Boring (manufacturing)
In machining, boring is the process of enlarging a 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 cannon barrel. Boring is used to achieve greater accuracy of the diameter of a hole, and can be used to cut a tapered hole.
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. 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).
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[edit] Machine tools used
The boring process can be executed on various machine tools, including (1) general-purpose or universal machines, such as lathes (/turning centers) or milling machines (/machining centers), and (2) machines designed to specialize in boring as a primary function, such as jig borers and boring machines or boring mills, which include vertical boring mills (workpiece rotates around a vertical axis while boring bar/head moves linearly; essentially a vertical lathe) and horizontal boring mills (workpiece sits on a table while the boring bar rotates around a horizontal axis; essentially a specialized horizontal milling machine).
[edit] Boring mills and milling machines
The dimensions between the piece and the tool bit can be changed about two axes to cut both vertically and horizontally into the internal surface. The cutting tool is usually single point, made of M2 and M3 high-speed steel or P10 and P01 carbide. A tapered hole can also be made by swiveling the head.
Boring machines come in a large variety of sizes and styles. Boring operations on small workpieces can be carried out on a lathe while larger workpieces are machined on boring mills. Workpieces are commonly 1 to 4 metres (3 ft 3 in to 13 ft 1 in) in diameter, but can be as large as 20 m (66 ft). Power requirements can be as much as 200 horsepower (150 kW). Cooling of the bores is done through a hollow passageway through the boring bar where coolant can flow freely. Tungsten-alloy disks are sealed in the bar to counteract vibration and chatter during boring. 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]
Various fixed cycles for boring are available in CNC controls. For mills, these are called using G-codes such as G76, G85, G86, G87, G88, G89, and other codes specific to particular control builders or machine tool builders.
[edit] Lathes
Lathe boring[2] is a cutting operation that uses a single-point cutting tool or a boring head to produce conical or cylindrical surfaces by enlarging an existing opening in a workpiece. For nontapered holes, the cutting tool moves parallel to the axis of rotation. For tapered holes, the cutting tool moves at an angle to the axis of rotation. Geometries ranging from simple to extremely complex in a variety of diameters can be produced using boring applications. Boring is one of the most basic lathe operations next to turning and drilling.
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.
The geometry produced by lathe boring is usually of two types: straight holes and tapered holes. Several diameters can also be added to each shape hole if required. To produce a taper, the tool may be fed at an angle to the axis of rotation or both feed and axial motions may be concurrent. Straight holes and counterbores are produced by moving the tool parallel to the axis of workpiece rotation.
The three most commonly used workholding devices are the three-jaw chuck, the four-jaw chuck, and the face plate. The three-jaw chuck is used to hold round workpieces because the work is automatically centered. The four-jaw chuck is used to hold irregular shapes because of its independent action on each jaw. The face plate is also used for irregular shapes that need to be through-bored.
For most lathe boring applications, tolerances are held within ±0.002 in (±0.05 mm) for deep holes. For precision applications, tolerances can be held within ±0.0005 in (±0.013 mm) only for shallow holes. Surface finish may range from 8 to 250 microinches, with a typical range between 32 and 125 microinches.
[edit] See also
[edit] References
[edit] Bibliography
- Kalpakjian, Schmid (2001), Manufacturing Engineering and Technology, Upper Saddle River, NJ, USA: Prentice Hall
- Todd, Robert H.; Allen, Dell K. (1994), Manufacturing Processes Reference Guide, New York, NY, USA: Industrial Press
