Get to know... Drill Bits

To drill a hole in any material, the correct type of drill bit must be used. For basic requirements, a set of high-speed steel twist drills and some masonry bits will probably be sufficient for the average handyman. But for more sophisticated jobs/material, others bits will be required - perhaps larger, or designed for a specific material/purpose.

 

Good quality drill bits can be expensive, so take care of them, keep them in a case or box if possible, rather than allowing them to roll around loose in a toolbox where the cutting edges may be damaged.

Twist bits
These are the most common bits used by the handyman for drilling. The front edges cut the material and the spirals along the length remove the debris from the hole and tend to keep the bit straight. They can be used on timber, metal, plastics and similar materials. Special care is required when using the smallest sizes - always hold the drill square to the work and apply only light pressure when drilling.

Screwdriver bit drills
Designed to fit in rechargeable screwdriver these bits have a hexagonal shank. They are ideal for drilling pilot holes but are limited by the low power of these type of screwdrivers and the limited size of small bits available.

Masonry bit
As the name suggests, these are designed for drilling into brick, block, stone, quarry tiles or concrete. The cutting tip is made from tungsten carbide bonded to a spiralled steel shaft. Most masonry bits can be used with a hammer action power drill, but always check as the action is quite punishing on the bit and cheaper bits have been known to shatter when subjected to the pounding. Always use a slow rotational speed for drilling into harder materials to avoid overheating of the tip, and frequently withdraw the bit to remove dust. Long Masonry bits (300 to 400mm) are available for drilling through masonry walls.

Spur point bit
Also known as a wood or dowel bit, they have a central point and two raised spurs that help keep the bit drilling straight. The bit cuts timber very fast when used in a power drill and leaves a clean sided hole. They are ideal for drilling holes for dowels as the sides of the holes are clean and parallel. Spur point bits should only be used for drilling wood or some plastics.

Countersink
Although not a true 'drill', it is used in a power drill to form the conical recess for the heads of countersunk screws. These bits tend to be designed for use on soft materials such as timber and plastics, not metals. When used with a power drill to counter sink an existing hole, the bit tends to 'chatter', leaving a rough surface. Better results be will obtained if the countersink bit is used before the hole is drilled, then take care to ensure that the hole is in the centre of the countersunk depression.

Tile Bit
A bit for drilling ceramic tiles and glass, it has a ground tungsten carbide tip. Best used in a variable speed power drill on a slow speed. When drilling glass, some form of lubricant (i.e. turpentine or white spirit ) should be used to keep the tip cool. Ceramic tiles can also be drilled using a masonry bit only if used at slow speed and without hammer action.

Spade bit
Intended for power drill use only, the centre point locates the bit and the flat steel on either side cuts away the timber. These bits are used to drill fairly large holes and they give a flat bottomed hole (with a central point) so are ideal where the head of a screw/bolt needs to be recessed into the timber - always use this bit before drilling the clearance hole for the bolt.
The larger bits require a fairly powerful drill to bore deep holes. The bits cause a lot of splintering as they break out the back of the workpiece - using a sacrificial backing board will reduce this. Flat wood bits are not really suitable for enlarging an existing hole.

Hole saw
Used for cutting large, fixed, diameter holes in wood or plastic. They will usually cut up to a depth of 18mm - deeper versions are available. Best used in a power drill at low speed as the blade saws it's way through the material.

Combination hole saw
Like the Hole Saw above, these combination saws can cut large holes but they consist of a number of different sized round saw blades, usually ranging from about 25 to 62mm in diameter. Normally the blade are secures by a radial screw in the 'head', all blades other than the desired sized being removed before the screw is inserted to secure the required diameter blade. Best used in a power drill at low speed as the blade saws it's way through the material.

Forstner bit
Used to form holes with a flat bottom, such as for kitchen cupboard hinges. Best used in a power drill held in a drill stand as there's little in the way of a central point. If used freehand, the positioning is difficult to control as there is no central pilot bit.

Wood Auger bit
This is ideal when drilling large-diameter, deep holes in wood or thick man-made boards. Generally an Auger bit should only be used in a hand brace. The bit will cut a clean and deep, flat bottomed holes. The single spur cuts and defines the edge of the hole while the chisel-like cutting edge removes the waste within the previously cut circle. The threaded centre bites into the wood and pulls the bit into the timber. This 'pulling' action means that the bit is really unsuitable for use in a power drill.

Most twist bits are made from either:
- high speed steel (HSS) - suitable for drilling most types of material.
- carbon steel - specially ground for drilling wood and should not be used for drilling metals.