Machine tools are operating machines designed for processing of metallic and non-metallic materials by chip removal. Modern machine tools, thanks to the high power and sturdiness with which they are equipped, allow the use of high cutting speeds, compatibly with the characteristics of the materials that make up the tools.
Chip removal machine tools can be of two categories depending on whether their main movement be it cutting or advancing. You have so:
- rectilinear motion machines, such as planers, shapers, grinders [fig. 1], slotting machines, some types of gear cutting machines and broaching machines;
- rotary motion machines with chip separation by non-rotating tools such as lathes and machines that use rotating tools such as boring machines, drills, the milling machines and the disk saws.
Chip removal: temperatures in cutting
As in all metal working processes where plastic deformation is involved, the energy dissipated in the cut is converted into heat which, in turn, increases the temperature in the cutting area. The increase in temperature is a very important factor in processing due to its main adverse effects, such as the following:
- Excessive temperature reduces resistance, hardness, rigidity and wear resistance of the cutting tool; the instruments can also soften and undergo plastic deformations; therefore, the shape of the tool is changed.
- The increase in heat causes irregular dimensional changes in the part being processed, making it difficult to control dimensional accuracy and tolerances.
- An excessive increase in temperature can cause thermal damage and metallurgical changes in the machined surface, negatively affecting its properties.
Since the sources of heat generation in the processing are concentrated in the primary cutting area and in the chip-tool interface, it is foreseeable that there are high temperature gradients in the cutting area.
Tools and materials
Chip removal by machine tools happens through special tools, which can be single-edged or single-cutting or multiple cutting edge.
The most common types of tools used in machinery for chip removal are:
- helical drill bit;
- single-cutting tool for the lathe;
- milling cutter for the milling machine;
- grinding wheel for the grinding machine;
- filing tool for the filing machine;
- planing tool for the planer.
During their use, the tools are subjected to various stresses, which can also be of considerable intensity: consequently they must be constructed of material with high hardness at high temperatures and high mechanical strength. The materials most used in the construction of the cutting edge of the tools are high-speed steels, sintered metal carbides (Widia) and the reinforced ceramic. The performance of the tool during machining varies greatly depending on the material with which it is made.
High speed steel tools (HSS)
High speed steel tools (HSS) are so called because they were developed to work at higher speeds than was previously possible. Produced for the first time in the early 1900s, high-speed steels are the most highly alloyed tool steels. They can be hardened at various depths, have a good resistance to wear and are relatively inexpensive. Because of their toughness (hence high fracture resistance), high speed steels are particularly suitable for (a) high positive rake angle tools (ie those with small angles included), (b) interrupted cuts, (c ) machine tools with low rigidity subject to vibrations, and (d) complex, monoblock instruments, such as drills, reamers, taps and cutters. Their most important limit (due to their low hot hardness) is that their cutting speeds are low compared to those of carbide tools. There are two basic types of high speed steels: molybdenum (M series) and tungsten (T series).
Sintered metal carbides (or hard metals)
In the first decades of the last century, the iron and steel industries developed techniques to produce extremely hard materials through a particular process (sintering) that allowed mixing a metal, called a matrix, with very hard and very fine carbide particles. The best known of these metals, commonly known as hard metals, is Widia, produced by the German industry Krupp. Hard metals are mainly used for the production of tool cutting edges suitable for machining operations that require materials of great hardness, such as those for chip removal, because they allow higher processing speeds and resist higher temperatures than the same high speed steel, compared to which, however, have less tenacity.