Today I replaced the two electric screwdriver motors on my robot with a pair of electric window motors, recovered from a Jeep at a local scrap car place. These are better than the screwdriver motors because:

• They are a mirror image of each other, so suit a differential drive perfectly. The screwdriver motors had to be mounted in opposite directions, using up more space
• They use a worm gear, run slower than the electric screwdriver motors, and have higher torque
• They have a proper shaft coming out which I was able to couple with my axles using standard shaft couplers. I was never able to remove the drill chucks on the other motors, so had to use the chucks to couple with axles, which inevitably led to some slippage
• They are a lot quieter than the drill motors, presumably because they are better quality motors and use a worm gear instead of what I'm guessing is maybe a dozen conventional gears
• The window motors have proper mounting holes, making mounting onto the robot chassis fairly easy. The screwdriver motors were an extremely awkward shape, with no mounting holes or anything, and the way I was finally able to mount them was neither elegant nor especially robust

Also, previously I had my optical quadrature encoders mounted directly onto the shafts from the motors, and I was never happy with this arrangement. I have now coupled the encoders using timing belts and pulleys, which is much more satisfactory.

I still have a small about of backlash between the wheels and axles. This is due to the fact that the wheels aren't really ideal for driving from a drive shaft (they were intended as free-wheeling wheels). If anybody knows where I can get a pair of wheels of 4-6" diameter with a good solid coupling to 1/4" shafts (set screw or pin or whatever), please let me know at neil@octrix.com.