Design and Construction
This is the KISS design of version 1
with some improvements.
The original cheap electric driver star tip (T10, right) has been replaced by a T9 (left) from Wickes.
Rather than using an electric screwdriver motor, I have re-used the original BfB extruder motor and gearbox. Note that this actually provides more torque than the original screwdriver. To provide a temporary coupling, a pair of soldering handy hands was sacrificed to the RepRap god and one of the clips provides a way of driving a hex screwdriver bit directly from the motor coupling.
The PEEK insulator has been drilled using a standard electric drill, and ended up a little off-centre at the top. This means that a 3mm hole drilled perpendicular to the filament channel has enough material to support the back of the hole. The star screwdriver bit can be inserted directly into the hole and bites directly into the filament.
Testing - ABS - 220C - score 7/10
Testing with ABS at 220C produced a few mm/sec - but with only a little additional force produces lots of filament. It's likely that with a slightly better drive arrangement - maybe lateral force to help the bite - much quicker extrusion rates should be possible.