Characteristics of workpieces to be machined on a lathe

The machining carried out on lathes is characterised by swarf removal, that is, the surplus part of a workpiece, until it takes the desired shape. This type of machining is called turning, but what are workpieces to be machined on a lathe like? What characteristics must they have?

Machining on a lathe

The lathe is the first machine tool ever invented and over the years it has adapted to new technologies and to the needs of the machining industry. Different types of work can be carried out on this machine tool, and certain stages are followed to achieve quality results. If the right tools are used, it may not be necessary to complement turning with other types of machining on the finish.

Workpieces to be machined on a lathe can undergo many different procedures to shape them. Among the operations a lathe can perform are:

Straight turning	Workpieces to be machined on a lathe that need to reduce their diameter, must be worked by straight turning.
Facing	Front-end machining to remove imperfections and reduce length.
Drilling	Operation carried out to make holes in the surface of the workpiece with the help of a drill bit.
Reaming	Used for surface finishes or to create holes with greater accuracy than drilling.
Boring	Method for enlarging existing holes.
Chamfering	It consists of breaking the edges of a material.
Parting off	Used to separate the finished workpiece from the surplus material.
Threading	It can be carried out on conventional centre lathes (with the Norton gearbox) and on CNC lathes.
Grooving	Machining that makes grooves in the surface of workpieces.
Taper turning	It consists of machining workpieces into a cone shape.
Knurling	This operation marks the surface of the workpiece to increase its grip by hand.

Characteristics of workpieces to be machined on a lathe

In industry you can find different types of lathe, including those that work by CNC computer numerical control; but all types of lathe offer great advantages for metalworking. For example, a lathe that offers a good surface finish requires less energy than other processes and makes it easy to control the swarf.

So what are workpieces to be machined on a lathe like? Above all, a lathe is very versatile and can accept workpieces of different sizes, shapes and materials. It all depends on the type of lathe used because, for example, CNC lathes accept workpieces with much more complex designs than a conventional lathe can produce.

Materials that lathes accept

Lathes allow the machining of parts made of different materials:

• Stainless steel.
• Carbon steel.
• Aluminium.
• Copper.
• Iron.
• Zinc.
• Magnesium.
• Lead, nickel, ceramics, titanium and tin.
• Wood.

On computer numerically controlled lathes, other materials can be worked, such as iron, acrylic, glass and other raw materials.

It is also worth pointing out that most lathes today work with workpieces made of stainless steel (SUS). This material has the advantage of high strength after cold working; stainless steel is also used in many industries: to make vehicles, conveyor belts, screws and nuts, sheet metal and so on.

Stainless steel can be:

• AISI 304: this is the most common form of stainless steel because it resists corrosion from the most oxidising acids, but also because it is quite inexpensive. However, it does not resist corrosion by chloride solutions.
• AISI 316: unlike 304, this steel contains between 2 and 3% molybdenum, which makes it much more corrosion-resistant, even to chloride solutions.
• AISI 430: this is a ferritic stainless steel and contains chromium and carbon among its components. This type of steel is widely used to make kitchen utensils. Titanium or niobium is usually added to this type of steel to stabilise the carbon, although aluminium can sometimes also be used as a stabiliser.

The size of workpieces to be machined on a lathe

The lathe accepts workpieces of different sizes. Their size depends on the type of machining to be carried out, since facing a workpiece is not the same as straight turning it, for example. Before the workpiece is placed on the lathe, it needs an initial rough treatment and to be given the dimensions required for machining. The aim of this preparation is to ensure parallelism between its ends, as well as perpendicularity to its axis. It is also necessary to rough the raw workpiece to remove oxides or surface imperfections.

The operations for preparing the roughing are:

• Facing: to obtain the final length of the workpiece.
• Centre drilling: allows the centre and the tailstock to be used to hold the workpiece with the chuck. This makes the machining much more stable.
• Straight turning: to obtain the outside diameter of a workpiece.