Turning operation

There are three principals of machining process which are turning, drilling and milling. Other operations fall into miscellaneous categories such as shaping, planning, boring, broaching and sawing. The focus of this thesis is on turning operation. Turning operation is one of the oldest and most versatile conventional ways of producing parts that are basically in round shape. Turning means that the work piece is rotating while it is being machined. The starting material is usually a work piece that has been made by other processes such as casting, forging or extrusion.

A conventional lathe which normally turning is performed is illustrated in Figure 2.1. One end of the work piece is fixed to the spindle by chuck and the other end is pin mounted to the tails stock as can be seen in Figure 2.1. The machine consists of a headstock which is mounted on the lathe bed. The headstock contains the spindle that rotates the cylindrical work piece that is held in the chuck. The single point cutting tool is placed at the tool holder that is mounted on the cross slide. The cross slide is in turn mounted on the carriage.

13 Figure 2.1: Conventional lathe machine at University of Liverpool

On a lathe, the tool is held rigidly in a tool post and moved at a constant rate along the axis of the work piece, cutting away a layer of metal to form a cylinder as shown in Figure 2.2. The tool is fed either linearly in the direction parallel or perpendicular to the axis of rotation of the work piece. The work piece will experience a rotary motion whereas the cutting tool will experience a linear translation. The three components of the cutting force acting on the rake face of the tool are also depicted in Figure 2.2. Normal to the cutting edge is called the tangential force, Py. This usually is the largest of the three components

and acts in the direction of cutting velocity. The force component acting on the tool, parallel with the direction of feed, is referred to as feed force, Px. This force

acts in the normal direction to the main cutting forces Py. The third component,

Pz , tend to push the tool away from the work in the radial direction, is the

smallest of the force components in simple turning operation.

Figure 2.2 also shows the cutting parameters involved in turning operation such as depth of cut, feed rate and cutting speed. A thorough

14 knowledge of the variable factors of cutting speeds, feed rate and depth of cut must be understood (Trent and Wright, 2000) and below are the definitions for each of the turning process parameters.

The cutting speed (V) is the rate at which the uncut surface of the work passes the cutting edge of the tool, usually expressed in units of m/minor ft/min. The cutting speed of a tool is the speed at which the metal is removed by the tool from the work piece. Cutting speed is usually between 3 and 200 m/min(10 and 600 ft/min) (Trent and Wright, 2000). The cutting speed can be calculated using the equation 2.1 below:

𝑉 =

where V is the cutting speed (m/min), N is the spindle speed (rev/min) and d is the work piece diameter. Since 𝜋𝑑 is constant, thus the cutting speed depends on the spindle speed in which it is usually being determined first before actual turning operation according to Trent and Wright (2000).

The feed rate (f) is the distance moved by the tool in an axial direction at each revolution of the work piece. The feed rate may be as low as 0.0125 mm (0.0005 in) per revolution and with very heavy cutting, it can go up to 2.5 mm (0.1 in) per revolution as mentioned by Trent and Wright (2000). Equation 2.2 is normally used to calculate the feed rate;

𝐹𝑒𝑒𝑑 𝑅𝑎𝑡𝑒 = 𝑓𝑒𝑒𝑑 x 𝑁

The depth of cut (w) is the thickness of the metal removed from the work piece, measured in radial direction. A depth of cut is the perpendicular distance measured from the machined surface to the uncut surface of the work piece. A

15 depth of cut may vary from zero to over 25 mm (1 in). Equation 2.3 is sometimes used to define a depth of cut;

𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝑐𝑢𝑡 =

2

where 𝑑₁ is diameter of the work surface before cutting and 𝑑₂ is the diameter of the machined surface. The unit of a depth of cut is in mm.

The rotational speed () or sometimes called speed of revolution is the number of complete rotations, revolutions, cycles, or turns per time unit. It is a cyclic frequency, measured in radians per second or in hertz or in revolutions per minute (rev/min or min-1) or revolutions per second in everyday life. Equation 2.4 is used to define a rotational speed;

𝜔 = 𝑣

𝑟 (2.4)

where v is a tangential speed and r is a radial distance.

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