Here are some picture of the QU-BD extruder and how they look on my printer. It is actually behind a wood block that holds it on the default tool holder that comes with the ShapeOko CNC, so it is not clearly visible.
Here is the final working extruder on my printer. For the fun of it, I would call it K-head. It is held by an aluminum holder. Also, more details can be seen with the fan off on the second picture.
Here are a few most important improvements I have made to it:
1. The hot-end tube
The original hot-end of QU-BD uses a stainless steel tube. This is actually one of their selling point: a complete metal extruder. However, I find out that even though stainless steel has pretty low thermo conductivity, that is still way to much for a hot-end. Indeed, the majority of the filament jam is caused by too much heat is going up the hot-end tube. I have tried to put extra heat sinks on the tube, has drill out the inside to a larger size, and tried to print at high speed trying to get cooler filament into the melting portion. None works until I changed the hot-end tube to a different material.
The tube you see here in the final working K head is made of PEEK. Indeed, it is called "Self-lubricating Carbon-Filled PEEK" (1/4" D, 1 ft, 1595A11, for under $8.75). Because I don't own a lathe, I like it very much the factor that PEEK is much easier to drill and tap than stainless steel.
With the new PEEK tube, I can touch the aluminum holding plate and only feel warmth even when the hot-end is heated to and held at 210C for a long time. The PEEK tube not only have a low thermo conductivity, it is also self-lubricating which makes jamming very unlikely.
2. Thermo shield of the heating blockThis is with the original QU-BD but it is not shown in their instructions. It took me a long time to realize what the white piece is for (it is a ceramic heat shield tape). And doing that greatly cuts down heat radiation loss, and makes the hot-end heating and temperature much more stable.
3. Aluminum filament tensioner
The original QU-BD uses a HDPE block with a set screw to hold the filament to the driving wheel. I found that hardly working. Maybe I am too cheap, and the filament I bought (from Amazon) has poor uniformity of diameter. There are a lot of designs on the Thingiverse on the tensioner for direct drive cold-ends, but it takes a working printer to print them. Even when I finally to get my printer work just enough to print a piece with PLA, I found out that it did not hold well as the motor is getting hot from driving current. So I finally just decided to make it in aluminum so it works under any temperature.
A small trick also used here which I learnt from one of the Thingiverse designs. The trick is to use binder clip instead of steel springs to supply the tension. Not only the binder clip is easily strong enough, it can also be held in place very easily by drilling small holes in the holder. The binder clips are very easy to find, which is another advantage too.
So here is the K-head that is finally working reliably for me. Through the process, I had a lot first hand experience about the extruders, particularly what works and what does not. The QU-BD extruder is really not a good design. It is not that theirs is not working. It might be, but with that design, there is a very narrow band of setting that it might work (specific filament, specific stepper motor, specific temperature, specific print speed, etc.) But real world reality is never always in the ideal case, and anything different will throw off the balance. In that sense, we can clearly see that the QU-BD is not a good extruder design.
My K-Head is a lot more reliable, and works in a wide range of settings. I will see if I can keep on making it better. For now, I will go print some fun stuff.