a machine for boring (manufacturing) holes
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 gun barrel or an engine cylinder. Boring is used to achieve greater accuracy of the diameter of a hole, and can be used to cut a tapered 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.
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.
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).
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 apply. 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—are preferably avoided. The boring bar is the protruding arm of the machine that holds cutting tool(s), and must be very rigid.
Because of the factors just mentioned, deep-hole drilling and deep-hole boring are inherently challenging areas of practice that demand special tooling and techniques. Nevertheless, technologies have been developed that produce deep holes with impressive accuracy. In most cases they involve multiple cutting points, diametrically opposed, whose deflection forces cancel each other out. They also usually involve delivery of cutting fluid pumped under pressure through the tool to orifices near the cutting edges. Gun drilling and cannon boring are classic examples. First developed to make the barrels of firearms and artillery, these machining techniques find wide use today for manufacturing in many industries.
Various fixed cycles for boring are available in CNC controls. These are preprogrammed subroutines that move the tool through successive passes of cut, retract, advance, cut again, retract again, return to the initial position, and so on. These are called using G-codes such as G76, G85, G86, G87, G88, G89; and also by other less common codes specific to particular control builders or machine tool builders.