Milling in relation to turning is characterized by much greater variation. The following are the basic descriptions and dependencies of the machining parameters related to milling. This is the first of a series of articles about machining parameters.

This method is a machining in which the main rotary motion is made by a tool and the feed movement performs the workpiece. This machining process is characteristic of conventional milling machines (fig. 1).

Fig.1 Tool milling machine JFN 36 from JAZON.

In the case of CNC machining centers (fig. 2) one should speak about the relative motion between the tool and the workpiece. The work table moves with the workpiece and the carriage moves with the main drive spindle (with the tool) in relation to the work table. 5-axis CNC machining centers simultaneously use the carriage movement with the spindle and the movement of the working table.

In this machining method, the tool’s work is not continuous. The tool is a rotating solid body. Depending on the location (workmanship) of the blades, such cutters as: plain milling cutters, end milling cutters, face mill. Conical, spherical and shaped tools are also made. On the cylindrical surface, the blades can be made as straight or helical.

Non-continuous machining means that only part of the cutter blades are working at the same time. On the one hand, this is a beneficial feature due to cooling conditions. On the other hand, the thickness of the cutting layer (cross-section of the cutting layer) is variable, which affects the course of the cutting process. As a consequence, the load changes and the durability of the tool decreases.

Technological parameters

The technological parameters of milling include:

• rotational speed n [rpm];
• diameter of the tool D_c [mm];
• cutting speed v_c [m / min] – equation 1;
• feed speed v_f [mm / min] – equation 2;
• feed per revolution f_f [mm / rev];
• feed per blade / tooth f_z [mm / tooth] – equation 3;
• width a_e and depth of cut a_p [mm].

Table 1 presents the basic computational relationships.

Table 1.
Number:	Technological parameter:	Equation:	Component description:
(1)	cutting speed vc		Dc – diameter of the tool n – rotational speed
(2)	feed speed vf		ff – feed per revolution n – rotational speed
(3)	feed per tooth fz		ff – feed per revolution n – rotational speed vf – feed speed

Fig. 2. CNC milling center (HASS – Abplanalp trade fair stand).

The width and depth of milling

Depending on the used cutter, it is necessary to correctly interpret the width a_e and the depth of the cutting a_p. This is shown in figure 3. In the following case, face milling (figure 3), the cutting width a_e is equal to the diameter of the tool D_c.

Fig. 3. On the left side face milling, on the right side – interpretation of the milling width and depth.

In the case of face milling the following types are distinguished: – figure 4: a.) full, b.) incomplete symmetrical c.) incomplete asymmetrical which results from the ratio of milling cutter diameter to cutting width a_e and workpiece width in the case of machining the entire entire surface.

Fig. 4. Face milling: a.) full, b.) incomplete symmetrical and c.) incomplete asymmetrical.

The presented above-mentioned issues do not fully cover the subject of milling. One of the technological parameters is the tool path. In the case of milling, the use of CAM software plays an important role.

Selection of machining parameters

The choice of machining parameters (fig. 5) is similar to turning.

Fig.5 Diagram of selection of machining parameters for milling.

Differentiation of milling varieties prevents clear definition of general values of machining parameters. An example is a high-speed machining, whose defining process is multifactorial and in addition variable over time.

Workpiece material

In order to facilitate the selection of parameters, the ISO material groups are used. The same division applies to all machining methods. There are 6 material groups – table 2.

Catalogs, whether in paper or electronic versions (eg Sandvik Coromant or ISCAR or SECO), constitute a significant user support in making the first choice and matching the tools and parameters to the individual technological task.

Tool path

The tool path in the case of surface machining with complex shapes (fig. 6) and also when machining pockets is a key issue of the technological operation. Milling is characterized by the necessity of dynamic changes of machining parameters during machining. CAM software plays a very important role in this machining method.

Fig. 6. Example of machining surfaces with a more complex shape – MAZAK CNC center.

Power and torque

When milling, the main drive power requirements depend on factors such as:

• the amount of material to be removed;
• average chip thickness;
• tool (insert) geometry (cutter);
• cutting speeds.

In the case of high-performance machining (HPM), which involves removing as much material as possible in a unit of time, more power is needed. The relatively low speed of the main drive for roughing requires a certain power and torque. If the main propulsion power is too low and the moment is not correct, we will have to deal with the loss of machining stability (chip of varying thickness).

Contemporary development trends lead to an increase in the range of available ranges and maximum rotational speeds. Spindles are driven directly. Consequently, less torque is obtained at higher speeds and less power at lower speeds of the main drive.

Machine tools with very high speeds (e.g. for high speed machining) are not recommended for roughing (high performance) where larger milling cutters (low speed and high power) are used.

For high speed machining, smaller cutter diameters, small depth and cutting width with high working feed are used.

Sources

• Materials by SANDVIK Coromant
• ISCAR materials
• SECO materials
• Didactic materials of Warsaw University of Technology and Rzeszów University of Technology
• Author’s own notes.