Wednesday, 11 May 2011

A few microsteps forward... and then a jump, for Joy?

The ScrewBall-Strap Bowden extruder with 1.75mm PLA is working, but with one small problem, I'll explain below.

ScrewBall-Strap is Assembled and working* (almost).
I'll take a better set of photo's of the completed build, but notice the built-in Filament holder at the top. I just need to make some reel spools to hold all the PLA and ABS Filament now, it's designed to feed down and enter the back of the extruder's and come out via a Bowden cable to the many hot ends I hope to have fitted.


Path through the extruder - 1.75mm Natural PLA

PTFE Tube 4mm Outer and 2mm Inner.

The PTFE tube is push-fitted into the Pneumatic connector, this holds very tightly, no play at all but it still allows rotation of the tube (o-ring).

And at the hot-end the PTFE tube goes through a guide (not sure I really needed this) and then into the Pneumatic Push-fit connector.

The Bowden extruder configuration looks like this while the Z axis is in the lowered state (home).

And this is it with the Z Axis raised by about 130mm

So my very first print on ScrewBall-Strap could start. I'm still using the official 5D firmware and Java Host program for now until I get everything running well, I'm then going to look at alternative firmwares and take the plunge with Skeinforge.

Not a bad start, the extruder is working well and it seems to be sticking down well, but could do with some more filament flow.

Then...

After the first layer the extruder made a mechanical sounding 'BANG' and the flow stopped but the print and the extruder motor all continued.

I could not see what was going on but the filament was not being drawn into the extruder so I wondered if I had the pressure set to high on the idler bearing and maybe the filament had snapped? I loosened the bolts but the filament was intact.

I tightened up the idler and started the print again, the same thing happened but this time I spotted that when the 'BANG' noise happened the idler block moved (jumped) towards the drive (by about 2mm).

I stopped the print and started to think about it.

What had happened was the filament has slipped off the drive wheel into a gap, these pictures should explain.

The drive wheel. (underside of extruder) 

The green section is where the idler bearing sits over the drive wheel.
The red lines show where the filament was ending up and causing the bearing to jump (BANG) into the drive wheel.
The bearing block is smaller than the drive wheel width. I'm not sure why I didn't see this when I was building it.

I know the a Hobbed filament drive would stop this from happening, but I specifically did not want to use one in this design as I want to be able to load different sizes of filament and just change over the Pneumatic connectors and PTFE cable, so a specific Hobbed bolt would need to be made for either 1.75mm or 3mm Filament.

I'm going to make a new idler block with some guide wheels to stop the filament jumping out.and see if that's the only problem before I can successfully print out something.

I now think ScrewBall-Strap is going to work, so that's a good reason to celebrate, hopefully in the next few days I can see how well it will perform. 

Rich.