The Locraker is a computer-controlled device that can crack a three digit rotary combination lock in about half an hour. Since the device totally encloses the lock, it is only possible to crack locks that aren't attached to something (ie. locks whose combinations have been forgotten). It was originally constructed as a project for a high school computer course.
The picture above shows a lock [bottom, left] under the stepper motor [top, left]. The stepper motor uses a pair of pinchers to grab the lock's knob. In this way the knob may be accurately rotated under the control of a computer. Two heavy-duty solenoids [right] attached to the clasp can attempt to open the lock after a combination has been dialled in. The superstructure is made of Meccano. The picture below shows the interface circuitry that connects the Locraker to a computer's parallel port.
Its strategy is essentially the brute-force approach of trying all combinations until the lock opens; however it uses several short cuts. A standard combination lock does not have 60 possible divisions (as its dial might suggest), but rather more like 15. To be thorough, the Locraker assumes 20 divisions. Thus the number of possible combinations is 20*20*20 or 8000. Another shortcut is that it doesn't have to dial in all three numbers separately for each try. Instead, it can dial in the first two numbers, then try all the possible third numbers in one pass by continually jerking the clasp as it spins the dial around once. The lock doesn't know it is being repeatedly polled. This reduces the number of passes to 20*20 or 400. Finally, the average lock will open after half the combinations have been tried, so the expected number of passes would be 200. At six and a half passes per minute, it can open a lock in about half an hour.
For more information, see the Locraker's plans and schematics.
The brute-force strategy isn't the only option. With a little bit of skill one doesn't need a complicated robot to open a combination padlock. Nathan Hillson describes a more sophisticated approach.