Turning is one of the most common of metal cutting operations. In turning, a workpiece is rotated about its axis as single-point cutting tools are fed into it, shearing away unwanted material and creating the desired part. Turning can occur on both external and internal surfaces to produce an axially-symmetrical contoured part.
Parts ranging from pocket watch components to large diameter marine propeller shafts can be turned on a lathe. The capacity of a lathe is expressed in two dimensions. The maximum part diameter, or "swing," and the maximum part length, or "distance between centers."
The general-purpose engine lathe is the most basic turning machine tool. As with all lathes, the two basic requirements for turning are a means of holding the work while it rotates and a means of holding cutting tools and moving them to the work.
The work may be held on one or by both its ends. Holding the work by one end involves gripping the work in one of several types of chucks or collets. Chucks are mounted on the spindle nose of the lathe, while collets usually seat in the spindle. The spindle is mounted in the lathe's "headstock," which contains the motor and gear train that makes rotation possible. Directly across from the headstock on the lathe is the "tailstock." The tailstock can hold the work by either a live or dead center. Work that is held at both ends is said to be "between centers." Additionally, longer workpieces may have a "steady rest" mounted between the headstock and tailstock to support the work. Typically workpieces are cylindrical, but square and odd shaped stock can also be turned using special chucks or fixtures.
Lathe cutting tools brought to the work may move in one or more directions. Tool movement on the engine lathe is accomplished using a combination of the lathe's "carriage", "cross slide", and "compound rest".
The carriage travels along the machine’s bedways, parallel to the workpiece axis. This axis is known as the "Z" axis.
Motion perpendicular to the work is called the "X" axis. On an engine lathe this motion is provided by the cross slide mounted on the carriage. Atop the cross slide is the "compound rest," which can be rotated to any angle and secured. The compound rest also holds the "tool post," where tools are mounted. Tools may also be mounted in the tailstock for end-working operations. External turning can be broken down into a number of basic operations. "Straight turning" reduces the work to a specified diameter equally along the work’s axis. "Taper turning" produces a taper along the axis of the workpiece. Tapers are produced by either offsetting the tailstock from centerline or by using a "taper attachment." Some short, steep tapers can be obtained by using the compound rest alone. "Contour turning" or "profiling" uses a single-point cutting tool to reproduce a surface contour from a template. This operation has been almost entirely replaced by numerically controlled or "NC" programming. "Forming" uses a cutting tool ground with the form or geometry of the desired shape. This forming tool is advanced perpendicular to the axis of the work to reproduce its shape on the workpiece.
Other external lathe operations include "chamfering" to remove sharp edges, "grooving" to produce recesses and shoulders, "facing" to finish the ends of a workpiece, "parting" to cut off finished pieces from the stock, and "thread chasing" with tools to produce the desired thread form.
The most common method of internal turning on the lathe is to present the rotating end of a workpiece to the point of a non-rotating drill bit mounted in the tailstock. Roughly drilled holes are finished to exact size by using a reamer which also mounts in the tailstock. Large diameter holes are made by boring. A boring bar with a cutting tool attached is moved along the work’s axis as in surface cutting, but inside a previously drilled hole. Internal threads are obtained by using tapping tools mounted in the...
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