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).
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.
The technological parameters of milling include:
- rotational speed n [rpm];
- diameter of the tool Dc [mm];
- cutting speed vc [m / min] – equation 1;
- feed speed vf [mm / min] – equation 2;
- feed per revolution ff [mm / rev];
- feed per blade / tooth fz [mm / tooth] – equation 3;
- width ae and depth of cut ap [mm].
Table 1 presents the basic computational relationships.
The width and depth of milling
Depending on the used cutter, it is necessary to correctly interpret the width ae and the depth of the cutting ap. This is shown in figure 3. In the following case, face milling (figure 3), the cutting width ae is equal to the diameter of the tool Dc.
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 ae and workpiece width in the case of machining the entire entire surface.
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.
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.
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.
|Workpiece material||ISO marking||Sample material:|
|stainless steel||M||austenitic stainless steel|
|cast iron||K||grey cast iron, ductile cast iron|
|heat-resistant alloys||S||high-alloys based on iron, nickel, cobalt
|hardened steel||H||hardened and tempered steel|
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.
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.
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.
- Materials by SANDVIK Coromant
- ISCAR materials
- SECO materials
- Didactic materials of Warsaw University of Technology and Rzeszów University of Technology
- Author’s own notes.