Here is a collection of 3D printed Gear systems I am currently working on.

Project (1) :- create an in-finger joint stepper motor using these mini steppers 

Pros :- light,surprisingly very accurate, cheap,fast, very robust (easily tolerated 8.4volt lipo on coils)

cons :- No torque, no static hold "limp biscuit" , tiny 1mm screw shaft.

Verdict :- Fail, maybe only useful as indictor arms

Project (2) :- Create a linear actuator using these standard metal geared motors 200:1

2cm linear movement / revolution.... total 4cm movement (enough for a finger I was thinking). Design needs a feedback potentiometer, also I will include a pulley like wheel fixed to the bottom of the motors base for facilitating  tendons/fishing line.

Pros :- light, metal, good torque for motor size, 200:1 ratio, 6 Volt

Cons :- my 3D screw was binding in the Nut mech. so needs redesign, will have to be careful with the stabalising support rod fixture (you can see the 3mm hole above the "L" on the keyboard) I will use a stainless steel bike spoke though it and attach this to the base of the motor.

Verdict :- Win! (awaiting redesign)

Project (3) :- Electronic Di2 gear shifters for my road bike...

Pros :- very cost effective, easy to implement.

Cons :- Needs lots of Torque (needs metal gears), needs good fixing mech, needs water proofing.

Verdict :- Win! (test proves its worth perusing , so awaiting redesign maybe with linear actuator)

Project (4) :- Linear actuator lightweight Stepper motor

Using a $1 stepper 28BYJ48, my files can be downloaded here :- http://www.thingiverse.com/thing:308576

Pros :- cheap, small, works, easy 2kg push

Cons :- sooo... slow, requires 4 digital pins (and driver chip ULN2003AN) and one analog pin (if you want feedback of position)

Verdict :- Win
 

Project (5) Linear actuator common Gear motor

This is using an easily obtainable (yellow) Geared motor and cermet feedback potentiometer.

Can be downloaded from here :- http://www.thingiverse.com/thing:317192

Pros :-

   Specs :-
   Stroke :- 45mm
   Speed :- Circa 1cm/sec using a 2 cell Lipo battery
   Force :- Easily 3Kg (my kitchen scale max_ed out so could not measure over 3Kg)

Cons :- requires H-bridge and analog input for feedback positioning

Verdict :- Win