This story’s for those who like getting something for nothing. Or, who hate to see the massive waste of useful things being thrown away.
Cordless drills are now among the most frequently discarded of power hand tools. At the tip, at garage sales - even in kerbside rubbish pick-ups – there are always plenty of defective battery-powered electric drills.
So why would you bother salvaging one of these? There are about five good reasons. But first, what’s inside a cordless drill?
Cordless drills are usually slow rotating drills with a maximum speed of 1000 rpm or even less. To reduce the speed of the DC electric motor – and to increase the torque – a planetary gearbox is used. In fact, most often there are 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).
The torque multiplication might be achieved by a cute little gearbox but if you want to be able to quickly drill holes – or screw screws – you need power. It’s provided by a pretty high current DC brushed motor. Typical motor-stalled DC current draws are around 10A at 12V.
Many cordless drills have an electronic variable speed function, achieved by pulse-width modulating the power feed to the motor. The switching transistor is mounted on a separate interior heatsink and the rest of the control electronics are integrated with the trigger switch. A reversing switch is often included.
Most of these drills have an adjustable slipping clutch that allows the peak torque to be set before drive ceases. A keyless chuck is usually fitted.
There are plenty of uses for these bits and pieces.
1. Hand-Cranked Generator
One of the easiest is to simply pull the body of the drill apart (because they are low voltage devices, tamper-proof screws aren’t fitted, making it really easy) and cut the wires at the motor. Bend a piece of steel rod into a crank-shaped handle and lock one end in the chuck. Connect the load to the wires and turn the handle and - hey presto! – you have a pretty grunty small DC electric generator!
How grunty, then? Well, on one unit I measured it was quite easy to run a half-amp load at 5 volts – that’s 2.5 watts! And two-and-a-half watts is plenty to run a torch bulb, or two high efficiency Luxeon LEDs. It’s also easily enough to charge two 1.2V rechargeable cells or recharge a mobile phone or the like.
If you pick a drill that has two user-selectable gear ratios, it works even better. In one ratio, turning the handle is easy but the amount of power generated is lower. That’s the ‘topping up’ setting, if you like. Or you can slide over the gear selection lever and have around twice the power output at the same rotational speed – but of course it’s much harder to turn the handle.
2. Planetary Gearbox
For their size, the planetary gears used in cordless drills 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.
Uses? If you teach automotive mechanics or any type of engineering, these gearboxes are a perfect demonstration teaching tool to show how planetary gearboxes work – something lots of people find difficult to get their heads around.
If you are into model robots or any miniaturised device needing a gearbox, the typical 30:1 reduction ratio found in these gearboxes allows either a lot of torque multiplication, or speed multiplication, in a very small space.
3. Motor and Gearbox
The motor/gearbox/clutch/chuck assembly can also be used wherever a high torque output, low voltage mechanical drive is needed.
For example, two of the assemblies can easily be combined to form the individual wheel traction motors for a small robot (or use four for the ultimate in manoeuvrability!) or an assembly can be used as a small winch, eg to hoist something up to near the roof of your shed for storage. In these applications, the built-in slipping clutch is a real asset as it stops the motor being overloaded when the output is stalled.
Adding an axle and wheel (or a winch drum) is easy - you just lock the shaft in the chuck.
4. Variable Speed Control
The electronic variable speed controller is a mixed blessing. While it is capable of handling high currents (for short periods, very high!) the physical layout of the module lends itself to applications only where a squeeze or push trigger is needed.
Unless you have lots of spare ones to play with, don’t pull apart the module to adapt a rotary potentiometer to replace the slide type or to make some other modification as, once apart, they’re very hard to get back together.
A better approach is to build a mechanical system that gives the progression of trigger movement needed in the application. For example, by using a coarse-thread screw and fixed nut mounted on a bracket, the original trigger can be progressively and finely moved by rotating the screw.
The speed controller can be used wherever low voltage DC motor speed control or filament light dimming is needed - one application is in motor speed control of miniature 12V lathes. You could also use it to control the speed of 12V fans and pumps.
One unit I pulled apart was rated at 10 amps at 12V – 120 watts!
5. Handheld Chuck
Finally, there’s the ability to use the chuck as a handheld drill. For my (lack of) money, for this use alone it’s worth collecting any discarded cordless drills you find.
After the drill has been disassembled and the chuck removed, the drill-bit goes into the chuck in the normal way. You then use it as an easily-held drill. It’s ideal for plastics, de-burring holes in metal (by using a larger drill-bit) and even where really fine control is needed.
Because most of these keyless chucks have large, knurled collars, it’s very quick and easy to insert and remove drill-bits.