easily removed by the flutes of the drill. A high tech monitoring system is used to control force, torque, vibrations, and acoustic emission. Vibration is considered a major defect in deep hole drilling which can often cause the drill to break. A special coolant is usually used to aid in this type of drilling. Gun drilling Gun drilling was originally developed to drill out gun barrels and is used commonly for drilling smaller diameter deep holes. The depth-to-diameter ratio can be even greater than 300:1. The key feature of gun drilling is that the bits are self-centering; this is what allows for such deep accurate holes. The bits use a rotary motion similar to a twist drill; however, the bits are designed with bearing pads that slide along the surface of the hole keeping the drill bit on center. Gun drilling is usually done at high speeds and low feed rates. Trepanning Trepanning is commonly used for creating larger diameter holes (up to ) where a standard drill bit is not feasible or economical. Trepanning removes the desired diameter by cutting out a solid disk similar to the workings of a drafting compass. Trepanning is performed on flat products such as sheet metal, granite (curling stone), plates, or structural members like I-beams. Trepanning can also be useful to make grooves for inserting seals, such as O-rings. Microdrilling Microdrilling refers to the drilling of holes less than . Drilling of holes at this small diameter presents greater problems since coolant fed drills cannot be used and high spindle speeds are required. High spindle speeds that exceed 10,000 RPM also require the use of balanced tool holders. Vibration drilling The first studies into vibration drilling began in the 1950s (Pr. V.N. Poduraev, Moscow Bauman University). The main principle consists in generating axial vibrations or oscillations in addition to the feed movement of the drill so that the chips break up and are then easily removed from the cutting zone. There are two main technologies of vibration drilling: self-maintained vibration systems and forced vibration systems. Most vibration drilling technologies are still at a research stage. In the case of self-maintained vibration drilling, the eigenfrequency of the tool is used in order to make it naturally vibrate while cutting; vibrations are self-maintained by a mass-spring system included in the tool holder. Other works use a piezoelectric system to generate and control the vibrations. These