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Salvage It!
BY JULIAN EDGAR
Getting the good bits from cordless drills
– and putting them to work
Cordless drills are now probably the most
frequently discarded power tools. Whether
it’s at the tip, at garage sales or even in
kerbside rubbish pick-ups, there are
always plenty of defective battery-powered
(cordless) drills available.
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
Cordless drill chucks usually have
a maximum rotational speed of 1000
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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!
84 Silicon Chip
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 “motorstalled” 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!
Many cordless drills have an electronic variable speed function, achiev
ed by pulse-width modulating the
power fed to the motor via a switching
transistor. This transistor is mounted
on a separate interior heatsink and
the rest of the control electronics are
integrated into a housing with the
trigger switch.
A reversing switch is also often
mounted directly above the speed control. So even if you salvage just these
parts, you have a handy high-current
electric motor speed control (or a 12V
DC light dimmer, etc).
Finally, most of these drills have an
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A discarded cordless drill can provide a compact and powerful drive assembly
for nothing! The motor/gearbox/clutch/chuck combination can be used to drive
robots, power small winches – or even as a portable drill for use on car power.
Cordless drills use planetary
gearboxes to reduce the chuck speed
and increase torque. Usually two
back-to-back geartrains are used –
this view shows the motor input side.
These gearboxes are compact, have a
high reduction ratio and are strong for
their size.
adjustable slipping clutch that allows
the peak torque to be set before drive
ceases.
Using the parts
There are plenty of uses for these
bits and pieces. 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 supply wires at the motor. Bend a
piece of steel rod into a crank-shaped
handle and lock one end in the chuck.
Turn the handle and – hey presto! –
you have a pretty grunty small DC
electric generator!
How grunty? Well, on one unit I
measured, it was quite easy to run a
half-amp load at 5V – that’s 2.5W! And
2.5W is plenty to run a torch bulb or
two high-efficiency Luxeon LEDs. It’s
One of the easiest uses of the innards of a discarded cordless electric drill is
as a hand-cranked DC generator. In this application, the gearbox steps-up the
rotational speed of the chuck, allowing up to 0.5A at 5V to be generated with
ease – quite good for such a small generator!
also quite enough to charge two 1.2V
rechargeable cells or a mobile phone
battery, etc.
If you pick a drill that has two userselectable 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). Alternatively, you
can slide over the gear selection lever
and have around twice the power output at the same rotational speed – but
it’s much harder to turn the handle.
To protect it and allow it to be easily
held, the generator/gearbox/clutch/
chuck assembly is best squeezed
inside a length of PVC pipe (again
picked up for nothing, this time from
the rubbish pile of a building site). If
it needs to be semi-weatherproof, just
add PVC end caps with appropriate
holes drilled for the crank handle and
power wire exits.
Mechanical drive
The motor/gearbox/clutch/chuck
assembly can also be used wherever a
high-torque output, low-voltage mech
Fig.1: these scope shots show a typical speed control output for low PWM (left) and high PWM (right) duty cycle settings.
siliconchip.com.au
April 2006 85
Fig.2: here’s how to adapt the control unit to allow low-voltage light dimming or speed control of DC motors. The
threaded bolt goes through a captive nut and is turned to smoothly deflect the speed control trigger. Above right is a
typical pulse-width modulated (PWM) control unit, complete with its heatsink-mounted switching transistor.
anical drive is needed. For example,
two of these assemblies can easily
be combined to form the individual
wheel traction motors for a small robot
(or you can use four for the ultimate in
manoeuvrability!). Another possibility is to use one of these assemblies
to drive a small winch – eg, to hoist a
model railway baseboard up near the
ceiling when it isn’t being used.
In these applications, the inbuilt
slipping clutch is a real asset, as it
stops the motor from being overloaded
when the output is stalled.
And 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),
the salvaged cordless drill is easily
equipped with a long cable with some
battery clips to allow it to be powered
by a car battery.
Variable speed controller
The variable speed controller is a
mixed blessing. While it is capable of
handling high currents (very high for
short periods!), the physical layout of
the module lends itself only to those
applications where a squeeze or push
trigger is needed. Unless you have
lots of spare units to play with, don’t
pull the module apart in an attempt to
substitute a rotary potentiometer for
the slide type – once it’s apart, it can be
very hard to put back together.
Rat It Before You Chuck It!
Whenever you throw away an old TV (or
VCR or washing machine or dishwasher
or printer) do you always think that surely
there must be some good salvageable
components inside? Well, this column is
for you! (And it’s also for people without a
lot of dough.) Each month we’ll use bits
and pieces sourced from discards, sometimes in mini-projects and other times as
an ideas smorgasbord.
And you can contribute as well. If you
have a use for specific parts which can
86 Silicon Chip
easily be salvaged from goods commonly
being thrown away, we’d love to hear from
you. Perhaps you use the pressure switch
from a washing machine to control a pump.
Or maybe you have a use for the highquality bearings from VCR heads. Or
perhaps you’ve found how the guts of a
cassette player can be easily turned into
a metal detector. (Well, we made the last
one up but you get the idea . . .)
If you have some practical ideas, write
in and tell us!
A better approach is to build a
mechanical system that can vary and
maintain the trigger movement needed
in the application. For example, by
using a coarse-threaded bolt and a
fixed nut, the original trigger can be
progressively moved by rotating the
bolt – see Fig.2. The unit can then be
used wherever low-voltage DC motor speed control (eg, for a miniature
12V lathe) or filament light dimming
is required.
Finally, the electric motor itself is
ideal for driving a fan. Small fan blade
assemblies can be salvaged for nothing
from microwave ovens.
Be careful with microwave ovens
though – they can pack a lethal
punch, even with the power switched
off. Make absolutely certain that all
high-voltage capacitors inside the
oven have been discharged before
attempting to salvage any parts. Don’t
think of even opening up a microwave
oven if you don’t know what you are
doing.
Conclusion
When you see a cordless drill, salvage it and strip it back to the internals.
The resulting bits take up very little
storage room and can be used to make
a hand-cranked generator or as a powerful low-voltage motor/gearbox unit
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with variable speed control.
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