Has anyone experimented with using a speedometer cable to drive an extruder? How about a second belt?  

Has anyone experimented with using a speedometer cable to drive an extruder? How about a second belt?

Advantage:

Lighter than a stepper motor, similar to why some choose Bowden drive, while preserving the advantages of direct drive, mostly.

Disadvantages:

Rotary "slop" in the cable, making precisely metering filament more difficult.

Stiffer than a set of four control wires, and could cause "bucking" when the thing rotates, triggering crash detection.

Mitigation:

Could be mitigated with a bit of gearing so slop amounts to a tiny fraction of a millimeter of filament. Rotary drive cables, such as speedometer cables and rotary tool flex cables, are designed for speed, so if you need to gear it up 1:100:1 between the motor and the drive gear, so that the cable within the cable is rotating 100 rotations for every 1 that either the motor or hob rotates, it should work. This mitigates the first issue at the expense of the second one, (more bucking for more control of extrusion).

I'm just thinking this might allow the E-axis stepper to be mounted somewhere on the frame, saving weight on the X-axis arm, but without taking on the disadvantages of a Bowden drive of having inertia in the extrusion, making over-extrusion and under-extrusion an issue.

Another approach might be to mount the E-axis stepper on the X-axis assembly, opposite or beside the X-axis stepper, synchronize the two, use a differential, (like in a car), to determine how much more, or less, the E-axis stepper has turned to drive the extruder hob. This would require a second timing belt, and would still be a load on the Z-axis, but still lighter on the X-axis.

The differential could tap the X-axis belt where it passes below the print head. There, it'd get 2x the X-axis motion, in the direction opposite of the extruder head's motion.

Disadvantages:

Difference in tension between the existing X-axis belt and the new E-axis belt could cause calibration issues with the extruder.

More wear on the E-axis stepper, since it'd be rotating for both X-axis and E-axis moves. I don't consider this a major disadvantage.

By synchronize the two, I mean that for X-axis moves, both the X-axis and E-axis steppers would have to turn the same number of rotations, (or possibly double the number of rotations, since tapping the X-axis belt below the extruder head would give double the X-movement across the X-gantry and in the opposite direction), while for extrusions, the E-axis stepper would either have to rotate faster/more than the X-axis, or slower/less. Probably the former for extrusion and the latter for retraction.

Thoughts?