A boring machine is a device for accurately enlarging and finishing holes in tools, dies, and machine parts. Boring machines are also used for hole-originating operations such as drilling. Holes are bored with a cutting tool that is either rotating or stationary. If stationary, the piece being machined is rotated on a table. A boring machine is similar in construction to a drill press, but it has a more rigid spindle (the shaft that drives the cutting tool) and a table that can be very accurately moved in two directions for exact positioning of holes.
There are two main types of boring machines, horizontal boring mills and jig borers, in addition to many special-purpose types. Horizontal boring mills have a large, rigid spindle with a horizontal axis of rotation. The spindle, which can be accurately raised and lowered, drives a long boring bar that carries the cutting tool. The most common cutting tool used in horizontal boring mills is single pointed. Drills, end mills, and face milling cutters are also used. The work-piece is clamped to a vertical table that can be accurately moved horizontally in two directions. A rotary table is frequently mounted on the basic table for the angular alignment of holes. As boring proceeds, the cutting tool is "fed" into the hole by moving the spindle in the direction of its axis. Horizontal boring mills are commonly used for machining the accurately aligned holes that serve as bearing or shaft supports in cast or welded housings.
Jig Borers have an extremely precise spindle that rotates about a
vertical axis. The workpiece is mounted on a horizontal table that can
be moved horizontally in two directions by means of lead screws. The
screws are turned each with the aid of a large drum micrometer. They
are used to locate, or position, the table to an accuracy of .0001 inch
(.00025 cm). The screws of some jig borers have a cam that turns the
micrometer to compensate for errors caused by the pitch of the screw.
These modified machines can locate the workpiece to an accuracy of
.00005 inch (.000125 cm). Jig borers are used to drill, ream, and bore
holes in jigs, fixtures, dies, and tools.
Both horizontal boring machines and jig borers require operators who are highly skilled machinists. Special boring machines require a skilled machinist to make the original setup, but a trained operator, who need not be a machinist, can load and unload the parts and manipulate the levers.
There are several types of special purpose boring machines. One is a vertical boring mill which is used for boring and turning (machining the outside diameter of a cylinder) large work-pieces. The equivalent operations on small work-pieces can be done on a lathe. The vertical boring mill has one or two vertical columns that support a horizontal rail to which the toolhead is attached. Additional horizontal rails can be provided when two or more cutting tools are used simultaneously. The vertical boring mill has a large table that rotates about a vertical axis. The horizontal table makes it relatively easy to set up large workpieces. Some vertical boring mills contain automatic control devices to cycle the tool-heads so that all the operator needs to do is to load and unload the parts.
Another type of special-purpose boring machine is the precision boring machine, which is used for the mass-production machining of parts. It may be either vertical or horizontal and may have one or more parallel spindles. Consequently, instead of a movable table to hold the workpiece, precision boring machines contain special fixtures that accurately and rapidly clamp and locate the part. The cutting tools are usually single-pointed and tipped with sintered carbide, ceramic, or industrial diamonds. Precision boring machines bore holes slowly, while the cutting tool rotates at high speed, to produce accurate and smooth holes in such parts as engine blocks, connecting rods, and bushings.