several hundred psi of pressure in the extruder barrel with the pump that threaded rod pushes out of the pump with considerable force when you get a break-loose event. If you've locked down the gear motor all that thrust goes into the GM3's plastic gearbox. That's not a good situation to create.
If, however, the gear motor ISN'T locked down that thrust just pushes the gearmotor harmlessly up till it falls off of those #4-40 studding rods. That doesn't hurt anything. I use similar techniques which I called "fall-apart" safe design. It's saved me a lot of broken parts and burned out motors.
That's how I did it to get it running. It works pretty good although it looks really nasty. That's also why I intend to redesign the Mk 2 so that it can handle that situation a lot better than regular Mk 2 parts can.