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?
Internals
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.
Uses
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.
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Car-Powered Drill
Since nearly all these motors will happily work for short periods on 12V
(even when the nominal battery voltage of the drill might be only 9.6V), a
salvaged cordless drill is easily equipped with a long cable and some battery
clips to allow it to be powered by a car battery – an ideal
drilling/cutting/grinding tool for remote travellers or working where there
isn’t mains power available.
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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.
Other Parts
It’s not worth salvaging a cordless drill for these parts (and not all
cordless drills have them anyway) but here are some other bits I usually keep.
From top-left then clockwise: three compression springs, three ball bearings,
two washers, a sealed ball bearing and a needle roller thrust bearing.
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