Choosing drill bits

The effectiveness of any electric drill is to a large extent governed by the drill bit you use. Unless it is correctly matched to the material you are drilling, and is well maintained and sharpened, the motor will be under strain and the job may take a lot longer than you bargained for.

Choosing the correct bit

Most electric drill bits are manufactured from either high-speed or carbon steel. Chrome vanadium bits— usually used in hand-operated drills— are not suitable for electric drills since they quickly blunt and wear out under high speed.

You can tell a great deal about the quality of a twist drill bit just by looking at it, so examine the bit carefully before buying. The angle of the point is all-important: for wood it should taper at an angle of about 60°; for thick steel 118°; and for thin steel 130°. Most general purpose drills have a point angle of 120° which allows the bit to be used for almost any type of work. Bits designed for masonry work have a specially strengthened, square-shaped point suitable for this type of work.

The edges of the spiral around the point and shank also merit careful inspection. They should stand out from the body of the drill bit so that it can turn more easily and cut smoothly through whatever material is being drilled.

The bit should have a shank which is deeply spiralled—or fluted. This allows waste material to be carried away during drilling so that the bit does not jam. Make sure that the flutes are smoothly cut without sharp prominences.

Bits are available in a large range of diameters and lengths, in both imperial and metric sizes. The diameter of the bit is usually marked on the top end of the shank and you should check this before you buy.

Bits can be bought separately or in sets containing various commonly-used sizes. Metric size drills in sets usually range in diameter from 1.5mm to 6.5mm. Try to purchase a good quality set suitable for most jobs and then supplement this with more specialized bits as the need arises.

Most drill bits are ten times as long as their diameter. This ratio—known as the jobber length—ensures that the bit never snaps under pressure. More specialized jobs may require different size bits—for especially deep holes or when working in a confined space, for instance.

Drill bits for wood

The material you are likely to want to drill most often is wood, and a whole range of bits is available to make various types of holes: Twist bits: These are the most common type of drill bit used to make small holes in wood. The most commonly used sizes range from 1.5mm to 6.5mm, but larger and smaller bits available for special jobs. Wood bit: These make holes up to 10mm in diameter. They are very much like the twist bit in shape but have a centre point which helps to position the bit more accurately. Many have shanks with a reduced diameter top end so that they can be fitted into a drill with a small chuck size. Countersink bit: Most screws have countersunk heads so that they can be positioned at or below surface level. To allow you to do this without having to force the screw, a countersink bit should be used. This has a short shank and a tapered cutting point which drills a hole in the surface just large enough to accommodate a screw head.

Countersink bits are available in a number of different sizes, so you should make sure that you have the correct size for the size of screws you are using. Use a countersink bit only when you have drilled a pilot hole and a second hole for the screw itself. Do not allow the bit to drill any deeper than is absolutely necessary or too deep a hole will result.

Once the screw has been positioned, plug the hole with wood filler, or—for a really professional finish—use a plug cutter to drill a rounded sliver of wood to fill the gap.

As an alternative to using a countersink, the holes can be both drilled and countersunk at the same time using a combination screw sink. However, since you need a different one for each diameter and length of screw, it is not worth buying one specially unless you have a lot of screws of the same size on one job. Flat bit: This is used for drilling large holes in wood. The bit has a flat, spade-shaped cutter with a central point to help position it when starting the hole.

Flat bits are especially useful for making rough holes in relatively thin timber and they are cheap when compared to other bits of similar sizes. Unfortunately, they tend also to be less accurate than the other types. Auger bit: For accurate or deep holes of a wide diameter, auger bits should be used. These have a deeply fluted shank with a wide cutting edge and in front of this a spiralled point which allows the bit to be positioned very accurately.

When using an auger bit in a power drill, make sure it is designed for high speed work and not for a hand drill. Look at the top end of the shank where it fits into the chuck—a square-ended shank shows that it is made for a hand-powered drill with a rounded shank for an electric drill. Hole saw: This is a piece of saw blade wrapped in a circle around an arbor with a twist pilot drill at its centre. Once you have located the centre of the hole with the twist drill, the saw attachment allows you to cut around the outside. Using this tool, you can cut holes up to 75mm in diameter in material up to 12mm thick.

Most hole saws are pre-fixed to cut holes of only one diameter but on some the blade can be adjusted to tackle a number of different sized holes. On others a number of saws can be fitted to the same arbor so that a hole of almost any size can be drilled. Extra large holes: If you want to drill holes more than 75mm in diameter—to accommodate pipework for instance—this cannot be done with a hole saw. Instead you should use a power jig saw and then a rasp or sanding tool to smooth around the edges of the cut.

Special bits: A number of drill bits designed for specialized jobs are now on the market: for example, the hinge boss used to make the housings for self-closing chipboard hinges. For this reason, it is always worth enquiring at a large hardware store if you have a particularly unusual hole to drill and no suitable bit.

Bits for metal

Only the best quality drill bits are suitable for drilling holes in metal. Made from high speed steel, they are often coated and tempered to make them more effective.

The size of holes you can make will vary according to the thickness of the metal you are drilling and the power of the drill. But if you use a good quality bit, holes up to 20mm in diameter should be possible.

To increase the effectiveness of the bit keep the drill at its lowest possible speed and lubricate the point with light machine oil. Make sure you wear goggles or safety glasses.

For larger holes use a hole saw with high speed blades. Try not to apply too much pressure to the saw but let it slowly find its own way through the metal. Use a tank cutter for thin sheet.

Drilling masonry

If you want to make holes in brick or blockwork it is essential to use special bits for the job. The bit you choose depends on whether you want to drill a small or a large hole: Masonry bits: These are rather like the twist bits used for drilling wood except that the drill point has a square shaped tip—made of tungsten carbide —slightly proud of the spiral. This enables the bit to cut into the masonry which would blunt an ordinary twist drill.

Some masonry bits are described according to their diameter in millimetres; others have a number corresponding to the size of screw they will accommodate. Most basic electric drills should be able to drill holes up to 15mm in diameter—especially if they have a hammer action. For larger jobs it is worth hiring a heavy-duty industrial drill with a larger power output and the necessary bigger chuck size.

Core drill: This is similar in appearance to a hole saw used for drilling wood or metal. It can cut holes to almost any depth—even the toughest masonry—and up to 50mm in diameter; a heavy industrial drill will be able to cope with 100mm holes. Hammer bits: Most makes of hammer drill require special bits strong enough to stand up to the constant pounding which the drill imparts. These are capable of drilling holes to similar dimensions as twist bits and can be purchased where you bought the drill.

Other materials

Power drills can be used to make holes in other materials—such as ceramics and glass—but they must be capable of being run at very low speeds. A drill with a very low speed setting—or even better a variable speed drill—is essential. If you have neither, avoid accidents by using a hand drill.

For drilling glass or ceramics you need a tungsten-tipped spear or spade-point bit. First lay the item to be drilled on a horizontal surface covered with some type of padding— such as a blanket. Then surround the area with a circle of putty or Plasticine and into this pour some type of lubricant to aid drilling. Break the glaze with the point of the bit to stop it skating across the surface. You can then accurately drill the hole in the normal way.

Caring for bits

You should maintain and store your bits carefully so that they remain effective and can be picked out easily when you need them.

During drilling, retract them from the hole frequently so that the flutes have a chance to clear. Never exert undue pressure on the bit in an effort to drive it through the material: apart from straining the motor, you are likely to bend the bit and make it unusable in the future.

After each hole has been drilled examine the bit to see whether it is clogged with the material you have just been drilling and clean out waste with a nail or awl before continuing. This will not only aid drilling but help the bit to maintain its cutting edge.

After use, rub down the bit with steel wool and then apply a thin coating of lubricating oil. Occasionally sharpen the bits yourself or hand them into your local hardware shop to be re-sharpened professionally.

Try to keep bits in a place where they can be traced easily without being knocked onto the floor. A small rack—made from a block of wood with holes of various diameters drilled in it—is easy to make and ensures that each bit is stored in its correct place. Alternatively, there are plenty of proprietary drill carriers on the market and you often get one when you buy a set of bits from a hardware store.

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