The production process of mechanical manufacturing involves a lot of mechanical processing, especially the size, shape, position accuracy and surface morphology of metal cutting products are related to tools. The performance, quality and management of cutting tools directly affect whether the required qualified products can be successfully processed, the processing rhythm and production efficiency, the control and reduction of manufacturing costs, and the core competitiveness of manufacturing enterprises, which is more prominent in automatic mechanical processing.
(1) New Features of Development and Application of NC Machine Tool
Modern machining and its tool technology have developed rapidly in recent years, and high-tech has been widely used. Modern machining is quite different from traditional machining in processing technology, cutting mode, tool structure, tool material and surface engineering. First, flexible machining production lines are widely used, high-efficiency and high-speed cutting is adopted, and various CNC machining center machines are widely used. Accordingly, superhard tools such as CBN (cubic boron nitride), PCD (polycrystalline diamond) tools and new cemented carbide tools are widely used. Various new coatings obtained from the latest development of surface engineering and tribology technology have been applied to tool surfaces, and new tool structures and blades with new cutting edges and geometries have been continuously researched and developed. Tool clamping and its connection with machine tools, such as HSK tool holder, hot sleeve tool holder, hydraulic tool holder and other new tool holders, are widely used to meet the needs of CNC machine tools and high-speed machining. In order to improve production efficiency and reduce tool changing time, more and more compound tools are used in the processing of automobile parts, such as drilling and boring compound, drilling and hinge compound, drilling and threading compound and so on. The deployment tools with complex structure meet many special processing requirements, and some tools have even developed into mechanical, electrical and hydraulic integrated devices, which have gone far beyond the traditional concept of tools. Tools need to be maintained, pre-adjusted and tested, tool life needs to be controlled, and a perfect system and a series of management are needed to ensure that the production line can get enough tools in time, and can get quick response and support when processing problems or tool problems occur, so as to quickly analyze and solve problems and make production run normally. Manufacturing costs including tool costs should have sufficient market competitiveness, which puts forward new requirements different from the past.
Facing the rapid development of machining and tool technology, the challenge of flexible and efficient production of CNC equipment, the increasingly fierce market competition and the pressure of reducing manufacturing costs, tool management has become a hot spot of increasing concern in all walks of life.
(2) The influence of cutting tools and their management on production efficiency
The performance and quality of the cutter directly affect whether the required qualified products can be successfully processed, whether the cutter can meet the requirements of high-speed cutting, how long the life of the cutter is, and the frequency of cutter replacement directly affects the operating rate, processing rhythm and production efficiency of the production line. Whether the adjusted or ground cutter can be provided to the production line on time and with good quality directly affects whether the production can be carried out normally. Flow production is widely used in machinery industry, and the production of the previous process directly affects the production of the next process. In order to improve production efficiency and reduce production cost, a large number of combined tools and non-standard tools are used. Therefore, if a key tool, especially a non-standard tool, is not supplied on time, it will cause the whole circuit to stop production, just like the failure of a component in a series circuit. If there is no emergency measures or quick response, it may also cause the assembly line or even the automatic assembly line to stop production.
(3) Definition of service life of tools
After sharpening, the cutting time from the start of cutting to the wear amount reaching the dulling standard is called the tool service life, which is expressed by t, and it refers to the net cutting time, excluding the non-cutting time for tool alignment, measurement, fast forward and tool withdrawal.
It is also useful to use the cutting distance lm before reaching the passivation standard. Define the service life of the tool. Lm is equal to the product of cutting speed Vc and tool service life (time) t, that is
lm=Vc T (6—3)
It should be pointed out that in the past, the tool service life defined above was called tool durability. In the past, tool service life and tool durability had different meanings. Tool service life refers to the total cutting time before new tools (such as welding turning tools or twist drills) are scrapped, including multiple regrinding. Therefore, the tool life is equal to the tool durability multiplied by the number of regrinding times, but according to the current recommended standard spirit, it should be called the total tool life.
The service life of tools is very important data. When cutting the same workpiece material under the same conditions, the cutting performance of different tool materials can be compared by the service life of the tool. The same tool material can be used to cut a variety of workpiece materials, and the machinability of workpiece materials can be compared by the service life of the tool. Tool life can also be used to judge whether the geometric parameters of the tool are reasonable. The performance of workpiece material and tool material has the greatest influence on the service life of tool. Among cutting parameters, cutting speed is the most important factor affecting the service life of tools, followed by feed and cutting depth. In addition, the tool geometric parameters also have an important influence on the tool service life.
(A) the relationship between cutting speed and tool life
The relationship between cutting speed and tool life is obtained by experimental method. Before the experiment, select the grinding standard of the tool flank. In order to save materials and reflect the wear strength of the tool under normal working conditions, according to the provisions of IS0, when the middle part of the main cutting edge is evenly worn, the passivation standard is VB = 0.3mm；; When the wear is uneven, take VB max = 0.6 mm [106].
After selecting the passivation standard, it is only necessary to change the cutting speed (e.g. V=Vc 1, Vc2, Vc3, Vc4, …). ) When other cutting conditions are fixed, the tool wear curves at various speeds are obtained (Figure 6-11); Then, the tool service life T 1, T2, T3, T4, … etc. The corresponding cutting speed is calculated according to the selected grinding standard VB. Then determine on logarithmic coordinate paper (T 1, VC1); (T2，Vc2)； (T3，Vc3)，(T4，Vc4)； ... equipotential lines (Figure 6- 12). In a certain cutting speed range, these points are basically distributed in a straight line. This straight line on the log-log coordinate graph can be expressed by the following equation:
lgVc=-mlgT+lgA
Where m = tgφ, that is, the slope of the straight line; A is the intercept of a straight line on the ordinate when T= 1s (or 1min). Both m and a can be measured from the diagram. Therefore, the relationship between VC and t (or T-VC) can be written as follows:
Vc=A/Tm (6—4)①
or
(z = 1/ m) (6-5)
(2). Relationship between feed, cutting depth and tool life.
According to the method of finding the VC-T relationship, the relationship between F-T and AP-T can also be found:
f=B/Tn (6—6)
ap=C/Tp (6—7)
Where b and c are coefficients;
N, p index.
Synthesizing Equation 6-4, Equation 6-6 and Equation 6-7, we can get the three-factor formula of tool life:
(6-8a)
or
(6-8b)
Where CT and CV are the coefficients related to workpiece materials, tool materials and other cutting conditions;
Exponent XV = m/p and yv = m/n.
For different workpiece materials and tool materials, under different cutting conditions, the coefficients and exponents in Equation 6-8 can be found in Reference [73]. In fact, Equation 6-8 is a prediction equation of tool life or cutting speed under a certain tool life. For example, when the carbide cylindrical turning tool is used to cut carbon steel with σ b = 0.75 GPA (75 kgf/mm2), when f > 0.75 mm/r, the empirical formula is
As can be seen from the above formula, cutting speed has the greatest influence on tool life, followed by feed rate and cutting depth. Therefore, when optimizing cutting parameters to improve productivity, the selection sequence should be as follows: first, try to choose a larger cutting depth ap, then choose the allowable maximum feed f according to machining conditions and requirements, and finally choose the maximum cutting speed Vc when the tool life or machine power allows.
As can be seen from the above formula, cutting speed has the greatest influence on tool life, followed by feed rate and cutting depth. Therefore, when optimizing cutting parameters to improve productivity, the selection sequence should be as follows: first, try to choose a larger cutting depth ap, then choose the allowable maximum feed f according to machining conditions and requirements, and finally choose the maximum cutting speed Vc when the tool life or machine power allows.
(C) the hump of the T-VC relationship
Equation 6-5 shows that the empirical formula of T-VC relation is only applicable in a certain cutting speed range. If the tool life experiment is carried out in a wide cutting speed range, the obtained T-VC curve is often not a monotonous function, but a hump curve (Figure 6- 13). In the lower speed range, when Vc increases, T not only does not decrease, but increases. When a certain speed is reached, t has a maximum value. As the speed continues to increase, t monotonically decreases. The descending part of the corresponding curve is the effective speed range of Taylor formula. Similarly, the LM-VC relationship also has a hump.
The tool at the hump has the longest service life or the longest cutting distance. Can we say that the cutting speed here is the "optimal cutting speed"? No, the cutting speed here is low, and the removal rate of gold chips is also low, which is often of no practical value in production. In general production, the cutting speed on the right side of the hump is usually selected.
(4), the distribution of tool life
The lower surface introduces the tool service life distribution under normal wear conditions. When the workpiece, cutter and cutting conditions are fixed, the service life of the cutter is not constant. If cutting experiments or machining are repeated, the service life of the tool will change according to certain rules within a certain range. Because the manufacturing quality, microstructure, mechanical (mechanical) properties, geometric parameters, grinding quality, machine tool movement and other technological conditions of workpiece and tool materials are randomly changed. The change of various factors can not affect the service life of the tool, so the service life of the tool is also a random variable. The research of mathematical statistics shows that under certain cutting conditions, the change law of tool life obeys normal distribution or lognormal distribution [146].
The probability density function of normal distribution is
Where μ is the average value;
σ-standard deviation.
The probability density function of lognormal distribution is
(when T>0)
Where μ is the position parameter,
σ-scaling parameter.
The author turned 38CrNi3MoVA quenched and tempered steel with P 10 cemented carbide blade, and took AP = 1mm, F = 0.2mm/r, VC = 150m/min, grinding standard VB = 0.2mm ... and repeated cutting for 60 times with 60 cutting edges. Count the frequency of tool life in different intervals and draw the tool life distribution curve (Figure 6- 14). After testing, it is considered to obey the normal distribution. μ= 15.284,σ= 1.34。
Mastering the life distribution of tools is of guiding significance to production. In modern machining, it is necessary to manage the service life of tools scientifically, change tools regularly and understand the distribution of tool service life. For example, under the cutting conditions used in Figure 6- 14, the probability that the P 10 tool is required to meet the tool service life T≥ 12mm is
This probability shows that the P 10 tool can meet the machining requirements of this process.
In the process of NC machining, the quality of tools and effective management in the process of tool use play a vital role in machining quality. In the field of automobile manufacturing, the quality process control system (TS 16949 quality certification system) also has very specific and clear requirements for tooling management. HARDINGE VT2 CNC vertical lathe is produced by American HADRGINE Company, and its CNC system is FANUC 18T system. This paper introduces how to use the tool life management function in FANUC 18T system to manage the use of tools to ensure the quality of processed products. 1 Add tool life management function In order to manage tool life, numerical control equipment is needed to realize automatic counting of tool usage times.
The above selection is taken from tool management.
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