What is a Tapped Hole?
A tapped hole is a hole drilled prior to a machine screw being inserted.
A tapped hole is similar to a pilot hole, except that it is usually larger, and has threads cut into the inside surface of the hole.
Tapped holes are used in metals where a nut and bolt cannot be used.
Why Create a Tapped Hole?
Unlike other screws, machine screws cannot cut their own threads when being driven into material. For example, when drilling a screw into a material like wood, the screws thread buries itself in.
As a result of metal being a stronger material, the screws thread is unable to cut into it. This means that they must be inserted into a tapped hole. Tapped holes provide extra grip to ensure that the screw stays in place.
Tap Drill Bits
A tapped hole is formed using a drill bit and a tap drill bit.
The pilot drill bit forms the initial hole…
…and then a tap drill is used to cut the threads.
When a screw is inserted, its external threads wind down into the internal threads created by the tap bit.
Please note: The tap bit and screw must have matching threads, or you could end up damaging the threads of the screw or the tapped hole.