With the incredibly low price of new cordless drills, it’s really not worthwhile repairing a defective drill – especially when they’re usually discarded because the battery pack is defunct. For the price of a new battery pack (or even less), you can buy a complete new drill.

What about in your own garage? Likely as not, you’ve got one or more broken cordless drills tucked away at the back of the workbench. If not, there are lots of cordless drills that can be picked up for nothing. And there are several useful items that can be made from the components inside them!

It just takes a little salvage work to retrieve those bits.

Internals

Nearly all cordless drills look like this inside: (1) chuck, (2) clutch, (3) planetary gearbox, (4) DC motor, (5) speed control, (6) speed control switching transistor and heatsink. Note that in this drill, there is a misalignment between the motor and gearbox, caused by some loose screws. Tightening them subsequently fixed it!

Cordless drill chucks usually have a maximum rotational speed of 1000 rpm or less. However, the motor speed is much higher than this. To reduce the speed of the DC electric motor (and to increase the torque), a planetary gearbox is used. In fact, there are usually two planetary gearsets back-to-back – rather like the gear systems used in automotive automatic transmissions. And like auto transmissions, some cordless drills let you select between ratios – more on this in a moment.

For their size, planetary gears are very strong and, especially when two sets are used, allow high reduction ratios to be achieved in small volumes. Considering their size and torque capacity, these are really nice little gearboxes.

The torque multiplication might be achieved by the gearbox but if you want to quickly drill holes or drive screws, you need real motor power. This is provided by a high-current DC brush motor. Typically, the "motor-stalled" DC current is around 10A at 12V and considering that the motors are about the size of a "D" cell, that’s makes for quite a powerful (and useful) motor – especially since it hasn’t cost anything!