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Turning

This article is about the machining term. For the generic use of the word, see rotating. This article or section is in need of attention from an expert on the subject.
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A cabinet maker turning a piece of wood in Tanzania

Turning is the process whereby a centre lathe is used to produce "solids of revolution". It can be done manually, in a traditional form of lathe, which frequently requires continuous supervision by the operator, or by using a computer controlled and automated lathe which does not. This type of machine tool is referred to as having computer numerical control, better known as C.N.C. and is commonly used with many other types of machine tool besides the lathe.

When turning, a piece of material (wood, metal, plastic even stone) is rotated and a cutting tool is traversed along 2 axes of motion to produce precise diameters and depths. Turning can be either on the outside of the cylinder or on the inside (also known as boring) to produce tubular components to various geometries. Although now quite rare, early lathes could even be used to produce complex geometric figures, even the platonic solids; although until the advent of C.N.C it had become unusual to use one for this purpose for the last three quarters of the twentieth century. It is said that the lathe is the only machine tool that can reproduce itself.

Facing is part of the turning process. It involves moving the cutting tool across the face (or end) of the workpiece and is performed by the operation of the cross-slide, if one is fitted, as distinct from the longitudinal feed (turning). It is frequently the first operation performed in the production of the workpiece, and often the last- hence the phrase "ending up".

The bits of waste metal from turning operations are known as chips (North America), or swarf in Britain. In some locales they may be known as turnings.

ADDITIONAL INFORMATION

Turn turning.gif

Typical turning process

Turning machine.jpeg

Common turning machine

Facing.gif

Typical facing process

Engine lathe.gif

Engine lathe

Turning is one of the most basic machining processes. That is, the part is rotated while it is being machined. The starting material is generally a workpiece generated by other processes such as casting, forging, extrusion, or drawing.


TURNING OPERATIONS

The turning processes are typically carried out on a lathe, considered to be the oldest machine tools, and can be of four different types such as straight turning, taper turning, profiling or external grooving. Those types of turning processes can produce various shapes of materials such as straight, conical, curved, or grooved workpiece. In general, turning uses simple single-point cutting tools. Each group of workpiece materials has an optimum set of tools angles which have been developed through the years.


LATHES

A lathe is a machine tool used principally for shaping pieces of metal, sometimes wood, or other materials by causing the workpiece to be held and rotated by the lathe while a tool bit is advanced into the work causing the cutting action. Lathes can be divided into three types for easy identification: engine lathe, turret lathe, and special purpose lathes. Some smaller ones are bench mounted and semi-portable. The larger lathes are floor mounted and may require special transportation if they must be moved. Field and maintenance shops generally use a lathe that can be adapted to many operations and that is not too large to be moved from one work site to another. The engine lathe is ideally suited for this purpose. A trained operator can accomplish more machining jobs with the engine lathe than with any other machine tool. Turret lathes and special purpose lathes are usually used in production or job shops for mass production or specialized parts, while basic engine lathes are usually used for any type of lathe work.


Turret.jpeg

Turret machine

TOOL GEOMETRY

The various angles in a single-point cutting tool have importance functions in machining operations. Different types of angle such as rake angle, side rake angle, cutting-edge angle, relief angle, nose radius exist and may be different with respect to the workpiece.

MATERIAL

Aluminium, copper alloys, steels, stainless steels, high-temperature alloys, refractory alloys, titanium alloys, cast irons, thermoplastics, thermosets, etc… are examples of different type of materials used.


MATERIAL REMOVAL RATE

The material removal rate (MRR) in turning is the volume of material removed per unit time in mm3/min. For each revolution of the workpiece, a ring-shaped layer of material is removed.

MRR = pi×Davg×d×f×N where

Davg: Average diameter
N: Rotational speed of the workpiece
f: Feed
d: Depth of cut


TURNING FORCES

The forces acting on a cutting in turning are important in the design of machine tools. The machine tool and its components must be able to withstand these forces without causing significant deflections, vibrations, or chatter during the operation. There are three principal forces during a turning process: cutting force, thrust force and radial force.

•The cutting force acts downward on the tool tip allowing deflection of the workpiece upward. It supplies the energy required for the cutting operation.
•The thrust force acts in the longitudinal direction. It is also called the feed force because it is in the feed direction of the tool. This force tends to push the tool away from the chuck.
•The radial force acts in the radial direction and tends to push the tool away from the workpiece.

Although it requires less-skilled labor, the engine lathes do need skilled labor and the production is somewhat slow. Moreover, it can be accelerated by using a turret lathe (In a turret lathe, a longitudinally feedable, hexagon turret replaces the tailstock. The turret, on which six tools can be mounted, can be rotated about a vertical axis to bring each tool into operating position, and the entire unit can be moved longitudinally, either mannually or by power, to provide feed for the tools) and automated machines.

REFERENCES

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See also

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