Here is how I converted an rc servo for continuos motion. I will use two of these servos as the main drive train for an upcoming little robot.
RC servos can be regarded as two main types, analog servos and digital servos, the ones modified in this instructable are of the analog type, I don't know if the process applies to digital ones too. The working principle of these servos is based on a variable resistor that senses the angular position of the main gear and tells to the control circuit when to stop to power the servo motor. The main mod we will do is to break the link between the resistor and the main gear so that when the servo receives a signal to rotate in some direction it will never stopto rotate until a new middle position signal will be received. The way the resistor and the gear are linked can vary from one servo to another so you should be creative based on your particular servo, in some case there could be a shaft that must be shortened or cut away.
The servo I will describe in this tutorial is a cheap 25g one, it has plastic gear train and dual ball bearing, I think it is the reasonable size for my next project, smaller ones can be tricky to mod and have less power for your needs.
Step 1: Open the Servo
Most servos can be opend by removing 4 long screws from their base, they hold both the lower cover and the upper gearbox, in most cases we wont mod the control circuit so it's better we don't move a lot the little wires on bottom side.
Here is how appears the inside of the upper gearbox, you can see that some gears are still on the servo base and some others went with the gear box, you should take note of every gear position if you want to put them all together and have a fully working servo.
You can also see one of the bearings into the gearbox and the main gear on the left on top of the base.
Step 2: Unlink the Variable Resistor
At this point you can carefully take away the main gear, in some cases you can help with a flat screwdriver.
With great surprise I found that there is no shaft linking the gear and the resistor but only that little white plastic stuff, don't know how to name it... The resistor mod is just a matter of removing it. After that you can see the lower bearing.
You should also set the resistor for its middle position, this will be the position the servo will not rotate. in my case I could simply align the resistor handle along the servo body, see the white line. Someone removes the variable resistor for two fixed resistor but I don't want to do any mod to the electronics, I will leave with a little possible error and than correct it trimming the middle position from the software.
Step 3: Check the Gear
No one gives you something for nothing, I found a bigger problem on the gear. Since these servos are made to rotate at most for a fraction of turn, they can have some way to prevent them from turning more than needed. You can see a little teeth that will stop on a corresponding on the case. But and in this case there are also some teeth missing or more in detail it misses the space between two teeth!!
Depending on every particular servo you should be really creative in modding gears like these.
I used a thin cut-off wheel with my dremel and a magnifying glass to shape the space between the missing teeth.
This operation is a little delicate but not impossible to do, the shape should allow the corresponding teeth from the pinion to work correctly but fortunately some degree of error should be forgiven. The servo could become a little noisy but should work fine.
At the end I cut out the limit teeth on the upper side, this last one is a more easy operation.
In the last photo you can see the final shape of the main gear.