Boring (mechanical)
Encyclopedia
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).
The boring process can be executed on various machine tool
s, including (1) general-purpose or universal machines, such as lathe
s (/turning centers) or milling machine
s (/machining centers), and (2) machines designed to specialize in boring as a primary function, such as jig borer
s 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).
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 m (3.3 to 13.1 ) in diameter, but can be as large as 20 m (65.6 ft). Power requirements can be as much as 200 hp. 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.
Various fixed cycles for boring are available in CNC controls. For mills, these are called using G-code
s such as G76, G85, G86, G87, G88, G89, and other codes specific to particular control builders or machine tool builders.
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.
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...
, boring is the process of enlarging a hole that has already been drilled
Drilling
Drilling is a cutting process that uses a drill bit to cut or enlarge a hole in solid materials. The drill bit is a multipoint, end cutting tool...
(or cast), by means of a single-point cutting tool
Tool bit
The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or...
(or of a boring head containing several such tools), for example as in boring a cannon
Cannon
A cannon is any piece of artillery that uses gunpowder or other usually explosive-based propellents to launch a projectile. Cannon vary in caliber, range, mobility, rate of fire, angle of fire, and firepower; different forms of cannon combine and balance these attributes in varying degrees,...
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).
Machine tools used
Machine tool
A machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
s, including (1) general-purpose or universal machines, such as lathe
Lathe
A lathe is a machine tool which rotates the workpiece on its axis to perform various operations such as cutting, sanding, knurling, drilling, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation.Lathes are used in woodturning,...
s (/turning centers) or milling machine
Milling machine
A milling machine is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines...
s (/machining centers), and (2) machines designed to specialize in boring as a primary function, such as jig borer
Jig borer
The jig borer is a type of machine tool invented at the end of World War I to make possible the quick-yet-very-precise location of hole centers. It was invented independently in the United States and Switzerland...
s 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
Horizontal boring machine
A horizontal boring machine or horizontal boring mill is a machine tool which bores holes in a horizontal direction. There are three main types — table, planer and floor. The table type is the most common and, as it is the most versatile, it is also known as the universal type.A horizontal...
(workpiece sits on a table while the boring bar rotates around a horizontal axis; essentially a specialized horizontal milling machine).
Boring mills and milling machines
The dimensions between the piece and the tool bitTool bit
The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or...
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
Cemented carbide
Cemented carbide, also called tungsten carbide, hardmetal, or widia, is a hard material used in machining tough materials such as carbon steel or stainless steel, as well as in situations where other tools would wear away, such as high-quantity production runs. Most of the time, carbide will leave...
. 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 m (3.3 to 13.1 ) in diameter, but can be as large as 20 m (65.6 ft). Power requirements can be as much as 200 hp. 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.
Various fixed cycles for boring are available in CNC controls. For mills, these are called using G-code
G-code
G-code is the common name for the most widely used computer numerical control programming language, which has many implementations. Used mainly in automation, it is part of computer-aided engineering. G-code is sometimes called G programming language...
s such as G76, G85, G86, G87, G88, G89, and other codes specific to particular control builders or machine tool builders.
Lathes
Lathe boring 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.