Choosing
an electric drill
An electric drill must be the most common tool owned and used by the diy person
and is probably the first electric tool purchased by the new diy'er. Modern electric
drills have a variety of 'bells and whistles' which can be confusing to anyone
looking to buy one.
The main 'bells and whistles' are:
Corded or cordless
Corded
electric drills have a dangling lead (with plug attached) which needs to be plugged
into an electrical supply. This does limit the distance you can work from an electric
socket (although a suitable extension can be used) but where you are working in
range of a socket, it's just a case of 'plug it in and start drilling').
Corded drills tend to be more powerful and have higher speeds
than cordless electric drills. Except for the smallest ones, they do tend to be
too bulky to use as simple screw drivers.
Cordless
electric drills have made great advances in recent years and can have similar
performance to most corded drills. Their great advantage is that they can be used
almost anywhere until the battery pack is exhausted. Even then, most models have
replaceable battery packs and having a spare, fully charged, battery pack will
enable the electric drill to come back to life.
Power
With corded electric drills, power is normally quoted as wattage in the UK
(horsepower in the USA), the higher the wattage, the more powerful. Most diy models
are in the range 500 to 1000 W. The wattage should normally reflect the robustness
of the whole drill, a drill with higher should have more robust bearings and gearbox
etc.
A low wattage drill used hard will be working at maximum power all the time
working its bearings etc. nearer to their limits - while a higher wattage tool
will work less hard to do the same job, with a consequent reduction in wear and
tear.
In simple terms, the more you intend to use a drill, the higher the input wattage
should be.
Cordless drills are generally rated by voltage, the higher the figure, the
more powerful. Most diy models have battery pack from 9.6 to 16 volts, while some
with upto 32 volts are aimed at the professional user. It is a distinct advantage
to have at least one replaceable battery pack.
Different types of battery are used on different models, the two common types
being:
- Nickel Metal Hydride (Ni-Mh)
- Nickel Cadmium(Ni-Cd)
Speed
The ideal speed of a drill bit depends upon the size of the bit and the material
being drilled. Larger bits need to turn slower than smaller bits otherwise they
may overheat and lose their cutting edge. They can also put a lot of strain on
the motor as it usually runs fast to compensate for its lack of power.
Single speed drills (2000/2500rpm) are available but lack versatility.
One step up from single speed drills are those offering fixed speeds, generally
two, which are selected by the operator, the slower speed being about 500rpm less
than the fast speed. The switch between speeds may be electronically or mechanically
(by changing the gearbox ratio).
One step up again offers variable speed control where the speed is infinitely
variable upto a pre-set limit. The speed is usually controlled by a trigger, the
further it is pulled back, the faster the speed of the drill. Some electric drills
have an adjustable trigger stop which can be set so that the drill is limited
to a speed suitable for the job in hand. Without this pre-set stop, it is very
easy to unconsciously over drive the drill while concentrating on drilling
Most variable drills provide a high torque, slow-start; this enables a hole
to be started carefully with reduced risk of the bit slipping and damaging the
workpiece. Long periods of use at low speeds should be interspersed by bursts
of high-speed running in order to keep the motor cool.
A slow-speed also allows the use of a screwdriver bit in the chuck to drive
screws.
Some variable speeds drills have two (or more) speeds ranges (such as 0 to
1100 and 0 to 3000 rpm). A separate switch on the body of the drill switches between
ranges.
Generally corded drills offer higher maximum drilling speeds than cordless
ones.
Chuck size and type
Chuck size determines the size of shaft which can be driven by the drill, with
HSS drill bits, the shaft size is usually the same as the diameter of the bit;
with hole cutters, the shaft is generally a lot, lot smaller.
Normally the larger the chuck size, the more powerful motor is fitted, this
generally won't have an effect except when larger tools are fitted.
The common chuck sizes are 3/8 and 1/2 inch (10 and 13 mm).
- 3/8 inch (10mm) - light duty, suitable for most 'every day' tasks, will accept
most accessories with shafts upto 3/8 inch;
- 1/2 inch (13mm) - heavy duty, will accept bits upto 1/2 inch shaft
Traditional drill chucks need a key (chuck wrench) to open and close their jaws
to fit/remove the drill bits or attachments. With the keyless chuck, drill bits
and other attachments can be fitted/removed with the flick of a wrist. Keyless
chucks make like easy, you won't be forever searching for the lost key.
SDS chucks require special drill bits best avoided by the diy'er.
Hammer action
The hammer action function moves the drill chuck rapidly in and out, causing
the drill bit to hammer its way into bricks and masonry. On most drills the hammer
action is created by a ratchet mechanism, which makes the chuck jump up and down
as it rotates. This type of action requires pressure to be applied by the user
- the harder the material, the more pressure - and that can be tough on the drill
bearings after prolonged use.
Most models with hammer action have a switch to select/deselect the hammer
action, this adds to the versatility of the drill.
When using a hammer action, a drill bit specially produced to take the punishment
of the hammering at a few thousand revs per minute must be fitted.
Handles
Most drills have a pistol grip with the speed control trigger built in to it.
Heavier drill also need a second handle just behind the chuck so that the drill
can be held steady when in use. Ideally it should be possible to change the position
of the second handle so that it is comfortable to hold (left handed ?? check the
position of the second handle).
Some drills are designed to be held with the hand around the main body of the
drill, thumb one side, fingers the other; with the third and little fingers operating
the trigger. The advantage of this hold is that the forearm is in line with the
drill bit, allowing more pressure to be put behind the bit, and it's easier to
keep the tool steady. Such drills have a curved recess in the body, but some are
more comfortable than others. User with small hands will find it awkward to hold
some drills and operate the triggers.
Always pick up a drill and try it in your hands for comfort before buying -
are the trigger and other controls easy to operate?
Depth stop
Some electric drills incorporate a drill depth stop which prevents the drill
bit from going in too far. Such depth stops are often useful but it helps if it
can be removed from the drill when not required.
Screwdriver function
For an electric drill to action as a screwdriver, it needs to have variable
speed plus two functions - torque control and a reverse drive.
When using a drill as a screwdriver, a fairly low speed is necessary so it
is essential that the drill has a variable speed control.
Torque controls are normally found on drills with a screw-driving function,
torque control shuts off the drill when a certain turning force is reached. The
size of screw together with the material accepting the screw will influence the
torque required for a particular job. Normally the adjustment is by a numbered
dial - often upto 16 positions. This all helps ensure that screws are not driven
in too far; or over tightened.
So that an electric drill can be used as a screwdriver it needs a reverse facility
to remove screws. Selection of the reverse drive is normally by a switch selector.
