Machinability

The cutting is a term used in manufacturing technology and describes the complex behavior of a material during the machining. It represents one of the most important characteristics of engineering materials represents the DIN 6583 defines the machinability as " [ ... ] the property of a workpiece or material, to let or machined under given conditions ."

By machining, such as turning, milling and drilling, workpieces are changed in shape. Abrasive material show smooth surfaces after machining, resulting chips that do not hinder the production process, the cutting forces are low and the life of the tools high. Depending on the production method can be advantageous or disadvantageous quite different behavior.

• 3.1 cutting speed and feed
• 3.2 Depth of cut
• 3.3 Coolant

• 4.1 steels 4.1.1 carbon content
• 4.1.2 structural constituents of the steel

Factors

The machinability of a material depends on many parameters. Mostly, strength and toughness are important factors. For example, materials with high strength is less easy to machine, because it requires higher cutting forces and hence higher energy for machining. Strain hardening negatively affects the machinability, since the deformed during the cutting zone and thus the chips harden and break later. In addition, the solidified material increases the cutting forces in front of the cutting edge. One speaks of the built-up edge.

Other factors, such as the thermal conductivity of the material are important. Therefore, materials are of low thermal conductivity, such as plastics, less easy to machine, because the frictional heat can not be dissipated quickly enough.

The quantitative evaluation of the machinability is difficult because the machinability depends not only on the material, but also on the cutting conditions. The cutting conditions are the geometry and material properties of the tool, the machine settings, such as cutting speed and feed rate. The use of cooling and lubricating fluid also has an influence. Furthermore, the different cutting method require different cutting conditions.

Quantify the machinability

Not only the determinants of machinability are complex, and the possibility to quantify the machinability itself is not trivial. Thus there are several variables which can be used to quantitatively describe the machinability.

Tool life

As a tool life of a tool is defined as the distance that can chipping a material under given conditions a tool before it must be replaced. If the tool life of a drill 100 meters, this means that you can drill in the relevant material for the condition are 1000 holes of 10 cm length with this drill. Industrially, the tool life is thus important that he has an important significance to the maintenance intervals for the machine, and use of tools and thus on the costs of processing.

However, the tool life is not an absolute measure of the machinability of a material, because it depends not only on the material but also on the cutting conditions and the tool.

Tool wear

Another criterion for assessing the machinability of tool wear. He has a direct influence on the service life. However, the cutting force is influenced by the tool wear because a worn blunt tool requires a higher cutting force. The surface finish of the material decreases with increasing tool wear. As a measure of tool wear, the wear mark width is used for a specific Zerspanpensum.

Cutting force

The cutting force is on the one hand for the efficiency of the cutting process is important because it is directly related to energy consumption. But the tool wear is a function of the cutting force.

Surface finish

An important criterion of quality of the finished workpiece is its surface quality. Thus the surface finish is also a criterion for the machinability. As values ​​for the surface finish the usual roughness parameters are used.

Chip shape

The chip shape allows direct conclusions on the cutting process, which affects the tool wear and surface finish. What is desired is an optimum balance between short and compact chips that allow easy removal and long, even chips that allow a higher surface quality of the workpiece. If the chip is too long, consisting for example when drilling is a risk that the chips get stuck and block the chip removal, causing tool breakage or at least to increased wear on the tool. Chips that are aufwendeln spiral cheaper than what that fold leporelloförmig, as the latter involve a high Verklemmrisiko.

Influence of cutting conditions

Cutting speed and feed

In principle, it is desirable to machine the highest possible cutting speeds and feed rates fast. Characterized cycle times can be minimized. However, poor machinability sometimes requires a drastic reduction of this rate parameter when excessive speed would have an unacceptably high tool wear and thus a low tool life and poor surface quality result.

Depth of cut

The cutting depth is determined in the face turning of the delivery, the grooving of the cutting width. In the pre-turning the depth of cut for economic reasons ( larger chip volume ) should be chosen as large as possible. It is limited by the performance of the rotary machine, the size of the insert and of the stability of the workpiece. When you finish turning the cutting depth corresponds to the allowance.

Coolant

All assessment criteria of machinability can be improved by the use of a cooling lubricant. As the name suggests, are the main tasks of the coolant cooling and lubrication of the cutting. The cooling causes the tool and workpiece is not locally heat up too much. Due to this reduced cutting temperature, there is less wear and tear. Greasing leads by enabling lower cutting forces also less wear and less energy consumption. In addition, the lubrication improves the surface roughness.

As coolant often comes an emulsion of water and oil are used. Because of their white appearance, this is popularly called " Bohrmilch ".

Machinability of different materials

Steels

Steels are a very complex material system. According to the complexity of the influence of parameters on the machinability. There are only the most important are described below:

Carbon content

Steels with low (< 0.2%) carbon content have a high proportion of ferrite. Ferrite is a relatively soft phase of the steel, but has a high potential of strain hardening. Therefore, at very low carbon contents, the risk of construction cutting. Steels with high carbon content, which have a pearlite, pose in terms of machinability is the problem that the carbides due to their high hardness lead in the pearlite to high wear and thus to lower level distances and lower surface qualities. The best are steels with medium carbon content (0.2% -0.5 %) to machine.

Structural constituents of the steel

In steel, there are the following important structural constituents: ferrite, austenite, pearlite, martensite and bainite. The individual structural constituents have an important influence on the machinability of the steel.

Ferrite is the softest fabric component, which also has a high ductility. The high ductility is problematic because it allows long chips. Furthermore, tends ferrite during machining for adhesion, which in turn affects interfere with the cutting process.

Perlite is a mixed structure of ferrite and eutectoid cementite. The latter is hard and brittle and thereby worsened the machinability pearlitic steels for a through increased cutting forces and increased wear. On the other hand, the brittleness of the pearlite leads to earlier chip breaking. As a result, there is less body shorter chips and cutting; a desirable effect on machinability.

Austenite is very malleable because of his face-centered cubic lattice and has a high work hardening potential. This results similar to the ferrite at a high inclination to the cutting structure and a high load on the cutting tool. Also the tendency to adhesion is strong. Further, the lower thermal conductivity of the austenite has an adverse effect on the machinability.

Martensite is the hardest and most brittle phase of the steel. This martensitic steels are difficult to machine. The abrasive wear and thermal loading of the tool are very high.

Bainite called the intermediate structure between pearlite and martensite. Depending on the transition temperature, it occurs in different variants. One speaks of Upper bainite ( perlitnah ) and Low bainite ( martensitnah ). With respect to machinability similar applies respectively for perlite as martensite.