In machining, Boring is used to achieve greater accuracy of the diameter of a hole, and can be used to cut a tapered hole that has already been drilled (or cast), by means of a single-point cutting tool (or of a boring head containing several such tools), for example as in boring a gun barrel or an engine cylinder.

Boring is the process of enlarging a hole Boring can be viewed as the internal-diameter counterpart to turning, which cuts external diameters.

There are various types of boring. The boring bar may be supported on both ends (which only works if the existing hole is a through hole), or it may be supported at one end (which works for both through holes and blind holes). Lineboring (line boring, line-boring) implies the former. Backboring (back boring, back-boring) is the process of reaching through an existing hole and then boring on the “back” side of the workpiece (relative to the machine headstock)

Because of the limitations on tooling design imposed by the fact that the workpiece mostly surrounds the tool, boring is inherently somewhat more challenging than turning, in terms of decreased toolholding rigidity, increased clearance angle requirements (limiting the amount of support that can be given to the cutting edge), and difficulty of inspection of the resulting surface (size, form, surface roughness). These are the reasons why boring is viewed as an area of machining practice in its own right, separate from turning, with its own tips, tricks, challenges, and body of expertise, despite the fact that they are in some ways identical.

Lathe boring usually requires that the workpiece be held in the chuck and rotated. As the workpiece is rotated, a boring bar with an insert attached to the tip of the bar is fed into an existing hole. When the cutting tool engages the workpiece, a chip is formed. Depending on the type of tool used, the material, and the feed rate, the chip may be continuous or segmented. The surface produced is called a bore.

Boring and turning have abrasive counterparts in internal and external cylindrical grinding. Each process is chosen based on the requirements and parameter values of a particular application.

Broaching is used when precision machining is required, especially for odd shapes. Commonly machined surfaces include circular and non-circular holes. Broaching is a machining process that uses a toothed tool, called a broach, to remove material.

Source Wikipedia