So my buddy machined the parts and I put them together, full of confidence that it would just work out of the box. Such a simple design, how can it go wrong- right?
Without even having the hotend hooked up to my Reprap yet, the first problems immediately became obvious. Let's take a look at the design:
The melting zone, where the filament is heated by the aluminum block, is 5mm long. This only works for VERY slow extrusions. otherwise even the 1.75mm filament does not have sufficient time to reach temperature and melt while it travels down the block.
Fail 2.
I have no recollection how this happened, but somehow I specified a 7.0mm diameter hole to fit a 4mm diameter resistor as a heater. There was lots of slop that needed to be filled with carefully wrapped aluminum foil around the resistor.
Fail 3.
And there's something else wrong with the design. It shows the thread in the block bottoming out. You can't machine a hole like that with a perfectly cut thread all the way to the bottom. The effect was that when you screw the barrel into the heater-block, there is a gap between the end of the barrel and the beginning of the melting zone. In my mind this was just going to fill up with molten filament, but have no real effect on the extrusion.
That appears to not be the case. It caused a certain springiness of the filament feed. Pushing the filament in by hand, I could actually feel it springing back, when I let go.
As you can see in the drawing, the one end is threaded and that's all the machining I had planned. But in order to avoid the gap between the barrel and the melting zone, we need to turn down the last few millimeters of the barrel to a diameter smaller than the diameter of the core drill for the internal thread on the heater block. Easy enough, just put it in a drill, spin it and apply a file to the end.
And there's more fail to come!
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