Below are the instructions to convert a Futaba 3003 standard servo motor to a continuous geared DC motor. The process is fairly similar with other standard servos, but differences will be found between the motors. You may have to account for that, but the general process is pretty much the same.
STEP 1
Remove the servo horn or any linkage that might be connected to it.
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STEP 2
Turn the servo motor over and unscrew the bolts from the backing plate. You don't need to remove the plate at this time.
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STEP 3
Turn the servo motor over again and remove the front plate. Be carefully to remember not to remove too much lubricant from the gears, and not loose any parts from the gear train. This step can easily find you on the floor for hours looking for some pinion that dropped when you pryed open the plate.
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STEP 4
Take a note of the orientation of the gears and remember how they are positioned. You'll need to remember this when you reassemble it.
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STEP 5
Remove the outer gear from the gear train. It is usually the larger gear and it is connected to the potentiometer below on the circuit board. Most notably there is a tab on the gearing that is used to prevent the gear from continious rotation.
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STEP 6
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Using a hobby blade knotch the tab to help remove it. I found that if you use a blade first to help with the cutting, then you have less chance of cracking the gear as you snip off the tab. Once the tab is properly knotched, take a diagonal cutter and snip off the tab. If your hobby blade is sharp enough and you have the patience you can just cut it out with that. Although I found this to be more time consuming, and just used my cutters. Be careful not cut yourself when doing that.
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STEP 7
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Then take a small file and smooth down the tab. Try not to file down any of the gearing. Try to get the outer gear to look as if it never had a tab at all.
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STEP 8
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Turn the outer gear over. You'll notice that there is a knotch for the potentiometer to sit in. You'll need to drill that out. Take a 3/16" drill bit and drill out the knotch. The picture above on the left is the before picture. The picture above on the right is the after picture. If you have a drill press it will probably make things a little easier, but I just reattached the horn to the outer gear and placed it in a vice. This was to insure that the teeth on the gears didn't get messed up. And then just used a regular cordless drill. You'll have to be careful not to drill all the way through. Use a stopper on the drill bit if you're worried about it. The stopper could be anything from a piece of plastic or tape about a 1/4" up from the tip of the bit.
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STEP 9
Replace the modified outer gear on the servo. And reassemble the gear train as it was before. Then replace the front panel on the servo to hold all the gears in place as you move on to the next step.
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STEP 10
Turn the servo over and remove the back plate.
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STEP 11
Take a small flathead screwdriver and pry out the circuit board from the servo housing.
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STEP 12
Desolder the power, ground, and control wires from the circuit board. These are usually colored red, black, and white or brown respectively.
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STEP 13
Completely tear away the control wire, since you won't be needing this. If the wire is attached to the other wires, like in a ribbon cable, just use your diagonal cutters and make a small incision between the control wire and it's neighbor. Once that is done you should be able to easily pull off the remaining wire.
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STEP 14
Solder the red and black wires to the tabs on the DC motor. While the motor will function regardless of which wire you solder to which tab, it'll be important that make a note of it if you plan to modify a number of these for a project. As the different wire orientations will change the direction of the motors. It's kind of a pain to test the modified servo everytime you want to use it just to figure out which way is clockwise rotation and which way is counter clockwise.
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STEP 15
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Take the other ends of the wires you just soldered onto the motor and test motor. Make sure that you are getting full rotation and the gearing isn't snagging on anything. If everything looks and sounds good, then you're pretty much done.
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STEP 16
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Place some heatshrink tubing on the end of the loose wires from the DC motor. Grab a female header (or any kind of header you may want to use) and solder the ends of the wire to the header.
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STEP 17
Push the heatshrink tubing down over the solder joints you maded on the header and wire. And use a heat gun to heat shrink the wires, to help prevent any short circuits.
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STEP 18
Take the circuit board with motor and potentiometer and place it back into the servo housing. And then replace the back plate and screw back in the bolts.
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STEP 19
You now have yourself a modified servo motor. Be aware that you have now voided your warranty, but you probably already knew that.