Since the one on wall chasers seemed to arouse so little comment, I
thought I may as well do another of the missing ones since that one was
so easy ;-)
Impact Drivers this time. Comments welcome as usual:
There are a variety of different tools that get given the title “impact
driver”. The original use of the term applied to a sturdy, hand held,
metal barrel shaped device, that was designed to loosen (or tighten)
stubborn screws, this it achieves by being clobbered with a hammer –
each blow turning the bit holder a small fraction of a rotation. Still a
favourite in mechanics workshops, but not what we are discussing here.
The term is also applies to a range of powered rotary tools that feature
an angular percussive action designed to increase the torque applied to
a fixing by applying high levels of it in discrete blows, rather than as
a continuously applied force. The term covers both electric and
pneumatic tools, with a range of bit coupling interfaces. The larger
units (also often called impact wrenches), typically have a square drive
output shaft designed to connect directly to conventional hex sockets
(you most commonly see these used on service forecourts for removing
wheel nuts on cars). The smaller tools typically have a hex drive
designed to take ordinary screwdriver bits or the type used with
cordless drills and screwdrivers.
For the purposes of this document we are discussing the smaller cordless
electric screw driving tools only.
What does it do?
Drives threaded fastenings basically. In use it initially behaves rather
like a powered screwdriver, however as the resistance to the screw
turning increases, the driver will automatically switch to its impact
mode. This continues to drive the screw (far slower than initially) but
with much greater torque.
These drivers very good at coping with screw driving tasks like the
penetration of metal panels or studwork using self taping (or self
drilling and tapping) screws where you often encounter a point where the
screw requires a massive increase in torque to start cutting the thread.
They are good for moving heavy coach screws (aka lag screws) that
usually require a spanner or socket set to drive fully home. Also any
long or large screw, or screws driven into hard materials without pilot
The nature of the drive also tends to reduce “cam out” (i.e. the
tendency of the bit to slip in the head of the screw, causing damage to
both the head and the bit and making further driving more difficult). As
a result you can often drive screws while needing less pressure applied
to the driver to keep the bit engaged in the screwhead. Coupled with the
fact that many impact drivers are lighter and smaller than many
comparable cordless drill drivers and combi drills, this makes driving
fastening in awkward positions (over head, at arms reach, up ladders
The maximum torque obtainable is also higher than with an equivalent
Even the best impact drivers lack the finesse of a drill/driver. They
usually have no torque limiter, and hence rely on the operator to set
the screw at the desired depth. If you are not paying attention they
will happily drive a screw right through many materials to the point
where it strips all the thread cut so far, snaps the screw, or makes a
hole the size of the screws head right through the material!
They are not so good at small screws that require low torque – partly
because their rotation speed is typically much higher than a drill
driver with a low speed setting on its gear box (the drill driver would
typically have a variable speed in the 0 – 600 or possibly even 0 – 300
rpm range. Impact drivers are usually geared more highly when in
rotation only mode – typically in the 0 – 3000+ rpm range). Hence even
delicate use of the trigger speed controller results in less accurate
speed and torque delivery.
Noise, impact drivers are loud! When operating in impact mode they not
only emit significant noise locally (sounding like a demented
woodpecker), they also send far more shock noise through the fabric of
the building. This may be of particular importance when using them in
inhabited workspaces, or homes.
Bit wear rates can be high – especially if not applying sufficient
pressure to the driver to keep it well seated on the fastening. Allowing
the bit to “rattle” in the screw head can result in useless screwdriver
bit in the space of one longish screw! Even high quality diamond
impregnated bits are not immune.
Is a impact driver a suitable replacement to a drill driver or combi drill?
In the vast majority of cases, no. You would normally purchase one in
addition to a more conventional drill. Not having a chuck makes holding
drill bits more difficult (although you can get add on chucks designed
for hex bit mounts, and there are also a limited range of drill bits
available with hex mounts), and the impact mode of driving is not well
suited to many drilling operations (tending to screw the drill bit into
the work, rather than have it cut a nice round hole and clear the swarf
in the process). They can work well for some drilling operations which
require very high torque and don’t require rapid bit rotation to clear
swarf – e.g. driving auger bits, or an expansive bit.
A significant cost saving can also be had by purchasing the driver “body
only” (i.e. not including a charger or batteries) from the same maker,
and in the same voltage as your main drill driver or combi drill. This
will allow dual use of these potentially expensive items (for example, a
18V Makita kit containing three batteries plus charger and a carry case
can easily cost £230+, whereas the same driver bought body only can
typically be had for £70 - £80
What jobs are they good for?
Driving medium sized screws (typically 8 gauge screws and up), without
the need for a pilot hole in many cases. Turning big screws, or screws
into stiff wall plugs, or through metal studding, Sticking coach screws
in joists, and decking. Freeing seized screws and bolts.
Any jobs they are usable for, but offer no special advantage?
Armed with the correct shrouded bit they can drive dry lining screws for
fixing plasterboard etc, but they usually do this no better than am
ordinary drill and certainly less well than a dedicated autofeed dry
lining driver, and depending on the screw length, may end up making lots
of noise into the process. Without a shrouded bit to pop the driver off
the screw head at the right depth, they are a liability.
 fixing plasterboard to the metal studs as is often used for shop
fitting etc can be significantly easier with an impact driver.
Jobs they are poor at:
Those needing very delicate control, especially driving small and or
light screws and bolts. Drilling (generally), running any of the other
attachments that might typically be used to augment the capabilities of
a drill (sanding, mixing paddles, and wire brush attachments etc).
Not much choice here – most have a single speed gearbox with reversing
action. Some have a small LED light to illuminate the head of the tool,
A belt hook can be useful.
As with any cordless tool, the quality of the batteries and charger
dictates to a large extent the overall performance of it. The best being
streets ahead of the ordinary. Good speed control is vital in this type
of tool since it is the only form of torque limiter you get (it controls
the number of impacts per min in impact mode). Pro tools will have
motors rated for continuous operation as well (the bulk of the tools
currently available typically from the brands and makes oriented toward
the professional user end of the market). Also be prepared to buy top
quality screw driving bits.
Ear protection may be needed in some cases or with prolonged use. Eye
protection is also advisable since the chance of shattering a bit or a
fixing is greater.
Second hand tools
Not commonly available in large numbers on the second hand market as
yet. If buying a used tool allow for the possibility that the batteries
may be nearing replacement time. Check that the charger is supplied.
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