Turning operation

What is Turning operation?
The process in which the material of the workpiece is removed with the help of a tool is known as turning operation.

Describe the tool geometry associated with turning operation?
In machining operations the angles associated with a single point cutting tool has a lot of important functions. .
These angles are measured in a coordinate system consisting of the three major axes of the tool shank.
Rake angle helps to control the flow of chip and the tool tip strength.
Reducing forces and temperatures helps in improving in the cutting operation which is mainly due to positive rake angles. .
But the positive angles also leads to a small included angle of the tool tip, which results in premature tool chipping and failure.

Right hand cutting tool
Fig. 1 Right hand cutting tool

Side rake angle has greater importance than the back rake angle, although the back rake angle controls the direction of chip flow. These angles are in the range from -5° to 5° for machining metals and using carbide inserts.
Cutting-edge angle mainly influence formation of chip, strength of tool, and cutting forces to various degrees.
Typically, the cutting-edge angle is around 15°.
Relief angle helps in controlling interference and friction at the tool workpiece interface. The tool tip may chip off if the angle is too large and the flank wear may be excessive if the angle is too small.
Relief angles typically are 5 °.
Nose radius influences surface finish and strength of tool tip.
The effect of smaller nose radius (sharp tool) is that it provides surface finish with greater roughness and reduces the strength of tool.

What is material removal rate?
The material-removal rate (MRR) in turning is the amount of material removed from workpiece per unit time, with the units of mm3/min.
For every revolution of the workpiece, a circular layer of material is removed which has a cross-sectional area that is equivalent to the product of the distance the tool covers in one revolution (feed, f) and the depth of cut, d.
The volume of this ring is the product of the cross-sectional area (f)(d) and the average circumference of the ring, where

For gentle cuts on greater diameter workpieces, the average diameter may be replaced by Do.
The rotational speed of the workpiece is N, and the material removal rate per revolution is (1-r)(Davg)(d)(f).
As there are N revolutions per minute, the removal rate is

The dimensional accuracy of this equation can be checked by substituting dimensions into the right-hand side. For instance, (mm)(mm)(mm/rev)(rev/min) = mm3/min, which indicates volume rate of removal.

Here V is the speed of cutting and MRR has the unit of mm3/min.
The cutting time, t, for a workpiece of length I can be calculated by noting that the tool travels at a feed rate of fN = (mm/rev)(rev/min) = mm/min. Since the distance traveled is I mm, the cutting time is

Describe the forces produced during turning operation?

Forces acting on cutting tool
Fig. 2 Forces acting on cutting tool.

The three principal forces acting on a cutting tool are shown in Fig. 2.
In design of machine tools these forces plays an important role and also in the deflection of tools.
The machine tool and its components must be able to withstand such forces without causing significant deflections, vibrations, and chatter in the overall operation.
As the cutting force, Fc, acts downward on the tool tip and hence this leads to deflection of the tool downward and the workpiece upward.
The energy necessary for the cutting process is provided by the cutting force.
The torque on the spindle is the product of cutting force and the radius of workpiece.
The power required in the process is equivalent to the product of torque and spindle speed.
The thrust force, Ft, acts in the longitudinal direction.
As the thrust force is along the feed direction of the tool it is also known as feed force.
The speciality of this force is that it pushes the tool towards the right and away from the chuck.
As the name suggests the radial force, Fr, acts in the radial direction and tends to push the tool away from the workpiece.

What is roughing and finishing cuts?
In machining, the usual procedure is to first perform one or more roughing cuts at high feed rates and large depths of cut, but with little consideration for dimensional tolerance and surface roughness.
Finishing cuts are done after the roughing cut at a lower feed and depth to produce a good surface finish.


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