Drilling science and technology
by Chris Woodford. Last updated: December 30, 2016.
Imagine spending your entire life worrying about nothing. If drilling is your
business, that's exactly how you pass your time—figuring out how to
spin wood, metal, rock, or some other obstinate material into nothing
but a hollow shaft of empty space. You might think drilling sounds
like an immensely dull subject, but it's a scientific and
technological challenge that's been taxing people since the first
drills were invented around 4000BCE. It's become vastly more
important in the last 100 years or so for, for if there were no drilling,
we'd have no petroleum—and no cars or trucks—and the modern world
would look much more like the horse-drawn buggy era of the 19th
century. Without drills, how would you cure a toothache, build
yourself a home, or extract long "cores" of ice to confirm that
climate change really is happening? This stuff is actually
pretty important... so let's take a closer look!
Photo: Drilling with a coring rig on a coffee plantation in Guatemala.
Photo by Sue J. Goff courtesy of Los Alamos National Laboratory Publications and US Department of Energy.
What is a drill?
Photo: Precision drilling to bore out the center of a pump shaft. The "engine" of this drill is a powerful electric motor. Photo by Kilho Park courtesy of
Easy! A drill is a machine that makes holes in something —but is it
as simple as it sounds? It's a machine in two senses of the word. The
kind of drill people use for household DIY has an electric motor, so
it's a machine in the sense of being a useful, everyday, electrical
appliance. But it's also a simple machine in the strict
scientific sense, where a machine can be literally anything from a
can opener to a monkey wrench—anything, in other words, that helps a person to
multiply the force they can produce with their own muscle power.
We'll come back to the science of drills shortly, but let's consider for a
moment the vast range of machines buzzing, whirring,
and—yes—drilling away on Earth as we speak. Drills range in size
from gigantic oil rigs towering hundreds of feet above the sea to
tiny handheld machines that blast decayed material from your teeth.
Even so, virtually all of them have three key parts:
Every drill needs some sort of a power source, which we'll call the engine.
In a brace-and-bit (a simple woodworking drill),
the engine is an off-centered crank that you turn by hand to make the
drill rotate using the muscles in your arm. A modern household DIY drill is usually
driven by a powerful electric motor with a gearbox that can increase
or decrease its speed. At the opposite end of the scale, outdoor
drilling rigs are powered by giant diesel engines or pneumatic
compressors (machines that drive a drill using the force of
compressed air blowing down a pipe), which are themselves typically powered by diesel engines.
Photo: A typical steel "twist" bit from a hand drill. Note the sharpened end and the auger screw along the length of the bit to carry waste material up and away from the cutting edge.
Apart from the power source, the other key part is the drill bit,
the rotating, rod-shaped tool that cuts or bores into the material
you're drilling. In something like a DIY electric drill or a bench
drill in a workshop, the drill bit is connected to the engine (motorized part)
of the drill by a vise-like clamp called a chuck; you tighten
or loosen the chuck's jaws to change one bit for another, either to make
different sized holes or when the bit has worn out through use.
Most drill bits have two features: a sharpened or tapered end that cuts or
scrapes the wood, metal, or other material out of the way and a spiral screw thread (called an auger) that lifts the material
up the hole and out of the way. Sometimes drill bits are coated with
super-hard materials such as tungsten carbide (a compound of tungsten metal and carbon) to stop them wearing away quickly; often, though, the
drill bit and shaft are made from a single piece of very hard steel
capable of withstanding high temperatures—typically high-speed steel (HSS) or carbon steel.
The temperature resistance is really important because (for reasons explained in the box below) drilling generates a huge amount of heat, which can damage the material being drilled as well as the drill itself. That's why drill bits are often designed to allow cooling fluid (sometimes plain old water) to be piped to the cutting
edge, which has an important side benefit: as the cooling fluid arrives, it
connects with the waste material being drilled and helps to carry it away and clear of the bit, easing the bit's onward passage.
It's sometimes called a cutting fluid.
Okay, so this isn't exactly a part of the drill! Even so, the hole—the empty, end-product of
drilling—is the most important part of all. Unless you're making a
very rough-and-ready hole (say, drilling quickly through the center
of a new wooden front door so you can push a saw blade through and
cut yourself a letterbox), the size of the hole you're drilling is
often critically important. It might have to admit, very snugly, a
precisely machined bolt, screw, or other part so it will have to be
exactly the right diameter (neither too big nor too small);
that's why drills have interchangeable bits that come in a huge range
of different and very precise widths. Less obviously, most holes have
to be absolutely straight and perpendicular (at right angles) to the
material you're drilling into. That's partly why metal drills in
workshops are mounted on little towers called presses and operated by
a hand lever.
Photo: Are holes really so important? Yes! Tunnels are examples of large-sized
holes made by machines like this, called tunnel boring machines (TBMs or "moles"),
which are essentially horizontal drills driven along tracks. The cutting head is on the right hand side (furthest
away from us in this picture). Photo courtesy of US Department of Energy.
Often drilling a hole is only the first stage in a process.
Sometimes the hole has to be enlarged with other tools (a process
known as boring) or finished off in various ways. When you
drill a hole through metal, for example, burrs (sharpened shards of
metal) are pushed through the hole and emerge at the exit hole, and
they have to be removed with a file. If you're drilling a horizontal
hole underground for a subway tunnel, you generally have to reinforce
it with concrete to stop it from collapsing. The same applies to
vertical shafts drilled through the ground into oil wells, which are
immediately reinforced with steel pipes and cement. Exactly the same
can apply to simple holes you drill in your walls at home. If you're
making a hole in a plaster wall to support something like a bookshelf
or a heavy mirror, you typically have to line the hole with a plastic plug so it holds a screw snugly and securely.
The science of drilling
What's science got to do with making holes? Quite a lot, actually. Here are some of the
more obvious ways that science comes into drilling. Note how I'm applying general scientific principles to understanding a very ordinary, everyday aspect of the world—that's the essence of science!
Photo: Crystals of quartz are surprisingly hard—harder than many steels, in fact. But they'd be no use if you wanted to drill something harder than quartz. Photo by courtesy of US Geological Survey.
Materials science is a crucial aspect of drilling science: it's no good trying to
drill with a bit made from cheese (unless of course you want to drill something even softer, like butter)! Scientists rank the hardness of different materials from 1–10 on the Mohs harness scale, with the softest ones at the top:
Any material is harder than (and can scratch) another material with a lower Mohs hardness number.
It comes as no real surprise that drill bits are made from materials high up the Mohs scale. Diamond
(the industrial kind, not the ones you wear on your fingers) is the hardest material, right at the top of the scale.
Tungsten carbide, another popular drilling material, has a Mohs hardness of 8–9 (compared to ordinary
tungsten, which is about 7–8), whereas high-speed and carbon steel used in everyday DIY drill bits comes in at about 6–7. By comparison, fingernails rank about 2–3 on this scale (maybe slightly less when you've just crawled out of the bathtub), which is why you'll often break your nail if you do something silly like trying to undo a screw (Mohs 6–7) with your fingers.
It takes the same amount of energy to make, say, a 5cm (2inch) deep screw hole in a plaster wall, no matter how
you do it: in scientific terms, you have to "do work" on the wall by applying a force for a distance of
5cm. You can drill a hole like this more quickly with an electric drill than by hand because the electric drill supplies energy to the wall at a faster rate—it's more powerful, in other words. If you use a drill with a higher power rating (with, say, an 800-watt electric motor) it will make the same hole more
quickly than a drill with a lower power rating (maybe a 650-watt motor): the hole takes the same
amount of energy to create, but the powerful drill supplies that energy at a faster rate—in less time.
The work you do in drilling is converted mostly into heat and lost forever. Notice how hot your drill and drill bit
have got when you've finished? That's a measure of how much energy you've had to expend to drill your hole (and
some—because your drill isn't 100 percent efficient).
Photo: Conservation of energy: When you use a cordless drill like this, energy flows from the rechargeable battery pack on the base of the handle into the electric motor inside the case, the spinning drill bit, and the wall itself, heating all of them up in the process. Your body supplies extra energy by pushing the drill against the wall. When the drilling's done, the electrical energy lost from the battery pack (and your body) is equal to the heat energy gained by these other things, plus the sound energy wasted in making that awful whiny, screeching noise! Photo by Dennis Schroeder, courtesy of US DOE/NREL (Department of Energy/National
Renewable Energy Laboratory).
Think about drilling a hole into your wall with an electric
drill. If you've ever tried doing this, you'll know that it's quite
hard work, even with a powerful electric motor helping you out.
Pushing a drill into a wall is a bit like pushing a thumbtack
(drawing pin) into a noticeboard, with everything scaled up in size.
The fact that the drill body is wider than the drill bit means the
pushing force you apply is concentrated into a much smaller area, so
it bites into the wall with greater pressure. In other words, it's easier to push the
drill into the wall than if the bit were wider than the drill.
Photo: Science of pressure: Drilling into a wall is like pushing in a thumbtack—scaled up in size!
Before power steering came along, trucks and buses had enormous
steering wheels to make them easier to turn round corners. As we explain in
our main article about how wheels work,
a wheel mounted on an axle is another kind of simple machine.
If you turn the rim of a large wheel, the axle at the center turns more slowly but with
much more force. In other words, a wheel acts like a lever
and the bigger its diameter, the more leverage it provides. The same
thing happens with a drill. If it has a large chuck that rotates a
tiny bit at its center, you get leverage, and the bit powers into the
wall with more force than if the chuck and the bit were exactly the
Photo: Turning the edge of a wheel (red arrow) produces a bigger force at the center (blue arrow).
What do you do when your hands are freezing? Rub them
together—because rubbing things converts the kinetic energy (the
energy of movement) into heat energy that warms you up. When you're
drilling, friction and heat become a problem. Using a simple
hand drill for just a couple of minutes can make the drill bit too
hot to touch—don't try, because you might burn yourself. Using an
electric drill that spins many times faster will generate much more
heat. Generally speaking, the faster the drill turns and the longer
it operates, the more heat it generates. Where does the heat come
from? It's back to energy again. An electric drill converts electricity into mechanical energy
(in the rotating drill bit) using an electric motor. As the drill bit
bites into the material you're drilling, the mechanical energy is
converted into heat energy. Unless you get rid of the heat somehow
(by pumping in a cooling fluid or pausing your drilling every so
often to let things cool down), it builds up—and there's a risk you
might melt or burn the material you're drilling or damage the drill
Most of the drilling we've considered so far has been the kind of stuff you
might do at home as part of a DIY project: small-time drilling, in
other words. But what about big-time drilling—getting oil from the
ground and so on? Broadly speaking, there are two kinds of
industrial-strength, outdoor drilling called rotary drilling and
Photo: A new section of drill string is winched into place on a typical rotary drilling rig. Photo courtesy of Craig Miller Productions and US Department of Energy.
Rotary drilling is what you'll have seen people doing on oil rigs. They make
a hole in the ground and gradually push a very long drill further and
further down. Unlike a household DIY drill, the drill bit used in
rotary drilling isn't a fixed length. To go deeper and deeper, the
drillers have to keep stopping and adding new sections of drill bit
(which look like pipes) on to the top as they go, making the bit into
an ever-lengthening piece of metal (sometimes kilometers/miles long) called a
drill string. The string is turned by a kind of spinning
collar called a rotary table, powered by a diesel engine, and
typically spinning at over 100rpm (roughly how fast a clothes washer goes
when it's spinning very slowly just before a really fast spin). At speeds like this, and great
distances underground, heat buildup is a major problem so cooling/cutting
fluid called drilling mud is constantly pumped down into the
hole (and used to carry the waste, known as cuttings, back up
again through the hollow inside of the string). Drill bits have to be
incredibly tough to survive in these conditions, so they're made from hardened materials like
tungsten carbide or industrial diamond.
You've probably heard the word "percussion" being used to describe
instruments such as drums, but it's used in a more general sense to talk about
things that we hit. So a percussive drill is one that's driven
into the ground not by an engine that spins it around but by repeated
blows from something like an air compressor. A jackhammer (pneumatic
drill) is the smallest kind of percussive drill; at the other end of
the scale, construction work often involves much larger kinds of
percussion drilling, such as using a piledriver, where foundations
for a building or bridge
are smashed into the ground with repeated blows from a huge machine. Depending on the
terrain, this kind of work can be amazingly slow and laborious: it can take hours to drill piles
down only a few meters (feet).
Photo: This diamond-studded drill bit (technically known as a polycrystalline diamond compact or PDC bit) is typical of the bits used for rotary drilling through hard rock.
Photo courtesy of Sandia National Laboratories and
US Department of Energy.
On an even bigger scale, percussive drilling can be used to drill holes down over 1km (0.6 miles) into solid rock. When the going gets really tough, big-time drilling uses super-heavy-duty tools that combine the spinning power of a rotating drill string with percussive blows applied to a hammer-type bit at the bottom: think of a mini-jackhammer, powered by compressed air, spinning round on the bottom of an oil-rig-type drill string and you're in the right neighborhood.
This is called air drilling and comes in numerous different types and variations, largely governed by the type of ground, the depth and diameter of the hole that needs to be drilled, and the reason the hole is being drilled in the first place. In rotary air percussion drilling (also called rotary air blasting (RAB) and "down-the-hole" (DTH) drilling) the drill string spins around as usual but, at the bottom, instead of a simple drill bit, there is a much more complex, pneumatically-powered drill mechanism called the hammer. It has a strong steel casing with lots of small tungsten rods called buttons that constantly move in and out, like small jackhammers, as the drill string spins around, chipping and blasting away at the rock. In conventional down-the-hole air drilling, the drill string is rotated at the top and compressed air is fed all the way down through the drill string so the percussive blows are applied only at the very bottom, immediately behind the drill bit. That saves wear-and-tear on the long drill string and all the joints that hold it together. Tophammer drilling is similar, but the percussive blows are applied at ground level to the top of the rotating drill string.
Where does the waste material go when you're drilling miles underground? In conventional air drilling, the compressed air goes down the very center of the drill string to power the the hammer bit at the bottom, blowing the cuttings back up the outside of the string. Another method called reverse circulation forces air down through the rim of the drill string, which blows the cuttings back up the middle of the string.
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