What are the parts of a spade bit?
Spade bit centre point
The central point of a spade bit is sharp and acts as a pilot that keeps the bit from wandering off course while drilling.
In some cases, the centre point is completely replaced with a guide screw tip. This minimizes the effort the DIYer needs to put into pushing the drill into the workpiece by drawing it through as the bit spins, at the expense of a degree of control (the bit wants to keep digging through the workpiece aggressively and is more difficult to stop). This type of bit is called a 'self-feeding' bit.
Some self-feeding spade bits have centre points with threads cut on the edges rather than a full guide screw.
Others may have the thread cut further down the bit. Once the threaded part of the bit engages with the workpiece, they will work in a similar way to a bit with a guide screw, but the threadless centre point allows for more control until that moment.
Spade bit lips
The lips on a spade bit shave material from the bottom of the bore hole as the bit spins. They are responsible for the bulk of the cutting.
Spade bit spurs
These sharp points pre-cut a circle in the wood fibre as the bit moves down through the workpiece, creating a much neater hole by shearing neatly through fibres in wood, and by cutting cleanly around the edge of the hole before waste material is removed.
Some spade bits are made without spurs. Instead, their lips are angled slightly so that their furthest corners are closer to the workpiece. These bits bore much rougher holes as they lack the precise shearing quality of the sharp points on the spurs.
Spade bit shank
Spade bit shanks are usually hexagonal.
This is because the long, flat cutting edges on a spade bit require much more torque (turning force) than most other types of drill bit. A cylindrical shank could spin in the driver's chuck jaws, even if it is tightened up well.
For more information on the different types of shank, see: What are the different types of drill bit shank?
Spade bit cable hole
The hole on a spade bit is used to pull electrical cables back through boreholes once they have been drilled. At a push, they can also be used to hang the spade bit on a wall-mounted rack.
For more information, see: