Tool life (machining)

The tool life is an important tool life parameter in machining , it indicates the time that a cutting tool can actually be used until it has to be reground or replaced. Only the time that the tool actually performs cutting work is measured, workpiece change times and other times during which the tools are not used do not count towards the service life. In modern industrial production, typical downtimes are 15 to 30 minutes. Further stand sizes are the stand quantity, distance and volume. They only apply to certain stand conditions . The most important are the material to be machined, the material of the tool (so-called cutting material ) and the cutting speed . The service life is a criterion for assessing the machinability of materials. In principle, increasing cutting speeds reduce tool life, processing time and proportionate machine and wage costs. However, the tool costs and the number of necessary tool changes increase. With the Taylor line named after him, Frederick Winslow Taylor found a simplified mathematical relationship between the tool life and the cutting speed in the form with the constants and . With it it is possible to find optimal tool life and cutting speeds that either lead to minimal unit costs or minimal processing times. ${\displaystyle T}$ ${\displaystyle v_{c}}$ ${\displaystyle T=c_{v}\cdot v_{c}^{k}}$ ${\displaystyle c_{v}}$ ${\displaystyle k}$

Typical values ​​of the service life

With CNC machines , which typically have short set-up times , the downtime is 15 to 30 minutes. For machines with medium set-up times, however, they are around 60 minutes. These include automatic turret lathes . For machines with long set-up times such as cam-controlled automatic lathes or transfer lines , it is 240 minutes (4 hours).

The service life and the various other stand sizes are always assigned associated stand criteria and stand conditions. for example, means that the service life until a wear mark width of 0.2 mm is reached at a cutting speed of 200 m / min is equal to 60 minutes. ${\displaystyle T_{VB0,2;vc200}=60\ min}$ ${\displaystyle T}$ ${\displaystyle VB}$ ${\displaystyle v_{c}}$

Influences

Schematic progression of wear during machining over time

Influences of cutting speed and material on tool life

Influences on the service life

A distinction can be made between two groups of sizes that influence tool life: those that can no longer be changed during machining and those that can be changed. The first group includes the cutting material, the material, the tool geometry , the cooling lubricant and the dynamic behavior of the tool, workpiece and machine tool. The cutting speed, the feed rate and the cutting depth are adjustable.

The time course of the wear variables is often determined experimentally and shown in wear-service life curves. At the beginning, a strong increase in wear can be determined, but this increases more slowly over time ( degressive wear). After that there is a longer range in which the wear is approximately proportional to the time (linear range), and finally increases more and more ( progressive range). This test is carried out particularly for hard cutting materials such as hard metal , cutting ceramics and boron nitride . These are the cutting materials for which the temperature does not lead to the tool stopping, but increasing wear.

The temperature-service life test, on the other hand, is carried out when the thermal load causes the tool to soften, which is the case with tool steels from 300 ° C and high-speed steel from 600 ° C. At these temperatures the cutting edges soften and lead to so-called bright braking .

Influences of tool life on other variables

As the tool life increases, the tool costs and the tool changing costs decrease, since fewer tools are used and therefore have to be changed less frequently. With given tools, however, longer service lives are also associated with lower cutting speeds and thus higher machining times. Especially the main time is increasing. Associated with this, the proportionate wage and machine costs also increase.

Tool life calculation

Taylor straight line (red) and real tool life curve (black)

Frederick Winslow Taylor found out around 1900 that with given workpiece-material pairings and given tools, the cutting speed has the greatest influence on the tool life. If the values ​​from the tool life-cutting speed tests are entered in a double-logarithmic diagram , an approximately linear curve results in a wide range, which is approximated by the Taylor straight line. She has the shape

${\displaystyle v_{c}\cdot T^{-{\frac {1}{k}}}=c_{T}}$ or

${\displaystyle T=c_{v}\cdot v_{c}^{k}}$.

With:

${\displaystyle v_{c}}$ - cutting speed

${\displaystyle k}$ - Constant indicating the slope of the straight line

${\displaystyle c_{v}}$ - theoretical tool life at a cutting speed of one meter per minute

${\displaystyle c_{T}}$ -Theoretical cutting speed for a tool life of one minute.

In the double logarithmic system, the relationship applies

${\displaystyle \log T=k\cdot \log v_{c}+\log c_{v}.}$

The following relationship exists between the constants:

${\displaystyle c_{v}={\frac {1}{c_{T}^{k}}}}$

The constants can be read from tables or determined experimentally. The steeper the straight line of a certain material / cutting material combination, the more sensitive the tool life is to changes in the cutting speed. If there are several straight lines for different materials for a tool, they are shifted to the left and right against each other. With hard, particularly wear-resistant cutting materials, it runs flatter. The straight line for flank wear as a standstill criterion is flatter than that for crater wear. If both signs of wear occur, then flank wear dominates at low cutting speeds and crater wear at high speeds.

However, the Taylor straight line only offers sufficiently accurate results within certain limits. At low cutting speeds it deviates from the real curve because of the built- up edge formation , at high cutting speeds because of increasing wear. It is therefore necessary in practice to know these limits in order not to obtain unrealistic values. On the other hand, their simple mathematical handling is advantageous. In addition, they can be determined very easily experimentally in practice, since only two points on the straight line need to be known.

After Taylor discovered the basic relationships, they were made accessible for practical application through further work by Max Kronenberg . He also showed that applying the similarity mechanics leads to the same results.

The Taylor straight line is also referred to as a simple tool life function, since it only contains the cutting speed as a variable. The extended shape also takes into account the influence of the feed rate and the cutting depth : ${\displaystyle f}$ ${\displaystyle a_{p}}$

${\displaystyle T=C\cdot v_{c}^{k}\cdot f_{z}^{k_{fz}}\cdot a_{p}^{k_{a}}}$

with as tooth feed (feed per tooth) and the constants and . A slightly different representation is ${\displaystyle f_{z}}$ ${\displaystyle k_{fz}}$ ${\displaystyle k_{a}}$

${\displaystyle T=c_{v}\cdot v_{c}^{k}\cdot c_{f}\cdot f_{z}^{k_{fz}}\cdot c_{a}\cdot a_{p}^{k_{a}}}$

In industrial practice, however, when optimizing the cutting values, first the cutting depth and then the feed are selected so that these are already fixed, which allows the use of the simple equation.

optimization

Since a longer tool life or a lower cutting speed has a positive influence on some costs and a negative one on others, a U-shaped total cost curve results depending on the tool life or cutting speed. Therefore, the Taylor line can also be used to determine optimal values ​​for the total costs. A distinction is made between several objectives:

• Minimizing manufacturing costs
• Minimizing production time

Under normal circumstances the cost will be optimized. However, the tool, labor and machine costs must be known for this. If there are production bottlenecks, the shortest production time is sought. It is made up of the main time during which the tool is doing work and the tool change time , which is used to change the worn tools. ${\displaystyle t_{H}}$ ${\displaystyle t_{W}}$

The manufacturing costs result from

${\displaystyle K_{F}=K_{ML}\cdot t_{H}+{\frac {t_{H}}{T}}\cdot (K_{ML}\cdot t_{W}+K_{WT})}$.

This includes the machine and labor cost rate , i.e. the cost of personnel and machines per hour, and the tool costs per tool life. The minimum is then there ${\displaystyle K_{ML}}$ ${\displaystyle K_{WT}}$

${\displaystyle T_{ok}=-(k+1)\left(t_{W}+{\frac {K_{WT}}{K_{ML}}}\right)}$.

The time-optimal service life results from the same approach

${\displaystyle T_{ot}=-(k+1)\cdot t_{W}}$.

The time-optimal tool life is generally associated with higher costs than the cost-optimal tool life. So both goals cannot be achieved at the same time.

Tool life variation

The actual idle times are subject to a certain spread. This arises even if the process parameters are precisely adhered to. The scatter can be traced back to geometric deviations in the raw parts and chemical or physical deviations in the properties of the material or cutting material.

See also

• Cutting force
• Energy conversion and heat during machining

literature

• Alfred Herbert Fritz, Günter Schulze (ed.): Manufacturing technology. 11th edition. Springer Vieweg, Berlin / Heidelberg 2015, ISBN 9783662465554 .
• Berend Denkena, Hans Kurt Tönshoff: Machining - Basics. 3. Edition. Springer, Berlin / Heidelberg 2011, ISBN 978-3-642-19771-0 .
• Fritz Klocke, Wilfried König: Manufacturing process 1 - turning, milling, drilling. 8th edition. Springer, 2008, ISBN 3-540-23458-6 .
• Herbert Schönherr: Machining production. Oldenbourg, Munich / Vienna 2002, ISBN 3-486-25045-0 .
• Heinz Tschätsch: Practice of machining technology. 7th edition. Vieweg.

Individual evidence

1. ^ ^{A b} Alfred Herbert Fritz, Günter Schulze: Manufacturing technology. 11th edition. Springer, 2015, p. 289 f.
2. ↑ Heinz Tschätsch: Practice of machining technology. 7th edition. Vieweg, p. 27.
3. ↑ Berend Denkena, Hans Kurt Tönshoff: Spanen - basic . 3. Edition. Springer, 2011, p. 149.
4. ^ Fritz Klocke, Wilfried König: Manufacturing process 1. 8th edition. Springer, 2008, p. 263.
5. ^ Fritz Klocke, Wilfried König: Manufacturing process 1. 8th edition. Springer, 2008, p. 262 f.
6. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 50 f.
7. ^ ^{A b} Alfred Herbert Fritz, Günter Schulze: Manufacturing technology. 11th edition. Springer, 2015, p. 307.
8. ↑ ^{a b} Fritz Klocke, Wilfried König: Manufacturing process 1 - turning, milling, drilling. 8th edition. Springer, 2008, p. 379.
9. ↑ Berend Denkena, Hans Kurt Tönshoff: Spanen - basic. 3. Edition. Springer, 2011, pp. 149–152.
10. ^ Alfred Herbert Fritz, Günter Schulze: Manufacturing technology. 11th edition. Springer 2015, p. 306.
11. ↑ ^{a b} Fritz Klocke, Wilfried König: Manufacturing process 1 - turning, milling, drilling. 8th edition. Springer, 2008, p. 265.
12. ^ Alfred Herbert Fritz, Günter Schulze: Manufacturing technology. 11th edition. Springer, 2015, p. 307 f.
13. ^ Alfred Herbert Fritz, Günter Schulze: Manufacturing technology. 11th edition. Springer, 2015, p. 309.
14. ↑ Berend Denkena, Hans Kurt Tönshoff: Spanen - basic. 3. Edition. Springer, 2011, pp. 153–156.