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.

Machine boring

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 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.

The boring machines (similar to the milling machines such as the classic Van Norman) 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. 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. 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

Lathe boring

Lathe boring² is a cutting operation that uses a single-point cutting tool to produce conical and cylindrical surfaces by enlarging an existing opening in a workpiece. The cutting tool moves parallel to the axis of rotation and will internal helical feed marks. To produce a taper, the cutting tool moves at an angle to the axis of rotation. Mechanical boring operations performed on a lathe are used to manufacture components for a number of industries ranging from automotive and aerospace to recreational equipment and medical suppliers. 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.

Tolerances and surface finish

For most lathe boring applications, tolerances are held within +/-0.002in. for deep holes. For precision applications, tolerances can be held within +/-0.0005in. only for shallow holes. Surface finish may range from 8 to 250 microinches, with a typical range between 32 and 125 microinches.

Workholding methods

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.

Effects on work material properties

Residual surface stresses may create microcracking. Dull tools may cause workhardening of some materials, and high temperatures may cause tempering of hardened materials. Lathe boring has little effect on either the physical or chemical properties.

Time calculations

Speed, feed rate, retract rate, and length of cut are major variables for economic machining. Feed Rate(ipr) = F. Retract Rate(ipm) = R. Length of Cut(in.) = L. Boring Time=L/F. Retract Time=L/R.

References

¹Kalpakjian, Schmid. Manufacturing Engineering and Technology. Upper Saddle River, NJ: Prentice Hall, 2001.
²Todd, Robert H.; Allen, Dell K. (1994). Manufacturing Processes Reference Guide. New York: Industrial Press.