Machining parameters, including turning parameters (fig. 1), are a challenge for every technologist.
The technological cutting parameters in the turning process include:
- cutting speed vc [m/min] (1) – D – diameter of the workpiece; n – rotational speed of the workpiece (main drive)
- main drive speed n [obr/min];
- independent minute machining feed rate f [mm/min] (3) and dependent [mm/obr] (4) – distance by which the tool will move during one full revolution of the workpiece;
- cutting depth ap [mm] – distance between the machining surface and the machined surface (4) – d – diameter of the workpiece; d1 – diameter of the workpiece after machiing
In the case of conventional lathes, the selection of parameters was started with the determination of the cutting speed vc. In the next step, the value of rotational speed n for a given procedure was determined from the classical equation (1). The diameter D in the formula indicates the machined diameter. In the case of turning the face, the maximum diameter was assumed.
The chosen speed value n was compared with the Renard R20 or R40 series. Conventional lathes (figure 2) do not provide a stepless speed change. Selection of rotational speed n of the main drive consisted in comparing the calculation value with R20 or R40 (depending on the lathe) and choosing the speed n nearest but lower. Very rarely, the value determined was in line with the value from the Renard’s range.
The key task was to choose the cutting speed vc, which was determined based on the empirical formula taking into account the type of machining, workpiece material and tool material. Coefficients were selected from tabular data. The modern application of this algorithm is difficult due to incomplete tabular data. This is due to the introduction of new tool and work materials. Works on supplementing tabular data are not taken. CAM systems, CAE and expert systems have changed the approach to the selection of cutting parameters.
Selection of turning parameters
Nowadays, the selection of machining parameters is in the first step based on the parameters suggested by the suppliers of cutting tools. In the case of difficult-to-cut materials, one should also base on research to determine the optimal parameters. The overriding criterion of optimization is the cost of machining. When machining difficult-to-cut materials, the tool life period as well as the main machine time are the basic criteria for optimization. Figure 3 shows the algorithm for selecting machining parameters.
In order to facilitate the selection of parameters, ISO material groups are used. There are 6 material groups – table 1.
|Workpiece material||ISO||Exemplary material (form)|
|stainless steel||M||austenitic stainless steel|
|cast iron||K||grey cast iron, ductile cast iron|
|heat-resisiting alloy||S||highly alloyed based on iron, nickel, cobalt and titanium|
|hardened steel||H||hardened and tempered steel|
Type of machining
The type of processing in the accepted context refers to the qualitative aspects. There are three basic types of processing: roughing, shaping and finishing. Table 2 presents illustrative descriptions of particular types.
|Machining type:||Technological characteristics:|
At this stage of selecting the machining parameters, we define the conditions of the technological operation (table 3) which affects the selection of the tool material (eg type of cemented carbide with specific coatings).
|Good||Continuous machining. High speed. Pre-prepared workpiece. Very good fixing of the workpiece.|
|Average||Profile machining at medium speeds. The workpiece is a forging or casting. Good fixing of the workpiece.|
|Hard||Intermittent machining at low speeds. Casting and forging defects on the surface of the workpiece. Necessary use of inferior fixing of the workpiece.|
The catalogs, whether in paper or electronic version (eg Sandvik Coromant), lead the user enabling the first selection and adjustment of machining parameters to the individual technological task. When selecting cutting tools, experience and improved engineering knowledge bases of tool suppliers play an important role. Despite the existence of classes of typical parts of machines and equipment, and due to the multifaceted nature of the cutting process itself, as well as various machining methods, there are many different cutting tools.
Turning is the basic method of machining, which at the same time well characterizes the generally understood cutting process. The constant technological progress causes changes in the selection of the values of the machining parameters. Table 4 shows the preset values of the dependent feed rate and the depth of cut depending on the type of machining.
|Type of turning:||Feed rate
|Depth of cut
|very rough||>0,7||8 – 20|
|rough||0,5 – 1,5||6 – 15|
|slightly rough||0,4 – 1||3 – 10|
|moderately accurate||0,2 – 0,5||1,5 – 4|
|finishing||0,1 – 0,3||0,5 – 2|
|very precise||0,05 – 0,15||0,25 – 2|
The presented algorithm of conduct depending on the supplier of cutting tools may be slightly different in details. All kinds of catalogs constitute the support of the technologist in the first selection. Even correctly selected machining parameters are subject to further optimization. Modern CAM systems in the development of the machining program also select machining parameters. Frequently, the technologist makes many iterations and analyzes of many variants of the same technological process.
- SANDVIK Coromant information materials.
- Teaching materials – Cracow University of Technology.
- Author’s own notes.