16 Feb 2012
An electric drill is an essential part of any toolbox. However, today there is such a wide variety to choose from, that it is often difficult to decide which one is most suitable for your needs.
When choosing and buying a drill, there are a number of important points to bear in mind. The drill must be safe and feel comfortable to use. It must be adaptable and capable of drilling through most materials around the home, including wood, masonry and mild steel.
If it is to be used to its fullest potential, it should also be capable of being fitted with a range of basic attachments, so that it can tackle a variety of jobs.
How an electric drill works
Cordless drills are powered by batteries, which can be a very convenient feature, as you can take and use the drill virtually anywhere. If you opt for a cordless, battery-powered drill, however, be sure to find out how long the battery lasts for, how powerful the drill is as they are often not as powerful as electrical drills, and also be aware that the drill itself will be substantially heavier to handle due to you having the carry around the power supply.
The majority of drills, however, are worked from the mains supply, giving them greater adaptability and range of speed.
The workings of a drill are pretty simple – at its centre, is a high-speed electric motor that drives a spindle through a set of gears designed to increase or reduce the speed at which the spindle turns. Screwed to the front end of the spindle is a chuck with a number of fingers that can be tightened to hold drill bits and other attachments.
The motor and gearbox are housed in a gun-shaped casing, normally made of high-impact plastic, which has a fat handle grip under or behind it. Mounted halfway down the handle, is a trigger that switches the motor on and off, and controls the speed at which the drill turns.
Power and efficiency
The power of any drill is all-important and should be one of the first things you check. This determines whether the drill can be expected to tackle tough materials and also how efficiently and quickly it can do the job.
Most manufacturers give the electrical input that the drill should be able to take without overheating – but often fail to quote its output. Try to find this out before making a purchase and if possible, try the drill out on a piece of tough masonry or brickwork.
Run the motor at a variety of speeds and if it shows signs of overheating or it stalls, especially at low speeds, take this as a sign that the drill output is too low for general domestic use.
The speed of a basic electric drill is controlled by the amount of pressure on the trigger mechanism. It is up to you to adjust the speed of the motor by increasing or lessening your hold on the trigger according to the type of material being drilled.
It is often difficult to do this accurately by “feel” and you may wish to buy a two-speed or variable power drill. These allow you to select one of a number of pre-set speeds suited to the material you are drilling.
Two-speed drill: Many drills are fitted with a lever or knob that allows you to pre-select a high or low speed setting; some heavy-duty models have as many as four separate settings. Follow the manufacturer’s instructions if you are in any doubt about which speed to select for a given material or task. When choosing a two-speed drill, check first that the speed changes are achieved by adjusting the gearing rather than by some electronic means. If not, the motor is simply speeded up or slowed down without the necessary changes in turning force, or torque, and this is unsatisfactory.
Variable speed drill: This incorporates a mechanism, within the body of the drill or connected to the power line feeding it, which allows the drill speed to be adjusted rather like a dimmer switch in a lighting circuit. Although this device does not change the gearing ratio in any way, to give greater torque, it is very useful in situations where you need to adjust the speed of the drill frequently. For instance, you may need to select a very low speed when you are starting to drill very brittle or slippery surfaces. Some drills also have a reverse facility, which can be very useful for removing screws or extracting the drill bit from hard materials, such as masonry and sheet metal for example.
Hammer drills: Most basic electric drills can cope with tough plaster or brickwork when fitted with a suitable masonry bit. But harder materials, such as concrete or stone for example, need a special hammer drill. As well as having a purely rotary action, these deliver rapid blows to the chuck and bit. This greatly speeds up the drilling process when used in conjunction with special hammer bits. If you need to drill hard materials regularly, it is well worth buying a hammer drill of your own. But for occasional use, it is better to hire one, preferably the heavy-duty industrial variety, which can cope with a far greater range of materials than almost any of the basic drills on the market. As an alternative, some drill manufacturers include a hammer attachment in their range of basic drill accessories and this may be another point you need to consider.
An important item to check when buying any drill is its chuck size. This gives a rough guide to the largest hole that can be made.
Most electric drills are fitted with either a 10mm or 13mm chuck. Tightened fully, this will hold a 1,5mm bit allowing you to cope with very fine work. The largest bit you can use varies according to the type of material you are drilling – in mild steel roughly the same size as the chuck; in hard masonry around one and a half times its size; and in soft materials about twice the chuck size.
All electric drills have a key that is used to adjust the chuck. This has a serrated end, which meshes with the teeth on the chuck, allowing it to be opened and closed. When deciding which drill to buy, check that the key engages smoothly with the teeth on the chuck and that it is comfortable to hold and use. This prevents long-term damage to the chuck, which is expensive to replace. Special rubber straps are available to hold the key on the drill lead and prevent it being lost. – Antonella Desi
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