The Motors and gearboxes from cordless drill have long provided 12lb and even 30lb robots with a cheap drivetrain. Originally these were mainly Handiwork 4.8v drills
from Walmart but that source has almost dried up. The Harbor Freight 7.2v drill looked like a possible replacement, but it too, is no longer available.
Harbor Freight continue to make available a series of drills. There were 9.6v and 12v versions but now only the 14.4v and 18v versions are readily available.
These are regularly on offer at $15 - $20 each.
Click on one of the options on the menu on the left to see how to strip down and modify one of these drills. Ready-to-go Gearmotors derived from some of these drills are
available in the Kitbots section of my site.
I wrote an article for Servo magazine on how to strip down and convert a cordless drill motor page 1 page 2
In 2009 I wrote a further article describing a similar process for a larger Speed 700 24v drill.
In 2011 I wrote about the search for a new source for cheap cordless drills PDF