Drill bits buying guide

Drill bits are to drills what blades are to saws: you can’t achieve much without them! There are a number of different bit types depending on the material you want to work on. Metal, masonry and wood can all be drilled with the use of specially designed bits. It is even possible to cut through glass or tiles using bits with diamond or tungsten carbide tips. The quality of drill bits is determined by the material used to make them which, in turn, controls their ability to drill into different materials. A commonly used material for drill bits is high-speed steel (HSS) used with different combinations of carbide, titanium, cobalt and so on.

The drill bit shank also comes in various formats, depending on the drill chuck used.

• Straight shanks are used for keyed or keyless chucks (from 10 to 13 mm).

• SDS shanks are used for SDS+ or SDS Max chucks.

• Hex shanks are used for screw guns or electric screwdrivers. 

Masonry and metal drill bits are twisted while wood bits can be twisted or flat. The twists, or flutes, of a drill bit, may also vary to allow for better removal of the drilled material. Further variations of drill bits include adjustable bits, Forstner bits, step or cone bits, and installer bits. They all come in different diameters and lengths. Extensions can also be used to increase the length of wood bits.

In order to select the right metal drill bit, you must first consider the type of metal you plan on drilling.

While all metals vary in weight, they also feature different levels of density that determine their hardness and resistance to drilling. For instance, lead is heavier than aluminium, but aluminium is a slightly harder metal. The density of a metal dictates the design of the drill bit as the bit must be harder than the metal it aims to drill into. When it comes to metal drill bits, one that can easily bore a hole in aluminium won’t make a scratch on an iron-carbon alloy.

With this in mind, metal drill bits are made of different materials and contain various elements, which are selected to enhance hardness and improve cutting ability. There are two main categories of metal drill bit:

• High-speed steel (HSS): an alloy steel (an alloy consisting of a metallic element combined with a chemical element to give the steel a certain feature);

• Hard metal or carbide: a sintered compound created through a manufacturing process whereby powdered forms of carbide tungsten are heat-treated with binder metals, such as titanium or cobalt, to make a solid.

Metal drill bits are categorised by their type, cutting method and, in the case of carbide bits, their binding element.

High-speed steel (HSS)

High-speed steel is the most commonly used alloy steel for metal drill bits. HSS bits feature a high level of hardness, are highly resistant and can be sharpened.

Cobalt

Cobalt, combined with an alloy steel like HSS (usually 5%), offers strength and resistance, and increases the hardness of the bit. Cobalt retains its properties at a higher temperature than titanium, meaning you can drill at a higher speed.

Titanium

Titanium-coated HSS bits provide a stronger and more resistant alloy. Nonetheless, it’s worth remembering that the element content of metals is measured as a percentage. There is no such thing as a ‘titanium bit’ and a few microns of titanium won’t necessarily improve the quality.

Tungsten carbide

Tungsten carbide lends an increased level of resistance and high hardness to drill bits. It is formed by a combination of tungsten and carbon, a very dense alloying element. This sintered compound can produce a harder or softer metal, depending on the percentage of carbon it contains.

Metal drill bits should be used with a cutting fluid, at low speeds using even and constant pressure. The metal bit should be sharpened appropriately and regularly. Even the most expensive and strongest metal drill bits will have a lifespan of just a few holes if used incorrectly.

Different mix ratios will generate metals with different properties. A low percentage (indicating a thin coating) will have little impact on the quality and resistance of drill bits. The quality of a drill bit is also affected by its treatment and manufacturing processes.

Several units of measurement are used to assess the hardness of a steel. The Brinell scale is often used with resistance measured by Newtons per square millimetre (N/mm²); the higher the number, the more resistant the drill bit.

HSS metal drill bits feature a helical or twisted shape and come in various materials providing different degrees of hardness and durability.

High-speed steel ground metal drill bits

HSS-Gbits are ground at the tip and along their flutes. The material of these bits is ground directly into shape with the use of a grinder. They are easy to spot as their cutting edges (flutes and lips) are white rather than grey and are perfectly symmetrical. Ground bits are suitable for sustained use and are efficient and precise.

High-speed steel rolled metal drill bits

HSS-R bits feature roll-forged flutes. These drill bits are formed by a heated rolling process by which the material is shaped between two parallel cylinders turning in opposite directions. HSS-R drill bits are suitable for multiple drilling tasks and offer high precision.

1. Point angle varies from drill to drill (90°, 120°, 135°, 140°, 145°) and should be tailored to the metal you plan on drilling. The harder the metal, the larger the point angle (and therefore the flatter the point). This factor also depends on the drill bit material.

Selecting a masonry bit is more straightforward given that your decision basically comes down to how soft or hard the drilling material is. Most masonry drill bits are made from tungsten carbide which further simplifies your decision.

Tungsten carbide drill bits work efficiently on concrete, granite, sand-lime brickwork and natural stone. The overall quality of concrete drill bits depends on the quality of their point and the number of cutting edges they feature (two, three or four).

For optimum efficiency when working with harder materials and reinforced concrete, a point with four cutting edges in solid carbide must be used. Lower quality drill bits generally only have two cutting edges. Drill bits made from other materials (for example, diamond bits) are more useful for drilling glass, tiles, ceramic, and so on.

Reinforced concrete or breezeblock should be drilled in hammer mode with a standard or hammer drill. Three types of shank are used for drilling concrete:

• Straight shank: used for hammer drills and drills with keyed or keyless chucks;

• Slotted Drive System (SDS): a drill bit with a grooved shank that clicks into the chuck. This shank is found on most hammer drills and various brands of standard drills. There are two types of SDS shank:

Most concrete drill bits feature SDS+ shanks. SDS Max shanks are used for heavy-duty jobs and with other hammer drill accessories such as flat chisels, moil points, and so on.

Some drill bits feature hex shanks, such as those used for electric screwdrivers. These drill bits are designed for use with screwdriver tools with a hammer mode.

Concrete drill bits also feature different flute designs in order to facilitate dust removal.

The four main flute designs seen in masonry drill bits are:

• L-type double flute: standard model;

• U-type double flute: for better dust removal;

• Four-flute model for better stability;

• Turbo design for optimum dust removal.

The flute design is directly linked to the quality of the point and the number of cutting edges the bit features. You do not necessarily have to choose the flute design: it simply comes as a result of these two factors.

Choosing a wood bit is easy: you simply have to decide between a flat bit or a twist bit. Most wood bits are made of tempered steel.

• Flat wood bits are used for drilling large diameters (generally 13 mm+). That being said, you can also find 6 mm flat wood bits.

• Twist bits are mainly supplied with diameters ranging up to 17 mm, although these can also go beyond 20 mm. 

• Auger bits, specially designed for removing shavings while drilling wide holes (in carpentry). Three-point bits are the most common and precise variety of wood bit; these are used in carpentry for drilling small holes. These bits have three points: one central point and two points on the sides that draw out the hole before drilling.

It should also be noted that twist bits are more appropriate for drilling thicker materials (such as a wooden post, for example), while flat bits should be used for shallower materials.

Like metal drill bits, wood bits can feature different shank types – the most common being the straight shank.

In addition to the many variations of metal, concrete and wood drill bits, a range of drilling accessories may also prove useful.

• Step bits (or unibits) are cone-shaped bits equipped with a range of different diameters to drill various hole sizes;

• Stub drill bits feature a short length for drilling into very hard metals and drilling out spot welds;

• The Forstner drill bit is mainly used in cabinet-making for drilling large holes (for hinges, for example), and is also known as a hinge bit;

• Extendable drill bits are wood bits that are used to drill several hole sizes (usually ranging from 13 to 45 mm) using just one bit. They are mainly used for drilling into plywood and thin composite panels such as chipboard with a power tool;

• Countersink drill bits are used to drill a countersink (for drilling pilot holes or entering a screw head);

• Installer drill bits are very long bits, designed to drill holes for pulling through wires;

• Hole saws are not necessarily categorised as bits but they can be fitted onto a drill to create different sized holes in a range of materials;

• Concrete hole saws, or diamond core drills, are a type of heavy-duty hole saw used to drill into concrete; 

• Extension rods are handy for wood drilling and allow you to drill deep holes (as long as the drill has a larger diameter than that of the extension bar).