For the purposes of this write-up
, I am going to use the word drill-bit
(or bit) for the thing which actually makes the hole, and the word drill
for the machine which holds and turns the drill-bit
A drill is a machine, designed to turn a drill-bit at an appropriate speed, to enable the operator to make a cylindrical hole in the workpiece. The hole is made by moving the drill-bit axially as it turns rapidly on its axis.
Drill-bits are designed to cope with moderate to high axial- and circumferential- forces. They are not designed to withstand lateral or bending forces, and will break if too much lateral force is applied. This is a bad thing.
Drills come in many forms. The most common is the hand–held power drill, but other types exist. The following list includes the main types, but is not exhaustive. If readers know of errors or omissions, pls /msg me, or e-mail to firstname.lastname@example.org.
Brace and bit
Portable power drill (Mains-powered)
Portable power drill (Battery powered)
There are many different types of drill-bit, including wood drills, high-speed drills, saw drills, and masonry drills. The materials used to make these bits can also vary, but typically the shank or shaft is made from tool steel, or even mild steel, while the tip (where the cutting takes place) is made from something harder: diamond in the case of heavy duty industrial drill-bits, tungsten carbide in up-scale hobbyist drill-bits and tool steel in most normal metal-cutting drill-bits. Users need to be aware that the properties of steel change as temperatures above around 300°C are reached, and once a steel drill-bit has been run above 500°C or more, it will become effectively useless for further work. Take care of your tools!
Brace and bit
This is the simplest of all drilling machines. It is powered only by the efforts of the operator, with no assistance from electricity. In essence, it is a cranked handle, with a chuck to hold the bit. There are bearings on the handle grip, and, in up-scale models, on the rest, which lies directly above the bit. This means you can turn the crank, and apply your weight to the rest, while turning the handle in order to push the bit more firmly into the workpiece.
They are not normally used for holes above a few millimetres in diameter in metals, or above perhaps a half-inch (10-15mm) in wood.
Portable power drill
This is an advance on the brace and bit, in that an electric motor is used to turn the chuck and, if fitted, the drill-bit. Early designs used a pistol grip, with a simple trigger, to switch the motor on. Releasing the trigger switched it off. Nowadays, however, most drills are designed to operate as both drilling machines and powered screwdrivers, which means the chuck must be capable of rotating clock-wise and anti-clockwise. Thus, an additional control is needed to reverse the direction of motion. In addition, the screwdriver function means the operator must be able to vary the speed of the machine, down to almost zero revs/min. This requires yet another control. Most modern hand drills have a pressure-sensitive trigger, in which greater pressure on the trigger delivers higher speeds at the chuck.
The trend toward battery-powered drills has helped in this area. It is much easier to control the speed of a direct current (DC) motor, than it is to control the speed of an alternating current (AC) motor. Batteries, of course, generate direct current, whereas the mains electricity is (almost) always AC.
A note on choosing a battery-powered hand drill. Broadly speaking, the higher the voltage on the battery pack, the more torque and power you will get from your drill. A 3.6 volt hand-held screwdriver is only good for light-duty work. On the other hand, a 14.4-volt drill will be able to handle most jobs up to a 10 to 12mm hole in either wood or soft metal.
A third issue concerning hand drills is the ‘masonry’, or ‘hammer’ function. Use this when drilling into bricks, not when drilling into glass or steel. It does what it says, generating a slight hammer action at the drill tip. A sharp drill-bit will actively cut into steel, aluminium or wood, and that is why the tip of the drill-bit designed for these materials is shaped like a knife, with a cutting blade (also, incidentally, why you need to keep your drill-bits sharp). However, it is all but impossible to cut into a brick, but brick is brittle, and will crumble if struck with a hammer blow. Therefore, the masonry bit does not have a cutting edge but a flat, hardened edge designed to hammer into the material, and shatter the area ahead of the bit.
The flutes (spiral shapes) up the shaft of a drill-bit do not cut material. They are designed to bring the debris out of the hole while maintaining good axial strength and stiffness down the shaft of the drill-bit. When drilling, it is always a good idea to withdraw the bit from time to time, in order to clean the debris out the hole.
Hand-held power drills can be used effectively up to around 12-15 mm (half-inch) in metals. With the correct type of bit, they can be used to cut almost any size of hole in soft wood, plaster or other soft materials.
This is the craftsman’s tool. The hand drill has a number of disadvantages:
It can wander when starting to drill a hole
It is hard to maintain the drill axis at the correct angle relative to the surface of the material
When widening a hole, the bit can snatch at the pre-drilled sides
It can be hard to tell how deep the hole is.
For larger hole sizes, big drills are very heavy and unwieldy.
The pillar drill is designed to overcome these issues. It does so very effectively, but at the expense of portability.
While the basic elements of this machine tool are similar to the brace-and-bit, it differs substantially in scale and robustness.
The pillar drill is usually mounted on a round steel pillar, typically 500mm or so in diameter, though large pillar drills can be even bigger. The pillar needs to be stiff enough to keep the drill precisely aligned with the workpiece, despite large applied forces.
Apart from a solid, heavy base and the pillar, the pillar drill comprises two main elements: the head assembly and the table.
The table is a solid steel construction, often 50mm or more in thickness, attached to the pillar by a collar. A screw and nut assembly holds this collar in place. If the nut is over-loosened, the whole heavy table will crash to the floor at speed. You have been warned!
It has a hole in the middle, to allow the drill-bit to penetrate the workpiece. Users are warned to ensure that drill and drill-bit are aligned with this hole. Pushing a spinning drill-bit into the table is considered extremely bad form. The table also has features designed to allow the operator to attach the workpiece and/or a jig firmly to the table bed, to prevent movement during machining.
The head assembly is essentially a large motor with gears and a shaft on which a chuck is mounted. This shaft is designed to move up and down under the control of a large handle. Pulling the handle brings the chuck vertically downwards against a spring. When the handle is released, the shaft returns to its original position. As the handle is pulled further, the drill-bit comes into contact with the workpiece. Further pressure on the handle pushes the bit into the workpiece. The mechanism is designed to have a large mechanical advantage, which means the operator can apply relatively large axial forces onto the drill-bit and workpiece.
The head assembly is also attached to the pillar with a collar and locknut. Again, if the locknut is loosened too much, the whole head assembly can slide down the shaft at speed. This is not recommended.
Additional features of the pillar drill include depth gauges, safety systems, speed adjustment and so forth. These vary substantially in detail according to manufacturer and model, so I make no attempt to describe them here, except to say that too many machinists think safety devices are for other people. Err, No. Not if you want to keep your sight and your fingers.