Trepanning

Trepanning, also known as trepanation, trephination, trephining or making a burr hole (the verb trepan derives via Old French via Medieval Latin from the Greek noun of relevant meaning trypanon, literally "borer, auger") is a surgical intervention in which a hole is drilled or scraped into the human skull, exposing the dura mater to treat health problems related to intracranial diseases. It may also refer to any "burr" hole created through other body surfaces, including nail beds. It is often used to relieve pressure beneath a surface. A trephine is an instrument used for cutting out a round piece of skull bone.

In ancient times, holes were drilled into a person who was behaving in what was considered an abnormal way to let out what they believed were evil spirits. Evidence of trepanation has been found in prehistoric human remains from Neolithic times onward. Cave paintings indicate that people believed the practice would cure epileptic seizures, migraines, and mental disorders. The bone that was trepanned was kept by the prehistoric people and may have been worn as a charm to keep evil spirits away. Evidence also suggests that trepanation was primitive emergency surgery after head wounds to remove shattered bits of bone from a fractured skull and clean out the blood that often pools under the skull after a blow to the head. Such injuries were typical for primitive weaponry such as slings and war clubs.

Drilling

Drilling is a cutting process that uses a drill bit to cut or enlarge a hole of circular cross-section in solid materials. The drill bit is a rotary cutting tool, often multipoint. The bit is pressed against the workpiece and rotated at rates from hundreds to thousands of revolutions per minute. This forces the cutting edge against the workpiece, cutting off chips (swarf) from the hole as it is drilled.

Exceptionally, specially-shaped bits can cut holes of non-circular cross-section; a square cross-section is possible.

Process

Drilled holes are characterized by their sharp edge on the entrance side and the presence of burrs on the exit side (unless they have been removed). Also, the inside of the hole usually has helical feed marks.

Drilling may affect the mechanical properties of the workpiece by creating low residual stresses around the hole opening and a very thin layer of highly stressed and disturbed material on the newly formed surface. This causes the workpiece to become more susceptible to corrosion and crack propagation at the stressed surface. A finish operation may be done to avoid these detrimental conditions.