 Deep hole drilling is a method which is used to drill holes with a high ratio between the holes' depth and diameter.
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.
Picture: Mollart Ltd
Fagersta
Kungälv
Borlänge
Värnamo
Essunga
Karlshamn
Vetlanda
Göteborg
Härryda
Malung-Sälen
Sandviken
Sandviken
Västervik
Färgelanda
Gislaved
Robertsfors
Kristinehamn
Finland-Region
Lettland-Region
Kalix
Boden
Ovanåker
Vaggeryd
Danmark-Region
Linköping
Västervik
Vetlanda
Smedjebacken
Karlskoga
Huddinge
Lysekil
Piteå
Skellefteå
Krokom
Östhammar