Lathe Intro

The Lathe
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 round or 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 tailstock.
Turning can produce long chips that may interfere with the work in progress. The right cutting tools and proper lubrication are used to control chip formation.

Very large work with swing dimensions beyond the capacity of a horizontal machine are turned on vertical lathes that can accept work two to six feet in diameter.
Raw stock is brought to the lathe in several ways. Long lengths can be fed through the head stock, short lengths or “slugs” can be manually or automatically chucked. Gantry systems are used to handle large, heavy pieces of stock.

Production lathes bring tools and arrangements of tools to the work by the use of turrets on larger machines and slide mounted “gang tooling” on smaller, more compact lathes.
Regardless of the type of lathe, three key parameters determine productivity and part quality.

These parameters are:

• the cutting speed
• the feed rate
• the depth of cut

The cutting speed is the speed of the work as it rotates past the cutting tool.

The feed rate is the rate at which the tool advances into the work.

The depth of cut is the amount of material removed as the work revolves on its axis.

Other factors include the machinability of the stock, the type and the geometry of the cutting tool, the angle of the tool to the work, and the overall condition and power of the lathe itself.

SAFETY

All lathe operators must be constantly aware of the safety hazards that are associated with using the lathe and must know all safety precautions to avoid accidents and injuries. Carelessness and ignorance are two great menaces to personal safety. Other hazards can be mechanically related to working with the lathe, such as proper machine maintenance and setup. Some important safety precautions to follow when using lathes are:

  • Correct dress is important, remove rings and watches, roll sleeves above elbows.
  • Always stop the lathe before making adjustments.
  • Do not change spindle speeds until the lathe comes to a complete stop.
  • Handle sharp cutters, centers, and drills with care. Remove chuck keys and wrenches before operating Always wear protective eye protection.
  • Handle heavy chucks with care and protect the lathe ways with a block of wood when installing a chuck.
  • Know where the emergency stop is before operating the lathe.
  • Use pliers or a brush to remove chips and swarf, never your hands.
  • Never lean on the lathe.
  • Never lay tools directly on the lathe ways. If a separate table is not available, use a wide board with a cleat on each side to lay on the ways.
  • Keep tools overhang as short as possible.
  • Never attempt to measure work while it is turning.
  • Never file lathe work unless the file has a handle.
  • File left-handed if possible.
  • Protect the lathe ways when grinding or filing.
  • Use two hands when sanding the workpiece.
  • Do not wrap sand paper or emery cloth around the workpiece.

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