Turning parameters: How to select the right one?

Technical matters related to the lathe machining – automatic, manual, or CNC – are called 'turning parameters'. Combined, this translates to matters such as time required to manufacture part, surface quality, precision of turning, and degradation of used bits. 

Each of these parameters should be taken into consideration during turning – to achieve the expected quality of the machined part and to provide safety to the operator. What are the turning parameters and how does the selection of cutting parameters for turning impact the final results?

What are the technological parameters of turning?

Just like in other machining processes, before turning on the lathe – traditional or numerically controlled – an operator should set work parameters.  Among the most important are:

• rotation speed (n) – measures the lathe spindle speed. It is important due to the placement of machined material – directly do the rotating spindle. That Turing parameter is measured in revolutions per minute (RPM), 
• feed (f) – measures the movement of a cutting bit in time of a full lathe spindle rotation. It is measured in millimeters/inches per revolution. That turning parameter is often called a working feed', 
• feed rate (vf, sometimes called 'feed velocity') – measures velocity of cutting bit alongside machined element, 
• depth of machining (ap) – describes the depth of penetration of machined material with a selected cutting tool. In other words – a difference between the diameter of machined and unmachined surfaces, described in millimeters, 
• power (Pc) – measures a force required to overcome friction forces and efficiently remove material by cutting tool or other cutting bit. That turning parameter is described in watts and kilowatts, 
• machining force (Fc) – measures forces acting on the turned block and the cutting bit, 
• torque (Me) – measured in Newton meters (Nm) or pound-foot of force (lbf).

Another parameter, often treated as secondary to the feed rate – is the machining time for lathe (measured in minutes). Describes time required to turn one element from turning on the lathe to detachment of a machined part. Turning time includes parameters such as:

• length of movement of the cutting tool, 
• feed rate, 
• RPM, 
• amount of moves of the cutting tool alongside the turned element. 

That parameter helps to describe the time required to machine the whole element (in other words: create the expected shape) or to complete the partial stage of machining (e.g. initial, forming, or finishing). 
Turning time helps to more efficiently use lathes and it is even more important for CNC lathes and estimation of time required to finish work. Each of the above-mentioned turing parameters can help to select the best type of machining.

How to match turning parameters with selected material specifications?

One of the most important steps in the turning process is a selection of turning parameters that are matched with the selected material. Proper settings are important from the perspective of operators' safety and the final product – parameters can impact the overall quality of machining. 

Experienced lathe operators should select parameters based on:

• type of turned material and its mechanical parameters, 
• overall machinability of selected materials (that also includes knowledge of known material hardness or the ISO material group), 
• selected cutting tools (there is a wide selection of turning bits, often with similar properties, that should be selected and matched not only to material but also to expected finish quality or other turning parameters), 
• turning environment (for example – machined material quality or stability of mounting on the spindle), 
• lathe parameters, 
• stage of machining.

In a process of selecting the right parameters or turning operators often used as a basepoint gatherer of earlier data – often called a first catalog selection'.  Such knowledge helps to speed up work, estimate initial parameters, and select the right tools to turn a particular part most efficiently. 

Calculation of cutting parameters during turning is often made in the initial minutes of the whole process. Operators observe machined areas and decide on final parameters like feed rate or rotation speed. 

It is worth mentioning the turning parameters for automatic lathes are often tweaked by hand by an operator. Such specialist combines catalog data with observation of machined element. On the other hand – while working with CNC lathes the biggest support for every CNC operator is CAM software, which helps to select the right parameters of work. 

Additionally – due to the collection of additional data during the machining process via sensors – such software can adjust turning parameters on the fly. Such flexibility helps to achieve the best possible turning parameters and parts quality.

Initial, shaping, and finish turning: What are the best parameters for each step?

Machining – as a group of processes that include shaping material with mills, lathes, or grinders is often divided into 4 key stages: 

• Initial machining – focused on removing most of the unwanted material and achieving the approximate shape of the processed part, 
• medium precision machining – focused on giving part almost final shape, 
• precise machining – focused on the final shape of the part, 
• finishing machining – that is mainly focused on removing tool marks from the surface. Often it is executed with grinding tools (CNC and automatic).

The stages described above are of course only an example. 
Workpiece material requirements can require more or fewer stages – the final decision should be made by an experienced machining operator. 

Each material requires slightly different turning parameters or cutting tools. That also impacts the quality of the achieved surface – if the initial machining tool marks are completely normal, such imperfections are unwanted after finishing machining and can signal problems with turning parameters that can decrease a tool's life. 

The most often selected turning parameters on stages can be estimated as (working feed and machining depth of cut):

• for 'very' initial machining – >0.7 RPM and 8-20 mm, 
• for initial machining – 0.5-1.5 RPM and 6-15 mm, 
• for 'light' initial machining – 0.4-1 RPM and 3-10 mm, 
• for 'medium' precision machining – 0.2-0.5 RPM and 1.5-4 mm, 
• for finishing machining – 0.1-0.3 RPM and 0.5-2 mm, 
• for very precise machining – 0.05-0.15 RPM and 0.25-2 mm. 

In other words – the more precision is required, the whole turning process should be executed slowly. 

Turning parameters and bits wear

The selection of right-turning parameters has a direct influence on the longevity of machining tools. On top of the typical wear of the turning bit (from grinding), there are important reasons for premature tool degradation, wear, and often – damage.  Among many causes of that it is worth highlighting the most important: 

• adhesion – in other words: build up on the blade made of chips of machined material. Decreases the effectiveness of machining (due to building up less material that can be removed) and the quality of the surface. That problem is often caused by low machining speeds, 
• oxidation and diffusion – a machining material atoms sedimentation on the cutting tool. Decreases the efficiency of machining and can lead to dulling bit. Often observed in situations of low turning speed or poor heat dispersion.

The selection of the right machining parameters or cutting bits – for CNC or traditional turning – is a key element to achieving the highest quality parts. Experienced lathe operators can easily set the right parameters of work – suited for selected material or required precision.

One order, many benefits - this is how you work with RADMOT

At RADMOT, we offer CNC milling services, CNC turning services, as well as many additional services, including washing, aluminum anodizing, laser marking and assembly. We have at your disposal over 80 modern machines in our machine park, all from renowned manufacturers. Download the presentation and check on which machine tools we produce CNC turned parts and CNC milled parts.

Contact us and tell us what you need. We have been providing CNC services for almost 40 years. Our valuation is completely free. And if you're in doubt about which technology will work best for you, our expertise is at your service.