What are the parts of a spade bit?

 
     
     
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 Diagram showing the location of the different parts of a spade bit 
     
   

Spade bit centre point

 
 Location of the centre point on a spade bit 

The central point of a spade bit is sharp and acts as a pilot that keeps the bit from wandering off course while drilling.

 
     
 An example of a self-feeding spade bit showing the location of the guide screw on the tip 

Self-feeding bits

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.

 
     
 Spade bits with thread-edged centre points 

Some self-feeding spade bits have centre points with threads cut on the edges rather than a full guide screw.

 
     
 A self-feeding spade bit with threads cut into the body of the bit instead of the centre point 

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

 
 Diagram showing the location of the lips on a spade bit 

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

 
 Diagram showing the location of the spurs on a spade bit 

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.

 
     
 An example of a spade bit with no spurs 

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

 
 An expansive bit with a hexagonal shank 

Spade bit shanks are usually hexagonal.

 
     
 Image to show that spade bits are not often made with round shanks due to the amount of force that a drill driver needs to turn them 

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

 
 A cable that has been pulled through a pre-drilled hole after being attached to the hole in the flat part of a spade bit 

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: How to use a spade bit to pull a cable through a wall

 
     
   Wonkee Donkee remarks on the fact that pulling a cable through a hole using a spade bit is much easier than trying to push it through yourself 
     
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