What are the parts of an auger bit?
Auger bit tip
|The tip of an auger helps to keep the bit centred so that it can accurately drill in a straight line. Auger bit tips are produced with spurs and either a guide screw or a gimlet point, each suited to a different purpose.
Guide screws draw an auger bit through a wooden workpiece as the bit turns, meaning the user does not have to exert a lot of downward force to bore a hole.
|Guide screws are produced with two different gauges of thread. The first, the coarse screw, is an aggressive thread that works best with soft woods. The wider thread allows for a faster feed rate – that is, you can drill through softwood at a faster speed with it. The wider space between the threads also means the guide screw is less likely to get clogged by waste wood.
|The second, the fine screw, is best suited to hard woods, which have more resilient grain and must be drilled at slower speeds. This fine thread provides improved grip.
When used in a drill press or power drill, guide screws can sometimes be too aggressive and really tug the bit through a piece of wood, causing damage by cutting too quickly. Auger bits with gimlet points are more suitable for this kind of application, as they allow the bit to be centred without the extra pull of the screw thread (although it is possible to use bits with guide screws – just be careful!).
Auger bits that have no gimlet point or guide screw at the tip are referred to as ‘barefoot’, or sometimes ‘bare faced’. They are uncommon, and rely on the flighting and cutting surfaces to guide them through the workpiece in a straight line. The lack of a guide screw also allows them to cut flat-bottomed holes, which is beneficial is the bottom of the hole will be visible in the finished workpiece (e.g. in a desk tidy).
Spurs, also known as ‘nibs’, are responsible for cutting around the outer perimeter of the hole before the lips begin boring the rest of it. This prevents splintering as the bit penetrates the surface of the wood, and keeps the edges of the borehole neat and smooth.
Auger bit lips
|The lips cut material out of the bore hole, lifting it and forcing it up and out of the hole along the flighting. They are sometimes referred to as ‘cutters’.
|The number of lips on the auger bit depends on whether the flighting has a single or double twist (see below). Single-twist augers have one lip, while double-twist augers have two.
Auger bit flighting
|The flighting of the auger bit is the spiral twist, or twists, through which the waste material escapes. Flighting can be single-twist or double-twist.
|Single-twist bits tend to be a little stronger and stiffer, and the wide, helical groove that runs up the length of the bit allows for a greater volume of chip ejection than a double twist bit. This means they do not need to be pulled out of the bore hole as often to keep it clear.
|Double-twist bits cut neater, smoother holes as they have a second lip that keeps the walls of the bore hole smoother. In addition, the second twist results in a greater surface area of the bit remaining in contact with the workpiece during drilling, which reduces the risk of harmful vibrations that could cause the borehole to be misshapen or too large.
Auger bit web
|The material that forms the spiral on an auger bit is sometimes referred to as the ‘web’. The thicker the web, the sturdier the auger.
Auger bit shank
|The shank is the part of the bit that fits into the drill driver. Auger bit shanks are usually square, as they are designed for use with hand braces. They can be be used in powered drivers, although you may find it difficult to secure them in a three-jawed chuck.