By referring to the literature, we can search for a lot of information about milling as a machining method. This article addresses contemporary aspects of this type of machining. In addition to machining with conventional parameters (conventional and CNC machines) I referred to high-speed machining and machining of difficult-to-cut materials. Currently, milling works as an alternative method for machining holes, threads, pockets and surfaces that previously required turning, drilling and tapping.
In the case of high-speed machining (high speed machining / HSM / HSC), milling plays the dominant role, and turning is much less common. The definition of high-speed machining is multi-criteria and I described it in the article HSM – High Speed Machining – basics. Milling is one of the types of machining in which multi-edge tools are used. This type of processing is used for machining (fig.1 – selected basics types):
- thread cutting,
- gear wheels,
- shaped surfaces, including free shapes (free-form surfaces).
Criteria for milling selection
Figure 2 shows the basic criteria for milling division.
Types of milling
The above division is a classic way of presenting types of milling machining. The development in the field of tool materials, machine control systems, and the design of machine tools itself resulted in unavoidable development in the field of this type of machining. Nowadays, adopting a technological criterion, milling is distinguished:
- profile and rolling,
- groove and thread,
- dedicated methods.
Figure 3 shows the division into out-cut and in-cut milling according to a kinematic criterion.
In the case of out-cut milling, the cutting layer at the beginning of the insertion of the tool. As the blade penetrates, the thickness of the cutting layer increases to its maximum at the exit of the milling cutter from the workpiece material. Thus, the cutting resistance at the beginning of machining is smaller and the highest when the blade comes out of the material. In out-cut milling, the cutting edge of the blade rubs against the machined surface, and as a result of this friction, with increasing cutting forces, friction significantly affects the tool wear process.
In the case of in-cut milling, the thickness of the cut layer is the largest when the cutter blade sinks into the workpiece, and the smallest when the blade leaves the material. As a consequence, cutting resistance decreases as machining progresses. With in-cut milling, no friction occurs before machining begins. A positive phenomenon is the collection of chips behind the cutter, which does not impede work and cooling. As a result, the treated surface is smoother than out-cut milling. The durability of the tool can be limited as a result of reduced machine rigidity, play in the kinematic system – feed movements.
Quality parameters (geometric accuracy and surface roughness)
Below (Table 1) the achieved roughness and geometric accuracy classes for selected types of milling are presented. Non-HSM data is typical for classical machining and has been used for many years in the engineering training process. They do not differ from reality, but it should be noted that in many cases technological progress is mainly of a qualitative nature and the currently achieved qualitative effects may be better than those declared below.
|Type of milling machining:||Ra||Grade
|End mill machining – rough||25||12 (11)|
|Slab mill machining – fine||3,2||11 (10)|
|Slab mill machining – finishing||1,6 (0,8)||8 (7)|
|Slotting mill machining – rough||6,3||12 (11)|
|Slotting mill machining – fine||3,2||11 (10)|
|Slotting mill machining – finishing||1,6 (0,8)||8 (7)|
|HSM mill dedicated||0,2 (0,1)||6 (5)|
Obtaining the 5th grade of machining accuracy, roughness at the level of Ra 0.1 even in the case of high-speed machining is the need to combine several factors and is not a standard option. This, however, shows the possibilities of this type of machining.
The main and general division of milling machines as machine tools is the orientation of the spindle (main drive): horizontal (figure 4) and vertical (figure 5).
Horizontal milling machining centers are used for machining workpieces with a larger mass and dimensions. This is due to the easier chip evacuation when machining the pocket. The design of these machine tools is characterized by the main drive body with a smaller mass which plays a role during acceleration and braking. For horizontal machining centers, pallet systems have been and are popular. These types of machine tools primarily use end-mill and face milling cutters. The disadvantage of horizontal milling machining centers is the surface they occupy. They are not large-scale machine tools (figure 6) although the ideas of horizontal spindle orientation are often used.
Vertical machining centers dedicated to machining small objects can be successfully introduced through typical 90 cm wide doors. Heavy machining centers with large dimensions ensure high rigidity, and thus also machining stability. The workpiece is fixed on the machining table or today more often on a pallet as part of a pallet system. Stability of machining affects the quality of the machined surface but when it is disturbed it significantly shortens the tool life.
Conventional milling machines (figure 6) are still being manufactured, often equipped with measuring scales and digital position reading.
From a structural point of view, machine tools for large-scale machining (fig. 7) constitute a separate area of issues.
High Speed Machining
HSM – High Speed Machining – the basics are the result of the qualitative nature of technological progress and significantly expanded our technological capabilities, including about machining of hard and difficult-to-cut materials. In my articles I often deal with issues related to HSM.
Another article about milling: Milling – selection of machining parameters.
Machining, including the variety discussed, is the dominant production technique in the broadly understood machine industry.
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