I needed a servo for the robotic finger so that it could go forwards and backwards in a controlled manner to push the switches. Servos normally have limited rotation e.g. 180 degrees but I accidentally ordered a continuous servo which means that the motor spins continuously and instead of controlling the angle of the motor, you control the speed of rotation. This is obviously not OK because I would not be able to move the finger reliably. However I realised that it was well suited to the motor driving the threaded rod.
It was well suited due to its square shape which meant that it was easy to mount, it ran on 6V and I could easily control the speed forwards or backwards. The only problem is that it did not spin fast enough to move the finger at a reasonable speed as servos are typically very heavily geared to increase torque. Nonetheless, the good outweighed the bad and I decided it was worthwhile modifying the servo to increase the speed at the expense of torque.
Originally there were four stages of gear reduction:
Gear 1 -> Gear 2 -> Gear 3 -> Gear 4 -> Gear 5 -> Output
By removing Gear 4 and gluing Gear 3 to Gear 5, the gear reduction became:
Gear 1 -> Gear 2 -> Gear 3 -> Output
This made the servo fast enough to drive the threaded rod.