Machining and turning

What are machining and turning? Are they two similar operations? When talking about machining and turning, you should know that one cannot exist without the other, because turning is one of the three types of machining that exist. What are the characteristics of each of these procedures? How do they differ from one another?

What are machining and turning?

In the metalworking industry, there is one operation for manufacturing metal parts, which is machining. Machining means shaping a piece of metal until it becomes a part with specific characteristics. When we talk about machining, we are referring to a series of industrial processes carried out on the raw material (metal, wood, ceramic or plastic), consisting of cutting, pressing, marking, turning, milling, drilling, and so on. When machining, the part is given specific dimensions.

And what is the relationship between machining and turning? Today there are three different types of machining. Depending on the machine tool used, they are:

• With swarf removal. This is the type of machining carried out with machines such as lathes and milling machines, in which an amount of material is removed from the workpiece and swarf in the form of small flakes is generated.
• Without swarf removal. This is a type of machining in which the machines do not remove material in the form of swarf. It is generally carried out by machines such as presses, which form parts such as sheet metal. That is, material is not removed but deformed.
• Abrasion. This is a type of machining carried out by machine tools such as grinding machines. Abrasion machining uses specific cutting tools known as grinding wheels. These make a kind of pass over the surface of the part and remove small amounts of material.

On the other hand, turning is a type of machining by swarf removal in which large amounts of material are removed from the surface of the workpiece. Therefore, machining involves removing material, while turning is the machining operation in which surplus parts of a workpiece are removed by means of a machine tool called a lathe.

What is a lathe?

When talking about machining and turning, we inevitably have to refer to the cutting tool that carries out these turning operations. The lathe is a group of machine tools that not only turn (material removal by cutting swarf) but can also perform other operations. Depending on the lathe chosen, it is possible to thread, part off, drill, straight-turn, groove and rough parts.

The parts worked on the lathe are geometric parts of revolution. They are said to be parts of revolution because they are placed in the headstock of the machine tool and rotate on their own axis at high speed. Machining by turning is characterised precisely by the fact that it is the workpiece that rotates and approaches the tip of the cutting tool. The cutting tool receives a controlled pushing or feed movement towards the surface of the workpiece. When the two meet, swarf removal takes place.

On lathes, the cutting tool (these can be inserts or drills, as the lathe performs different operations) is mounted on a carriage that holds the tool and moves along guideways parallel to the axis of rotation of the workpiece (Z axis). On this same carriage there is another that moves along the X axis, that is, radially to the workpiece. On some lathes there is a third carriage, called the compound slide, which is used for machining and turning tapered parts.

Machining and turning: working movements

Different movements take part in the operations carried out on the lathe, depending on the machining and turning to be done. In general, all machine tools have different parameters so that the work is of high quality. These movements or parameters are:

• Cutting. The cutting movement in turning is made by the workpiece rotating about the main axis and is provided by the motor, which transmits the rotation to the spindle where the workpiece is held. This movement is transmitted through pulleys or gears. On conventional lathes, there is a fixed range of cutting speeds.
• Feed. The feed movement is made by the cutting tool directly on the workpiece. When it is not parallel to the workpiece, tapered shapes are machined. For taper work, it is the compound slide that must swivel.
• Depth of cut. This parameter determines the depth to which the cutting tool must penetrate the workpiece to remove the swarf. The depth of cut depends on the material of the workpiece, the cutting tool, the speed, the power and the feed rate.

Some machining and turning operations

Straight turning

Straight turning consists of machining cylindrical parts externally or internally. The cross and longitudinal slides are used to set the depth of cut and the diameter of the workpiece.

Grooving

An operation by which cylindrical grooves of varying width and depth are created on the workpiece to be machined.

Threading

Some lathes can cut threads. Centre lathes, by means of the Norton gearbox, are able to create threads on parts. It is also possible to carry out this type of operation on CNC lathes. Threads can be internal or external.