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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 ➏ ➍ ➌ ➋ ➊ ➎ 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 siliconchip.com.au 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 SC with variable speed control. siliconchip.com.au