The term boring is also sometimes used for drilling a hole, especially with respect to tunnels and wells in the earth.
The boring process can be carried out on a lathe for smaller operations, but for larger production pieces a special boring mill (work piece rotation around a vertical axis) or a horizontal boring machine (rotation around horizontal axis) are used. The dimensions between the piece and the tool bit can be changed about two axes to cut both along and into the internal surface. A tapered hole can also be made by swiveling the head.
The boring machines (similar to the milling machines such as the classic Van Norman) come in a large variety of sizes and styles. Work piece diameters are commonly 1-4m (3-12 ft) but can be as large as 20m (60ft). Power requirements can be as much as 200 hp. The control systems can be computer-based, allowing for automation and increased consistency.
Because boring is meant to decrease the product tolerances on pre-existing holes, several design considerations must be made. First, large length-to-bore-diameters are not preferred due to cutting tool deflection. Next, through holes are preferred over blind holes (holes that do not traverse the thickness of the work piece). Interrupted internal working surfaces—where the cutting tool and surface have discontinuous contact—should be avoided. The boring bar is the protruding arm of the machine that holds cutting tool(s), and must be very rigid.1
Boring is used for a wide variety of applications in geology, agriculture, hydrology, civil engineering, and oil and natural gas industries. Today, most earth drilling is done in order to do one of the following things:
In the 1970's and early 1980's the USSR attempted to drill a hole through the crust, to sample the Mohorovicic Discontinuity. The deepest hole ever drilled failed not because of lack of money or time, but because of the physics of rocks within the crust. The hole achieved approximately 12,000 metres depth, a depth at which rock begins to act more like a plastic solid than a rigid solid. The rock also approached temperatures of several hundred degrees Celsius, requiring that the drilling fluid was refrigerated before being sent to the cutting face of the drill. As the drill bits burnt out and were removed for replacement, the hole simply flowed closed, and the rock had to be re-drilled. Due to the temperature, the drill bits burnt out before achieving any headway. The hole was scrapped.