Deep hole drilling is a method which is used to drill holes with a high ratio between the holes' depth and diameter.
It is not about the depth of the hole, but the correlation to the diameter. Sandvik Coromant's definition of deep hole drilling is when the depth is 10 to 150 times greater than the diameter.
Deep hole drilling is generally a good method of drilling holes, but it requires a skilled operator and good equipment. Advanced systems monitor the process and measure the drill's force, torque, acoustics and vibrations. The drill breaks easily if something goes wrong, and it may even damage the component. In addition, the tolerances are affected if drilling incorrectly. The cooling is also a bigger issue compared to conventional drilling.
Deep hole drills are quite uncommon in workshops. However, some that do large components have deep hole drills.