Let’s first work to give an understanding of turning. Turning is a machining process in which the workpiece (like a metal part) is rotated at a high speed, while a cutting tool is used to cut away the parts of the workpiece which aren’t needed. The cutting tool does not rotate, and it moves in a linear pattern. But, the cutting tool may move along a set of angles or curves.

Turning is normally done on the exterior surface of the workpiece, but sometimes it is also carried out on the inside. At that point, the process is known as boring. Similarly, cutting faces on the external side of the workpiece while it is rotating is known as facing.

Why is Turning Important?

In the manufacturing sector, the importance of turning is paramount. This is because engineers have to build complex designs which would not be possible without the process of turning. If you have ever inspected an assembly, you will understand why. In an assembly, there are literally thousands of parts interconnected with one another that also interact with each other to create a system. For example, an automobile can contain as many as 10,000 different parts.

All these parts are made of different types of materials and are of different sizes and shapes. Even the quality of the material varies with each part. All these parts are built by machinists using techniques like turning, casting, milling, welding, etc. Turning is carried out on a lathe machine.

How is it done?

As mentioned earlier, turning is done on a lathe machine. Being the oldest type of machine tool, a lathe machine is used to shape pieces of wood, metal, plastic, etc. During the process of turning, the workpiece which is usually rigid and made of either metal, wood, stone or plastic is rotated, while a cutting tool is moved along the workpiece in 3 different axes of motion in order to generate accurate depths and diameters.
There are four types of turning; profiling, straight turning, external grooving, and taper turning. These processes produce different types of shapes from a single workpiece like conical, curved, grooved or straight.

The Process

Turning can be done in either of two ways; manually or through the use of a computer. In the older days, turning was done by hand and the operator had to constantly monitor the process. It was time-consuming and inaccurate. With advancements in electronics and information technology, the use of computer-numerical control (CNC) became widespread and machines equipped with microcomputers were and are being used to conduct turning.

In CNC turning, a computer program is written by the operator with all the necessary dimensions and variables and the file is transmitted to the lathe machine. Using stepper motors to control the cutting tool, the machine automatically yields the best results. It is highly accurate, flexible, and quicker than doing it manually.
 

Sources
https://www.worldskills.org/what/career/skills-explained/manufacturing-and-engineering-technology/cnc-turning/
http://www.slideshare.net/APM_Longmont/what-is-cnc-turning
http://custommachining.ickler.com/blog/whats-the-difference-between-cnc-milling-and-cnc-turning
http://www.mnbprecision.com/what-is-cnc-turning/