Depth of Cut on a Lathe

In operations carried out on the lathe, the workpiece is held in the headstock or chuck and rotated at high speed, while the cutting tool is pushed against the surface of the workpiece in a feed motion to shape it. The result is the removal of surplus material by metal cutting. But how important is the depth of cut on a lathe for achieving the results you want?

Cutting parameters in turning

Today you can find many different types of lathe in industry: centre lathes, vertical lathes, copying lathes, automatic lathes, turret lathes, computer numerical control lathes and so on. But what do these lathes have in common? Besides carrying out turning operations (some have more functions than others), they are all governed by the same cutting parameters in order to machine.

Without the right parameters, machining operations would simply be impossible to carry out, or the results would not be what you want. What are these parameters and how are they calculated?

• Cutting speed, expressed in metres per minute.
• Revolutions per minute of the spindle (rpm).
• Feed rate.
• Depth of cut on a lathe or pass depth.
• Outside diameter.
• Cutting force.
• Choosing the right cutting tool.
• Type of lathe.

Cutting speed

This is the linear speed of the workpiece surface that comes into contact with the tool. It is expressed in metres per minute and must be chosen before machining starts. This parameter depends on the type of tool, the pass depth, the hardness of the material, its machinability and, of course, the type of work you want to do. In addition, the feed rate must also be taken into account when choosing the cutting speed.

From the cutting speed you can work out the revolutions per minute.

Spindle revolutions per minute

The revolutions per minute (rpm) express the rotational speed, that is, the number of turns an object makes about its own axis in one minute.

Feed rate

The feed rate is the relative speed between the workpiece being machined and the cutting tool, that is, the speed at which the cut takes place. It is expressed as feed per revolution and depends on the diameter of the workpiece, the depth of cut on a lathe and the tool chosen.

Outside diameter

For any type of machining it is essential to know the design and shape you want to give the workpiece. In turning operations, you need to know the outside diameter of the workpiece in order to determine the pass depth and the feed rate.

Cutting force

The cutting force is a parameter you need in order to calculate the power required for machining. It is calculated from the tool feed, the cutting speed and the pass depth.

Choosing the right cutting tool

Since a lathe can carry out many different machining operations, it is essential to choose the right tool. For example, for drilling you will need a drill bit and for grooving a slotting tool, and so on.

Type of lathe

As we have already mentioned, there are many types of lathe in industry, but they do not all have the same functions. A centre lathe, for example, does not do the same jobs as a copying lathe or a numerically controlled CNC lathe. That is why, to machine properly, it is essential to choose the machine tool that best suits your machining needs.

What is depth of cut on a lathe?

The depth of cut on a lathe, or pass depth, is a parameter of this machine tool that determines how far the tool penetrates the workpiece on each machining pass. In other words, it is the depth of the layer removed from the surface of the workpiece in one pass of the tool. This parameter is usually expressed in millimetres, and the width of cut, which is the width of the part of the workpiece being machined, is also taken into account.

Why is depth of cut on a lathe so important? It plays a key role in calculating the cross-section of the swarf and, as a result, the force that must be applied to machine. Pass depth can therefore be defined as the distance the tool penetrates the surface of the workpiece each time it makes contact with it.

It has the following characteristics:

• It has a permissible maximum value.
• If you want deep cuts, you will need to make several passes in succession over the same point on the surface of the workpiece.
• It depends on the tool and its rigidity, as well as on the power of the machine, the feed rate, and the material and design of the workpiece.
• The pass depth value must be greater than the radius of the nose of the cutting tool.

Each turning operation requires a different depth of cut. Grooving, where swarf is removed with a slotting tool, is not the same as grinding, where machining is done by abrasion with a grinding wheel.